Zagada, Timothy M. 1-B1 Medical Physiology A SGD- June 29,2011

1. Cell membrane is impermeable to some ions, glucose but is permeable to alcohol and urea. Explain the variation in permeability of the membrane to these agents. The variation in permeability of the cell membrane is because of its lipid bilayer, which is a thin, double-layered film of lipids. Each layer only one molecule thick, is continuous over the entire cell surface. The basic lipid bilayer is composed of phospholipid molecules. One end of each phospholipid molecule is soluble in water; that is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic. The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.

2. Describe and discuss the importance of integral proteins found in the cell membrane. Channels- integral proteins provide structural channels (or pores) through which water

molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others.

Carrier proteins- for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their natural direction of diffusion, which is called “active transport.” Still others act as enzymes.

Receptors- for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein. This, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers, thereby relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior.

3. Enumerate the major differences between passive and active transport activities.

Passive Transport Active Transport Subtypes Simple

diffusion Facilitated diffusion

Primary active transport

Secondary active transport Cotransport Contertransport

Electrochemical Gradient

downhill downhill Uphill Uphill Uphill

Carrier- Mediated

No Yes Yes Yes Yes

Metabolic Energy (ATP)

No No Yes Indirect Indirect

Na Gradient No No _ Yes, same direction

Yes, opposite direction

4. Enumerate 3 major cellular functions which use ATP.

1. Transport of substances through multiple membranes in the cell. Example; To supply energy for the transport of sodium through the cell membrane.

2. Synthesis of chemical compounds throughout the cell. Example; To promote protein synthesis by the ribosomes.

3. Mechanical work Example; To supply the energy needed during muscle contraction.

Zagada, Timothy M. 1-B1 Medical Physiology A SGD- June 29,2011

5. Describe the process of diffusion. What are the functions that determine the net rate of diffusion? Diffusion means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter (by which there is a net flow of matter from a region of high concentration to a region of low concentration). Factors that affect the net rate of diffusion:

1. Concentration difference. -The rate of net diffusion through a cell membrane is proportional to the difference in concentration of the diffusing substance on the two sides of the membrane.

2. Electrical potential. -If an electrical potential is applied across a membrane, the ions will move through

the membrane because of their electrical charges. 3. Permeability - Membrane permeability is enhanced by increases in the lipid solubility of the

diffusing molecule, the number of channels per unit area of membrane, and temperature. It is decreases by the increase in membrane thickness as well as increase in molecular weight of the diffusing substance.

4. Pressure difference - When a pressure is higher on one side of a membrane than on the other, increased

amounts of energy are available to cause net movement of molecules from the high-pressure side toward the low-pressure side.

6. What happens to human red blood cells when suspended in a solution of 3% glucose? Will

it undergo crenation or hemolysis? Explain. What is isotonic. Human RBC when suspended in a 3% glucose will undergo hemolysis (red blood cells will swell or rupture) because the glucose concentration in the intracellular (5%) is more than that of extracellular glucose (3%). The RBC is placed in a hypotonic solution. If a cell is placed into a hypotonic solution that has a lower concentration of impermeant solutes, water will diffuse into the cell, causing it to swell; water will continue to diffuse into the cell, diluting the intracellular fluid while also concentrating the extracellular fluid until both solutions have about the same osmolarity. Isotonic solution is when the concentration in the intracellular and extracellular fluids is equal and the solutes cannot enter or leave the cell. The cell will neither shrink nor swell. These solutions are important in clinical medicine because they can be infused into the blood without the danger of upsetting osmotic equilibrium between the intracellular and extracellular fluids.

7. Differentiate voltage gating from ligand gating mechanism. Voltage gating- the molecular conformation of the gate or of its chemical bonds responds to the electrical potential across the cell membrane. For example, a strong negative charge on the inside of the cell membrane causes the sodium gates to remain tightly closed. When the inside membrane loses its negative charge, these gates suddenly, allowing sodium to pass inward through the sodium pores. Chemical (ligand) gating- Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein; this causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate.

8. Explain why facilitated diffusion, a carrier mediated transport, considered a passive transport activity. Facilitated diffusion which is carrier mediated transport is considered a passive transport activity because it follows an electrochemical gradient (downhill) and does not require any energy (ATP). The energy that causes diffusion is the energy of the normal kinetic motion of matter.

Zagada, Timothy M. 1-B1 Medical Physiology A SGD- June 29,2011

9. Differentiate filtration from diffusion. Filtration- is the process by which fluid is forced through a membrane or other barrier because of a difference in pressure on the two sides. Diffusion- is caused by kinetic motion of the molecules in both the plasma and the interstitial fluid.

10. Why is the sodium-potassium pump referred to as an electrogenic pump? Why is the pump

important in human physiology? Sodium-Potassium pump as an electrogenic pump and its importance to human physiology.

-It is referred to as an electrogenic pump because more positive charges are pumped to the outside than to the inside (three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative potential inside the cell membrane. - The sodium potassium pump performs a continual surveillance role in maintaining the normal cell volume. It is responsible for maintaining the sodium and potassium concentration differences across the cell membrane. When the cell begins to swell, the Na+-K+ pump is automatically activated since the continual loss of ions (3 Na ions out for 2 K ions inside) initiates an osmotic tendency to move water out of the cell thus, without it (Na+-k+ pump) most of the cells of the body would swell until they burst.

11. What limits the rate of active transport to maximum rate? Explain why we observe such

limitation. Active transport saturates in the same way that facilitated diffusion occurs. When the concentration of the substance to be transported is small, the rate of transport rises approximately in proportion to increases in its concentration. At high concentrations, the rate of transport is limited by the rates at which the chemical reactions of binding, release, and carrier conformational changes can occur.

12. Differentiate synaptic from endocrine communication.

Synaptic Endocrine Message transmission Across synaptic cleft By circulating body fluids Local or general Local General Specifity depends on Anatomic location and

receptors Receptors

13. What are the responsible for the establishment and maintenance of the resting membrane

potential? a. Potassium diffusion potential b. Sodium diffusion potential c. Membrane permeability d. Electrogenic nature of Na+-K+ pump

14. Discuss the different periods of the action potential.

Resting stage- The resting membrane potential before the action potential occurs (–90 millivolts) Depolarization stage- The membrane suddenly becomes permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to flow to the interior of the axon, and the potential rises rapidly in the positive direction. Repolarization- Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the potassium channels open more than normal. Then, rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential.

References: Textbook of Medical Physiology / Arthur C. Guyton, John E. Hall.—11th ed.

Review of Medical Physiology/ William F. Ganong- 18th ed Laboratory Manual and Lecture Guide in Medical Physiology, Part 1/ IM, FEU-NRMF