(3.2.3) transport across cell membranes

Question 1

how do proteins arrange themselves in the membrane?

Answer 1

• hydrophobic parts of helix to the outside to sit within the hydrophobic fatty acid tails of the phospholipids
• hydrophilic parts of helix to the inside as ions are charged, polar and water soluble;

Question 2

what is the role of cholesterol?

Answer 2

cholesterol stabilises the membrane and restricts the movement of molecules, and phospholipids making up the membrane

Question 3

name and describe five ways substances can move across the cell-surface membrane into a cell

Answer 3

• the simple diffusion of small and non-polar molecules down a concentration gradient
• facilitated diffusion down a concentration gradient via protein carrier or channel
• osmosis of water down a water potential gradient
• active transport against a concentration gradient via protein carrier using ATP
• co-transport of 2 different substances using a carrier protein;

Question 4

how is the movement of substances across cell-surface membrane affected by membrane structure?

Answer 4

• phospholipid bilayer allows diffusion of non-polar and lipid-soluble substances AND prevents diffusion of polar or lipid-insoluble substances
• carrier proteins allow active transport for polar substances
• channel proteins also allow facilitated diffusion or co-transport for polar substances
• shape of channel or carrier determines which substances move
• number of channels/carriers determines how much movement; * cholesterol affects permeability

Question 5

give two similarities in the movement of substances by diffusion and by osmosis

Answer 5

• both arrange movement down a concentration gradient (from higher to lower)
• both are passive and not active processes so don’t require ATP from aerobic respiration