CH03 Membrane Transport

Exam 1 Review: Chapter 3 - Membrane Transport

channel - Any of the integral proteins of cell membranes which assist in the passage of specific substances by providing a passageway through their interior from one side of the membrane to the other; most are regulated to control the rate of flow permitted; such proteins are examples of facilitated diffusion because they do not require an expenditure of cellular energy and cannot oppose the concentration gradient of the substance being transferred.

transporter - Any of the integral proteins of cell membranes which assists in the passage of specific substances across the membrane by changing shape or orientation within the membrane; some such proteins are examples of facilitated diffusion because they do not require an expenditure of cellular energy and cannot oppose the concentration gradient of the substance being transferred; others are “active” and require ATP energy but can oppose the concentration gradient of the substance being transferred.

membrane receptor - Any of the integral or outer peripheral proteins of cell membranes which have the capacity to bind reversibly to certain types of ligands, signal molecules (e.g., neurotransmitters, hormones, local hormones), and, as a result of a conformational change in the protein, communicate information to the cytoplasm of the cell which results in the turning on or turning off or the changing of the rate of an internal metabolic pathway or process.

ligand - A very general term for any ion, molecule, or molecular group which binds to another chemical entity to form a larger complex; such binding is usually non-covalent and reversible; one example of ligands are the signal compounds which bind to membrane receptors; another are various non-protein groups which must attach to enzymes in order for the enzyme to have its catalytic function active.

electrochemical gradient - The relative proportions or differences between the combined concentration of dissolved ions ("electrolytes") and the magnitude of any difference in net charge when comparing the effect of solute solutions in two fluid compartments on opposite sides of a semipermeable membrane such as a cell membrane.

membrane potential - The voltage difference (the difference in the net charge based on the total number of negative and positive charges) inside a cell membrane measured relative to the fluid just outside; it is negative under resting conditions and becomes positive during an action potential; it is maintained by the action of the sodium- potassium pump = Na⁺/K⁺ pump = Na⁺/K⁺ ATPase.

selective permeability = semipermeability - The property of biological membranes which allows passage of some substances through the membrane while restricting the passage of other substances.

passive transport - The movement of a chemical substance across a cell membrane without expenditure of energy by the cell, and, therefore, only in the direction favored by the concentration gradient; the movement occurs due to the inherent kinetic energy of the substance molecules themselves; e.g., simple diffusion and facilitated diffusion; the special case of the passive transport of water across a membrane is called osmosis.

bulk flow - The movement of large quantities of molecules, often in solution, by processes which require an expenditure of cellular energy; within individual cells, this is achieved by vesicular transport (vesicular transport is the form of active transport in which masses of molecules, as opposed to individual molecules, are moved across a cell membrane by the formation of a transport vesicle which carries the transported substances from one side of the membrane to the other by merging with or separating from the outer cell membrane; such movements can be either transport into or out of the cell; like all active transport processes, they require a significant input of chemical energy); within tissues and organs, such movements often occur within special passageways such as the movement of blood through blood vessels, or the movement of air through the respiratory tree, or the movement of glandular secretions through duct systems; in general, all these movements are powered by some form of mechanical energy such as the push-pull mechanisms of the cytoskeleton with a cell or the hydrostatic forces produced by muscle contractions around a vessel or duct system.

filtration - The physiological process of mechanically separating a liquid from the some or all of the undissolved particles suspended in it by passing the liquid through a semipermeable membrane with pores small enough to prevent the passage of some or all of the undissolved particles; e.g., the formation of a plasma filtrate at the glomerulus of the nephron in the kidney.

facilitated diffusion - Any of the passive transport mechanisms in which the movement of molecules across a cell membrane is assisted by the presence and action of specific membrane proteins, e.g., channel proteins and transport proteins, which provide a route or action which allows molecules which are not lipid-soluble to cross through the hydrophobic interior of the cell membrane; the molecules move across the cell membrane without expenditure of energy by the cell, and, therefore, only in the direction favored by the concentration gradient; the movement occurs due to the inherent kinetic energy of the substance molecules themselves.

List:

5. three passive transport processes.

(1) simple diffusion, (2) facilitated diffusion (which relies on channel proteins and other membrane proteins which are passive transporters), (3) the special case of the passive transport of water across a membrane which is called osmosis.

18. three aspects of a substance (molecular type) that might prevent it from being moved across a cell membrane by passive transport processes.

If it were (1) a very large molecule, (2) a very hydrophilic molecule with very low lipid solubility, or (3) a charged particle, e.g. ions and electrolytes.

Sketch and Label:

2. an electrochemical gradient.

An electrochemical gradient reflects the relative proportions or differences between the combined concentration of dissolved ions ("electrolytes") and the magnitude of any difference in net charge when comparing the effect of solute solutions in two fluid compartments on opposite sides of a semipermeable membrane such as a cell membrane. In the figure below, there are 10 cations (8 Na⁺ and 2 K⁺) and 10 anions (10 Cl^-) indicated in the extracellular fluid. There are 10 cations (1 Na⁺ and 9 K⁺) and 5 anions (1 Cl^-,2 PO₄^3-, and 2 proteins^3-) indicated in the cytosol = intracellular fluid. However, because the phosphate ions and proteins have charges of -3, there are more negative charges inside the cell (-13) than outside the cell (-10). Thus, there is a charge gradient as well as a series of concentrations gradients for Na⁺, K^+,Cl^-,PO₄^3-, and proteins^3-. This combination of an unequal charge distribution and unequal ion concentration distributions forms the electrochemical gradient. The gradient exerts the greatest force on sodium which is being drawn to the cytosol by both the charge difference and the concentration gradient.

3. an illustration of osmosis.

4. an illustration of facilitated diffusion.