MCAT Biochemistry Review

Chapter 8: Biological Membranes

8.3 Membrane Transport

The cell membrane functions to control movement of substances into and out of the cell; however, it varies in its selectivity for different substances. Transport of small nonpolar molecules occurs rapidly through the cell membrane via diffusion, while ions and larger molecules require more specialized transport processes. The different membrane traffic processes are classified as either active or passive, and are driven by concentration gradients or intracellular energy stores.

CONCENTRATION GRADIENTS

Transport processes can be classified as active or passive depending on their thermodynamics. Spontaneous processes that do not require energy (negative ΔG) proceed through passive transport, while those that are nonspontaneous and require energy (positive ΔG) proceed through active transport. Diffusion, facilitated diffusion, and osmosis generally increase in rate as temperature increases, while active transport may or may not be affected by temperature, depending on the enthalpy (ΔH) of the process. The primary thermodynamic motivator in most passive transport is an increase in entropy (ΔS).

MCAT EXPERTISE

An important point to keep in mind is that all transmembrane movement is based on concentration gradients, which are an MCAT favorite; understanding concentration gradients will net you points on Test Day. Remember that the gradient will tell us whether this process will be passive or active.

PASSIVE TRANSPORT

Passive transport processes are those that do not require intracellular energy stores but rather utilize the concentration gradient to supply the energy for particles to move.

Simple Diffusion

The most basic of all membrane traffic processes is simple diffusion, in which substrates move down their concentration gradient directly across the membrane. Only particles that are freely permeable to the membrane are able to undergo simple diffusion. There is potential energy in a chemical gradient; some of this energy is dissipated as the gradient is utilized during simple diffusion. We can liken this process to a ball rolling down a hill: there is potential energy in the ball when it sits at the top of the hill, and as the ball spontaneously rolls down the hill, some of the energy is dissipated.

Osmosis

Osmosis is a specific kind of simple diffusion that concerns water; water will move from a region of lower solute concentration to one of higher solute concentration. That is, it will move from a region of higher water concentration (more dilute solution) down its gradient to a region of lower water concentration (more concentrated solution). Osmosis is important in several places, most notably when the solute itself is impermeable to the membrane. In such a case, water will move to try to bring solute concentrations to equimolarity, as shown in Figure 8.8. If the concentration of solutes inside the cell is higher than the surrounding solution, the solution is said to be hypotonic; such a solution will cause a cell to swell as water rushes in, sometimes to the point of bursting. A solution that is more concentrated than the cell is termed a hypertonic solution, and water will move out of the cell. If the solutions inside and outside are equimolar, they are said to be isotonic. A key point here is that isotonicity does not prevent movement; rather, it prevents the net movement of particles. Water molecules will continue to move; however, the cell will neither gain nor lose water.

Figure 8.8. Osmosis Water moves from areas of low solute (high water) concentration to high solute (low water) concentration.

REAL WORLD

Osmolarity explains why pure water should never be given intravenously for resuscitation. Red blood cells have an osmolarity around  while pure water has an osmolarity of  Water would rush into the red blood cells, causing them to burst. To avoid this, saline or dextrose-containing solutions are used.

MNEMONIC

To remember that water flows into a cell placed in hypOtonic solution, imagine the cell swelling to form a giant letter O.

One method of quantifying the driving force behind osmosis is osmotic pressure. Osmotic pressure is a colligative property: a physical property of solutions that is dependent on the concentration of dissolved particles but not on the chemical identity of those dissolved particles. Other examples of colligative properties include vapor pressure depression (Raoult's Law), boiling point elevation, and freezing point depression.

To illustrate osmotic pressure, consider a container separated into two compartments by a semipermeable membrane, just like the membranes in our cells. One compartment contains pure water, while the other contains water with dissolved solutes. The membrane allows water but not solutes to pass through. Because substances tend to flow, or diffuse, from higher to lower concentration (which results in an increase in entropy), water will diffuse from the compartment containing pure water into the compartment containing the water–solute mixture. This net flow will cause the water level in the compartment containing the solution to rise above the level in the compartment containing pure water, as shown in Figure 8.9.

Figure 8.9. Change in Water Level Due to Osmotic Pressure

Because the solute cannot pass through the membrane, the concentrations of solute in the two compartments can never be equal. However, the hydrostatic pressure exerted by the water level in the solute-containing compartment will eventually oppose the influx of water; thus, the water level will only rise to the point at which it exerts a sufficient pressure to counterbalance the tendency of water to flow across the membrane. This pressure, defined as the osmotic pressure (II) of the solution, is given by the formula:

Π = iMRT

Equation 8.1

where M is the molarity of the solution, R is the ideal gas constant, T is the absolute temperature (in kelvins), i is the van 't Hoff factor, which is simply the number of particles obtained from the molecule when in solution. For example, glucose remains one intact molecule, so iglucose = 1; sodium chloride becomes two ions (Na+ and Cl), so iNaCl = 2. The equation clearly shows that osmotic pressure is directly proportional to the molarity of the solution. Thus, osmotic pressure, like all colligative properties, depends only on the presence and number of particles in solution, but not their actual identity.

In cells, the osmotic pressure is maintained against the cell membrane, rather than the force of gravity. If the osmotic pressure created by the solutes within a cell exceeds the pressure that the cell membrane can withstand, the cell will lyse. Generally, osmotic pressure is best thought of as a “sucking” pressure, drawing water into the cell in proportion to the concentration of the solution.

Facilitated Diffusion

Facilitated diffusion is simple diffusion for molecules that are impermeable to the membrane (large, polar, or charged); the energy barrier is too high for these molecules to cross freely. Facilitated diffusion requires integral membrane proteins to serve as transporters or channels for these substrates.

KEY CONCEPT

Unless otherwise specified, semipermeable membrane refers to a membrane governed by the same permeability rules as biological membranes: small, nonpolar, lipid-soluble particles (and water) can pass through freely, while large, polar, or charged particles cannot.

The classic examples of facilitated diffusion involve a carrier or channel protein. Carriers are only open to one side of the cell membrane at any given point. This model is similar to a revolving door because the substrate binds to the transport protein (walks in), remains in the transporter during a conformational change (spins), and then finally dissociates from the substrate-binding site of the transporter (walks out). Binding of the substrate molecule to the transporter protein induces a conformational change; for a brief time, the carrier is in the occluded state, in which the carrier is not open to either side of the phospholipid bilayer. In addition to carriers, channels are also viable transporters for facilitated diffusion. Channels may have an open or closed conformation. In their open conformation, channels are exposed to both sides of the cell membrane and act like a tunnel for the particles to diffuse through, thereby permitting much more rapid transport kinetics. The activity of the three main types of ion channels was discussed in Chapter 3 of MCAT Biochemistry Review.

ACTIVE TRANSPORT

Active transport results in the net movement of a solute against its concentration gradient, just like rolling a ball uphill. Active transport always requires energy, but the source of this energy can vary. Primary active transport uses ATP or another energy molecule to directly transport molecules across a membrane. Generally, primary active transport involves the use of a transmembrane ATPaseSecondary active transport, also known as coupled transport, also uses energy to transport molecules across the membrane; however, in contrast to primary active transport, there is no direct coupling to ATP hydrolysis. Instead, secondary active transport harnesses the energy released by one particle going down its electrochemical gradient to drive a different particle up its gradient. When both particles flow the same direction across the membrane, it is termedsymport. When the particles flow in opposite directions, it is called antiport. Active transport is important in many tissues. For instance, primary active transport maintains the membrane potential of neurons in the nervous system. The kidneys use secondary active transport, usually driven by sodium, to reabsorb and secrete various solutes into and out of the filtrate.

Figure 8.10. Membrane Transport Processes The movement of solutes across the cell membrane is mediated by concentration gradients.

Figure 8.10 shows simple diffusion, facilitated diffusion, and active transport. Table 8.1 summarizes these types of movement as well as osmosis.

 

Simple Diffusion

Osmosis

Facilitated Diffusion

Active Transport

Concentration gradient of solute

High → Low

Low → High

High → Low

Low → High

Membrane protein required

No

No

Yes

Yes

Energy required

NO—this is a passive process

NO—this is a passive process

NO—this is a passive process

YES—this is an active process; requires energy

Example molecule(s) transported

Small, nonpolar (O2, CO2)

H2O

Polar molecules (glucose) or ions (Na+, Cl)

Polar molecules or ions (Na+, Cl, K+)

Table 8.1. Membrane Transport Processes

ENDOCYTOSIS AND EXOCYTOSIS

Endocytosis

Endocytosis occurs when the cell membrane invaginates and engulfs material to bring it into the cell. The material is encased in a vesicle, which is important because cells will sometimes ingest toxic substances. Pinocytosis is the endo-cytosis of fluids and dissolved particles, whereasphagocytosis is the ingestion of large solids such as bacteria. Substrate binding to specific receptors embedded within the plasma membrane will initiate the process of endocytosis.

Exocytosis

Exocytosis occurs when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment. Exocytosis is important in the nervous system and intercellular signaling. For instance, exocytosis of neurotransmitters from synaptic vesicles is a crucial aspect of neuron physiology. Both endo- and exocytosis are illustrated in Figure 8.11.

Figure 8.11. Endocytosis and Exocytosis

MCAT Concept Check 8.3:

Before you move on, assess your understanding of the material with these questions.

1.    What is the primary thermodynamic factor responsible for passive transport?

2.    What is the relationship between osmotic pressure and the direction of osmosis through a semipermeable membrane?

3.    Compare the two types of active transport. What is the difference between symport and antiport?