Transport Across Membranes

Question 1

why is the cell surface membrane classed as a fluid-mosaic model (2)

Answer 1

FLUID
the molecules that it consists of are constantly moving around relative to one another

MOSAIC
the protein molecules are unevenly distributed throughout the membrane, forming a mosaic

Question 2

what does the cell surface membrane consist of? (7)

Answer 2

• phospholipids
• channel proteins
• carrier proteins
• enzymes
• receptors
• glycolipids and glycoproteins
• cholesterol

Question 3

channel proteins (3)

Answer 3

• have a specific tertiary structure that span the membrane, forming hydrophilic tunnels
• channels are selective and will only allow one type of molecule
• small, charged molecules pass through channel proteins via facilitated diffusion

Question 4

carrier proteins (4)

Answer 4

• have a specific tertiary structure
• allow the transport of ions and polar molecules by facilitated diffusion and active transport
• can change their shape in response to the binding of their target molecule, moving it through the membrane
• typically selective for one or a few substances

Question 5

enzymes (2)

Answer 5

• the shape of the enzyme’s active site is complementary to its substrate

maltase is found in the cell membrane of the small intestine where it hydrolyses maltose into glucose

Question 6

receptors (2)

Answer 6

• hormones attach to binding sites on the receptors and allow a cell to respond

a synapse in the nervous system; there are receptors on the postsynaptic membrane that allow the nerve to respond to the neurotransmitter

Question 7

glycolipids and glycoproteins (2)

Answer 7

glycolipids composed of carbohydrates attached to a phospholipid within the membrane
- they are important in cell recognition

glycoproteins are on the outer surface of the membrane
- they are also important in cell recognition, some acting as self-antigens

Question 8

cholesterol (3)

Answer 8

• increases stability of the membrane by restricting the movement of other molecules
• decreases permeability by causing fatty acid chains to pack closer together
• different types of cells have different amounts of cholesterol

Question 9

what is the name of the specific channel protein that allow water to cross the cell-surface membrane?

Answer 9

aquaporin

Question 10

what does fick’s law state?

Answer 10

diffusion rate =

surface area x conc. gradient
divided by
diffusion distance/pathway

Question 11

define diffusion

Answer 11

the net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient

Question 12

define facilitated diffusion

Answer 12

the net movement of molecules from high to low concentration through a protein down a concentration gradient

Question 13

what type of process is both diffusion and facilitated diffusion?

Answer 13

a passive process
they do not require ATP (energy)

Question 14

which proteins play a significant role in facilitated diffusion?

Answer 14

• channel proteins
• carrier proteins

Question 15

define active transport

Answer 15

the movement of molecules from low to high concentration against a concentration gradient

Question 16

what proteins play a significant role in active transport?

Answer 16

ONLY carrier proteins

Question 17

in what way is active transport different to diffusion and facilitated diffusion?

Answer 17

it requires ATP (energy)

Question 18

how exactly does ATP help with active transport? (3)

Answer 18

• the ATP binds to the allosteric site and hydrolyses
• the phosphate molecule remains attached to the carrier proteins
• this changes the shape of the carrier protein (phosphorylation)

Question 19

define osmosis

Answer 19

the net movement of water molecules from high water potential to low water potential across a partially permeable membrane using channel proteins

Question 20

what is the water potential of pure water?

Answer 20

0

Question 21

different solutions have different water potentials

list the three ways in which this can affect cells

Answer 21

• hypertonic
• hypotonic
• isotonic

Question 22

hypertonic (3)

Answer 22

• wherein the cells themselves have a higher water potential than the solution they’re in
• as water moves from high to low, more water is leaving the cells than entering
• this causes the cells to shrivel up

Question 23

hypotonic (3)

Answer 23

• wherein the solution has a higher water potential than the cells
• more water is entering the cell through osmosis
• the cell may burst (osmotic lysis)

Question 24

isotonic (2)

Answer 24

• wherein the water potential within the cells and the water potential in the solution are at equilibrium
• water is entering and leaving at the same time

Question 25

some molecules can pass through the cell membrane without any help

what three qualities will these molecules have?

Answer 25

• small
• non-polar
• lipid soluble