membranes and transport

Outline four different processes, with examples, that allow substances to pass through the plasma membrane. (4)

1. simple/passive diffusion down a concentration gradient / from high concentration to low concentration (without the use of channels/proteins); (e.g., CO₂ / O₂ / H₂O / steroid hormones)

2. osmosis is the diffusion of water from an area of high water potential / low solute concentration to low water potential / high solute concentration;

3. facilitated diffusion is passive transport/diffusion through a protein channel; (e.g., glucose)

4. active transport requires energy/ATP to move the molecules through a protein channel (e.g., Na-K pump / sodium potassium pump) against a concentration gradient/from low solute concentration to high concentration;

5. endocytosis is the infolding of membranes to form a vesicle and take in a large molecule; (e.g., macrophages engulfing pathogens)

6. exocytosis is the fusion of vesicles with membranes to release a large molecule; (e.g. neurotransmitters)

Describe the functions of proteins in cell membranes (4)

a. facilitated diffusion by channel proteins ✔

b. active transport by protein pumps
OR
protein pumps eg sodium-potassium ✔

c. cell recognition by glycoproteins/protein receptors ✔

d. communication/receptors for hormones/signal molecules ✔

e. cell adhesion ✔

State the role of cholesterol in animal cell membranes. (1)

a. controls/regulates/reduces fluidity of membrane / prevents crystallisation of phospholipids;

b. reduces permeability to some substances.

Outline four types of membrane transport, including their use of energy (4)

a. simple diffusion is passive movement of molecules/ions along a concentration gradient ✔

b. facilitated diffusion is passive movement of molecules/ions along a concentration gradient through a protein channel «without use of energy» ✔

c. osmosis is the passage of water through a membrane from lower solute concentration to higher ✔ OWTTE

d. active transport is movement of molecules/ions against the concentration gradient «through membrane pumps» with the use of ATP/energy ✔

e. endocytosis is the infolding of membrane/formation of vesicles to bring molecules into cell with use of energy
OR
exocytosis is the infolding of membrane/formation of vesicles to release molecules from cell with use of energy ✔

Explain how hydrophobic and hydrophilic properties contribute to the arrangement of molecules in a membrane (7)

a. hydrophilic is attracted to/soluble in water and hydrophobic not attracted/insoluble ✔

b. hydrophilic phosphate/head and hydrophobic hydrocarbon/tail in phospholipids ✔

c. phospholipid bilayer in water/in membranes ✔

d. hydrophilic heads «of phospholipids» face outwards/are on surface ✔

e. hydrophobic tails «of phospholipids» face inwards/are inside/are in core ✔

f. cholesterol is «mainly» hydrophobic/amphipathic so is located among phospholipids/in hydrophobic region of membrane ✔

g. some amino acids are hydrophilic and some are hydrophobic ✔

h. hydrophobic «amino acids/regions of» proteins in phospholipid bilayer «core» ✔

i. hydrophilic «amino acids/regions of» proteins are on the membrane surface ✔

j. integral proteins are embedded in membranes due to hydrophobic properties/region
OR
transmembrane proteins have a hydrophobic middle region and hydrophilic ends ✔

k. peripheral proteins on are on the membrane surface/among phosphate heads due to being «entirely» hydrophilic
OR
«carbohydrate» part of glycoproteins is hydrophilic so is outside the membrane ✔

l. pore of channel proteins is hydrophilic ✔