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I've always wondered why cells have only one nucleus, as having multiple would seemingly prevent mutation. Are there examples of organisms with multiple nucleuses? If not, is there a reason?
Are there examples of cells with more than one nucleus?
Yes, they are called Multinucleate cells. There are two types of multinucleated cells
I highly recommend having a look at this answer for the definitions.
Examples of Syncytia include
- Skeletal muscle fibers (thanks @kmm)
Examples of Coenocytes include
- Codium (Thanks @GerardoFurtado; see picture below)
- Blastoderms early in the development of a fruit fly
Are there examples of organisms with multiple nucleuses?
Side note: The plural of nucleus is nuclei
In many fungi, during sexual reproduction, a fusion of cytoplasm happen early in the mycelium but a fusion of the nucleus happens only very late (just before sporulation). This is a type of Coenocytic mycelium. In these species, non-negligible fractions of their cells are multinucleated.
There are endosymbiotic and endoparasitic eukaryotes in other eukaryotes that would result in a cell containing several nuclei but that would not count I would guess as the nuclei belong to different species.
Picture of Codium. The entire algea is a single multinucleated cell.
There is a branch of life called the Diplomonads, most of which have two nuclei. They are single cell organisms and an early offshoot of the eukaryotic linage. A good example is Giardia lamblia. https://en.wikipedia.org/wiki/Diplomonad Giardia lamblia
According to this article The hairy beast with seven fuzzy sexes
Tetrahymena thermophila has two: a large macronucleus and a small micronucleus. The macronucleus controls the everyday functions of the cell, while the micronucleus deals with its complicated sex life. In fact this is true for all ciliates.
When a "slime mold" enters the "plasmodium" phase n cells merge together to form one cell with n nuclei. This means plasmodiums can have thousands or tens of thousands of nuclei…
Skeletal muscles (striated) are multi-nucleated-have more than one nucleus per cell as they are formed as a result of the fusion of myoblasts.
Reference- Skeletal Muscle: Form and Function (2006) By Brian R. MacIntosh, Phillip F. Gardiner, Alan J. McComas
The cell is the basic functional and structural unit of all living organisms. It is often referred to as the ‘building block of life.’ Robert Hooke discovered the cell in 1665. Although the natural living cells exist on this planet for at least 3.5 billion years, scientists have made efforts and succeeded in synthesizing a cell synthetically. This article is about one of the recent developments in synthetic biology named synthetic cells. Let us start from basics about cells and develop the concept of artificial cells. Let’s see if it is possible to make cells synthetically.
The cell is the organization of many biomolecules (proteins, lipids, sugars, nucleic acids, etc.), organelles, and cytoplasm( which consists of water as its primary component) within a membrane. These are a lot of terminologies. Let’s break the cell into its parts and read about them. It will help us to know the requirement to make cells artificially and create life.
18 Questions on the Function of the Cell Nucleus
The mains elements of the nucleus are chromatin (made of DNA molecules), the nucleolus, the karyolymph, or nucleoplasm, and the nuclear membrane (or karyotheca).
More Bite-Sized Q&As Below
2. Do all eukaryotic cells have only one nucleus?
Some eukaryotic cells do not contain a nucleus and others contain more than one. For example, osteoclasts, the cells responsible for resorption of bone matrix, are multinucleate cells, meaning they have more than one nucleus. Striated muscle fibers are also multinucleate. Red blood cells are an example of enucleated (no nucleus) specialized cells.
Chromatin, Heterochromatin and Euchromatin
3. What substances is chromatin made of?
Chromatin is made of DNA molecules bound to proteins called histones.
Cell Nucleus Review - Image Diversity: chromatin
4. What are heterochromatin and euchromatin?
Chromatin is uncondensed nuclear DNA, the typical DNA morphology during interphase (the phase of the cell cycle in which the cell does not divide). During this phase of the cell cycle, which is the chromatin can be found as heterochromatin, more condensed portion of DNA molecules and which appears darker under electron microscopy, and as euchromatin, which is less condensed and composes the lighter portions of DNA molecules.
Since it is less condensed, euchromatin is the biologically active part of the DNA, that is, the region that contains active genes to transcribe into RNA. Heterochromatin makes up the inactive portions of the DNA molecule.
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5. What is the relationship between the concepts of chromatin and chromosomes? Are euchromatin and heterochromatin a part of chromosomes?
Every filament of chromatin is a complete DNA molecule (a complete double helix), or rather, a complete chromosome. A DNA molecule may contain euchromatin and heterochromatin portions and, as a result, both are a part of chromosomes.
6. During the phase when the cell is not dividing (interphase), is there activity within the cell nucleus?
During interphase, there is intense metabolic activity in the cell nucleus: DNA is duplicating, euchromatin is being transcripted and RNA is produced.
7. How are the concepts of chromosomes, chromatin and chromatids related? During which phase of the cell cycle does DNA replicate?
Chromatin is a set of filamentous DNA molecules dispersed in the karyoplasm, made up of euchromatin and heterochromatin portions. Each chromatin filament is a complete chromosome (a DNA molecule, or double helix). The chromatin of the human somatic cell is formed by 46 DNA molecules (22 homologous chromosomes and 1 pair of sex chromosomes).
During interphase, the cell prepares itself for division and the duplication of DNA molecules occurs. The duplication of every DNA molecule forms two identical DNA double helices bound by a structure called the centromere. During this phase, each identical chromosome of these pairs is called a chromatid. It is also during interphase that chromatids begin to condense, taking on the thicker and shorter shape typical of chromosome illustrations. Therefore, the phase of the cell cycle during which DNA duplicates is interphase.
Some biology textbooks refer to a chromosome as a unique filament of chromatin as well as the condensed structure made of two identical chromatids after DNA duplication. The pair of identical chromatids bound in the centromere is always made up of two copies of the same chromosome, therefore, they are two identical chromosomes (and not only one).
8. What structure maintains the binding of identical chromatids?
The structure that maintains the binding of identical chromatids is the centromere.
9. What is the name given to region of the chromosome where the centromere is located? How are chromosomes classified in relation to the position of their centromere?
The region of the chromosome where the centromere is located is called primary constriction. Under a microscopic view, this region is narrower (a stricture) than most parts of the chromosome.
Depending on the position of the primary constriction, chromosomes are classified as telocentric, acrocentric, submetacentric or metacentric.
10. What are the primary and secondary constrictions of a chromosome? What is the other name given to the secondary constriction?
Primary constriction is the narrower region of a condensed chromosome where the centromere, the structure that binds identical chromatids, is located. The secondary constriction is a region similar to the primary constriction, narrower than the normal thickness of the chromosome, and which is normally related to genes that coordinate the formation of the nucleolus and which control ribosomal RNA (rRNA) synthesis. For this reason, secondaryonstrictions (there can be one or more in a chromosome) are called nucleolus organizer regions (NOR).
11. What are homologous chromosomes? Which human cells do not have homologous chromosomes?
Chromosomes contain genes (genetic information in the form of nucleotide sequences) that control protein synthesis, thus regulating and controlling cell activities. In the nuclei of somatic cells of diploid beings, every chromosome has its corresponding homologous chromosome, both of which contain alleles of the same genes related to the same functions. This occurs because one chromosome of one pair comes from the father and the other comes from the mother of an individual. Chromosomes that form a pair with alleles of the same genes are called homologous chromosomes. In humans, there are 22 pairs of homologous chromosomes plus one pair of sex chromosomes (sex chromosomes are partially homologous).
The only human cells that do not have homologous chromosomes are gametes, as during meiosis, the homologous chromosomes are separated.
Karyotypes and Genomes
12. What is the difference between karyotype and a genome?
A genome is the set of DNA molecules that characterizes each living being or each species. This concept includes the specific nucleotide sequence of the DNA molecules of each individual or species. A karyotype is the set of chromosomes of a given individual or species, and focuses on the number of pairs of chromosomes as well as their morphology.
13. Can two normal individuals of the same species with sexual reproduction have identical genomes and identical karyotypes? How is the human karyotype usually represented?
Except for clones (individuals created from nucleus transplantation, like Dolly the sheep) and monozygotic twins, it is very improbable that the genomes of two individuals of the same species generated by sexual reproduction will be identical. Nevertheless, the karyotypes of two normal individuals of the same species and of the same sex are always identical. The normal human karyotype is represented by the formula 44+XX for women and 44+XY for men.
Alosomes and Ploidy
14. What is the other name given to sex chromosomes? What is the function of sex chromosomes?
Sex chromosomes are also called allosomes (other chromosomes that are not sex chromosomes are called autosomes).
Sex chromosomes take their name from the fact that they have genes that determine the sex (male or female) of an individual. Sex chromosomes also contain genes related to other biological functions.
15. How many chromosomes does a normal human haploid cell have? How many chromosomes does a normal human diploid cell have? How many sex chromosomes do each of them contain?
The human haploid cell is the gamete (egg cell and sperm cell). The human gamete has 22 autosomes and 1 allosome, i.e., 23 chromosomes. The diploid cell is the somatic cell and it has 44 autosomes and 2 allosomes, i.e., 46 chromosomes.
Gametes have one sex chromosome and somatic cells have two sex chromosomes.
16. Do phylogenetically close species have cells with similar chromosome counts?
The number of chromosomes typical of each species is similar for phylogenetically close species (for example, orangutans, gorillas, chimpanzees and humans). However, it is not impossible for evolutionarily distant species, such as rats and oats, to have similar karyotypes and the same total number of chromosomes.
Even if they present equal number of chromosomes, evolutionarily distant species have radically different characteristics, since the quantity and the sequence of nucleotides that make up their DNA molecules are quite different.
The Nucleolus and the Nuclear Membrane
17. What is the nucleolus?
The nucleolus is a small and optically dense region in the interior of the cell nucleus. It is made of ribosomic RNA (rRNA) and proteins. One nucleus can have one or more nucleolus.
18. What structures make up the nuclear membrane?
Eukaryotic cells have a nucleus that is enclosed by two juxtaposed membranes that are a continuation of the membrane of the endoplasmic reticulum. The nuclear membrane, or karyotheca, contains pores through which substances pass. In addition, its external surface contains ribosomes.
Now that you have finished studying Cell Nucleus, these are your options:
Cell is the smallest unit of an organism that can function independently. All living organisms are made of cells and nothing less than a cell can truly be said to be alive. Some microscopic organisms, such as bacteria and protozoa, are single cells whereas animals and plants are composed of many millions of cells built into tissues and organs. Although viruses and cell-free extracts are able to perform many individual functions of a living cell, they lack the capacity shown by cells of independent survival, growth, and replication, and are therefore not considered to be living. Biologists study cells to learn how they are made from molecules and how individual cells cooperate to make an organism as complex as a human being. Before we can fully understand how a healthy human body functions, how it develops and ages, and what goes wrong with it in disease, we need to understand the cells of which it is made.
II HISTORICAL ORIGINS OF CELL BIOLOGY
Lymphocytes, or white blood cells, are produced in the marrow of bones. The cells are largely responsible for controlling infection within the body, directly attacking antigens, or foreign substances, in the tissues or circulatory system. Following organ transplants, lymphocytes often attack transplanted tissues, causing the transplant to be rejected.
The idea that cells are the fundamental building blocks of living organisms&mdashsometimes termed &ldquothe cell theory&rdquo&mdashis now universally accepted as true. However, this concept did not spring into existence fully formed, as the result of a single discovery. It took many years for the present view of cells to emerge, and there were many false turns and misapprehensions on the way. The word &ldquocell&rdquo comes from the Latin cellula, meaning a small room or cubicle, and was first used by Robert Hooke in his book Micrographia, published in 1665. Hooke was describing the air-filled spaces of dead cells in a slice of cork (bark from an oak tree) and certainly did not realize the general importance of his discovery. Nor did many other talented microscopists of the 17th and 18th centuries, such as Antoni van Leeuwenhoek, Nehemiah Grew, Marcello Malpighi, and Jan Swammerdam, who also saw cells in plant or animal tissues, or as free-living organisms. Indeed it was not until 1839 that the combined insight of a botanist, Matthias Schleiden, and a zoologist, Theodor Schwann, led them to pronounce that &ldquo&hellipall organisms are composed of essentially like parts, namely of cells&rdquo.
The cell theory was still far from complete and many curious notions remained, for example, about the origins of cells. It required the work of many other biologists, such as Bartholemy Dumortier and Robert Remak, to establish the fact that all cells are produced as a result of the division of existing cells. This notion, which has powerful implications for both cell biology and the origins of life, was famously articulated by German biologist Rudolph Virchow in the phrase &ldquoOmnis cellula e cellula&rdquo, that is, &ldquoall cells come from cells&rdquo.
Even then, many misconceptions still existed regarding the nature of the cell membrane, the cytoplasm, and the hereditary material, and it was not until well into the 20th century that the contemporary view of cells emerged. Indeed, even today, the amazingly complex internal structure and chemistry of living cells is still not fully understood and contains many secrets yet to be discovered.
III LOOKING AT CELLS UNDER THE MICROSCOPE
The invention of the microscope in the 17th century made cells visible for the first time and, for hundreds of years afterwards, all that was known about cells was discovered using this simple device. Light microscopes are still crucial to the work of cell biologists, although they have improved out of all recognition from the primitive instruments used by Hooke and Leeuwenhoek. Contemporary light microscopes incorporate sophisticated state-of-the-art devices, such as laser light sources, fluorescent optics, and computer-assisted image processing, which reveal detail at the very limit of resolution (down to 0.1 microns or micrometres (µm), each µm being a millionth of a metre). For even higher magnification, electron microscopes, invented in the 1930s, extend this limit by using beams of electrons instead of beams of light as the source of illumination. They greatly extend our ability to see the fine details of cells, and even make some of the larger molecules visible, although they cannot be used with living specimens.
What do you see if you look at cells under a light microscope? If you examine a very thin slice of a suitable plant or animal tissue, for example, you will see it is divided into thousands of small cells. These may be either closely packed or separated from one another by a material known as the extracellular matrix. Each cell will be about 20 µm in diameter. Under the right conditions, the cells in your section will show signs of life, with particles moving around inside them and individual cells slowly changing shape and dividing.
To see more of the internal structure of a cell you need to use special tricks, since cells are not only small but also transparent and colourless (being about 70 per cent water). One approach is to stain the cells in your section with dyes or specific molecular probes that colour particular components. Alternatively, you can exploit the fact that cell components differ slightly from one another in refractive index (just as glass differs from water) and these small differences can be made visible by means of special lenses. In either case, the contrast and resolution of the image can be stored, enlarged, and further enhanced by electronic processing.
IV GENERAL FEATURES OF CELLS
An animal cell typically contains several types of membrane-bound organs, or organelles. The nucleus controls the cell&rsquos activities and stores the genetic information that is carried from generation to generation. The mitochondria generate energy for the cell. Proteins are manufactured by the ribosomes that sit on the rough endoplasmic reticulum. The Golgi apparatus packages, distributes, and exports lipids and proteins, while lysosomes store enzymes for digesting food. The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm.
The microscope shows us that cells exist in many different sizes and shapes. Some of the smallest bacterial cells are short cylindrical objects less than 1 µm in length. At the other extreme, nerve cells have complex shapes including many long thin extensions, and may reach lengths of several metres (those in the neck of a giraffe provide a dramatic example). Between these extremes, plant cells are typically 20-30 µm long, polygon-shaped with box-like boundaries defined by rigid cell walls. Most cells in animal tissues are compact in shape, 10-20 µm in diameter with an irregular and often richly folded surface.
Plant cells contain a variety of membrane-bound structures called organelles. These include a nucleus that carries genetic material mitochondria that generate energy ribosomes and rough endoplasmic reticulum that manufacture proteins smooth endoplasmic reticulum that manufactures lipids used for making membranes and storing energy and a thin lipid membrane that surrounds the cell. Plant cells also contain chloroplasts that capture energy from sunlight and a single fluid-filled vacuole that stores compounds and helps in plant growth. Plant cells are surrounded by a rigid cell wall that protects the cell and maintains its shape.
Despite their many differences in appearance and function, all cells have a surrounding membrane (termed the plasma membrane) enclosing a water-rich substance called the cytoplasm. All cells carry out multiple chemical reactions that enable them to grow, produce energy, and eliminate waste, together termed metabolism (from a Greek word meaning &ldquochange&rdquo). All cells contain hereditary information, packed into a central nucleus and encoded in molecules of deoxyribonucleic acid (DNA), which directs the cell's activities and enables it to reproduce, passing on its characteristics to its offspring. These and other similarities too numerous to mention, and including many identical or nearly identical molecules, demonstrate that all modern cells are related to one another. In other words, there must have been an unbroken continuity between modern cells&mdashand the organisms they compose&mdashand the first primitive cells that appeared on Earth.
There is nothing in living organisms, or the cells from which they are made, that contravenes chemical and physical laws. However, the chemistry of life is based overwhelmingly on carbon compounds and depends almost exclusively on chemical reactions that take place in aqueous solution and in the relatively narrow range of temperatures experienced on Earth. The chemistry of living organisms is also very much more complicated than any other chemical system known. It is dominated and coordinated by enormous polymeric molecules (macromolecules) made from chains of linked chemical subunits. The unique properties of macromolecules endow cells with all of the properties we recognize as living, such as the ability to grow, to move, to reproduce, and to respond in an informed way to changes in their environment.
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) (see Nucleic Acids) are the information-carrying macromolecules that encode the complex form and composition of an organism and allow it to be perpetuated from one generation to another. Unique among molecules, they provide directions for their own replication, thereby providing a basis for the continuity of life. DNA molecules are extremely long polymers made from four nucleotide bases &mdashadenine, cytosine, guanine, and thymine&mdashoften represented by the letters A, C, G, and T, in a linear sequence. A typical chromosome in a human cell, for example, consists of two strands of DNA, each of which might contain 50 million to 100 million bases in a unique and precisely determined sequence. The cell has 46 chromosomes of this kind and the same set of chromosomes, each with an essentially identical base sequence, can be found in every one of the other hundreds of millions of cells in the body. One of the most exciting developments in recent years has been the elucidation of the complete sequence of nucleotides in the DNA of many organisms, including humans (see Human Genome Project).
RNA molecules are built to a similar plan as DNA, but they are much shorter and slightly different chemically their four bases are adenine, cytosine, guanine, and uracil rather than thymine (represented by A, C, G, and U, not T). RNA molecules are made as copies of selected regions for the DNA, usually for the purpose of making protein.
If we liken DNA to the program, or software, of the cell, then proteins are its hardware: the physical bricks from which the cell is built. Protein molecules perform a bewildering variety of functions. As well as providing building blocks, proteins also act as enzymes to catalyse the myriad reactions inside a cell. Proteins embedded in the plasma membrane form channels and pumps that control the passage of small molecules into and out of the cell. Some proteins carry messages from one cell to another while others act as signal integrators that relay sets of signals from the plasma membrane to the nucleus of individual cells. Yet others serve as tiny molecular machines with moving parts that propel organelles through the cytoplasm or untangle knotted DNA molecules. Highly specialized proteins act as antibodies, toxins, hormones, antifreeze molecules, elastic fibres, ropes, or sources of bioluminescence.
In chemical terms, proteins are long linear polymers of amino acids joined head to tail by peptide bonds. In contrast to DNA and RNA, which are made of nucleotide subunits that are chemically very similar to each other, proteins are built up from an assortment of 20 amino acids that differ greatly in their chemical &ldquopersonalities&rdquo. Each chain of amino acids folds into a particular shape, or conformation, in which some amino acids are buried on the inside, and other are exposed to the surrounding water. It is the chemistry of amino acids on a protein surface that specify its interactions with other molecules and determine its function, as an enzyme, or structural protein, or whatever. The sequence of amino acids in a protein, on which its properties depend, is itself specified by the sequence of nucleotide bases in one particular region of the DNA. A complicated machinery inside the cell first copies this region of DNA into a smaller RNA molecule, and then uses this RNA to direct amino acids to the machinery that links them together, in the correct sequence.
B Prokaryotes and Eukaryotes
A fundamental division, both in size and in internal organization, exists between prokaryotic cells and eukaryotic cells. Prokaryotic cells, found only in bacteria and cyanobacteria (formerly known as blue-green algae), are relatively small (1-5 µm in diameter) and simple their genetic material (DNA) is concentrated in one region of the cytoplasm but no membrane separates this region from the rest of the cell. Eukaryotic cells, from which all other living organisms are made, including protozoa, plants, fungi, and animals, are much larger (typically 10-30 µm in linear dimension) and their genetic material is enclosed by membrane, forming a conspicuous spherical body termed a nucleus. In fact, the name 'eukaryotic' comes from Greek words meaning 'true kernel, or nucleus' 'prokaryotic' means 'before nucleus'.
A thin oily skin termed the plasma membrane encloses the contents of all living cells, defining the boundary between the contents of the cytoplasm and the surroundings. The plasma membrane is a continuous layer of lipid and protein molecules about 5 nanometres (nm) thick that acts as a selective barrier to regulate the cell's chemical composition. When lipid molecules such as phospholipids and cholesterol are mixed with water they spontaneously form thin sheets, or bilayers, that close up into vesicles. Water-soluble ions and molecules are unable to cross a bilayer unaided, but can cross if specific carrier proteins or channels made of protein are embedded in the lipid. In this way the cell is able to maintain concentrations of such ions and small molecules that are different from those of their surroundings. A different mechanism, involving small membrane vesicles that add to or bud from the plasma membrane, allows animal cells to secrete or uptake macromolecules and even particles the size of bacteria across their membranes.
Many other proteins are present in the plasma membrane, the actual mixture being a distinctive feature of a particular cell type. Membrane proteins can work as enzymes to catalyse specific reactions or serve to link the membrane to the matrix of proteins on the outside of the cell, or to the cytoskeleton on the inside. Still other membrane proteins function as receptors that detect substances such as growth factors in the cell&rsquos environment and signal their presence to the cytoplasm or nucleus.
The cells of bacteria, yeasts, fungi, and plants usually have, in addition to a plasma membrane, a relatively thick and sturdy cell wall made of polysaccharides (predominantly cellulose in the case of higher plants). The cell wall, which is external to the plasma membrane, maintains the shape of the cell and protects it from mechanical damage, but it also restricts the movements of the cell and limits the entry and exit of materials.
Although animal cells lack a rigid cell wall, they often secrete a tough &ldquoexoskeleton&rdquo which can have a major influence on the form of the cell. In most tissue of the human body, for example, cells are enclosed in an extracellular matrix composed of tough fibrous proteins such as collagen and variable amounts of proteoglycans, made of proteins linked to long, highly charged polysaccharides. The extracellular matrix is especially abundant in connective tissues, where it forms the basis of bone and cartilage, but can also be found in endothelia, epithelia, nerve, and muscle.
Nucleus of a Cell
The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, the genetic blueprint of the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, the nucleus contains a cellular material called nucleoplasm. Nuclear pores, around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm.
The most conspicuous organelle in most plant and animal cells is the nucleus, typically a membrane-enclosed, roughly spherical body about 5 µm in diameter. Within the nucleus, molecules of DNA and proteins are organized into long chromosomes, which usually occur in identical pairs. Chromosomes are too stringy and intertwined to be identified separately unless the cell is dividing. Just before a cell divides, however, its chromosomes become condensed and thick enough to be seen as separate structures. The DNA inside each chromosome is a single very long, highly coiled molecule containing a linear sequence of genes. Genes contain the coded instructions for the assembly of RNA molecules and proteins needed to produce a functioning copy of the cell.
The nucleus is enclosed in a two-layered membrane and interaction between the nuclear contents and the cytoplasm is permitted through holes, called nuclear pores, in this membrane. A specialized region, the nucleolus, is the site of assembly of particles containing RNA and protein, which migrate through the nuclear pores to the cytoplasm and are modified to become ribosomes.
The nucleus controls protein synthesis in the cytoplasm by sending molecular messengers in the form of RNA. This messenger RNA (mRNA), as it is called, is made in the nucleus according to instructions in the DNA, after which mRNA conveys the messages to the cytoplasm via the nuclear pores. Once in the cytoplasm, the mRNA attaches to ribosomes and a genetic message is translated into the primary structure of a specific protein.
This freeze-fracture transmission electron micrograph of a yeast cell, Rhodosporidium toryloides, shows a number of organelles suspended in its cytoplasmic matrix: a dark, round lipid body occupies the bottom of the cell, with the large nucleus above and to the right, and a curved mitochondrion at the top of the cell. High-voltage magnification reveals that the cytoplasm, here a viscous gel, contains a three-dimensional lattice of protein fibres. Called the cytoskeleton, these filaments interconnect and support the &ldquosolid&rdquo substances mentioned above.
The entire volume of a cell, excluding the nucleus, is called its cytoplasm, which includes many specialized structures and organelles, as described below. The concentrated aqueous solution in which these organelles are suspended is termed the cytosol. This is a water-based gel containing a host of large and small molecules and in most cells it is by far the largest single compartment (in bacteria it is the only intracellular compartment). The cytosol is the site of many of the most important housekeeping functions of the cell, including the early stages of breakdown of food molecules and the synthesis of many of the large molecules from which a cell is built. Whereas many molecules in the cytosol exist in true solution, moving rapidly from one location to another by free diffusion, other molecules are more highly ordered. These ordered structures give the cytosol an internal organization that provides a framework for the manufacture and breakdown of large molecules, and they channel many of the chemical reactions of the cell along spatially restricted pathways.
A network of protein fibres criss-crosses the cytoplasm of eukaryotic cells, providing shape and mechanical support. The cytoskeleton also functions as a monorail to transport substances around the cell. A cell such as an amoeba changes shape by dismantling parts of the cytoskeleton and reassembling them in other locations.
The cytoskeleton is a cohesive meshwork of protein filaments extending throughout the cytoplasm of plant and animal cells. The cytoskeleton is the primary determinant of cell shape and movement and to this end it operates according to its own functional 'logic' or set of rules. From a mechanical standpoint, the cytoskeleton acts like a set of struts and girders that support the form of the cell, constraining movements according to engineering parameters, such as elasticity and bending modulus. From a biochemical standpoint, the cytoskeleton undergoes a repertoire of characteristic reactions that take place over and over again as living cells move. These reactions include the polarized growth and shrinkage of protein polymers, their association through multiple linking proteins into larger structures, and the directed motion of motor proteins walking along protein polymers.
In a typical animal cell, the peripheral region, just beneath the plasma membrane, consists largely of a dense three-dimensional meshwork of thin filaments. This mesh is composed of actin filaments (microfilaments), which are thin, flexible filaments with a diameter of about 8 nm made of the protein actin. Microfilaments are often cross-linked into a dense, three-dimensional weave and they can also be more regularly arranged into parallel bundles, such as those that form the core of filopodia on the cell surface. The cortical meshwork contains small amounts of other material, thought to represent soluble proteins of the cytoplasm, but relatively few membrane-bounded organelles. Large bundles of actin filaments are found in muscle cells where, together with the protein called myosin, they produce forceful contractions.
By contrast, the central region of most cells, close to the nucleus, has a more open construction with fewer actin filaments and a smaller number of long microtubules, interspersed by abundant granular material and membrane organelles, including mitochondria. Microtubules are long polymers of the protein tubulin that stand out because they are relatively thick and inflexible. In thin sections they are even more conspicuous because of their hollow cylindrical form: a microtubule has an outer diameter of 25 nm, and a central canal, or lumen, of about 15 nm diameter. Many cells possess flexible 'hairs' on their surface, called cilia or flagella, which contain a core bundle of microtubules capable of regular, energy-driven bending movements. Sperm cells swim by means of flagella, for example, and cells lining the intestine or other ducts in the vertebrate body carry fields of motile cilia on their surfaces that sweep fluids and particles in a specific direction.
A third category of protein filament, widely found in animal cells and often located in the central region together with microtubules, is intermediate filaments. Their distribution is often similar to that of the cell&rsquos microtubules, but they can be distinguished from the latter by their smaller diameter (about 10 nm) and absence of a hollow lumen. Intermediate filaments terminate on the matrix of protein that encloses the nucleus and radiate from there into the surrounding cytoplasm, often impinging on membrane-associated junctions with neighbouring cells. They are irregular, flexible ropes, composed of a diverse family of proteins and their principal function appears to be to confer mechanical strength to living cells.
Cell movements in eukaryotic cells almost always depend on actin filaments or microtubules. In broad terms, they are produced by one of two mechanisms. In the first, arguably the most primitive, the cell produces movement by controlling the polymerization of actin filaments or microtubules at specific locations. For example, the nucleation and growth of actin filaments at one location of a cell drives its plasma membrane forward, causing the boundary of the cell to extend over a solid surface. Similarly, the controlled polymerization of microtubules in a nerve cell axon helps to elongate and support the growing axon. The second general mechanism of movement is driven by motor proteins, which utilize the energy in ATP to move stepwise along a protein polymer. For example, growth and survival of the nerve axons just mentioned depends on motor proteins called dynein and kinesin. These move along microtubules inside the nerve axon, ferrying proteins, vesicles and organelles from the cell body to the synapses or in the reverse direction. Eukaryotic cells also contain a variety of myosins, which are motor proteins that move along actin filaments. Myosins not only transport materials such as RNA molecules and organelles from one location to another, but also are responsible for contractions and other movements of different parts of the cell (the best-known example being that of muscle cell contraction).
G Mitochondria and Chloroplasts
Mitochondria, minute sausage-shaped structures found in the hyaloplasm (clear cytoplasm) of the cell, are responsible for energy production. Mitochondria contain enzymes that help to convert food material into adenosine triphosphate (ATP), which can be used directly by the cell as an energy source. Mitochondria tend to be concentrated near cellular structures that require large inputs of energy, such as the flagellum, which is responsible for movement in vertebrate sperm cells and single-celled plants and animals.
Mitochondria are among the most conspicuous organelles in the cytoplasm, and, like the nucleus, they are present in essentially all eukaryotic cells. They have a very distinctive structure when seen in the electron microscope: each mitochondrion is usually sausage-shaped, several micrometres long, and enclosed in two separate membranes, the inner one being highly folded. Mitochondria are energy-producing organelles. Cells need energy to grow and replicate and mitochondria supply most of this energy by performing the last stages of the breakdown of food molecules. These stages involve the consumption of oxygen and the production of carbon dioxide the entire process is called respiration, because of its similarity to breathing. Without mitochondria, animals and fungi would be unable to use oxygen to extract the full amount of energy from the food they consume to fuel their growth and replication. Various organisms that live in environments that lack oxygen are said to be anaerobicand they all lack mitochondria.
An examination of leaves, stems, and other types of plant tissue reveals the presence of tiny green, spherical structures called chloroplasts, visible here in the cells of an onion root. Chloroplasts are essential to the process of photosynthesis, in which captured sunlight is combined with water and carbon dioxide in the presence of the chlorophyll molecule to produce oxygen and sugars that can be used by animals. Without the process of photosynthesis, the atmosphere would not contain enough oxygen to support animal life.
Chloroplasts are large green organelles that are found only in the cells of plants and algae but not in the cells of animals or fungi. They have an even more complex structure than mitochondria: in addition to the two surrounding membranes, they have multiple sacs in their interior formed from a membrane that contains the green pigment chlorophyll. From the standpoint of life on Earth, chloroplasts carry out an even more essential task than mitochondria: they perform photosynthesis&mdashthat is, they trap the energy of sunlight in chlorophyll molecules and use this energy to drive the manufacture of small, energy-rich, carbon-containing molecules. In the process they release oxygen. Thus chloroplasts generate both the food molecules and oxygen that mitochondria use.
Rough Endoplasmic Reticulum
The major site of protein synthesis within the cell is on the surface of the rough endoplasmic reticulum (RER). Characterized by a stacked, sheet-like appearance dotted with small dark structures called ribosomes, the RER synthesizes proteins on its outer surface, then secretes these proteins to the outside of the cell. The ribosomes dotting the surface of the RER are also sites of protein synthesis, however these proteins are retained within the cell to perform metabolic functions.
Nuclei, mitochondria, and chloroplasts are not the only membrane-bounded organelles inside eukaryotic cells. The cytoplasm also contains a complex profusion of other organelles, each enclosed by a single membrane, that perform many distinct functions. Most of these functions are concerned with the cells&rsquo need to import raw materials and export manufactured substances and waste products. Thus organelles of one class are enormously enlarged in cells that are specialized for secretion of proteins organelles of another class are especially plentiful in cells in higher vertebrates that capture and digest viruses and bacteria that have invaded the body.
The golgi apparatus, a minute cellular inclusion in the cytoplasm, is a series of smooth, stacked membranous sacs. The golgi apparatus directs newly synthesized proteins to the correct destination in the cell.
An irregular three-dimensional network of spaces enclosed by membrane, called the endoplasmic reticulum (ER), is the site at which most cell membrane components are made, as well as materials destined for export from the cell. Stacks of membrane-bounded flattened sacs constitute the Golgi apparatus, which receives the molecules made in the endoplasmic reticulum, processes them, and then directs them to various locations in the cell. Lysosomes are small, irregularly shaped organelles that contain stores of enzymes responsible for the digestion of many unwanted molecules in cells. Peroxisomes are small, membrane-bounded vesicles that provide a contained environment for reactions where dangerously reactive hydrogen peroxide is generated and degraded. Membranes form numerous other small vesicles of many different types involved in the transport of materials between one membrane-bounded organelle and another. In a typical animal cell the membrane-bounded organelles may occupy up to a half the total cell volume.
I Secretion and Endocytosis
One of the most important functions of vesicles is to carry materials to and from the plasma membrane, thereby providing a means of communication between the interior of the cell and its surroundings. A continual exchange of materials takes place among the endoplasmic reticulum, the Golgi apparatus, the lysosomes, and the outside of the cell. The exchange is mediated by small membrane-bounded vesicles that pinch off from one membrane and fuse with another.
At the surface of the cell, portions of the plasma membrane continually bud inwards and pinch off to form vesicles. These carry material captured from the external medium to the cell interior&mdasha process termed endocytosis. The reverse process, called secretion, which takes place when vesicles from inside the cell fuse with the plasma membrane and release their contents into the external medium, is also common in many cells.
Several kinds of endocytosis can be distinguished, the most dramatic being the ingestion of large particles such as bacteria and cell debris, called phagocytosis or &ldquocell eating&rdquo. Carnivorous amoebae perform phagocytosis as a means of feeding, whereas certain types of cells in the human body, such as macrophages, defend us against infection by ingesting micro-organisms that have invaded the body. An even more widespread form of uptake is termed pinocytosis, or &ldquocell drinking&rdquo. Most types of animal cells, for example, continually ingest fluid and bits of plasma membrane by means of small vesicles that bud inwards and are taken into the cytoplasm. The rate of uptake by pinocytosis can be surprisingly large, and some cells can ingest 10 per cent or more of their own volume every hour.
Finally, there is a much more selective kind of uptake that depends on the cell having specific types of receptors in its membrane. Receptor-mediated endocytosis provides a concentrating mechanism by which a cell can take up essential macromolecules from the surrounding fluid without having also to ingest a huge volume of fluid. This is the mechanism by which cholesterol, an essential component of membranes, is taken up from the bloodstream, and also the method of uptake of vitamin B12, iron, and other essential metabolites. Unfortunately, receptor-mediated endocytosis can also be exploited by viruses, and provides the route by which the influenza virus and HIV, which causes AIDS, gain entry into cells.
V CELL GROWTH AND DIVISION
This interactivity outlines the stages involved in mitosis, the division of a cell to produce two identical cells.
Living cells reproduce by carrying out an orderly sequence of events in which they duplicate their contents and then divide into two. In unicellular organisms, such as bacteria and yeasts, each cell division produces a complete new organism, whereas many rounds of cell division are required to make a new multicellular plant or animal from a single-celled egg. Once a plant or animal has reached its fully mature state, then many of its cells cease to divide and remain in their differentiated state for the life of the organism. However, there are also cells that die and need to be replaced. In adult humans for example, liver cells divide once every year or so, whereas some of the epithelial cells lining the gut and many of the blood cell precursors in the bone marrow divide more than once every day. In fact, each of us must manufacture many millions of cells every second simply to survive.
Cell division is at its simplest and most rapid in bacteria, which do not have a nucleus and contain a single chromosome. In the common gut bacterium Escherichia coli, for example, the whole cell cycle can take as little as 20 minutes under favourable growth conditions. Its circular chromosome, containing a single DNA molecule, is attached to the plasma membrane and remains attached while it replicates. The two new chromosomes then become separated as the cell grows. When the cell has approximately doubled in size, cell division occurs by simple fission as new cell wall and plasma membrane is laid down between the two chromosomes to produce two separate daughter cells.
Division in eukaryotic cells is significantly more complicated than in bacteria. The cells are many thousand times larger in volume and contain many organelles and other cytoplasmic structures, all of which have to be duplicated and then segregated into one of the two daughter cells. In particular, the DNA of eukaryotic cells is enormously long and requires an intricate intracellular apparatus for its maintenance and replication. For instance, all 46 chromosomes of a human tissue cell have to replicate individually and then physically move into one or another newly formed daughter cell. These elaborate rearrangements require many coordinated movements and major structural changes in the cytoskeleton.
The Cell Cycle
The cell cycle here shows the phases that a cell undergoes from the moment of its formation to its division into two daughter cells. It comprises interphase, when the cell is metabolically active, and mitosis, in which the cell gives rise to two daughter cells that are genetically identical to it and each other. The stages of mitosis are prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.
Progression from one cell cycle stage to the next is controlled, as if by clockwork, by a series of biochemical reactions. These occur in a single direction and if necessary can be halted at critical steps, sometimes called checkpoints. One point of no return, for example, is when the nuclear membrane breaks down and the cell becomes committed to division of the nucleus, known as mitosis. A second irreversible step is the separation of chromosomes, which marks the exit from mitosis. This ability to arrest the cycle is crucial for the cell, since it allows division to respond to external conditions, such as the presence of growth factors. It is also necessary to synchronize nuclear events with cytoplasmic events, for example ensuring that mitosis does not begin before the DNA has been completely copied.
Summarizing a great deal of complicated biochemistry, it can be said that progression through the cell cycle is driven by the episodic synthesis and precipitous breakdown of small proteins, called cyclins. The cyclins activate specific protein kinases, called cyclin-dependent kinases, which phosphorylate many different target proteins and thereby control events such as DNA synthesis and entry into mitosis.
As it grows and divides, a eukaryotic cell passes through a number of stages, driven by the biochemical clockwork just mentioned. These are characterized by distinct events affecting both DNA and the cytoskeleton. The complete sequence of stages is referred to as the cell cycle, since it repeats over and over for as long as cell division continues. Division of the nucleus, or mitosis, is actually a relatively short episode in the cell cycle. It is preceded by a prolonged interphase, during which the cell duplicates its contents, both nuclear and cytoplasmic, and is closely followed by cytokinesis&mdashthe process by which the two newly produced cells are physically separated. Mitosis itself is traditionally subdivided into a sequence of stages according to the behaviour of the chromosomes as seen in the light microscope, the principal stages being prophase, metaphase, anaphase, and telophase.
Sperm and eggs are produced by a special kind of cell division called meiosis, in which the number of chromosomes is precisely halved. The actual process is complicated and entails first the duplication of chromosomes, then the association of maternal and paternal chromosomes to form &ldquotetrads&rdquo (which allow the exchange of genetic material by chromosomal crossovers). Two successive divisions then produce the final haploid (that is, containing a single&mdashnot double&mdashset of chromosomes) egg or sperm cell.
Are there examples of cells with more than one nucleus? - Biology
According to cell theory, every living organism&mdashplant, animal, or otherwise&mdashis made of cells, and all cells arise from preexisting cells. The simplest organisms have one cell they are unicellular. More complex organisms are made of many cells they are multicellular. In this chapter, we will explore the structure of cells&mdashthe basic building blocks of life.
EUKARYOTIC CELL STRUCTURE
Look at the following diagram, and think of a cell as having three main areas: the cell wall and/or membrane, the cytoplasm, and the nucleus.
1. The cell wall and/or membrane forms the outer layer.
2. The cytoplasm or cytosol contains the organelles.
3. The nucleus, which is bounded by a nuclear membrane, contains chromosomes.
Notice that in the list, number one says “the cell wall AND/OR membrane.” It turns out that plants, bacteria, and fungi have both a cell wall and a membrane (the cell wall is the outermost portion), whereas animal cells have only a cell membrane.
More about Cell Walls
Plants, bacteria, and fungi all have cell walls, but their cell walls are made up of different substances.
• Plants have cell walls made of cellulose. Remember that cellulose is a polysaccharide.
• Bacteria have cell walls made of peptidoglycan. Peptido refers to protein, and glycan refers to sugar, so bacterial cell walls are made of protein and sugar.
• Fungi have cell walls made of chitin. Chitin is a polysaccharide that”s similar to cellulose.
More about the Cell Membrane
Remember: The cell membrane is made of
We learned about these molecules in the last chapter when we covered basic biological chemistry. The primary lipids found in cell membranes are phospholipids do you remember their special characteristics? If you don”t remember, go back and review them now.
Phospholipids have both polar and nonpolar regions and form lipid bilayers:
Cell membranes are just lipid bilayers. They make excellent barriers, because the inside of the cell is aqueous (watery), and the external environment of the cell is usually aqueous so in effect, the lipid bilayer cell membrane forms an “oily” layer between the inside of the cell and the outside of the cell. Substances that are “happy” in an aqueous medium (in other words, hydrophilic substances) do not like to cross the oily (in other words, hydrophobic) barrier.
However, substances do need to get in and out of the cell&mdashoxygen, carbon dioxide, glucose and other nutrients, and waste products, to name a few. The cell membrane lets some things through and restricts the passage of other things. For this reason, it is said to be selectively permeable.
Four Ways That Substances Can Cross the Cell Membrane
1. Simple Diffusion
Diffusion is simply the movement of a substance from an area of high concentration to an area of lower concentration until a dynamic equilibrium is reached. This is called “moving down a concentration gradient.” So if there is more of something on one side of the membrane (for example, inside the cell) and less of that same thing on the other side of the membrane (in this case, outside the cell), that substance will simply move across the membrane to the area of lower concentration (again, in this case, outside the cell). Basically, our example substance, whatever it is, relieves some of the crowding inside the cell by moving outside the cell.
Lipid Soluble Substances Only
Because the cell membrane is a lipid bilayer, simple diffusion works only if the substance in question is lipid soluble (hydrophobic) and can interact with that oily barrier. Some examples of substances that cross the cell membrane by simple diffusion are oxygen, carbon dioxide, and cholesterol.
2. Facilitated Diffusion
Facilitated diffusion is similar to simple diffusion, except that the molecules that cross are not hydrophobic. Because they are not hydrophobic, they cannot interact with the oily barrier and cannot simply cross the membrane. They need help. The proteins that make up the cell membrane can help move substances across the lipid bilayer. Because they help, or facilitate, the movement of substances across the membrane, this type of movement is called facilitated diffusion.
Comparisons: Facilitated vs. Simple Diffusion
Facilitated diffusion is like simple diffusion in one important respect: A substance crosses the membrane only if there is a concentration difference on either side of the membrane. Some examples of hydrophilic substances that cross the membrane by facilitated diffusion are sodium, potassium, calcium (in fact, all ions, because they carry a charge, must cross membranes by facilitated diffusion), and glucose. Also, both facilitated diffusion and simple diffusion are considered forms of passive transport since no energy is required on the part of the cell for the process to occur.
The proteins form specialized channels, sort of like pores, across the membrane. The channels are highly specific for particular substances&mdashfor example, a channel might allow sodium to cross but not potassium. Some proteins do not form pores but instead act as carrier molecules that bind to a substance and “pull” it through the membrane.
3. Active Transport
In active transport, the cell must expend energy to move something across the membrane. Active transport is different from simple and facilitated diffusion in this important respect: It can move a substance across the cell membrane from an area of low concentration to an area of higher concentration. That”s why it requires energy. Another way to describe active transport is to say that it moves substances against their concentration gradients. Simple and facilitated diffusion are considered passive processes because they move substances down their gradients, and this doesn”t require energy.
Active transport also relies on membrane proteins to move substances, and it doesn”t matter whether the substance is hydrophobic or hydrophilic.
4. Bulk Transport
Bulk transport is what its name implies: the movement of large, bulky items across the cell membrane. There are two possible directions to move the substances: into the cell, called endocytosis, and out of the cell, called exocytosis.
In endocytosis, the cell takes in some particle by surrounding and engulfing it within a pocket known as a vesicle. Two examples of endocytosis are phagocytosis (cell eating: amoeba surrounding a particle of food with its pseudopods a white blood cell ingesting a pathogen) and pinocytosis(cell drinking).
Exocytosis is the exact opposite. A particle in a cell (in a vesicle) is released to the outside by fusing the vesicle with the cell membrane.
Osmosis is simply the movement of water across a cell membrane, down its concentration gradient. The thing to remember, though, is that water”s concentration gradient is opposite to the solute (dissolved particles) concentration gradient. In other words, if there is a lot of some particular substance inside a cell and less of that substance outside the cell, then there is less water inside the cell and more water outside the cell. Basically, the substance takes up some room where water could be, so if there”s a lot of the substance present, then there will be less water present. In our example, water will want to move down its concentration gradient, into the cell.
Water is hydrophilic (seems obvious, doesn”t it?), so it must cross the membrane by facilitated diffusion. Cells have many water channels in their membranes that allow water to cross easily. This can cause problems if the cells are placed into solutions that are more or less concentrated than they are.
Think about a cell placed into a concentrated solution (a hypertonic solution). There are more particles outside the cell than inside, so there is less water outside the cell than inside. Water will move down its gradient, from inside the cell to outside the cell, and the cell will shrivel up.
Now think about a cell placed into a dilute solution (a hypotonic solution). There are now fewer particles outside the cell than inside, so there is more water outside the cell than inside. Water will move down its gradient, from outside the cell to inside the cell, and the cell will swell up and may burst.
If the cell is put into a solution that is exactly the same concentration as the cell itself (an isotonic solution), then the cell will neither shrivel nor swell. Human cells are isotonic (equally concentrated) to a 0.9% sodium chloride solution.
Quick Quiz #1
Fill in the blanks and check the appropriate boxes:
1. Animal cells [ />do />do not ] have cell walls.
2. Bacteria have cell walls made of _________________________.
3. Engulfing large particles in a vesicle is known as ________________.
4. Facilitated diffusion is a way for [ />hydrophobic />hydrophilic ] substances to cross the cell membrane.
5. Fungi have cell walls made of _________________________, and plants have cell walls made of _________________________.
6. Simple diffusion [ />does />does not ] require energy.
7. Hydrophobic substances cross the membrane by ________________.
8. A type of movement that requires energy and moves substances against their concentration gradients is called __________________.
9. A human blood cell placed in a 10% solution of sodium chloride solution will
10. Which term best describes a cell membrane?
Correct answers can be found in Chapter 15.
More About the Cytoplasm
The cytoplasm is a semiliquid goo that contains a eukaryotic cell”s organelles. The organelles perform specific functions for the cell. Note that some cells (bacteria) do not have organelles and are prokaryotic. We”ll talk more about bacteria in Chapter 10.
All of the organelles except the ribosome are bounded by a membrane. Two of the organelles (the nucleus and the mitochondria) are bounded by two membranes.
Study this picture of a eukaryotic cell and its organelles, as well as the list of organelle functions that below.
Quick Quiz #2
Match the organelle on the left with its function/description on the right.
A. cellular transport system
B. stores waste and other substances
C. selectively permeable barrier that regulates what enters and exits the cell
D. site of ribosome synthesis in the nucleus
E. related generally to formation of the spindle during mitosis
F. cellular respiration and ATP production has double membrane
G. holds ribosomes that synthesize membrane or secreted proteins
H. contain hydrolytic enzymes digest foreign substances and worn organelles
I. contains genetic material (DNA) control center of the cell
J. sites of protein synthesis
K. sorts and packages membrane and secreted proteins
Correct answers can be found in Chapter 15.
What Goes On in the Cytoplasm: Chemical Reactions and Enzymes
Thousands and thousands of different chemical reactions take place in the cytoplasm. Here”s an example of a very generic chemical reaction:
In the above reaction, molecules X and Y are the reactants and molecule Z is the product. However, the reaction won”t happen unless X and Y get together.
X and Y need a mutual “friend” that will help them get together in the same place at the same time so they can react and form Z. The mutual friend is an enzyme. Here”s a picture of an enzyme that would work nicely to get X and Y together:
Enzymes: If Ya Can”t Stand the Heat &hellip
One way to accelerate a reaction is to heat up the substances involved. The problem with this is that the heat can potentially cause unintended reactions, which in turn damage the structure of the cell. A more specific way to accelerate a reaction within a cell depends on compounds that speed up chemical reactions. These compounds are called catalysts. Chemical reactions in cells are usually sped up by specific catalysts called enzymes.
We”ll call the enzyme molecule E. If E is around, the reaction between X and Y occurs much more quickly than if E is not around. We call E a catalyst. Catalysts simply make chemical reactions occur faster. E”s job is to catalyze the reaction between X and Y.
Notice that the spaces on the top of molecule E match up with molecule X”s shape. The spaces on the bottom match up with molecule Y”s shape.
So if molecule E is floating around in the cytoplasm of a cell, X and Y have a good chance of getting together. Once they do, they react to form molecule Z.
When the reaction is finished, molecule E is still around, and is unchanged. It”s now free to go and find another pair of X and Y to catalyze another reaction between them. So here”s the first important fact to remember about an enzyme:
When an enzyme catalyzes a reaction, it is not used up in the reaction and therefore is reusable.
Remember how precisely molecules X and Y fit into molecule E in our example? Real reactants fit into enzymes just as precisely. This is sometimes referred to as the “lock and key” theory. In other words, the reactants fit into the enzyme as precisely as a key fits into a lock. The places where the reactants bind are called the active sites of the enzyme.
Because the fit is so precise, a particular enzyme in a cell can only catalyze a particular reaction. We say that the enzyme is specific for a particular set of reactants and a particular reaction. In our example above, enzyme E is specific to reactants X and Y. E will only catalyze the reaction between X and Y to form Z.
Enzyme F is specific to reactants G and H. It catalyzes only the reaction in which G and H combine to form some product. So here”s the second important fact to remember about an enzyme:
Enzymes are specific for particular reactions.
One other thing: Reactants in an enzyme-catalyzed reaction are called substrates. So molecules X, Y, G, and H would be referred to as substrates.
Enzymes Are Proteins
Enzymes are nothing more than proteins, which means they are organic molecules. They have specific three-dimensional shapes (like all proteins), and their shapes are what make them specific for particular reactions. If they lose their shapes (become denatured) they can no longer run reactions. One thing that can denature enzymes is heat. Heat destroys an enzyme”s three-dimensional shape and prevents it from catalyzing its reaction. Other things that can denature enzymes are acids and bases.
Enzymes are important because they help determine which particular chemical reactions a cell is going to run. Cells are like little bags of chemicals. Enzymes help determine which chemicals will react with one another to carry out the particular functions of a cell.
Sometimes enzymes need help to catalyze reactions. Molecules that help enzymes are called coenzymes. Coenzymes can help enzymes work faster, and some enzymes can”t work at all without coenzymes. What, specifically, are coenzymes? Vitamins. Vitamins are coenzymes. Can you see why getting the right vitamins in your diet is important? Without vitamins, many enzymes would be unable to function properly, and many chemical reactions would not occur. If these chemical reactions were not to occur, your body”s cells (and your body!) would not be able to work properly, if at all.
Quick Quiz #3
Fill in the blanks and check the appropriate boxes:
1. The fact that enzymes interact with substrate by physically fitting together has given rise to the phrase “_________________________ and ______________” theory.
2. Enzymes are known as organic ____________________.
3. When an enzyme has catalyzed a chemical reaction and the products are formed, the enzyme itself [ is is not ] consumed and is [ unavailable available ] to catalyze additional reactions.
4. The location on an enzyme where substrate binds is called the ________________________.
Origins of cells
The origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.
Origin of the first cell
If life is viewed from the point of view of replicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If freely-floating DNA molecules that code for enzymes are not enclosed into cells, the enzymes that benefit a given DNA molecule (for example, by producing nucleotides) will automatically benefit the neighbouring DNA molecules. This might be viewed as " parasitism by default." Therefore the selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: All molecules in a given neighbourhood are almost equally advantaged.
If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it. (Note. This is more a metaphor given for simplicity than complete accuracy since the earliest molecules of life, probably up to the stage of cellular life, were most likely RNA molecules that acted as both replicators and enzymes: see RNA world hypothesis. However, the core of the reasoning is the same.)
Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth.
Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in one's mind as a hydrophilic head on one end, and a hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is being able to link together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multi-cellular organisms could be achieved. This theory is expanded upon in the book, The Cell: Evolution of the First Organism by Joseph Panno Ph.D.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell &ndash a theory termed the endosymbiotic theory.
There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells.
Are there examples of cells with more than one nucleus? - Biology
There are three types of cells: prokaryotic, eukaryotic, and mesokaryotic. Prokaryotic cells are generally single -celled organisms, eukaryotic cells are generally found in multi-cellular organisms, and mesokaryotic cells exhibit characteristics of both prokaryotic and eukaryotic cells.
Prokaryotic Cells are the simplest cells and scientists believe they were the first cells to inhabit our world. They are much smaller than eukaryote cells, as they lack a defined nucleus and most other organelles that are found in eukaryotes. In fact, the word "prokaryote" means "before the nucleus." There are two types of prokaryotes - bacteria and archaea.
The nuclear substance of prokaryotes consists on a single chromosome which is in direct contact with the cytoplasm of the cell. There is no defined membrane surrounding the nuclear region, which is called a "nucleoid" in these cells.
Three primary features found in prokaryotic cells are:
1. Flagella and Pili - protein structures which project from the cell's surface which are used primarily for movement. They are also used to help cells attach to one another. Flagella and pili are not found in all prokaryotes.
2. Envelope - a cell wall which covers a plasma membrane. The envelope of the cells is the containing structure of the cell, separating the interior of the cells from the outer environment. Some bacteria also have an additional outside later of cells, called a capsule.
3. Cytoplasmic Region - the area contained inside the cell envelope or capsule. The cell DNA and ribosomes are found in this area. The DNA of bacteria is generally circular in shape. Some prokarotes carry additional extrachromosomal DNA inclusions called "plasmids." Plasmids are also circular in shape, and the generally carry out additional functions within the cell, such as antibiotic resistance.
Eukaryotic cells, found in plants and animals, are more advanced than prokaryotic cells, and are more advanced in structure. They are about 15 times wider than the average prokaryote, and can have a cell volume as much as 1000 times greater.
The primary difference between peukaryotes and prokaryotes is that the eukaryotic cell has a defined cell nucleus, which contains the cell's DNA. The word "eukaryote" means "true nucleus."
Additional primary features of the eukaryotic cell are:
1. Cell Membrane - the plasma membrane that makes up the outer boundary of the cell. In addition to the cell membrane, plant cells also have a cell wall.
2. Chromosomes - the DNA in eukaryotes is organized into linear molecules called chromosomes. These chromosomes are stored in the nucleus in the cell.
3. Primary Cilia - protein structures on the outside of the cell which serve as sensory organs. Eukaryotes use these cilia to sense temperatures, movement and chemical makeup of their environment.
4. Flagella or Motile Cilia - more complete than those found in prokaryotes, but which perform similar functions in controlling the mobility of the cell.
5. Organelles - eukaryotic cells may also contain various "small organs" which perform specific cell functions. Various organelles include: nucleolus, ribosomes, vesicles, rough endoplasmic reticulum, Golgi body (or "apparatus"), cytoskeleton, smooth endoplasmic reticulum, mitochondrion, microtubules, vacuole, cytosol, lysosome, and centriole.
Mesokaryotic cells share characteristics of both prokaryotic and eukaryotic cells. The first part of the word, 'meso,' means 'in between', while 'karyon' means 'nucleus.'
Mesokaryotic cells exhibit a well-organized nucleus, as eukaryotes do, but it's nucleus divides through a process called amitosis, which more closely resembles the behavior of prokaryotic cells. The nucleus of a mesokaryotic cell duplicates itself with one nucleus going with each cell half when the rest of the cell divides.
A group of organisms known as "dinoflagellates", marine plankton and algae, are generally considered to be examples of mesokaryotes.
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During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide–phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.
Are there examples of cells with more than one nucleus? - Biology
The nucleus is a highly specialized organelle that serves as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).
Only the cells of advanced organisms, known as eukaryotes , have a nucleus. Generally there is only one nucleus per cell, but there are exceptions, such as the cells of slime molds and the Siphonales group of algae. Simpler one-celled organisms ( prokaryotes ), like the bacteria and cyanobacteria, don't have a nucleus. In these organisms, all of the cell's information and administrative functions are dispersed throughout the cytoplasm.
The spherical nucleus typically occupies about 10 percent of a eukaryotic cell's volume, making it one of the cell's most prominent features. A double-layered membrane, the nuclear envelope, separates the contents of the nucleus from the cellular cytoplasm. The envelope is riddled with holes called nuclear pores that allow specific types and sizes of molecules to pass back and forth between the nucleus and the cytoplasm. It is also attached to a network of tubules and sacs, called the endoplasmic reticulum, where protein synthesis occurs, and is usually studded with ribosomes (see Figure 1).
The semifluid matrix found inside the nucleus is called nucleoplasm. Within the nucleoplasm, most of the nuclear material consists of chromatin, the less condensed form of the cell's DNA that organizes to form chromosomes during mitosis or cell division. The nucleus also contains one or more nucleoli, organelles that synthesize protein-producing macromolecular assemblies called ribosomes, and a variety of other smaller components, such as Cajal bodies, GEMS (Gemini of coiled bodies), and interchromatin granule clusters.
Chromatin and Chromosomes - Packed inside the nucleus of every human cell is nearly 6 feet of DNA, which is divided into 46 individual molecules, one for each chromosome and each about 1.5 inches long. Packing all this material into a microscopic cell nucleus is an extraordinary feat of packaging. For DNA to function, it can't be crammed into the nucleus like a ball of string. Instead, it is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin.
The Nucleolus - The nucleolus is a membrane-less organelle within the nucleus that manufactures ribosomes, the cell's protein-producing structures. Through the microscope, the nucleolus looks like a large dark spot within the nucleus. A nucleus may contain up to four nucleoli, but within each species the number of nucleoli is fixed. After a cell divides, a nucleolus is formed when chromosomes are brought together into nucleolar organizing regions. During cell division, the nucleolus disappears. Some studies suggest that the nucleolus may be involved with cellular aging and, therefore, may affect the senescence of an organism.
The Nuclear Envelope - The nuclear envelope is a double-layered membrane that encloses the contents of the nucleus during most of the cell's lifecycle. The space between the layers is called the perinuclear space and appears to connect with the rough endoplasmic reticulum. The envelope is perforated with tiny holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. The inner surface has a protein lining called the nuclear lamina, which binds to chromatin and other nuclear components. During mitosis, or cell division, the nuclear envelope disintegrates, but reforms as the two cells complete their formation and the chromatin begins to unravel and disperse.
Nuclear Pores - The nuclear envelope is perforated with holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. Building blocks for building DNA and RNA are allowed into the nucleus as well as molecules that provide the energy for constructing genetic material.