7: Photosynthesis - Biology

7: Photosynthesis - Biology

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Learning objectives

  • To recognize how the first photosynthetic organisms changed the atmosphere of the earth.To describe the 2 phases of photosynthesis.
  • To test a hypothesis concerning the effect of different light conditions on the rate of the light dependent reaction of photosynthesis.
  • To collect data and generate a bar graph depicting the results of this experiment.

Thumbnail: Plant cells (bounded by purple walls) filled with chloroplasts (green), which are the site of photosynthesis. Image used iwth permission (CC BY-SA 3.0; Kristian Peters).

Course Description

In this course, you will journey through the web of physical, chemical, and biological reactions that collectively constitute photosynthesis. We will begin with light harvesting and follow photons to the sites of primary photochemistry: the photoreaction centers. A molecular-scale view will show in atomic detail how these protein complexes capture and energize electrons. Then we will follow the multiple pathways electrons take as they carry out their work. Consequent reactions, such as the synthesis of ATP and the reduction of CO2 during the synthesis of carbohydrates, will also be discussed in structural detail. Lastly, we will delve into the evolution of these systems and also discuss other photosynthetic strategies, such as light-driven proton pumps and anoxygenic photosynthesis. The course will include a visit to an electron microscope to allow students to directly observe proteins involved in photosynthesis.

This course is one of many Advanced Undergraduate Seminars offered by the Biology Department at MIT. These seminars are tailored for students with an interest in using primary research literature to discuss and learn about current biological research in a highly interactive setting. Many instructors of the Advanced Undergraduate Seminars are postdoctoral scientists with a strong interest in teaching.

Questions in Short Answer Questions (SA)

Q2) Which four of the following are needed for photosynthesis in a leaf:

  1. Carbon dioxide:
  2. Oxygen:
  3. Nitrates:
  4. Water:
  5. Chlorophyll:
  6. Soil:
  7. Light:

Q3) What is the source of energy for photosynthesis?

Q4) Which gas is taken in and which one is given out by leaf in bright sunlight?

Q5) Suppose we compare the leaf with a factory, match the items in column A with those in column B

Leaf Factory
(i) Cells in the leaf (a) Raw materials
(ii) Chloroplast (b) power
(iii) Sunlight (c) machinery
(iv) Oxygen and water (d) end product
(v) Carbon dioxide and water (e) by product
(vi) Glucose (f) work room

Q6) State whether the following statement is true or false

  1. Green plans prepare their food by using two raw materials, oxygen and watr.
  2. The chlorophyll enables the plants to use light energy.
  3. The free oxygen in the atmospheric air is the result of photosynthesis.
  4. Photosynthesis occurs only in chlorophyll-containing parts of the plant.

Q7) Difference between aerobic and anaerobic respiration. Write the overall chemical equations of the two kinds of respiration in plants.

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My AP Biology

Three questions about the chapter:
1.What process do plants use that eukaryote organisms do not, to get energy?
2.Where does photosynthesis occur?
It occurs in the chloroplasts in plant cells.
3.What are the two stages of photosynthesis?
The first one is the light reactions, and the second one is the Calvin cycle, or the dark reactions.

Five main facts from the reading:
1.Autotrophs are the producers of the biosphere.
2.Plants produce oxygen by splitting water.
3.Photosynthesis is a redox process,as is cellular respiration.
4.Photosynthesis uses light energy, carbon dioxide, and water to make food molecules.
5.Photosynthesis moderates global warming.

This diagram is a simple overview of the two stages of photosynthesis that take place in a chloroplast.

In the introduction of the chapter, we learned that scientists are trying to use plant power as fuel source. Photosynthesis is one of the oldest energy pathways on the planet. In this process, green plants, algae, and certain bacteria transform light energy to chemical energy stored in the bonds of the sugar they make from carbon dioxide and water. After this we learned that autotrophs are the producers of the biosphere. Producers are the organisms that produce their own food supply. All organisms that produce organic molecules from inorganic molecules using the energy of light are called photoautotrophs. Photosynthesis occurs in the chloroplasts in plant cells. Plants' green color is from chlorophyll, a light-absorbing pigment in the chloroplasts that plays a central role in converting solar energy to chemical energy. Chloroplasts are concerned in the cells of the mesophyll, the green tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits, by way of tiny pores called stomata. Water absorbed by the roots is delivered tot he leaves in veins. An envelope of two membranes encloses an inner compartment in the chloroplast, which is filled with a thick fluid called stroma. Suspended in the stroma is a system of interconnected membranous sacs, called thylakoids. In some places thylakoids are concentrated in stacks called grana. Plants produce oxygen by splitting water. Photosynthesis is a redox process, as is cellular respiration. It has two stages and they are linked by ATP and NADPH. The light reactions include the steps that convert light energy to chemical energy and produce oxygen. The reactants in this process are water sunlight energy, ADP, and NADP+ . The products are ATP, NADPH, and oxygen. This process takes place in the thylakoids in the chloroplast. The Calvin cycle occurs in the stroma of the chloroplast. It is a cyclic series of reactions that assembles sugar molecules using carbon dioxide and the energy-containing products of the light reactions. The reactants of this process are carbon dioxide, ATP, and NADPH. The products are NADP+, ADP, and sugar. The process takes place in the stroma in the chloroplast. Visible radiation drives the light reactions. An electromagnetic spectrum is the full range of electromagnetic wavelengths from the very short gamma rays to the very long-wavelength radio waves. The distance between the crests of two adjacent waves is called a wavelength. A photon is a fixed quantity of light energy. Photosystems capture solar power. A photosytem consists of a number of light-harvesting complexes surrounding a reaction center complex. The reaction center complex contains a pair of chlorophyll "a" molecules and a molecule called the primary electron acceptor, which is capable of acdpeting electrons and becoming reduced. There two photosystems in the light reactions process. Photosystem 2 (P680) and photosytem 1 (P700). The two photosystems are connected by an electron transport chain and generate ATP and NADPH. Chemiosmosis powers ATP synthesis in the light reactions. In photosynthesis the chemiosmotic production of ATP is called photophosphorylation. ATP and NADPH power sugar synthesis in the Calvin cycle. Adaptations that save water in hot, dry climates evolved in C4 and CAM plants. In most plants, initial fixation of carbon occurs when the enzyme rubisco adds carbon dioxide to RuBP. Such plants are called C3 plants because the first organic compound produced is the three-carbon compounds 3-PGA. In certain plant species, alternate modes of carbon fixation have evolved that save water without shuttling down photosynthesis. C4 plants are so named because they precede the Calvin cycle by first fixing CO2 into a four-carbon compound. When the weather is hot and dry, a C4 plant keeps its stomata mostly closed, thus conserving water. CAM plants are species adapted to very dry climates. A CAM plant conserves water by opening its stomata and admitting carbon dioxide only at night. Photosynthesis moerates global warming.

Key Terms:
1.Autotrophs - organisms that make their own food and thus sustain themselves without consuming organic molecules derived from any other organisms.
2.Mesophyll - the green tissue in the interior of the leaf.
3.Stomata - tiny pores by which carbon dioxide enters the leaf, and oxygen exits.
4.Stroma - a thick fluid filled in an envelope of two membranes in the chloroplast.
5.Thylakoids - a system of interconnected membranous sacs suspended in the stroma.
6.Grana - stacks in which thylakoids are concentrated.
7.Light reactions - include the steps that convert light energy to chemical energy and produce oxygen.
8.Calvin cycle - occurs in the stroma of the chloroplast, and it is a cyclic series of reactions that assembles sugar molecules using carbon dioxide and the energy-containing products of the light reactions.
9.Photosystem - consists of a number of light-harvesting complexes surrounding a reaction center complex.
10.Photon - a fixed quantity of light energy.


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Science Fair Project on Photosynthesis (With Experiments)

Do you want to create an amazing science fair project for your next exhibition? You are in the right place. Read the below given article to get a complete idea on photosynthesis with some experiments: 1. Meaning of Photosynthesis 2. Definition of Photosynthesis 3. Historical Perspective 4. Importance 5. Process 6. Mechanism 7. Site 8. Factors 9. Significance.

  1. Science Fair Project on the Meaning of Photosynthesis
  2. Science Fair Project on the Definition of Photosynthesis
  3. Science Fair Project on the Historical Perspective of Photosynthesis
  4. Science Fair Project on the Importance of Photosynthesis
  5. Science Fair Project on the Process of Photosynthesis
  6. Science Fair Project on the Mechanism of Photosynthesis
  7. Science Fair Project on the Site for Photosynthesis
  8. Science Fair Project on the Factors affecting Photosyn­thesis
  9. Science Fair Project on the Significance of Photosynthesis in Plants

Science Fair Project # 1. Meaning of Photosynthesis:

Photosynthesis is the process of manufacture of carbohydrate food matters like sugars and starch by the chloroplasts of the cells out of water and carbon dioxide gas in the presence of sunlight. As absorption of radiant energy supplied by sunlight is an indis­pensable factor, the process is called photosynthesis or carbon assimilation (photo = light, synthesis=construction).

It is a monopoly of the green plants. Though all the green aerial portions are capable of constructing food, the flat green leaves are best suited for the purpose. Their fineness, thinness and the natural exposure of their maximum surface to light and air as well as their internal structure are very much conducive to the process of photosynthesis.

The chloroplasts present abundantly in the mesophyll cells are instrumental to the absorption of radiant energy from the sunlight. It is transformed into chemical energy for bringing about a series of chemical reactions which culminate in the formation of sugar and starch with accumulated potential energy, capable of being oxidised during respiration.

Photosynthesis, as performed by green plants, is a very slow process utilizing about 0.5 to 3% of the total radiant energy falling upon the leaf surface. But this is the only mechanism of keeping the solar energy stored up for all out work on the surface of the earth.

If the green plants had not solved this little problem of con­verting solar energy into chemical energy of the carbohydrates, the continued existence of life would have been impossible on earth.

Theoretically there seems to be no difficulty for man, the most highly developed animal, to solve this little technical problem of converting the solar energy into some other potential form before the solar energy is converted into heat and lost beyond recovery. But man has not solved this problem yet, i.e. artificial photo­synthesis has not yet been achieved.

We still cannot prepare one molecule of sugar from its constituents, CO2 and H2O2 in spite of all our technical and scientific advance and attainments. The day artificial photosynthesis will be possible, mankind will discard the green plants and will have at their disposal an unlimited and in­exhaustible source of energy.

Science Fair Project # 2. Definition of Photosynthesis:

The process in which certain carbohydrates are synthesized from carbon dioxide and water by chlorophyllous cells in the presence of light, oxygen being a by-product, is generally called photosynthesis.

The summary equation of photosynthesis is as follows:

This equation, although properly balanced, gives an erroneous impression of the mechanism by which the reaction is accomplished. By the use of H2O and CO2 labelled with isotopic oxygen (O 18 ), biochemists have been able to demonstrate that the oxygen released in photosynthesis comes not from CO2 but from water.

“Photosynthesis (Photos = light synthesis = putting together) is an anabolic process in which green plants or green parts of the plants synthesize or manufacture complex carbonaceous organic food substances (carbohydrates) with carbon dioxide and water (respectively taken from air and water) in the presence of sunlight and evolve oxygen as a by-product. Thus, in this process radiant energy is converted into chemical energy.”

However, green plants apart from taking nutrients from soil, make their own food by utilising carbon dioxide, water and sunlight. This is basis of photosynthesis. During this process, oxygen is evolved and released to the atmosphere.

A simple equation given by C. B. van Niel of Stanford University, U.S.A. is as follows:

During photosynthesis, carbon dioxide CO2 is chemically reduced to carbohydrate (CH2O). Here, n is an integer.

In glucose, n is 6, i.e., formula of glucose is C6H12O6.

Water molecule splits in presence of light. This process is called photolysis and O2 is released.

Thus, released O2 comes from water and not from CO2.

Science Fair Project # 3. Historical Perspective of Photosynthesis:

Study on photosynthesis originated only about 300 years ago. For the first time Belgian physician, Jan Baptista van Helomont, on the basis of his simple experiment concluded that all the substance of the plant was produced from the water and none from the soil.

By the end of the eighteenth century, Joseph Priestley (1733-1804), in 1771-72, performed first experiment on photosynthesis, and showed that plants have ability to take up CO2 from the atmosphere and release O2.

Later, Dutch physician Jan Ingenhousz (1730-1799), in 1780 confirmed Prietley’s work, and made following important conclusions:

(i) The evolution of O2 took place only within a few hours and not weeks.

(ii) The evolution of O2 took place only during day time, i.e., in presence of sunlight.

(iii) Only green parts of plants could restore used air.

In 1804, a Swiss scholar Nicholas Theodore de Saussure observed that water is essential for photosynthesis. He also correlated the importance of light during intake of CO2 and evolution of O2.

In 1837, Dutrochet experimentally demonstrated that chlorophyll is necessary for photosynthesis.

In 1845, Liebig told that CO2 is the main source of all organic compounds synthesised by green plants.

In 1864, Sachs told for first time that organic matters produced by plants were carbohydrates.

Science Fair Project # 4. Importance of Photosynthesis:

Food is the source of energy needed by animal and plant life. And this food is manufactured by green plants from inorganic substances with the aid of sunlight energy during photosynthesis. Food represents the stored energy of sunrays.

Fuel in the form of wood is also the indirect result of photosynthesis activity of plants and it is also thought that coal, petroleum, etc., are also the remote consequences of photosynthesis. A number of other substances are also produced as the by-products, for example, the oxygen needed by animals and human being continues to be supplied by photosynthesis.

Science Fair Project # 5. Process of Photosynthesis:

Water absorbed from the soil is conducted upwards through the xylem vessels to reach the mesophyll cells of the leaf. Stomata open due to the turgor pressure of the guard cells and carbon dioxide gas comes in. It dissolves in imbibed water saturating the cell wall and diffuses in solution to the chloroplasts of the mesophyll cells.

The chemical reactions taking place in photosynthesis are not clearly understood. With the aid of radiant energy, six mole­cules of carbon dioxide unite with six molecules of water to pro­duce one molecule of simple sugar, glucose, and six molecules of oxygen as end product.

During the process energy is stored up in sugar molecule in potential form. Oxygen usually goes out through the stomata some may as well be used for respiration. It has been calculated that the volume of oxygen liberated is precisely equal to the volume of carbon dioxide absorbed.

6CO2+6H2O+light energy (674 cal.) = C6H12O6 + 6O2. The process is not actually so simple. For a long time it was taken , for granted without any conclusive experimental evidence that an intermediate product, formaldehyde (CH2O), is formed.

It was thought that as formaldehyde is very unstable and poison­ous, six molecules are quickly condensed into a molecule of glucose. This process of condensation is referred to as polymerisation. The accumulation of sugar results in too much concentration of sap which retards photosynthesis.

So sugar is converted by chloro­plasts into insoluble starch grains by condensation. A number of molecules of glucose condenses to form one molecule of starch by elimination of one molecule of water for each molecule of glucose used.

These starch grains, known as assimilatory starch, are very temporary bodies, as they are converted into sugar with nightfall so that the starch-free leaves can again carry on photo­synthesis next morning.

Carbohydrates are assimilated as sugar and travel to different parts probably through the sieve tubes in the phloem. Surplus sugar is converted into reserve starch and other complex carbohydrates in the storage regions.

The probable reactions have been represented thus:

CO2 + H2O = CH2O (formaldehyde) + O2.

(C6H12 O6)n – (H2O6) n = (C6H10O5)n(starch).

But it has been conclusively demonstrated recently that formaldehyde is never formed as an intermediate product in photosynthesis. The most pro­bable intermediate in photosynthetic process is a phosphate compound, actually a phospho-glyceric acid.

Though sugar is the first-formed carbohydrate of photosynthesis, insoluble starch is the first visible product. It can be demonstrated in the leaves with characteristic iodine reaction.

A leaf collected in the evening is decolorized by boiling for a few minutes in alcohol over a water bath. If the leaf is now treated with iodine solution, it turns blue indicating the presence of starch grains but a leaf collected early in the morning will not give iodine reaction, because it is starch-free.

The whole idea of photosynthesis has been that CO2 enters the plant and breaks down into C and O2. Carbon was correctly thought to be acquisition, (which is retained) and O2 came out. So the ratio O2/CO2 was unity. So far as the origin of oxygen in photosynthesis is concerned, it has been taken for granted that it came from CO2, after carbon has been retained in plants.

Several modern investigators have proved conclusively in 1941 that all the oxygen evolved in photosynthesis came not from CO2 but from H2O, and that photosynthesis is a purely oxidation-reduction process between CO2 and H2O—CO2 being reduced, i.e. hydrated by the reductant H2O, which is oxidised to oxygen by the removal of H2.

Removal of H2 is oxidation by all the sense of the term. Then the equa­tion of photosynthesis, according to recent clarification, should be either

CO2 + 2H2O = CH2O + H2O + O2, or CO2 + 4H2O = CH2O + 3H2O + O2.

All the oxygen evolved is derived from water and not from CO2. Recent evi­dences have conclusively demonstrated that formaldehyde is never formed as an intermediate product in photosynthesis. The point is again verified in the photosynthesis is pigmented sulphur bacteria which live in an atmosphere of sulphuretted hydrogen.

These bacteria use H2S in place of H2O and make carbohydrate food from CO2 and H2S in pre­sence of sunlight—CO2+2H2S=CH2O + H2O+2S. The reduction of CO2 here leads to the production of sulphur and not 1 part of sulphur and 2 parts of oxygen or of SO2, which really proves that O2 in photosynthesis of green plants comes from H2O and not from CO2.

The pigmented sulphur bacteria and the world of green plants may be thus called photoautotrophic (independent in presence of light) in contrast to the other groups of non-pigmented sulphur bac­teria which also utilise sulphur compounds like H2S.

These bacteria which have been termed chemoautotrophic (independent when there is supply to chemical energy) derive their energy from oxidation of inorganic compounds like H2S.

The reactions are exothermal and the energy thus liberated can be used up by these bacteria for the synthesis of carbohydrate food. The chemoautotrophic bac­teria thus can synthesise carbohydrates without chlorophyll and in complete ab­sence of light.

The abolition of formaldehyde hypothesis and the conclusive evidence about origin of O2 from H2O and not from CO2 have for the first time since the discovery of photosynthesis was announced, made the picture clear about the most fundamental biochemical reaction in the living cells.

The chemical mecha­nism of photosynthesis, as conceived today based on undeniable evidences and shorn of its formaldehyde-beclouded atmosphere, stands on solid foundations and can be summarised as follows:

It is an oxidation-reduction reaction between CO2 and H2O. In the first part of the reaction, photo-chemical dehydrogenation of water takes place at the expense of absorbed light energy by chlorophyll. Water loses its hydrogen, i.e. it is oxidised, and the liberated hydrogen forms a hydrogen donator with any compound which would take up hydrogen easily (A).

In the second reaction the hydrogen donator, i.e. a compound formed in the first reaction, which would readily give up hydrogen, would reduce CO2. The second reaction might take place in dark—a chemical reaction, but the formation of the hydrogen donator must be stimulated by light as dehydrogenation of water is a photo-chemical reaction.

It must be evident, here, that the present conception envisages that evolution of O2 and the reduction of CO2 take place in two separate steps, as can be seen from the following summary equations:

A=any compound which would readily take up hydrogen (hydrogen acceptor). CH20 here does not mean formaldehyde, but the simplest sugar.

That oxygen is given out during photosynthesis can be demonstrated by the following experiment.

Experiment: Photosynthesis- Evolution of oxygen (Fig. 173)

A few submerged plants like Elodea, Hydrilla are taken in a beaker half-filled with water and are covered by an inverted funnel. A test-tube filled with water is placed on the stem of the funnel. Now the beaker is ex­posed to sunlight for some time. It is found that bubbles of gas evolve and collect on the end of the test-tube dis­placing water.

By standard tests it can be determined that the collected gas is oxygen. The aquatic plants draw their CO2, supply from the large amount dissolved in water. If distilled water is used instead of tap water, there will be no evolution of O2, owing to lack of CO2. If the tap water is previously boiled all the dissolved CO2 will be driven off, and as a result there will be no evolution of O2 and no photosynthesis.

Science Fair Project # 6. Mechanism of Photosynthesis:

Carbon-Dioxide Aassimilation:

Arnon is 1954 found that isolated chloroplasts under suitable experimental conditions, could assimilate CO2. The enzymes involved in the reduction (assimilation) of carbon dioxide, therefore, must be present and perhaps produced within the chloroplast.

The reduction of carbon dioxide is accompanied by the evolution of O2. With the use of radioactive carbon dioxide and chromatographic techniques, D. I. Arnon and his associates could identify several soluble and insoluble products, such as—phosphate esters of glucose fructose ribulose sedoheptulose dihydroxyacetone and glyceric acid glycolic, malic and aspartic acids alanine, glycine free dihydroxy-acetone and glucose.

Stages in photosynthesis:

It is now generally agreed that photosynthesis consists of two stages:

The light is necessary for the light stage, whereas the dark stage is independent of the presence of light.

Evidences for the Existence of Light and Dark Stage:

Three important evidences have been considered here.

Flashing Light Experiments:

Warburg (1919) compared the rate of photosynthesis of a plant kept in continuous light for a certain time with the rate for the plant when supplied with alternate light and dark periods but receiving the same total amount of light. The rate of photosynthesis of the latter was found to be sufficiently greater than that of the plant kept in continuous light.

The explanation is as follows:

A → B → C (photosynthesis product).

In continuous light of high intensity the reaction A—B proceeds at a faster rate than B—C so that there is a tendency for some B to accumulate. The effect of intermittent light therefore is to enable B—C to proceed in the dark at a time when there is no production of B, as well as for B—C continue in the light. This way, for given total quantity of light, there is more C produced.

Temperature Experiments:

Over a temperature range of about 10°-25° C, if light intensity and CO2 concentration are relatively high, the Q10, (temperature coefficient) of photosynthesis is approximately two. Strictly chemical reactions have a Q10, of from two to three. This fact indicates that at least one of the reactions involved in photosynthesis is of purely chemical type.

Since this fact was first pointed out by Blackman, this reaction is known as Blackman reaction. This is also called the dark reaction, since it does not require light and therefore, may take place in either the light or the dark.

A chemical reaction which proceeds only at the expense of absorbed light is called a photochemical reaction. The Q10, of a photochemical reaction is approximately one.

Temperature Coefficient Q10:

The temperature coefficient of a physiological, chemical or physical process is the ratio of the process at any stated temperature of the rate at a lower temperature usually 10° C lower, and designed as the Q10 of the process. Thus, if a process is 2.3 times as fast at 25° as at 15° C, the temperature coefficient, Q10 is 2.3.

Temperature does not accelerate photochemical reactions. Under conditions of high light intensity, increased temperature accelerates the reaction B—C at the expense of the excess B (Q10 = 2).

When there is a low light intensity, A—B is slower than B—C will not affect the overall production of C. The limiting reaction A—B is photochemical (Q10 = 1):

Under conditions of intense illumination but lacking CO2, ‘B’ would accumulate and this would react with CO2 in the dark.

Science Fair Project # 7. Site for Photosynthesis.

Photosynthesis takes place only in the green parts of the plants, such as leaves, stems, etc. Within a leaf photosynthesis occurs particularly in mesophyll cells which have Chloroplasts.

Chloroplasts make actual sites for photosynthesis in green plants. Chloroplasts are located at the outer margins with their broad surfaces parallel to the cell wall of the mesophyll cells. This type of arrangement helps in easy diffusion of CO2 required for photosynthesis from atmosphere to the inside of chloroplasts.

The complex process of photosynthesis takes place from beginning to end in the chloroplast. The absorption of light energy and carbon dioxide and the conversion of the carbon dioxide to starch and evolution of oxygen all take place within the illuminated chloroplast.

Structure of the Chloroplast:

The contents of the fully developed chloroplast are enclosed in an envelope consisting of two membranes with an enclosed space. Each membrane being 40 to 60 Å thick and the space between them vary from 25 to 75 Å. Inside is filled with proteinaceous matrix called stroma contains starch grains and osmophilic droplets. Eye spots and pyrenoid bodies, often found in algal cells, are also found in the matrix (stroma).

Several membranes stacked on top of each other are exhibited in the cross section of the chloroplast. These membranes are paired, forming stacks of discs. On or within the lamellae the chlorophyll and other pigments are found.

In the lower forms of plant life, the pigments are evenly distributed over the entire surface of the lamellae, while in the higher forms of plant life, they are restricted to certain areas of lamellae.

If these concentrated areas are layered one on the top of the other, the complete stack is known as a granum. According to Wolken (1961) and Calvin (1959) the chloroplast is a lamellar structure composed of lipid and aqueous protein layers.

The thylakoids in the chloroplasts contain most of the machinery for photochemical reactions of photosynthesis they contain pigments required for capturing solar energy. The most important pigments are chlorophylls.

Chlorophyll Pigments:

The chlorophylls, the green pigments of plants, are the most important pigments active in the process of photosynthesis. There are at least eight types of chlorophyll pigments.

They are chlorophyll a, b, c, d, and e bacteriochlorophyll-a bacteriochlorophyll-b and chlorobium chlorophyll (bacterioviridin) of these chlorophyll a is most nearly of universal occurrence being present, in all photosynthetic organisms except the green and purple bacteria.

Chlorophyll b is found in all higher plants and in the green algae, but is not found in algae of most other classes (e.g., blue-green, brown and red algae).

The other chlorophylls (i.e., c, d and e) are found only in algae and in combination with chlorophyll a, bacteriochlorophyll (a and b) are present in the purple bacteria, whereas the green bacteria contain the pigment called bacteriovir-idin (chlorobium chlorophyll). All of the chlorophylls are very similar in chemical composition and all of them are compounds which contain magnesium.

Chlorophyll a and b are the characteristic chlorophylls of the higher plants. Neither of the two is soluble in water, but both are soluble in a number of organic reagents. Chlorophyll-a is readily soluble in absolute ethyl alcohol, ethyl ether, acetone, chloroform and carbon bisulfide. Chlorophyll-b is also soluble in above-mentioned reagents. Chlorophyll a is usually blue-green, while chlorophyll-b is yellow-green.

The chlorophyll a molecule consists of a cyclic structure composed principally of four pyrrolenuclei containing magnesium atom at its centre. Extending from one of the pyrrolerings is a long chain alcohol, the phytol part of the chlorophyll molecule. The molecular formulae for chlorophyll a and b are—C55H72O5N4Mg and C55A70O6N4Mg.

Chlorophyll b constitutes about one-fourth of the total chlorophyll, content and absorbs light of different wavelength than the chlorophyll a. On absorbing light, the chlorophyll b molecule is excited and transfers its energy to the chlorophyll a molecule. Finally, the chlorophyll a molecule converts the light energy into electrical energy by bringing about charge separation. Here, chlorophyll a molecules act as reaction centres.

The Carotenoids (Carotenoid Pigments):

The carotenoids are lipid compounds. They are found to be distributed widely in both animals and plants. They are red, orange, yellow and brownish pigments. They are present in variable concentrations in nearly all higher plants and many microorganisms such as — red and green algae, photosynthetic bacteria and fungi.

β-carotene, the major carotenoid is orange-yellow pigment and found in plant tissues. This is generally accompanied by a-carotene.

The carotenoids which consist of carbon and hydrogen are known as carotenes, while the carotenoids containing oxygen are called xanthophylls which are found more frequently in nature than the carotenes. The major xanthophylls are generally found in green leaves.

The carotenoids are located in the chloroplasts and chromatophores. They occur there as water-insoluble protein complexes.

Like chlorophyll carotenoids are also embedded in the thylakoid membranes of chloroplasts. These accessory pigments act as antenna complexes and procure light from different regions of the spectrum than the chlorophyll.

The light captured by these pigments is transferred to the reaction centres of chlorophyll-a for conversion into electrical energy. The accessory pigments (carotenoids) and the reaction centre (chlorophyll-a), together form photosystem.

The phycobilins are found only in algae. The red and blue phycobilins are called phycoerythrins and phycocyanins, respectively. They are active in the transfer of light energy to chlorophyll for utilization in the process of photosynthesis. The role of phycobilins and carotenoids is indirect in the sense that the energy they absorb is transferred to chlorophyll before it becomes active in photosynthesis.

Science Fair Project # 8. Factors affecting Photosyn­thesis:

Photosynthesis depends upon some external and internal factors. As a matter of fact, the factors jointly influence the rate of carbohydrate formation in plants. The external factors are carbon dioxide content of the atmosphere, supply of water, light and temperature.

As all these external factors act simultaneously on the photosynthetic process, the factor which is in the relative minimum will act as the limiting factor, i.e. will control the rate of the process. The important internal factor influencing the process is chlorophyll, the green-colouring pigment of the chloroplasts.

This gas is present in air in only 0.03%, i.e. only 3 parts in- 10,000. In industrial areas and above soil surface the percentage may be a bit higher. But it is the only source of carbon which, as we have seen, forms nearly half of the dry weight of plants. Many plants can utilise higher percentage of carbon dioxide. In fact, encouraging results were obtained by the application of extra carbon dioxide to crop plants.

Carbon dioxide absorbed from air is replaced–by the respiration of plants and animals, decomposition of organic matters, etc. The rate of photosynthesis increases when the concentration of carbon dioxide in air reaches near about 1%. As the percentage is very low this factor possibly decides the rate of photosynthesis under normal conditions.

It may appear that continued intake of CO2 from the atmos­phere for photosynthesis by the green plants, both aerial and aquatic, results in deficiency of that gas in the air. But there are various ways by which CO2 comes back to air to make up the deficiency.

During respiration, a process going on day and night, the plants and animals give out CO2 and take in O2. Plant and animal bodies after death and their refuses are gradually decomposed and broken down to simpler elements by a group of organisms—fungi and bacteria. These are referred to as decomposers and by their action ultimately CO2 is released to the air.

Moreover during combustion of coal and wood CO2 is given out. Thus the total volume of this gas remains constant in the air. It may be represented by a cycle (Fig. 174) —something like a wheel having no beginning or end, known as Carbon Cycle.

Science Fair Project # 9. Significance of Photosynthesis in Plants:

Photosynthesis is a vital process for life on this planet. This process links the physical and biological world. This process helps conversion of solar energy into organic matter that makes bulk of dry matter of any organism.

The plants biomass (dry matter), derived through photosynthesis supports humans and all other heterotrophic organisms living in biosphere.

Survival of all organisms is dependent upon food (carbohydrates, etc.), a photosynthetic product. All organisms need energy for different life activities which they derive from food (carbohydrates, fats and proteins).

Green plants are unique in the character that they are able to synthesise food for whole bio-kingdom.

During photosynthesis, O2 is released, which is helpful to living organisms mainly in two ways. Firstly, in efficient utilisation of the energy rich molecules, i.e., carbohydrates, in respiration.

Secondly, in making ozone (O3) in outer layer of atmosphere, which is required to stop harmful ultraviolet (UV) rays from reaching the earth.

During photosynthesis green plants take CO2 from atmosphere and release O2, thus purifying air. Without O2, life of all aerobic organisms including humans is not possible.

Crops are totally dependent on photosynthesis. This process feeds whole bio-kingdom directly or indirectly.

Importance of Photosynthesis

Plants play an integral role in the continuity of life on the planet, all thanks to photosynthesis. They achieve that through the following:

1. Atmospheric Gases

In photosynthesis, plants take in carbon dioxide and release oxygen as a by-product. Without the process of photosynthesis, it would be difficult to replenish all the oxygen being used in processes such as combustion and respiration.

Moreover, the amount of carbon dioxide in the atmosphere would rise to dangerous levels. Therefore, it suffices to say that photosynthesis helps to balance atmospheric gases.

2. Food and Energy

Green plants produce their own food through photosynthesis. They are called producers. Conversely, animals and humans are consumers.

They get all of their food from plants, either directly or indirectly. In fact, the larger part of the world’s population gets over 80% of its food directly from plants. The remaining source is obtained from animals.

And animals are part of the food chain, which normally starts with plants. Speaking of the food chain, the energy you get from the food you eat is also a product of photosynthesis, whether the food is from plants or animals.

3. Petroleum Products

Did you know that your car runs on what was once light energy? Or that your cooking gas is a product of photosynthesis?

We get petroleum from plants that stored light energy in their system millions of years ago, thanks to photosynthesis, as well as animals that ate those plants.

The petroleum came into being due to intense pressure applied to the plants and animals over millions of years. Coal and natural gas were also produced in the same way.

4. Wood and Other Side Products

We use wood for a wide range of purposes, including construction and combustion. Paper is also a product of wood.

Moreover, cotton and other natural fibers consist of cellulose produced virtually entirely by photosynthesizing plants. And while wood comes from the sheep, the sheep gets its food from the plants. It is, therefore, suffice to say that we wear clothes courtesy of photosynthesis.

5. Medicinal Products

Most medicines are manufactured using various chemicals extracted from plants. And thousands of plants have been confirmed to have medicinal properties. Aspirin, for example, is derived from salicylic acid.

Salicylic acid comes from the back of the willow tree. Aspirin is a popular painkiller. It is also used to minimize blood clotting in heart patients.

Much stronger analgesic drugs such as codeine and morphine are products of opium. Opium is extracted from the seeds of the poppy plant.

Tests are still ongoing to ascertain the medicinal status of thousands of other plants species, especially those found in the tropical rainforests. In the light of this, it is imperative that we protect the natural habitats of these plant species.

Bottom Line

Photosynthesis is the process used by plants, algae, and some bacteria to convert solar energy into chemical energy. Besides light energy, other photosynthesis ingredients are water and carbon dioxide. It is a complex, enzyme-controlled process that is vital for the existence of all lifeforms on Planet Earth. Namely, all living things are dependent on plants, directly or indirectly.

Photo by: Khanh , pexels

About Sonia Madaan

Sonia Madaan is a writer and founding editor of science education blog EarthEclipse. Her passion for science education drove her to start EarthEclipse with the sole objective of finding and sharing fun and interesting science facts. She loves writing on topics related to space, environment, chemistry, biology, geology and geography. When she is not writing, she loves watching sci-fi movies on Netflix.

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