Winter_2021_Bis2A_Facciotti_Reading_10_NB2

Learning Goals Associated with Winter_2021_Bis2A_Facciotti_Reading_10 Compare and contrast passive diffusion, facilitated diffusion, and active transport.  Discuss the advantages and trade-offs associated with passive diffusion, facilitated diffusion, and active transport.  Predict the properties of the R-groups on amino acids located in different locations within transmembrane transporters. Predict how changing a specific amino acid or group of amino acids within a transmembrane transporter protein might affect its function. Understand the role of the plasma membrane in maintaining chemical and electrical gradients.

 

Energetics of transport

All substances that move through the membrane do so by one of two general methods, which are categorized 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 is coupled to an exergonic reaction.

Passive transport

Passive transport does not require the cell to expend 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’s 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 type of 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’s 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 small in size, as with many prokaryotes, or be flattened, as with many single-celled eukaryotes.

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 expending cellular energy. If the materials are ions or polar molecules (compounds that are repelled 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(s). 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 aquaporins 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 “gated.” 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 are able to 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. (credit: modification of work by Mariana Ruiz Villareal)

Carrier protein play an important role in the function of kidneys. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed 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 is not reabsorbed and it is excreted from the body in the urine. In a diabetic individual, this is described as “spilling glucose into the urine.” A different group of carrier proteins called glucose transport proteins, or GLUTs, are involved 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’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.

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’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 directly depends on ATP. Secondary active transport describes the movement of material 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 specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters. 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. 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 are antiporter carrier proteins. Two other carrier proteins are Ca²⁺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 “Lupask”/Wikimedia Commons)

Primary active transport

In primary active transport, the energy is often - though not only - derived 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 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 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 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 it releases the two ions into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

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.

Link to learning

Visit the site 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 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.

 

Figure 7. An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport. (credit: modification of work by Mariana Ruiz Villareal)