Chapter 4 - Cell Membrane Transport

   Membranes, whether the cell membrane itself or membranes surrounding organelles, serve the vital function of forming a boundary. This boundary is dynamic in that it selectively permits certain molecules to cross it and others not to cross it. It also has the ability to move molecules across the boundary against a concentration gradient.
   Cell membrane transport is crucial to understanding how the diverse cells, tissues and organs in the body perform their functions. It is also key to understanding how drugs function to counteract disease and how various fluid compartments in the body are maintained.

Factors Affecting the Direction of Transport
   If energy is not necessary to move molecules across a membrane the transport is called passive transport. Passive transport may involve molecules capable of crossing the membrane without assistance and molecules requiring a transport protein. The energy driving the movement of molecules during passive transport comes from differences in the concentration of the molecules on either side of the membrane with the molecules tending to move from  the side of higher concentration to the side of lower concentration because of their net thermal movement.
   When the transport of a molecule across the membrane requires energy the transport is called active transport.
Driving Forces Acting on Molecules
   Molecules will move from an area of higher energy to a lower energy. The forces that create this energy may be chemical, electrical or electrochemical.
1. Chemical Driving Force
   When a molecule exists in a different concentration on either side of the membrane, a concentration gradient is said to exist across the membrane (DC symbolizes this difference in concentration).
   Molecules will move because of their net thermal motion from an area of higher concentration to an area of lower concentration. The net force driving molecules down a concentration gradient is the chemical driving force. This force is directly proportional to the concentration gradient. In other words, the greater the gradient the greater the force. If there are more than one kind of molecule across a cell membrane each molecule has its own concentration gradient or chemical driving force.
  Molecules move from a region of high concentration, spontaneously. In other words, this happens because it results in a release of energy. Molecules in this situation are described as going down their concentration gradient. In contrast, molecules going from a region of low concentration to a region of high concentration require energy. Molecules are then described as going up their concentration gradient.
2. Electrical Driving Force
   Ions, atoms or molecules that have a charge, can be affected by an electrical driving force. This force across a cell membrane is expressed as the membrane potential. This potential results from an unequal distribution of charges across the membrane.
   If ions that contain a positive charge (cations) balance the ions with the negative charge (anions) across the membrane, the membrane potential would be zero. This is not the case in the living cell. If we consider the cell membrane there is usually more anions on the intracellular side of the membrane than on the extracellular side. The excess charges on either side of the membrane are attracted to the separating membrane because the anions are attracted to the cations and vice versa.
   The cell membrane potential  is a reflection of the separation of charges by the membrane. The size of the membrane potential is measured in mV (one thousandth of a volt) and is proportional  to the number of opposite charges separated by the membrane. By convention the sign of the membrane potential refers to the net charge of ions inside the cell compared to the outside of the cell. In the resting cell there are more anions within the cell compared to outside the cell. Hence the potential is negative. For most cells the membrane potential is around -70mV.
  The direction of the electrical driving force results in the ion moving toward a region where the opposite charge exists. The simple rule is that like charges repel, and opposite charges attract, each other. Hence, in the resting cell, cations are attracted  to the interior of the cell and anions are attracted to the exterior of the cell. The magnitude of the electrical force depends upon the size of the membrane potential and the charge of the ion. The greater the membrane potential or the charge of the ion the greater the electrical driving force.
3. Electrochemical Driving Force
   The total forces acting upon ions across a membrane is a combination of both chemical and electrical forces and is referred to as the  electrochemical driving force. The net direction of this force is equal to the sum of both forces.
   If the chemical and electrical driving forces are in opposite directions, the point at which each equals the other is the equilibrium potential. The equilibrium potential of an ion depends upon its concentration gradient and valence (charge). The equilibrium potential can be determined using the Nernst equation:
  
E = 61 log Co
Z Ci
Z is equal to the valence of the ion. Co is its concentration outside the cell Ci is its concentration inside. (See page 100 for using this equation to calculate the equilibrium potential for Na+ and K+.) 
Determining the Direction of the Electrochemical Driving Force  
1. If the chemical and electrical forces on the ion are in the same direction no analysis is needed.
2. If the chemical and electrical forces on the ion are in opposite directions, and are equal, there is no net force on the ion and the equilibrium potential equals the membrane potential.
3. If the chemical and electrical forces are in opposite directions and the equilibrium potential > the membrane potential, the chemical force is larger and the combined electrochemical force is in the direction of the chemical force. If the membrane potential > equilibrium potential, the electrical force is larger and the electrochemical force is in the direction of the electrical driving force. 
     In other words, if the forces are in opposite directions think of the membrane potential as representing the electrical force and the equilibrium potential as representing the chemical force (the force due to the concentration of the ion). Then a comparison of the magnitude of each will tell you which force is greater the electrical force (membrane potential), or the chemical force (equilibrium potential).
Significance of Electrochemical Driving Force
     The electrochemical driving force will indicate in which direction across the membrane an ion can move passively or without the input of energy (spontaneously) and in which direction energy is required to move the ion across the membrane.
     An ion moving in the direction in which no energy is required is going down its electrochemical gradient and an ion moving in a direction in which energy is required is going up its electrochemical gradient. 
Flux
  The direction of movement of molecules or ions is actually the result of what is called the net flux. 
  Flux is the rate at which a substance is transported across a membrane. It is equal to the number of molecules crossing a membrane per unit time per unit surface area. The net movement is what is measured by flux because molecules will be moving in both directions.
  If we look at the flux in each direction in isolation we are considering the unidirectional or one-way flux. Looking at both unidirectional  fluxes together gives the net flux.
Passive Transport 
Simple Diffusion
   Diffusion is the result of the movement of molecules due to their thermal motion. Although the thermal motion of individual molecules is random, the molecules in a total population of molecules will tend to move down their concentration gradients.
Factors Affecting Rates of Simple Diffusion
   The rate of transport of molecules across a membrane by simple diffusion depends on three factors:
1. Size of Driving Force
   In simple diffusion, the net flux of a molecule is directly proportional to the size of the chemical driving force (concentration gradient). If an ion is involved, the net flux is proportional to both the chemical and electrical driving forces (electrochemical gradient). 
2. Membrane Surface Area
   The rate of transport of molecules across a membrane varies in direct proportion to the membrane's surface area. (The importance of this simple concept will become clear when we look at exchange across the surfaces of the pulmonary and intestinal epithelia and the capillary endothelium.)
3. Membrane Permeability
   The permeability of a membrane to a molecule depends upon the nature of the molecule and the various properties of the membrane that affect the ability of the molecule to cross it. Hence, permeability will always refer to a particular membrane and will vary with different types of molecules (permeants). The permeability of a membrane for a substance is Px with x representing the molecule under consideration. 
  
   All three factors are expressed mathematically by Fick's Law:

Net Flux

=

PA(DC)
with being the membrane surface area
DC concentration gradient
P permeability factor for molecule in question
 

 

Factors Affecting the Permeability of Membranes
1. Lipid solubility of the diffusing substance.
   Generally, the more lipid soluble a substance is the greater the permeability to that substance.
2. Size and shape of the diffusing substance.
   The larger and more irregular in shape the molecule the lower the membrane permeability.
3. Temperature.
   The higher the temperature the greater the permeability. This factor is rarely important because of the constancy of temperature in the human body. 
4. Membrane Thickness.
   The thinner the membrane the more permeable it is to various molecules.
   Lipid solubility has the strongest influence on permeability as most substances in the body are hydrophilic and don't easily cross the lipid bilayer.
Facilitated Diffusion
   If the transport of molecules across the membrane is mediated by a transmembrane protein, but the force driving transport is either a concentration gradient (chemical force) or an electrochemical gradient, the process is facilitated diffusion.
   Carriers.
   A carrier is transmembrane protein that binds specific molecules or classes of molecules and transports them to the other side by changing their shape (conformation). 
   The rate of transport with carriers depends on three factors:
1. The transport rate of the individual carriers.
2. The number of carriers in the membrane.
3. The size of the driving force acting upon the permeating molecules. This force again is the concentration gradient, the chemical force. 
   Carrier proteins can reach a saturation point when their binding sites are no longer available. This occurs when the concentration of the transported molecule reaches a certain point. When this point, is reached increasing the concentration of the diffusing molecules does not increase the rate of transport across the membrane. However, it is possible to regulate the number of carriers in a membrane and thereby increase the rate of transport. For example, one of the actions of insulin is to increase the number of glucose carriers in the cell membrane of selected cells. 
  Ion Channels

 
   Channels are transmembrane proteins that transport molecules through a passageway or pore extending from one side of the membrane to the other. Channels can be specific for certain ions. Ions can pass through the channel in both directions. In other words, conformation changes are not needed to transport the ions across the membrane as with carriers. 
  Channels called aquaporins also exist for water. We will encounter these when we study the kidney. 
  The rate of transport through channels depends on three factors:
1. The transport rate of the individual channels.
2. The number of channels in the membrane.
3. The size of the driving force acting upon the ions. This force is due to the electrochemical gradient. With uncharged molecules, such as water, the force is only chemical (concentration gradient). 
  Channels transport molecules at a much faster rate compared to carriers. Some channels show saturation behavior while other do not. Channels can be regulated more directly by mechanisms that cause the channels to open or close. We will look at various mechanisms that do this later. 
  As with carriers, the number of channels in the membrane can also be increased or decreased by hormones and drugs. 

Active Transport
Active transport involves the use of energy to move molecules against a driving force or, in other words, up an electrochemical gradient. There are two basic forms of active transport:
Primary active transport uses the energy source (ATP) to directly transport molecules.
Secondary active transport uses the energy of a concentration or electrochemical gradient created by primary active transport.
   Active transport is powered by protein molecules called pumps which are similar to carrier proteins except that they use energy to move molecules up an electrochemical gradient. Like carriers, pumps are specific for certain molecules, have a fixed number of binding sites and can become saturated.
Primary Active Transport
   Proteins involved in primary active transport act as transport proteins and as enzymes. As enzymes these proteins use the energy of ATP by catalyzing ATP hydrolysis. The proteins are frequently called ATPases
   The important and ubiquitous sodium-potassium pump is an example of an ATPase. The sodium-potassium pump, during each cycle of the pump, transports 3 Na+ out of the cell and 2 K+ into the cell. The energy for each cycle is derived from the hydrolysis of one ATP. Both Na+ and K+ are moved up electrochemical gradients. 
   The binding sites on the protein are specific for either Na+ or K+ and the affinity of the binding site for these ions depends upon whether it is facing the outside or inside surface of the membrane. The binding site for Na+ has a higher affinity for Na+ when it faces inward and the binding site for K+ has a higher affinity when facing outward. This ensures that the ions are always transported in the appropriate direction. 
   The Na+/K+ pump creates the concentration gradients of Na+ and K across the cell membrane.
Secondary Active Transport
   In secondary active transport the energy released when one molecule moves down an electrochemical gradient is coupled with the movement of another molecule up its electrochemical gradient. The movement of the two molecules can be either in the same direction or in opposite directions:
Cotransport (symport) refers to the case when the two molecules move in the same direction. Na-linked glucose transport is an example. 
Countertransport (antiport) or exchange is the case when the molecules move in opposite directions. Na+-proton exchange is an example.

Osmosis
    Water molecules follow the same laws of diffusion as any other molecule. Because water is so essential and abundant in cells it is given special attention and has its own terminology. The flow or water across a membrane down its concentration gradient is called osmosis.
   The concentration of water within cells needs to be maintained within narrow limits. If the cell could not maintain this concentration and the concentration of water in the extracellular fluid were increased relative to the concentration within the cell, the cell would swell. On the other hand, if the concentration of water in the cell were less than that of the extracellular environment, the cell would shrink.
   Water is unique among body fluids in that it is so abundant. It is by far the dominant molecule in any body fluid. At body temperature the concentration of pure water is 55.5 molar. The normal concentration of solutes in intracellular fluid is about 0.3 molar. Hence when a cell is placed into pure water there is a 0.3 molar higher concentration of water outside the cell than inside and water moves down its concentration gradient into the cell until the cell bursts. Conversely, if a cell is placed into water with a 1 molar concentration of sucrose so that the concentration of water outside the cell is 54.5 molar, the concentration of water inside the cell is higher than outside the cell and the cell shrinks as water leaves it.
   In the examples above the water concentration can always be expressed as the solute concentration because a change in one always means a change in the other. Physiologists will always use solute concentration to calculate water movement along its concentration gradient.
Osmolarity
   The total solute concentration is called osmolarity. Osmolarity expresses the concentration of a solute as the quantity of solute molecules expressed as moles or milliosmoles (one thousandth of a mole) per liter of water. The normal osmolarity of intra- and extracellular fluid is about 300 milliosmoles.
   Two solutions that have the same osmolarity are iso-osmotic. A solution that has a higher osmolarity than another is hyperosmotic and a solution that has a lower osmolarity is hypo-osmotic.
   A solution's osmolarity is related to the concentration of dissolved particles. If a solute ionizes (separates into ions) this will increase the number of dissolved particles. For example, 150 millimolar solution of NaCl is 300 milliosmolar because NaCl completely disassociates into Na+ and Cl- in water.
Osmotic Pressure
The osmotic pressure of a solution (p) is an indirect measure of its solute concentration. It is expressed in units of pressure (atmospheres or mmHg). As osmolarity increases osmotic pressure increases. Another way to look at this relationship is to say that as water concentration decreases osmotic pressure increases.
   When two solutions of different osmolarity are separated by a membrane, water will flow down its concentration gradient but up an osmotic pressure gradient.
Tonicity
   Tonicity is a measure of how a solution affects cell volume. It depends not only on solute concentration but also on the solute permeability of the cell membranes. A solution is isotonic when it does not alter the cell volume. A solution that causes cells to shrink is hypertonic, a solution that causes cells to swell is hypotonic.
   The tonicity of a solution depends upon the cell's final volume. For example, if a cell is placed in a 300 milliosmolar solution of urea, because the cell membrane is permeable to urea, the urea will flow down its concentration gradient and enter the cell. This will increase the osmolarity and the osmotic pressure within the cell. Water will then move down its concentration gradient and up the osmotic pressure gradient and enter the cell causing the cell to swell. Hence, this solution would by hypotonic to the cell.
   A solution's tonicity is determined solely by the concentration of impermeant solutes. If the concentration of impermeant solutes is greater than 300 milliosmoles, the solution is hypertonic. If the concentration of impermeant solutes is less than 300 milliosmoles the solution is hypotonic.
   When a cell comes into contact with solutions that are hypotonic and hypertonic the degree to which it swells or shrinks is determined by the initial concentration of impermeant solutes in intracellular and extracellular fluid.

Epithelial Transport
   As the cell membrane forms a barrier separating intracellular and extracellular environment, epithelial tissue forms a boundary between external environment and internal environment and between fluid compartments in the body.
   As part of forming a barrier epithelial tissue is also designed to transport materials across cells. The process of transporting material from the outside into the internal environment is absorption. The process of transporting material from the internal environment to the outside is secretion.
   Transport across the entire cell is possible because the membrane on one side of the cell has transport systems that are different from the membrane on the other side. This means that epithelial cells are polarized.
Epithelial Structure
   In epithelium specialized for absorption or secretion, the membrane that faces the lumen of a body cavity is the apical membrane and the membrane facing the interstitial fluid is the basolateral (base and sides) membrane. A basement membrane anchors the basolateral membrane and supports the epithelial layer.
   The cells forming the epithelium maintain the boundary by being joined by tight junctions that limit the passage of material through the spaces between the cells (paracellular spaces).
Epithelial Solute Transport
   The energy that drives the active transport of solutes such as Na+ and glucose is derived from the Na+/K+ pumps in the basolateral membrane. This pump maintains a low concentration of Na+ inside the cell. This creates an electrochemical gradient that favors the inward movement of Na+. K+ is pumped into the cell but also leaks out through K+ channels. 
   Because of the Na+ gradient created by the Na+/K+ pumps, cells can absorb Na+ across the apical membrane through Na+ channels. The net flow of Na+ is therefore across the apical membrane from the lumen through the cell and across the basolateral membrane into the interstitial fluid.
   Other molecules, such as glucose, can be actively transported across the apical membrane by coupling its movement across the membrane with Na+ by means of a Na+-glucose co-transporter. The Na+ gradient created by the Na+/K+ pump again provides the energy that moves the glucose up its concentration gradient. The glucose concentration inside the cell increases creating a glucose concentration gradient across the basolateral membrane. Glucose can then go down its concentration gradient across the basolateral membrane by means of a glucose carrier in the basolateral membrane.
Epithelial Water Transport
   Water transport is secondary to solute transport. This is because the movement of solutes across membranes results in changes in osmotic pressure gradients. If epithelial cells actively transport solute molecules across the basolateral  membrane this increases the solute concentration in the interstitial fluid and at the same time increases the osmotic pressure. Water then flows from the lumen through the cells and into the paracellular spaces up the osmotic pressure gradient.
   The epithelium can also secrete water by reversing the movement of solutes. This is seen in the epithelium lining the respiratory tract where Cl- is actively secreted into the lumen. Cl- brings with it Na+ because of electrical attraction. The increase of Cl- and Na+ in the lumen creates an osmotic pressure gradient that causes water to flow into the lumen.