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What is the purpose of co-transport?

What is the purpose of co-transport?


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My current understanding of co-transport is that, first, a substance is actively transported across a membrane, establishing a concentration gradient across said membrane. This same substance then diffuses down the established concentration gradient, effectively travelling back to where it began, but this time travelling via a transmembrane protein that transports another substance also, regardless of its concentration gradient. Thus, this second substance is transported to wherever it is necessary.

Would it not then make more sense for the latter substance to simply be actively transported to its destination, as opposed to involving another substance which has no net movement at the end of the process? What is the advantage of co-transport?


@Jam is correct - if a little long-winded - in pointing out that one of the benefits of co-transport (no need to involve evolution) is that the concentration gradient of one co-transported molecule can provide the thermodynamic energy for the transport of the other molecule against a concentration gradient. This is dealt with in a quantitative manner in Section 13.1.2 of Berg et al. and in a qualitative manner in Section 13.4.

However, there is another factor to consider. This is the maintenance of electrical neutrality. Typically it is charged ions that are transported (anions in the examples shown below, taken from Ch. 18 of Berg et al.) so that co-transport of a counter ion is needed to maintain electrical neutrality.

The diagram above also highlights another important point. You need to look at the biochemistry of the co-transporting systems, rather than just considering them in abstract. One of the sites of many co-transporters is the mitochondrial membrane, where transport is part of a co-ordinated process.

Consider for example ATP, ADP and phosphate. ATP is synthesized in the mitochondrion from ADP and phosphate, and then much of it must be transported into the cytoplasm. At the same time ADP and phosphate must enter the mitochondrion as substrates for the generation of more ATP. Active transport is clearly a non-starter here (it would use the ATP that is to be transported!) and co-transport ensures that the influx of ADP is balanced by the efflux of ATP. Phosphate influx (which is balanced by hydroxide efflux, to preserve neutrality) must be coupled to this process, although I am not aware of how this achieved (contributions welcome here).

Analogous considerations apply to NAD+ and NADH, but it is the electrons that are transported, rather than these compounds themselves, using surrogate oxidized or reduced compounds. Shuttle systems of this sort integrate the biochemical functions of the mitochondrion with those in the cytoplasm, and need to be studied individually to understand the choice of co-transporters. Section 18.5 of Berg et al. is a convenient starting point.


Would it not then make more sense for the latter substance to simply be actively transported to its destination, as opposed to involving another substance which has no net movement at the end of the process?

But where would the energy for the transport come from! Thermodynamics tells us that the universe tends to increase the amount of disorder (Entropy). If we have active transport of a solute, then by definition, this is going against a concentration gradient. So we could end up with a substance being entirely on one side of a plasma membrane, which wouldn't be disordered at all - clearly thermodynamics wouldn't let this happen by itself since we would be reducing entropy.
This is why energy is needed for active transport. We need to put in the leg-work by using some energy to locally reduce entropy, and ultimately move something to one side of the plasma membrane. Think of the concentration gradient like the pent up energy of a spring. We can release this energy when we want, in order to make it thermodynamically feasible to move a solute against its concentration gradient.

What is the evolutionary advantage of co-transport?

We've established that we need an energy source to drive active transport. But indeed, why would we use a concentration gradient? Couldn't we equivalently use a different energy source? Well yes, we could use ATP (for instance in Na$^+$/K$^+$-ATPase). But a concentration gradient might already be established over a plasma membrane, so its already there as an existing source of energy. Perhaps this is why co-transport evolved with this mechanism.


Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane.

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil ([link]).


In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin , which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.


Volume 1

Types of Transport and Transporters

Epithelial transport and specific transport proteins are covered in more detail in Chapters 1 and 2. In brief, transport across an epithelial cell can be divided into those that do not require a protein, and those that are protein-mediated. The former is termed simple diffusion and is passive, being dependent on the concentration gradient and surface area. If a molecule is capable of permeating the cell membrane, it will cross according to its chemical gradient. Molecules that move using this form of transport are exclusively small and non-polar, and are capable of crossing a lipid bilayer. Simple diffusion cannot be saturated and is not regulated. The movement of NH 3 in parts of the tubule is a good example of this form of transport. Mediated transport, in contrast, is capable of being saturated, and is dependent on the presence of specific proteins. Moreover, it often is regulated by cell signaling. Figure 8.4 shows models representative of the different classes of transport proteins common to renal epithelial cells. Forces driving protein-mediated transport are discussed in more detail below, but they can include the concentration difference of the molecule to be transported across the membrane or barrier, electrical potential differences, and the activity of transport proteins.

Figure 8.4 . Representative models of the different types of renal transport proteins.

Facilitative diffusion, similar to simple diffusion, is passive, allowing molecules to move down concentration gradients. Facilitative transporters translocate molecules across membranes. An example of a facilitative transport protein in the kidney is the proximal tubule glucose transporter, GLUT2.

Ion channels allow restrictive diffusion, a unique form of passive transport. Channels form selective pores in the membrane, allowing ions to cross the membrane through permeation rather than being translocated across the membrane. Restrictive diffusion through ion channels is driven by electrochemical gradients.

The two remaining types of transport proteins allow active transport: the active movement of molecules against gradients. This type of transport is directly, in the case of primary active transport, and indirectly, in the case of secondary active transport, tied to the consumption of energy. Primary active transporters require ATP to transport molecules against their concentration gradients. As discussed below, this ultimately energizes all transport across the renal tubule. The Na + ,K + -ATPase is a notable primary active transporter in the tubule and collecting duct system. Secondary active transporters use gradients established by primary active transporters. They couple the movement of one molecule against its gradient to the movement of another molecule down its gradient. NKCC2 in the TAL and the thiazide-sensitive Na + ,Cl − -co-transporter (NCC) in the DCT are prominent secondary active transporters in the kidney. The latter couples the inward movement of Cl − into the cell against its gradient to the inward movement of Na + into the cell with its gradient. Thus, Na + entering the cell downhill on this transporter pulls Cl − uphill with it.


5.3 Active Transport

Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Electrochemical Gradient

We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K + ) and lower concentrations of sodium (Na + ) than does the extracellular fluid. So in a living cell, the concentration gradient of Na + tends to drive it into the cell, and the electrical gradient of Na + (a positive ion) also tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other elements such as potassium. The electrical gradient of K + , a positive ion, also tends to drive it into the cell, but the concentration gradient of K + tends to drive K + out of the cell (Figure 5.16). The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient .

Art Connection

Injection of a potassium solution into a person’s blood is lethal this is used in capital punishment and euthanasia. Why do you think a potassium solution injection is lethal?

Moving Against a Gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy is harvested from ATP generated through the cell’s metabolism. Active transport mechanisms, collectively called pumps , work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Much of a cell’s supply of metabolic energy may be spent maintaining these processes. (Most of a red blood cell’s metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.) Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.

Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.

Carrier Proteins for Active Transport

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters (Figure 5.17). A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na + -K + ATPase, which carries sodium and potassium ions, and H + -K + ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca 2+ ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

Primary Active Transport

The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The second transport method is still considered active because it depends on the use of energy as does primary transport (Figure 5.18).

One of the most important pumps in animals cells is the sodium-potassium pump (Na + -K + ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na + and K + ) in living cells. The sodium-potassium pump moves K + into the cell while moving Na + out at the same time, at a ratio of three Na + for every two K + ions moved in. The Na + -K + ATPase exists in two forms, depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions. The process consists of the following six steps.

  1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
  2. ATP is hydrolyzed by the protein carrier and a low-energy phosphate group attaches to it.
  3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
  4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
  5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell.
  6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Watch this video to see a simulation of active transport in a sodium-potassium ATPase.

Secondary Active Transport (Co-transport)

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane (Figure 5.19). Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.

Art Connection

If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease?


Carrier Proteins for Active Transport

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters ([link]). A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na + -K + ATPase, which carries sodium and potassium ions, and H + -K + ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca 2+ ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.



Therapeutic Areas II: Cancer, Infectious Diseases, Inflammation & Immunology and Dermatology

7.13.1.5.2 M2 ion channel inhibitors

The transmembrane domain of the influenza A M2 ion channel is the target of the adamantane group of antivirals. Amantadine (Symmetrel), 1, was the first specific influenza antiviral drug licensed in the USA (1966) for the treatment and prevention of influenza A. Rimantadine (Flumadine), 2, was licensed in 1993. The adamantylamines block the ion channel activity of M2 through allosteric inhibition. M2 inhibition blocks viral uncoating and RNA release and results in inhibition of viral replication. These drugs are only effective toward influenza type A and not type B because M2 is specific for type A ( Scheme 1 ).

Amantadine inhibits influenza type A replication in infected MDCK cells with IC50 ranging from 1.1 to >25 μM. Rimantadine is 4- to 10-fold more active. Both drugs are orally bioavailable and exposure at the nasal mucus is the same as in circulation. Both are equally effective provided treatment is initiated within 48 h of the onset of symptoms. Most studies show a reduction in symptom scores and fever by 1 to 2 days as well as a decrease in viral shedding. They are also useful as prophylactics, although there is no convincing evidence that these drugs reduce complications of influenza infections.

The use of the amantadines is limited by the adverse effects and early development of drug resistance. 11,12 Both amantadine and rimantadine can cause severe central nervous system and gastrointestinal side effects. Although the incidence is less with rimantadine, usage in elderly patients is still limited to lower doses. Resistance to amantadine and rimantadine may emerge within the first 3–5 days in up to 50% of children, elderly people, and immune-compromised patients. The mechanism of resistance appears to be due to single amino acid changes in the M2 protein. The rapid and extensive emergence of resistance severely limits the use of these drugs as therapeutic and prophylactic regimens especially in close-contact environments. The resistant viruses are cross-resistant with both amantadine and rimantadine.


Active and Passive Transmembrane Transport

based on whether the transport process is exergonic or endergonic. Passive transport is the exergonic movement of substances across the membrane. In contrast, active transport is the endergonic movement of substances across the membrane that

Passive transport

Passive transport does not require the cell to

energy. In passive transport, substances move from an area of higher concentration to an area of lower concentration, down their concentration gradient

Depending on the chemical nature of the substance, we may associate different processes with passive transport.

Diffusion

Diffusion is a passive process of transport. A single substance moves from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle its lowest concentration is at the edges of the room. The ammonia vapor will diffuse, or spread away, from the bottle gradually, more and more people will smell the ammonia as it spreads. Materials move within the cell&rsquos cytosol by diffusion, and certain materials move through the plasma membrane by diffusion.

Figure 2. Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm). Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium.(credit: modification of work by Mariana Ruiz Villareal)

Factors that affect diffusion

If unconstrained, molecules will move through and explore space randomly at a rate that depends on their size, their shape, their environment, and their thermal energy. This

movement underlies the diffusive movement of molecules through whatever medium they are in. The absence of a concentration gradient does not mean that this movement will stop, just that there may be no net movement of the number of molecules from one area to another, a condition known as a dynamic equilibrium.

Factors influencing diffusion include:

  • Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
  • Shape, size and mass of the molecules diffusing: Large and heavier molecules move more slowly therefore, they diffuse more slowly. The reverse is typically true for smaller, lighter molecules.
  • Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion.
  • Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, rates of diffusion increase. Since cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm&rsquos density will decrease the rate at which materials move in the cytoplasm.
  • Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.
  • Surface area and thickness of the plasma membrane: Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it.
  • Distance traveled: The greater the distance that a substance must travel, the slower the rate of diffusion. This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the center of the cell, respectively. Therefore, cells must either be

, as with many prokaryotes, or

Facilitated transport

In facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that allows these materials to diffuse into or out of the cell without

cellular energy. If the materials are ions or polar molecules (compounds that

by the hydrophobic parts of the cell membrane), facilitated transport proteins help shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.

Channels

The integral proteins involved in facilitated transport are collectively referred

to as transport proteins, and they function as either channels for the material or carriers. In both cases, they are transmembrane proteins. Different channel proteins have different transport properties. Some have evolved to have very high specificity for the substance that is being transported while others transport a variety of molecules sharing some common characteristic

. The interior "passageway" of channel proteins have evolved to provide a low energetic barrier for transport of substances across the membrane through the complementary arrangement of amino acid functional groups (of both backbone and side-chains). Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma membrane that would otherwise slow or prevent their entry into the cell. While at any one time significant amounts of water crosses the membrane both in and out, the rate of an individual water molecule transport may not be fast enough to adapt to changing environmental conditions. For such cases, Nature has evolved a special class of membrane proteins called

that allow water to pass through the membrane at a very high rate.

Figure 3. Facilitated transport moves substances down their concentration gradients. They may cross the plasma membrane with the aid of channel proteins. (credit: modification of work by Mariana Ruiz Villareal)

Channel proteins are either open at all times or they are &ldquogated.&rdquo The latter controls the opening of the channel.

Various mechanisms may be involved

in the gating mechanism. For instance, the attachment of a specific ion or small molecule to the channel protein may trigger opening. Changes in local membrane "stress" or changes in voltage across the membrane may also be triggers to open or close a channel.

Different organisms and tissues in multicellular species express different channel proteins in their membranes depending on the environments they live in or specialized function they play in an organism. This provides each type of cell with a unique membrane permeability profile that is evolved to complement its "needs" (note the anthropomorphism). For example, in some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues a gate must open to allow passage. This occurs in the kidney where both forms of channels are found in different parts of the renal tubules. Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening and closing of these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting a change in electrical potential across the membrane that lead to message propagation with nerve cells or in muscle contraction with muscle cells.

Carrier proteins

Another type of protein embedded in the plasma membrane is a carrier protein. This aptly named protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. This selectivity adds to the overall selectivity of the plasma membrane. The molecular-scale mechanism of function for these proteins remains poorly understood.

Figure 4. Some substances

move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane.

Carrier protein play an important role in the function of kidneys. Glucose, water, salts, ions, and amino acids needed by the body

in one part of the kidney. This filtrate, which includes glucose,

in another part of the kidney with the help of carrier proteins. Because there are only a finite number of carrier proteins for glucose, if more glucose is present in the filtrate than the proteins can handle, the excess

from the body in the urine. In a diabetic individual,

as &ldquospilling glucose into the urine.&rdquo A different group of carrier proteins called glucose transport proteins, or GLUTs,

in transporting glucose and other hexose sugars through plasma membranes within the body.

Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.

Active transport

Active transport mechanisms require the use of the cell&rsquos energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient&mdashthat is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)&mdashthe cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Moving against a gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. Transporters harvest this energy from ATP generated through the cell&rsquos metabolism. Active transport mechanisms, collectively called pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Much of a cell&rsquos supply of metabolic energy may

maintaining these processes. (Most of a red blood cell&rsquos metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.) Because active transport mechanisms depend on a cell&rsquos metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.

Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which directly depends on ATP. Secondary active transport describes the movement of material

the electrochemical gradient established by primary active transport that does not directly require ATP.

Carrier proteins for active transport

An important membrane adaption for active transport is specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters. A

also carries two different ions or molecules, but in different directions. These transporters can also transport small, uncharged organic molecules like glucose.

These three types of carrier proteins are also found

in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are

-K + ATPase, which carries sodium and potassium ions, and H + -K + ATPase, which carries hydrogen and potassium ions. Both are

carrier proteins. Two other carrier proteins are Ca 2+ ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

Figure 5. A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. (credit: modification of work by &ldquoLupask&rdquo/Wikimedia Commons)

Primary active transport

In primary active transport, the energy is

directly from the hydrolysis of ATP. Often, primary active transport, such as that shown below, which functions to transport sodium and potassium ions allows secondary active transport to occur (discussed in the section below).

The second transport method is still considered

active because it depends on the use of energy from the primary transport.

Figure 6. Primary active transport moves ions across a membrane, creating an electrochemical gradient (electrogenic transport). (credit: modification of work by Mariana Ruiz Villareal)

One of the most important pumps in animal cells is the sodium-potassium pump (Na + -K + ATPase), which maintains the electrochemical gradient (and the correct concentrations of

K + ) in living cells. The sodium-potassium pump moves K + into the cell while moving Na + out

at a ratio of three Na + for every two K + ions moved in. The Na + -

exists in two forms depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions. The process comprises the following six steps.

  1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
  2. ATP

Several things have happened because of this process. There are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Visit the site to see a simulation of active transport in a sodium-potassium ATPase.

Secondary active transport (

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will return through the membrane down the gradient. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane. Many amino acids, and glucose, enter a cell this way. This secondary process is also used to store high energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.


24 Active Transport

By the end of this section, you will be able to do the following:

  • Understand how electrochemical gradients affect ions
  • Distinguish between primary active transport and secondary active transport

Active transport mechanisms require the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the substance’s concentration inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Electrochemical Gradient

We have discussed simple concentration gradients—a substance’s differential concentrations across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K + ) and lower concentrations of sodium (Na + ) than the extracellular fluid. Thus in a living cell, the concentration gradient of Na + tends to drive it into the cell, and its electrical gradient (a positive ion) also drives it inward to the negatively charged interior. However, the situation is more complex for other elements such as potassium. The electrical gradient of K + , a positive ion, also drives it into the cell, but the concentration gradient of K + drives K + out of the cell ((Figure)). We call the combined concentration gradient and electrical charge that affects an ion its electrochemical gradient .


Injecting a potassium solution into a person’s blood is lethal. This is how capital punishment and euthanasia subjects die. Why do you think a potassium solution injection is lethal?

Moving Against a Gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy comes from ATP generated through the cell’s metabolism. Active transport mechanisms, or pumps , work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances that living cells require in the face of these passive movements. A cell may spend much of its metabolic energy supply maintaining these processes. (A red blood cell uses most of its metabolic energy to maintain the imbalance between exterior and interior sodium and potassium levels that the cell requires.) Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to many metabolic poisons that interfere with the ATP supply.

Two mechanisms exist for transporting small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport does not directly require ATP: instead, it is the movement of material due to the electrochemical gradient established by primary active transport.

Carrier Proteins for Active Transport

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three protein types or transporters ((Figure)). A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na + -K + ATPase, which carries sodium and potassium ions, and H + -K + ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca 2+ ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.


Primary Active Transport

The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The second transport method is still active because it depends on using energy as does primary transport ((Figure)).


One of the most important pumps in animal cells is the sodium-potassium pump (Na + -K + ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na + and K + ) in living cells. The sodium-potassium pump moves K + into the cell while moving Na + out at the same time, at a ratio of three Na + for every two K + ions moved in. The Na + -K + ATPase exists in two forms, depending on its orientation to the cell’s interior or exterior and its affinity for either sodium or potassium ions. The process consists of the following six steps.

  1. With the enzyme oriented towards the cell’s interior, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
  2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it.
  3. As a result, the carrier changes shape and reorients itself towards the membrane’s exterior. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
  4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
  5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the cell’s interior.
  6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Several things have happened as a result of this process. At this point, there are more sodium ions outside the cell than inside and more potassium ions inside than out. For every three sodium ions that move out, two potassium ions move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Watch this video to see an active transport simulation in a sodium-potassium ATPase.

Secondary Active Transport (Co-transport)

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an electrochemical gradient. If a channel protein exists and is open, the sodium ions will pull through the membrane. This movement transports other substances that can attach themselves to the transport protein through the membrane ((Figure)). Many amino acids, as well as glucose, enter a cell this way. This secondary process also stores high-energy hydrogen ions in the mitochondria of plant and animal cells in order to produce ATP. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy then converts ADP into ATP.


If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease?

Section Summary

The combined gradient that affects an ion includes its concentration gradient and its electrical gradient. A positive ion, for example, might diffuse into a new area, down its concentration gradient, but if it is diffusing into an area of net positive charge, its electrical gradient hampers its diffusion. When dealing with ions in aqueous solutions, one must consider electrochemical and concentration gradient combinations, rather than just the concentration gradient alone. Living cells need certain substances that exist inside the cell in concentrations greater than they exist in the extracellular space. Moving substances up their electrochemical gradients requires energy from the cell. Active transport uses energy stored in ATP to fuel this transport. Active transport of small molecular-sized materials uses integral proteins in the cell membrane to move the materials. These proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with ATP to drive their action. In co-transport (or secondary active transport), energy from primary transport can move another substance into the cell and up its concentration gradient.

Visual Connection Questions

(Figure) Injecting a potassium solution into a person’s blood is lethal. Capital punishment and euthanasia utilize this method in their subjects. Why do you think a potassium solution injection is lethal?

(Figure) Cells typically have a high concentration of potassium in the cytoplasm and are bathed in a high concentration of sodium. Injection of potassium dissipates this electrochemical gradient. In heart muscle, the sodium/potassium potential is responsible for transmitting the signal that causes the muscle to contract. When this potential is dissipated, the signal can’t be transmitted, and the heart stops beating. Potassium injections are also used to stop the heart from beating during surgery.

(Figure) If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease?

(Figure) A decrease in pH means an increase in positively charged H + ions, and an increase in the electrical gradient across the membrane. The transport of amino acids into the cell will increase.

Review Questions

Active transport must function continuously because __________.

  1. plasma membranes wear out
  2. not all membranes are amphiphilic
  3. facilitated transport opposes active transport
  4. diffusion is constantly moving solutes in opposite directions

How does the sodium-potassium pump make the interior of the cell negatively charged?

  1. by expelling anions
  2. by pulling in anions
  3. by expelling more cations than are taken in
  4. by taking in and expelling an equal number of cations

What is the combination of an electrical gradient and a concentration gradient called?

  1. potential gradient
  2. electrical potential
  3. concentration potential
  4. electrochemical gradient

Critical Thinking Questions

Where does the cell get energy for active transport processes?

The cell harvests energy from ATP produced by its own metabolism to power active transport processes, such as the activity of pumps.

How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?

The sodium-potassium pump forces out three (positive) Na + ions for every two (positive) K + ions it pumps in, thus the cell loses a positive charge at every cycle of the pump.

Glucose from digested food enters intestinal epithelial cells by active transport. Why would intestinal cells use active transport when most body cells use facilitated diffusion?

Intestinal epithelial cells use active transport to fulfill their specific role as the cells that transfer glucose from the digested food to the bloodstream. Intestinal cells are exposed to an environment with fluctuating glucose levels. Immediately after eating, glucose in the gut lumen will be high, and could accumulate in intestinal cells by diffusion. However, when the gut lumen is empty, glucose levels are higher in the intestinal cells. If glucose moved by facilitated diffusion, this would cause glucose to flow back out of the intestinal cells and into the gut. Active transport proteins ensure that glucose moves into the intestinal cells, and cannot move back into the gut. It also ensures that glucose transport continues to occur even if high levels of glucose are already present in the intestinal cells. This maximizes the amount of energy the body can harvest from food.

The sodium/calcium exchanger (NCX) transports sodium into and calcium out of cardiac muscle cells. Describe why this transporter is classified as secondary active transport.

The NCX moves sodium down its electrochemical gradient into the cell. Since sodium’s electrochemical gradient is created by the Na+/K+ pump, a transport pump that requires ATP hydrolysis to establish the gradient, the NCX is a secondary active transport process.

Glossary


Co-transport

In co-transport (sometimes called symport) two species of substrate, generally an ion and another molecule or ion, must bind simultaneously to the transporter before its conformational change can take place. As the driving substrate is transported down its concentration gradient, it drags with it the driven substrate, which is forced to move up its concentration gradient. The transporter must be able to undergo a conformational change when not bound to either substrate, so as to complete the cycle and return the binding sites to the side from which driving and driven substrates both move.

Sodium ions are usually the driving substrates in the co-transport systems of animal cells, which maintain high concentrations of these ions through primary active transport. The driven substrates include a variety of sugars, amino acids, and other ions. During the absorption of nutrients, for example, sugars and amino acids are removed from the intestine by co-transport with sodium ions. After passing across the glomerular filter in the kidney, these substrates are returned to the body by the same system. Plant and bacterial cells usually use hydrogen ions as the driving substrate sugars and amino acids are the most common driven substrates. When the bacterium Escherichia coli must metabolize lactose, it co-transports hydrogen ions with lactose (which can reach a concentration 1,000 times higher than that outside the cell).


4 Main Stages of the Transportation Planning Process – Explained!

In most of the countries transport planning is treated as a part of general economic planning and no special attention has been paid, but now not only developed countries but developing countries have also realised the need for separate planning for the transportation, not only for the existing system but for the future development also.

The study of development and planning is basically a study of interaction between man, land and activity in the form of spatial organisation of economy. After industrial revolution and rapid growth of urbanisation, development in the field of transport is enormous both in infrastructures, speed as well as in transport technology. Nowadays every country of the world is having its own national transport system, not in isolation but as a part of international system of transportation. Transport now has, as ever, become an integral and essential part of the economy and requires a planned growth, which should be ‘sustainable’.

In fact, transport planning is the process of regulating and controlling the provision of transport to facilitate the efficient operation of the economic, social and political life of the country at the lowest social cost. In practice, this means assuring adequate transport capacity and efficient operations to meet the needs generated by the nation’s geographical array of activities.

The primary aim of transport planning is the identification and evaluation of the future transport needs. The basis of transport planning process has been depicted in Figure 9.1.

The four main stages of the transportation planning process are:

(i) Transportation survey, data collection and analysis

(ii) Use of transportation model

(iii) Future land use forecasts and alternative policy strategies and

Survey and Data Collection:

The entire planning process of transportation, may be local, regional or national, is based on survey and data collection. This includes all types of literature and data (both government and non-government) available on transportation, journey behaviour patterns, nature and intensity of traffic, freight structure, cost and benefits, i.e., income, employment estimates, etc.

The comprehensive knowledge of traffic flows and patterns within a defined area is essential. In addition to traffic data, planners also require land use and population data for their study area. In this connection West Midlands Transportation Study (1968) provides a format, which is useful for transport survey and data collection (Figure 9.2).

The survey should be well defined and be divided in ‘zones’ so that origins and destinations of trips can be geograph­ically monitored. The data collection regarding existing travel patterns is time consuming as well as a costly affair. It involves both ‘road­side-interview’ and ‘home-interview’. The variables for both types of interviews are given in the Table 9.1

The details-of existing transport network are an important source of information. In some cases, a very detailed description of links and nodes in terms of vehicle speed, carriage-way width and nodal type is collected. Travel times and network characteristics of public transport networks are simultaneously collected. Finally, data processing should be done. When this has been completed, planners can begin their data analysis.

The Transportation Model:

The second stage of the transportation planning process is to use the collected data to build up a transportation model. This model is the key to predicting future travel demands and network needs and is derived in four recognised stages, i.e., trip generation, trip distribution, traffic assignment and model split.

The first stage of model building process is that of trip gener­ation. Trips are made for a variety of purposes and for various land uses. For convenience, trips are often split into two groups:

Such trips have one trip end at the home of the person making the trip, which may be either the origin or destination of the given trip.

(ii) Non-home-based trips:

These have neither origin nor desti­nation trip-end at the home of the person making the trip.

This initial part of the transport model expresses trip-making relationships in a mathematical form so that ultimately we can calculate the total number of trips-ends origi­nating from the defined survey zones.

Multiple regression technique are often used to calibrate a trip-generation model incorporating the above household variables. This model takes the following general form:

where Y = number of trips (by mode and purpose) generated in a given zone

b1…bn = regression coefficients relating to independent variables (e.g. household income, car-owner- ship, house-hold structure, etc.)

New estimates of the independent variables are made and inserted into the equation in order to estimate future levels of trips generation. Multiple regression analysis, therefore, provides a suitable method for estimating future trip levels. Its main disadvantage, however, is that the original regression estimates have been established at a given point in time and are expected to remain constant over the period for which the forecast is required.

Consequently, a more recent approach to trip generation has been to use a technique known as ‘category analysis’. The trip-generation stage of the planning process estimates the total number of trips originating in the survey area at one or more future dates.

This is the next stage in the transportation model, it involves on analysis of trips between zones. Lane (1971) states the function of this stage of the model:

It is the function of trip distribution to calculate the number of trips between one zone and another, given the previously deter­mined numbers of trip ends in each zone together with further information on the transport facilities available between these zones.

For example, given that in zone I, gi trip ends are generated and that in zone j, ai trip ends are attracted, it is the purpose of the trip distribution model to determine the number of trips (tij) which would go from zone i to zone j. That is, the trip distri­bution model calculates the proportion of trip ends generated in zone i which would travel between i and j and so take up a certain proportion of the available attractions in zone j.

Overall, the distribution stage of the transportation model has received considerable attention and has been the main source of research over the last quarter of a century. The earliest attempts to produce a future trip distribution matrix used simple growth factor methods, taking the following general form:

where Tij = future flow from zone i to zone j

tjj = base year flow from zone i to zone j

E = agreed expansion factor

The value of the expansion factor can take various forms. For example, Bevis (1956) put forward the idea of a crude expansion factor of the following format:

Where Tij= future origin zone

ti = base year origins zone i ,

Tj = future destinations zone j,

tj = base year destinations zone j.

This simple model was further refined, but growth-factor techniques are now rarely used. The method is a crude one and has been superseded largely because it does not attempt to measure any future resistance to travel between zones. For this reason, synthetic models tend to be widely used to model trip distribution. The trip-distribution stage of the transport model has received much attention and has been the source of many new developments.

The third stage of the modelling process is that of traffic assignment, its aim being to stimulate route choice through a defined transport network. Traffic assignment may be considered in two parts.

First, it is necessary to define the transport network and determine criteria for route choice through the network.

Second, using the inter-zonal trip matrix as the input data, trips are assigned to this network.

When future trip levels are assigned it is possible to assess deficiencies in the existing transport network and so determine a list of construction priorities. Network description refers to the process where the highway network is broken down into links and nodes. For each link, data is required on its length, road type, vehicle travel time and traffic capacity. When coding the road network, links are usually identified by the node numbers at each of its ends. In addition to such route-intersection nodes, zone-centroid nodes are also defined. In the assignment process, all traffic originating in a particular traffic zone is assumed to be loaded on to the network at this latter type of node.

The early transportation studies used manual assignment techniques, but with the universal use of computer analysis, the transport network can be specified to the computer in a most detailed manner. Special data collection surveys (especially of journey times) are usually needed to provide this network specification information.

For deriving minimum route paths through the network, it is normally assumed that travellers choose the path, which minimises travel time. This applies for both private and public transport journeys. Travel time has been used in most transportation studies although it is usually used as an approxi­mation for minimising the travel costs of a journey.

A more recent and more realistic assignment procedure is that of capacity restraint. This may be used, with or without diversion curves, for assignments to road and public transport networks. After the initial assignment to the given network, new travel times are calculated for each link. New minimum path trees are then calculated and the assignment procedure reiterated. Further reiterations may follow until most or all of the future traffic volume has been assigned to the network.

This type of procedure has tended to supersede other assignment techniques and has been used in most of the second-generation transportation studies. The assignment stage of the transpor­tation model therefore is the process by which trips are assigned or loaded on to the road network. At the end of this stage, construction priorities can be established and alternative proposals put forward.

This term is used by transport planners to describe the phase where the choice of travel mode is incorporated into the model. The positioning of this stage is neither fixed nor singularly definable since elements of model split are part of the other stages. Its position within the transportation model differs between studies. It is either used at the trip generation stage by stratifying the total trips or at the assignment stage of the model. The main purpose of the model-split stage is to determine the trip shares of public, as against private, transport.

Future Land Use and Travel Demand Forecasting:

The forecasting of future land use inputs is a precarious task, for two important reasons. Firstly, transport planners have to rely on the judgment of to the types of planner for most of their land use forecasts. This information is vitally important since it has a profound effect upon travel forecasts. Secondly, long-term forecasting is beset with many statistical problems.

Since trans­portation planners are usually working at least 10, and sometimes 25 years ahead, their estimates are inevitably open to much criticism. Nevertheless, estimates of future travel demands have to be made using the best methods, which are available. Some of these forecasting problems are amplified below in the listing of the main land use inputs necessary for travel forecasts to be made.

The most important variables are:

(i) Population – its size, age structure and distribution.

(ii) Employment – as the journey to work is the greatest travel demand.

(iii) Personal income and expenditure.

The above groups of variables have a compound influence upon the overall level of demand for travel at some future date. Further complications arise when their impact upon the spatial pattern of this demand is assessed. So, forecasts of population and economic variables are an important input into the use of the transportation model for forecasting future travel demands.

The final stage of the transportation planning process is one of evaluating the alternative policies, which have been suggested. The evaluation stage is probably the most important of all, yet has received only limited research attention. An economic evaluation of transport proposals is necessary because vehicle-km and road space are commodities, which are not directly bought and sold.

The technique of cost benefit analysis has consequently evolved as an investment criterion in the public sector. As such, it provides an economic evaluation. On the cost side of the calculation, estimates are made for capital outlay, land purchase and maintenance.

The benefits are those accruing to users, e.g., savings in time, vehicle operation and accidents. The individual costs and benefits are assessed over a particular number of years and discounted back to the base year so that a rate of return can be calculated. On the basis of ‘transportation plan’, transport policies should be formulated and implemented properly so that systematic ‘sustainable’ development of transport can be done.

Nowadays every country is particular regarding the planned development of transport system, thus formulate their own transport policy, which depends upon their needs and resources. The nature of transport policy varies with time and space. In formulating transport policy, one should take into consideration the ‘coordination’ and ‘competition’.

The coordination involves the relationship between two or more different modes of transport. On the others hand, competition has occurred as a consequence of the public/private sector interaction. The transport policy also differs with the type of government, i.e., socialistic, democratic, etc. Inspite of varia­tions in policy, which are natural, there are certain points which are useful if incorporated in transport policy. The points are from transport policies of the countries like USA, UK, Netherland, and European Union. These are as follows:

Transport Demand Management in USA:

‘Transport Demand Management’ (TDM) system as a part of transport policy has been adopted in USA. TDM is the art of modifying travel behaviour in order to reduce the number of trips or modify their nature. It may be categorised according to whether it mainly affects trip generation, trip distribution, and model choice or route selection. As Table 9.2 shows, some implementation strategies rely on changes to the transport system, others on land use policies and still others on alter­ations to employment conditions and social values.

In the field of TDM, the USA has done considerable work. Persuading a number of large companies to introduce flexible working hours (‘flexitime’) is a logical way to reduce congestion at peak periods. The introduction of car-pooling is another step in this direction. The most TDM measures are ones that require employers to reduce the number of peak-period car trips made by their worker. In USA at least 20 suburban communities have enacted such programmes.

Netherlands’s Policy for ‘Sustainable Development:

National Environmental Policy Plan or NEPP of Netherland has been adopted in 1989. NEPP is an example of environ­mental protection as well as policy for the control of pollution created by transport. The NEPP recognises that safeguarding environmental quality on behalf of what it calls ‘sustainable development’ will be a process that will last for several decades. The NEPP is the first step in this process: it contains the medium-term strategy for environmental policy, which is directed at the attainment for sustainable development over the longer period.

The objectives of the NEPP are:

i. Vehicles must be as clean, quiet, safe and economical as possible

ii. The choice or mode for passenger transport must result in the lowest possible energy consumptions and the least possible pollution and

iii. The locations where people live, shop, work and spend their leisure time will be coordinated in such a way that the need to travel is minimised.

The approach of the NEPP is shown in Figure 9.3. As pollution from road traffic is seen as a three-step process, these objectives are to be met through a ‘three-track’ response, the tracks being those of technical vehicle standards, reducing ‘automobility’ and instigating urban traffic measures.

As shown in Figure 9.3, the three-track approach has been developed to the abatement of environmental pollution. The first track consists of a series of measures to convert the vehicle fleet into one that is the cleanest possible.

The second track, of reducing car use, aims to shift people from cars to public transport for the longer journeys and to cycling or walking for the shorter ones. This is to be achieved through provision of more and better facilities for cycling and public transport, more subsidies, better fare and ticket integration and publicity campaigns.

However, it is recognised that if the policy is to seek a balance between individual freedom, accessibility and the environment, the only way to achieve this is to control the use of cars. Therefore, the strategy is to increase variable motoring costs through fuel taxation and road pricing. Car commuting will be discouraged through a variety of TDM measures including ‘kilometre reduction plans’, whereby companies and institutions will have to draw up and then implement plans to reduce the distance travelled by employees in the course of work and in commuting to it.

Additionally, the second track will improve the transport of freight by rail and water and will tighten up physical planning policy, to ensure that businesses which are labour-intensive or amenities which attract numerous visitors are not permitted to locate at places which are not well served by public transport.

As well as having cleaner vehicles, which are used less, the NEPP recognises – the third ‘track’ – that further measures are necessary to alleviate the problems at a local scale. These include stricter enforcement of parking controls, traffic management to influence drivers’ choice of routes, circulation schemes to slow traffic and similar measures to improve road safety and increase environmental protection.

The most noticeable feature of the NEPP is the way that its individual measures reinforces each other, to produce an integrated package which links environmental, transport and land use policy. Yet even this impressive, comprehensive approach comes nowhere near solving the problems. Without the NEPP, car-kilometres had been expected to rise by 72 per cent over the period 1986-2010.

With the NEPP this increase is lowered to 48 per cent, a worthwhile reduction but still a very long way from a sustainable level of transport use. The NEPP must be seen only as the first stage in a long-term drive towards sustainability: it serves to illustrate what a difficult task lies ahead of the Dutch (and indeed all motorised countries).

Transport Policy in UK:

Due to geographical conditions, UK always remains very particular regarding its transport development and policy. Since 1945, UK has done considerable changes in its transport policy.

In general, three particular phases can be distinguished:

(i) 1945-51: The genesis of nationalised transport sector, with increasing regulation in order to restrict competition, coordination of transport services was envisaged through state (i.e., common) ownership.

(ii) 1951-68: A gradual relaxation of regulation and control, with the aim of allowing natural tendencies to determine the direction of transport policy.

(iii) 1968-77: Coordination through competition remains foremost, but more resources have been allocated to propping up and rationalising an ailing public transport sector.

The notable points of UK’s transport policy (1970) are:

(i) The transport infrastructure and services (rail, road, ports, etc.) must be modernised. Since total resources are limited, this means planning investment as a whole, increasing productivity and developing better criteria to assist choice.

(ii) The problem of traffic conditions in towns must be given greater priority,

(iii) The transport system must take account of the social as well as economic needs of the country.

(iv) Public transport must play a key role in solving the transport problems.

Five areas of concern are also identified:

(i) There is still no proper framework for the effective coordi­nation of transport policy,

(ii) The social problem of ensuring adequate public transport for those without cars is becoming more pressing.

(iii) Concern for the environment and the quality of life has increased dramatically in recent years.

(iv) The energy crisis of 1973 has necessitated a revision of car ownership forecasts.

(v) Overshadowing all the above developments is the need for public expenditure restrictions.

Following Britain’s accession to the EEC in 1973, the transport policy has also been changed accordingly but its basic features remained the same.

Example of the European Union (EU):

The EU has not yet been able to evolve a common policy for the development of transport. Within the EU there are differ­ences between members states in their philosophical position towards transport, with the ‘Anglo-Saxon’ approach focusing on economic efficiency and contrasting strongly with French-German-Scandinavian attitudes in which efficiency is more usually seen as secondary to the wider role of transport within economic and social planning.

This conflict between the interests of member states produces an unstable policy environment and one which is far from ideal for the task of producing profound insights or long-term goals. There are further complications as a result of the fact that the EU does not form a contiguous geographical space, with Greece physi­cally separate and routes having to pass through third-party countries such as Switzerland in order to connect two members such as Germany and Italy. Not surprisingly, by the time the Single Economic Market (SEM) came into existence in 1993 a common market in transport still had not been achieved.

The main policy objectives at the European level are now:

i. An economic and regulatory framework for transport, including harmonisation of fiscal policies and fair comparison and assessment of different transport projects

ii. New research and development initiative

iii. Standardisation and technical regulation, e.g., road pricing technology

iv. Development of trans-European networks and

v. Information exchange, including better quality transport statistics, which will assist the objective of ‘sustainable mobility’.

Policy for Sustainable Transport:

Sustainable means “that meets the needs of the present without compromising the ability of future genera­tions to meet their own needs”.

For transport to be sustainable, it must satisfy three basic conditions:

(1) Its rates of use of renewable resources do not exceed their rates of regeneration

(2) Its rates of use for non-renewable resources do not exceed the rate at which sustainable renewable substitutes are developed and

(3) Its rates of pollution emission do not exceed the assimilative capacity of the environment.

The following guiding principles have been listed by

Withelegg (1993) for sustainable transport development:

(1) Transport is a vital element in economic and social activ­ities but must serve those activities rather than be an end in itself.

(2) The consumption of distance by freight and passengers should be minimised as far as possible whilst maximising the potential for locally based social interaction and locally based economic activity.

(3) All transport needs should be met by the means that is least damaging to the environment.

(4) There should be a presumption in physical land use planning against those activities, which by nature of their size and importance attract car-based users from a large area.

(5) All through investment plans should be subjected to a full health audit notwithstanding the uncertainties surround­ing epidemiological proof. Proposals which are potentially health damaging should be rejected.

(6) All transport investment plans should have clear objective designed to cover social, economic and environment concerns and be evaluated by an independent authority with sufficient expertise to comment on value for money, costs and benefits and the availability of alternative strat­egies to achieve the same objectives.

(7) All transport investment should be monitored over their lifetime to check on the degree to which they meet their objectives and their contribution to environmental damage.

(8) All transport policy matter should be dealt with in a transport policy directorate that has no direct responsibilities for the management of individual modes. The responsibilities of the directorate are to deliver sharply focused polices that minimise danger, minimise air and noise pollution, maximise social interaction and urban quality of life and oversee the non-policy-making execu­tives (for road, rail and air) whose role is to implement the directives of the transport policy directorate (Whitelegg, 1993:157). These principles would represent a starting-point for a new approach to transport policy and set out an agenda for transport planners.