Although the internal fluid environment around the cells (extracellular fluid, ECF) is kept stable by homeostasis, the fluid inside the cell (intracellular fluid, ICF) is distinctly different from the ECF. Each cell type maintains an intracellular composition which is unique for that cell type. This unique difference between the composition of the ICF and ECF is due to the plasma membrane. The ability of the plasma membrane to regulate materials passing through it is a function of its chemical structure and physiological activity.
The plasma membrane consists of a thin layer of lipids (fats) and proteins with a limited number of carbohydrates. The most abundant membrane lipids are the phospholipids. Phospholipids are comprised of a hydrophobic (water repellant) component and a hydrophilic (water loving) component. The phosphate component of the phospholipid is hydrophilic and the fatty acid component is hydrophobic. The phospholipid can be illustrated with the phosphate component represented as a circle (O) and the fatty acid component as two tails. Thus a phospholipid molecule would be represented as:
When exposed to water the hydrophilic portion will orient toward the water and the hydrophobic component will orient away from the water. When placed in a water media the phospholipids will organize themselves into a bilayer with the hydrophilic heads facing the water and the hydrophobic tails facing w\each other.
The solvent of the ICF and ECF is water. The phospholipids of the plasma membrane form a lipid bilayer with the hydrophilic phosphate components facing the water of the ECF and ICF while the hydrophobic tails face each other in the interior of the lipid bilayer away from the ECF and ICF.
Cholesterol, also a lipid, can be found in the plasma membrane between the phospholipid molecules. Cholesterol prevents the phospholipids from crystalizing and provides rigidity which helps stabilize the plasma membrane. The more cholesterol in the plasma membrane, the less the membrane fluidity.
A second major component of the plasma membrane are the membrane proteins. Membrane proteins that extend through the phospholipid layer from the ECF to the ICF are termed integral proteins. Other proteins, termed peripheral proteins, are found in either the outer or inner phospholipid layer.
A consideration will now be given to how materials pass through the plasma membrane. Molecules are not static, rather, the energy in the molecules cause them to be in constant motion. This movement is referred to as Brownian movement. Molecules in a solution will come in contact with each other and collide due to the Brownian movement. When the molecules collide they are each knocked away as a result of the collision. The more concentrated the solution ( more molecules per area) the more collisions will occur. These collisions result in the molecules being knocked from an area of high concentration to an area of low concentration. Molecules collide randomly and are knocked in all directions but the net movement is from the area of high concentration to the area of low concentration. The net movement of molecules from an area of high concentration to an area of low concentration is termed DIFFUSION. A common example of diffusion is the coloring of eggs at Easter. During the coloring process a dye pill is placed in a cup containing water and vinegar. The dye pill contains dye molecules in very high concentration and the water and vinegar contain no dye molecules. After a period of time the dye pill will ÒdissolveÓ and the dye molecules will be equally distributed throughout the water and vinegar solution. The dye molecules have diffused from an area of high concentration (the pill) to an area of low concentration (the water and vinegar). The movement of molecules down a concentration gradient ( from a high concentration to a low concentration) does not require energy and is said to be a passive movement of molecules. Passive diffusion of molecules can occur through the plasma membrane. Materials that are soluble in the lipid bilayer can move through the membrane following their concentration gradients. Molecules that are lipid soluble such as: steroid hormones; fatty acids; glycerol, and cholesterol can diffuse across the plasma membrane. Non-polar molecules such as oxygen and carbon dioxide can diffuse through the membrane. Those substances which are not lipid soluble may pass through the membrane by utilizing certain integral membrane proteins which are referred to as channels. These protein channels possess a shape which allows small molecules - water, ions - to diffuse through them following their concentration gradient. These channels are termed ÒleakyÓ channels and are always open.
Molecules which are lipid soluble or small enough to pass through protein channels can diffuse through the membrane following their diffusion gradient. However, the rate at which substances can diffuse across the plasma membrane is influenced by several factors. Factors affecting the rate of diffusion are as follows:
1) Concentration gradient - as the difference in concentration of a substance between two solutions increases, the rate of diffusion increases.
2) Permeability of the membrane - the more permeable a membrane is to a substance the greater the rate of diffusion. Permeability is influenced by a substances permeability in the phospholipid bilayer or by the number of channels available for that substance.
3) The surface area of the membrane - The greater the surface area of the membrane the faster the rate of diffusion of a substance.
4) The molecular weight of a substance - diffusion of a substance is dependent on the collision of the molecules. When light weight molecules collide they are knocked further than heavy molecules. Thus, the heavier the molecules the slower the rate of diffusion.
5) The distance through which diffusion takes place - the greater the distance to diffuse the slower the rate of diffusion.
These factors affecting the rate of diffusion can be expressed by FicksÕ Law of Diffusion.
The net rate of diffusion (J) = C x A x P X x MW
C = the difference in concentration between two areas ( C1 C2 ) A = Surface area of the membrane P = permeability of the membrane X = Distance through which diffusion takes place MW= molecular weight of substance
The net diffusion of water down its concentration gradient is termed osmosis. The concentration of a solution is generally measured by the amount of a substance ( solute) dissolved in water (the solvent). The highest concentration of water would occur when there is no solute present ( pure water, 100%). As a solute is added to pure water the concentration of water becomes less. In osmosis then, water moves to the area of highest solute concentration ( lowest water concentration). When comparing the amount of solutes in solutions we refer to the solutions tonicity. Some examples of tonicity are given below:
A) If two solutions are separated by a semipermeable membrane (bag membrane) which allows the water to pass through but not the solute, and the two solutions have the same concentration , the solutions are said to be isotonic. Under isotonic conditions there will be an equal movement of water between the solutions.
B) If the solution in the bag has a higher concentration of solutes than the solution surrounding it, the net movement of the water will be into the bag. The fluid surrounding the bag is said to be hypotonic. Hypotonic (hypo means below) solutions have lower solute concentrations than the solution to which they are being compared.
C) If the solution surrounding the bag has a higher concentration than the solution in the bag it is said to be hypertonic (hyper = above, greater). The net movement of water would be from the bag to the surrounding hypertonic solution.
Through the process of osmosis water will move towards areas of solute concentrations. In the body water can move freely through the cell membranes but the water is not uniformly distributed. The distribution of water in the body is controlled by the movement of solutes. By moving the solutes from one solution to another the tonicity of the solutions is changed and water moves to the area of highest solute concentration. In the Kidney the movement of sodium out of the kidney tubules will result in water following the sodium and the urine becoming more concentrated. In the large intestine the movement of NaCl out of the large intestine results in water following the NaCl and the feces becomes concentrated.
In addition to molecules moving in response to concentration gradients, molecules may also move in response to electrochemical gradients. Electrochemical gradients exist as a result of some molecules having an electrical charge. How do molecules acquire an electrical charge? Recall that atoms are comprised of a nucleus which contains positively charged particles termed protons and neutral particles termed neutrons. In orbit around the nucleus are negatively charged particles termed electrons. Prior to any chemical reaction the atoms are electrically neutral with the number of protons (+) equaling the number of electrons (-). During chemical reactions the atoms may either gain or lose electrons. If an atom gains an electron it would then possess more electrons (-) than protons (+) and the atom would have a negative charge. The charged atom is said to be an ION and if it is negatively charged it is termed an anion. Examples of some anions are:
Cl-, PO4,HCO3-, OH-
If an atom losses electrons (-) it would then possess more protons (+) than electrons and the net charge of the atom would be positive. Ions with a positive charge are termed cations and some examples of cations would be: Na+, K+, H+, Ca++
Opposite charges attract each other, thus cations may move toward a negative solution and anions may move toward a positive solution. Ions will move due to the attraction of the opposing charge. This movement of ions in response to charge differences is termed the electrochemical gradient.
Those substances that are not lipid soluble and too large to pass through ÒleakyÓ channels move through the membrane by carrier mediated transport. Sugars and amino acids are two examples of carrier mediated substances. There are two types of carrier mediated transport - passive and active.
In passive carrier mediated transport, termed facilitated transport, the substance interacts with the outer surface of a specific carrier protein. The interaction of the substance with the carrier protein initiates a change in the carrier protein structure which transports the substance through the membrane to the opposite side. Because the substance in facilitated transport is following a diffusion gradient and the carrier protein only facilitates its movement across the membrane, no energy is needed to move the substance through the membrane.
A second type of carrier mediated transport is termed active transport. Active transport involves a specific carrier protein for a substance but in contrast to facilitated transport involves the use of energy to move the substance through the membrane. The use of energy is necessary because in active transport the substance is moved against the concentration gradient. In active transport the carrier protein contains ATPase, an enzyme which catalyzes the breakdown of ATP to ADP, P and energy. The substance to be moved binds to the inner surface of the carrier protein and the energy from the breakdown of ATP to ADP is used to phosphorylate the carrier protein. Phosphorylation of the carrier protein induces a change in the configuration which moves the substance through the protein to the other side of the membrane. Active transport systems are also termed "pumps". Some pumps move only one substance ( the H+ pump) and other pumps may move more than one substance ( the Na+- K+ pump).
Both Facilitated transport and active transport share three common characteristics:
1) Specificity - Because of their distinct shape, carrier proteins are generally specific for a substance or substances of similar structure.
2) Saturation - There are a limited number of carrier proteins for each substance and once all these carrier proteins are being utilized for transport, the system is saturated. The number of carrier proteins determines the rate at which substances can be moved across the membrane. When all the carrier proteins are utilized this is the maximum rate at which a substance can be be moved across the membrane ( Tm = maximum transport rate).
3) Competition - as mentioned under specificity, substances with similar structure may use the same carrier proteins. As the number of carrier proteins is finite, when two similar substances are present in the ECF they may compete for use of the available carrier proteins. In these cases, the Tm for each substance will be less than if the substance was by itself in the ECF.
In the kidney and intestine, glucose and amino acids are moved from low concentration in the intestinal lumen and kidney tubule lumen to high concentrations in the blood. They are moved against concentration gradients, but energy is not directly utilized for this process. Glucose and amino acids are moved across the membranes by secondary active transport. On the membrane facing the lumen there is a cotransport carrier for sodium (Na+) and the nutrient ( glucose or amino acid). This cotransport carrier has a site for Na+ and for the nutrient. When Na+ binds to the cotransport carrier this increases the carriers affinity for the nutrient. There is a Na+-K+ pump on the side of the cell which is pumping sodium out of the cell. Energy is required to run the Na+-K+ pump. There a higher concentration of Na+ in the lumen than in the cell because the pump is moving the Na+ out of the cell, so Na+ will diffuse from the lumen into the cell. In contrast the nutrient level is lower in the lumen than inside the cell. However, when the sodium binds with the cotransport carrier a nutrient also binds to the carrier and when the Na+ is transported into the cell because of the diffusion gradient cause by the Na+-K+ pump, the nutrient is also transported into the cell against the concentration gradient. On the cell membrane facing the bloodstream, there is a transport protein for the nutrient and the nutrient then moves by facilitated transport from the cell into the bloodstream. Notice that energy is not required to move the Na+ and nutrient by the cotransport protein. The energy is used in the Na+-K+ active transport to establish the Na+ diffusion gradient.
Some channels have a shape which prevents substances from passing through when these channels are at rest. However, when the appropriate chemical or electrical stimuli acts on these channels the channels change their shape allowing the substance to pass through. These channels are said to be gated. When the shape of the channel prevents the substance from passing through the ÒgateÓ is closed and when the shape of the channel allows the substance to pass through the ÒgateÓ is open. Gated channels are usually specific for a particular substance. In chemical gates a specific chemical in the extracellular fluid reacts either directly with the channel protein or with a receptor protein which then interacts with the channels protein. In either case the presence of the chemical induces a structural change in the carrier protein (opens the gate) which allows the substance to follow its diffusion gradient through the open channel. Voltage gates are similar to chemical gates except a change in the electrical potential in the membrane opens the gate rather than a chemical stimulus.
Nerve cells, muscle cells and other cells of the body have the ability to respond to stimuli. In addition, nerve cells and muscle cells have the capacity to conduct electrical charges along their membrane surfaces. These characteristics of cell irritability and conduction are a function of the cell membrane.
When these cells are at rest the distribution of ions in the intracellular and extracellular fluids are quite different. Sodium (Na+) and Chloride (Cl-) have a high concentration in the ECF and a low concentration in the ICF. In contrast potassium (K+) and intracellular anions (protein, organophosphates and organosulphates) are high in the ICF and either low (K+) or absent (intracellular anions) from the ECF. The relative distribution of Na, K and intracellular anions in a resting cell would be as follows:
The distribution and concentration of these ions results in a differential distribution of charges across the cell membrane. The ECF adjacent to the membrane has a positive charge and the ICF adjacent to the membrane has a negative charge. When opposite charges are separated they are said to be polarized. Thus, the resting cell membrane is polarized. When discussing electrochemical gradients, it was explained that opposite charges attract each other and like charges repel each other. If the opposite charges were allowed to move across the membrane they would create an electrical current. An electrical current is described as the movement of charged particles. In the resting cell when opposite charges are separated, they have the potential for creating an electrical current. This potential is referred to as the resting Membrane Potential. The size of the potential depends on the number of opposite charges separated by the membrane. The greater the number of charges separated the greater the resting membrane potential. The movement of charged particles is an electrical current and is therefore measured in volts. The voltage of the potential is small and expressed in millivolts (mV). A mV is equal to 1/1000 V . The potential is measured using electrodes inside and outside the membrane. If the potential is measured outside the membrane it would be positive and inside the membrane it would be negative. By convention the potential is expressed by the inside negative charge and would thus be expressed as - mV.
The distribution of the ions across the membrane is a function of the structure and function of the plasma membrane. Some of the characteristics of the membrane which determine the distribution of these ions are: 1) membrane permeability; 2) active transport mechanisms.
The membrane is differentially permeable to the ions involved in the membrane potential. The relative permeability of these ions would be as follows:
When the cell is at rest some sodium channels and potassium channels have a shape that allows these ions to pass through. These channels are sometimes termed ÒleakyÓ channels. As the table above shows the leaky channels are relatively impermeable to sodium but 50-75 times more permeable to potassium. The intracellular anions can not pass through the membrane.
The active transport system of the membrane involved in the resting membrane potential is termed the sodium-potassium pump ( Na+- K+ pump). This active transport system pumps sodium out of the cell and potassium into the cell. However, the pumping of sodium and potassium is not equal, instead three sodium are pumped out for each two potassium pumped in. As more positive ions are pumped out than pumped in, this active transport system results in establishing a membrane potential with the inside being negative and the outside positive. Experiments involving the Na+ - K+ pump demonstrate that the pump can only account for approximately 20% of the resting potential.
The remaining 80% of the resting potential is due to the diffusion of potassium and sodium. Comparison of the sodium and potassium concentration gradients indicates that following their gradients sodium would move into the cell and potassium would move out of the cell. The low permeability of sodium limits its ability to follow its diffusion gradient. In contrast, potassium has relatively high membrane permeability and can follow its diffusion gradient out of the cell. However, the movement of both ions is influenced by the electrochemical gradient. Sodium would be attracted by the negative charge inside the cell. Potassium would repelled by the positive charge outside the membrane and attracted by the negative charge inside the cell. The intracellular anions cannot move through the membrane due to the membrane impermeability, so the potential is dependent on the movement of potassium and sodium. The relative impermeability of the membrane to sodium eliminates this ion as a factor in establishing the membrane potential. The resting membrane potential is dependent on the movement of potassium. The factors acting on the movement of the potassium ion would be as follows:
If the factors influencing the movement of potassium out of the cell equaled the factors moving potassium into the cell were equal, the net charge would be neutral and the cell would not be polarized. However, the diffusion force is greater than the electrochemical force and potassium moves out of the cell. The anions can not move so as each potassium ion moves out of the cell the anions remain resulting in an increase in the net negative charge inside the cell. As the negative charge inside the cell increases, this increases the electrochemical gradient creating a greater attraction for the potassium. Eventually, the electrochemical force balances the diffusion gradient and even though a potassium concentration gradient occurs, no potassium would move out of the cell because the diffusion gradient is equally opposed by the electrochemical gradient. This point of equilibrium between the diffusion gradient and the electrochemical gradient is termed the equilibrium potential for potassium (the EK+). When this equilibrium potential is reached for potassium the resting potential in a nerve cell is approximately -70mV.
The equilibrium potential for an ion across a membrane can be determined using the Nernst equation:
EMF = 61 x Log ( Concentration outside, Co Concentration inside, Ci
EMF = equilibrium potential for an ion in mV
61 = constant Co = concentration outside in mM/liter
Ci = concentration inside in mM/liter
Using the concentration gradient for K+ ( Co = 5, Ci = 150). The Ek+ would equal 61 x log 1/30.
EK+ = 61 x ( -1.477) = -90 mV
Notice that the -90 mV derived from the Nernst equation is more negative
than the -70 mV of the resting potential for a nerve cell. This difference
is due to the leakage of sodium into the cell following its diffusion gradient.
Even though sodium leaks into the cell following its concentration gradient
the total concentration of sodium inside the cell does not increase due
to the sodium being pumped out by the active transport system. At the same
time, the potassium ion does not decrease inside the cell because it is
being pumped back into the cell by the pump. At the resting potential of
a cell there is no net movement of ions across the membrane, the passive
forces are balanced by the active forces. Even though movement across the
membrane is taking place due to passive leaks and active transport, the
movement out of and into the cell are exactly balanced establishing the
resting potential.