Chapter 7 – Membrane Structure and Function

I. Cellular Membranes Are Fluid Mosaics of Lipids and ProteinsA. The Fluid Quality of Membranes

1. membranes are made of phospholipids and proteins held together by weak interactions that cause the membrane to

be fluid. a. unsaturated hydrocarbon tails enhance membrane

fluidity, because “kinks” at the C-C double bond hinder close packing of the phospholipids

b. cholesterol modulates membrane fluidity by making the membrane less fluid at warmer temperatures by

restraining phospholipid movement and more fluid at lower temperatures by preventing close packing of the phospholipids.

2. the plasma membrane is selectively permeable

B. Cell Membrane is a Mosaic of Structure and Function 1. The membrane is an assortment of different

proteins embedded in the phospholipid bilayer

a. integral proteins – are those completely embedded in the membrane and span the plasma

membrane b. pherpheral proteins – are loosely bound to the plasma membrane

2. Membrane Carbohydrates and Cell-To-Cell Recognition a. cell-to-cell recognition – the ability of a cell to determine if other cells it encounters are alike or different from itself. It is the basis for:

1. sorting of animal embryo’s cells into tissues and organs

2. rejection of foreign cells by the immune system

b. cell surface carbohydrates are important cell markers used in cell-to-cell recognition because of their diversity

II. Cell Membrane’s Selective Permeability A. Gen Information: depends upon

1. membrane solubility characteristics of the phospholipid bilayer 2. presence of specific integral protein

B. Permeability of the Lipid Bilayer 1. Nonpolar (hydrophobic) Molecules

a. dissolve in the membrane and cross with ease(hydrocarbons and O2)

b. if 2 molecules are equally soluble, the smallest will cross the membrane faster

2. Polar (hydrophilic) Molecules a. small, polar uncharged molecules(H2O, CO2)

b. large, polar uncharged molecules(glucose) and all ions have difficulty penetrating the hydrophobic

C. Transport Proteins – integral membrane proteins that transport specific molecules or ions across membranes

1. aquaporins – special transport protein by which water enters the cell

III. Passive Transport A. Gen Information

1. diffusion of molecules from an area of high concentration to an area of low concentration (movement down the concentration gradient) without energy

2. concentration gradient – graded concentration change over a distance in a particular direction 3. net directional movement – overall movement away from the center of concentration

4. hydrocarbons, carbon dioxide, and oxygen are hydrophobic molecules that can pass across the membrane by passive diffusion

B. Osmosis – passive transport of water across a selectively permeable membrane.

1. Terms used to compare the osmotic concentration of a solution to the osmotic

concentration of a cell a. hypertonic solution – solution with high solute and

low solvent than inside the cell b. hypotonic solution – solution with low solute and high solvent than inside the cell

c. isotonic solution – solution with equal soluteconcentration compared to inside the cell

2. water diffuse down its concentration gradient 3. osmotic concentration – total solute concentration of a solution 4. water will diffuse from the hypoosmotic solution(low osmotic

concentration) to the hyperosmotic solution(higher osmotic concentration)

5. water molecules form a hydration shell around hydrophilic solute molecules, and this bound water cannot freely diffuse across membranes 6. direction of osmosis is determined by the difference in total solute concentration regardless of the type of solutes

7. in 2 isoosmotic solutions separated by a selective permeable membranes, water will diffuse across the membrane in both directions at the same rate. 8. osmotic pressure – measure of the tendency for a solution to take up water when separated from pure water by a selectively permeable membrane (the greater the solute concentration, the greater the osmotic pressure)

C. Water balance in cells without walls and with walls

1. in hypertonic environments, an animal cell will lose water and crenate(shrivel)

2. in hypotonic environments, an animal cell will gain water and lyse(burst)

3. in a hypertonic environment, walled cells will lose water and plasmolyze(shrivel)

4. in a hypotonic environment, walled cells will gain water, it will swell against the cell

wall until the internal pressure equals the osmotic pressure of the cytoplasm(turgid)

D. Facilitated Diffusion – diffusion of solutes across a membrane with the aid of transport

proteins. 1. helps the diffusion of hydrophilic(polar)

molecules and ions. 2. transport protein are specific for the

substances they transport. a. they provide a hydrophobic channel so the molecules can pass through b. they bind loosely to the molecule and carry them through the membrane.

IV. Active Transport – uses energy to move solutes against their gradient A. Gen. Information

1. helps cells maintain ionic gradients across cell membrane2. transport proteins harness energy from ATP to pump molecules against

their concentration gradients. B. sodium-potassium pump – this transmembrane protein pumps Na+ out of

the cell and K+ into the cell against their concentration gradients 1. the sodium-potassium pump is necessary for proper nerve transmission

2. Na+ K+ pump translocates three sodium ions out of the cell for every two potassium ions pumped into the cell.

V. Ion pumps and cell membranes A. Gen. Information

1. anions and cations are unequally distributed across cell membranes, all cells have voltages across the plasma membrane

2. membrane potential – voltage across membranes a. the cell’s inside is negatively charged with respect to the

outside b. favors diffusion of cations into cell and anions out of the cell

B. Forces that drive passive transport 1. concentration gradient of the ion

2. effect of membrane potential on the ion

C. Electrochemical gradient – diffusion gradient resulting from the combined effects of membrane potential and

concentration gradient. 1. ions always diffuse down their electrochemical gradients 2. uncharged solutes diffuse down concentration gradients

because they are unaffected by membrane potential 3. electrogenic pump – transport protein that generates

voltage across the membrane; examples a. sodium-potassium pump b. proton pump