3.2.3 - CELL MEMBRANE + TRANSPORT

Question 1

describe how phospholipids are arranged in a plasma membrane (2)

Answer 1

1. in a bilayer
2. hydrophobic fatty acid tails facing inward
3. hydrophilic phosphate head facing outward

Question 2

name and describe five ways substances can move across the cell-surface membrane into a cell (5)

Answer 2

1. simple diffusion of small/non-polar molecules down a concentration gradient
2. facilitated diffusion down a concentration gradient via a channel/carrier protein
3. osmosis of water down a water potential gradient (high to low)
4. active transport against a concentration gradient via a protein carrier using ATP
5. co transport of 2 different substances using a carrier protein

Question 3

explain the function of ATP hydrolase (2)

Answer 3

1. ATP –> ADP + Pi releases energy
2. energy allows ions to be moved against a concentration gradient/allows active transport of ions

Question 4

the movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum. explain how. (2)

Answer 4

1. maintains/generates a concentration/diffusion gradient for Na+ (from ileum into cell)
2. Na+ moving in by facilitated diffusion brings glucose with it OR Na+ moving in by co transport bring glucose with it

Question 5

describe and explain two features you would expect to find in a cell specialised for absorption (2)

Answer 5

1. folded membrane/microvilli so large surface area for absorption
2. large number of co transporters/channel proteins so fast rate of absorption
   OR for active transport OR for facilitated diffusion
3. large number of mitochondria so they make more ATP by respiration OR for aerobic respiration OR for active transport
4. membrane-bound (digestive) enzymes so maintains concentration gradient for fast absorption

Question 6

the movement of substances across cell membranes is affected by membrane structure. describe how. (5)

Answer 6

1. phospholipid bilayer allows diffusion of non-polar and lipid-soluble substances
2. phospholipid bilayer prevents diffusion of polar/ charged/lipid soluble substances OR membrane proteins allow polar/charged substances to cross the membrane
3. carrier proteins allow active transport and co-transport
4. channel and carrier proteins allow facilitated diffusion/co-transport
5. shape/charge of channel determines which substances move
6. number of channels/carries determine how much movement
7. membrane surface area determines how much movement/diffusion
8. cholesterol affects fluidity/rigidity/permeability

Question 7

give two similarities in the movement of substances by diffusion and by osmosis (2)

Answer 7

1. movement down a concentration gradient/from high to low concentration
2. passive processes/do not use energy from respiration/ATP/metabolism

Question 8

in mammals, in the early stages of pregnancy, a developing embryo exchanges substances with its mother via cells lining the uterus. at this stage, there is a high concentration of glycogen in the cells lining the uterus.

suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients. (2)

Answer 8

1. membrane folded so increased surface area/ membrane has large SA for fast diffusion/facilitated diffusion/active transport/co-transport
2. large number of protein channels/carriers for facilitated diffusion/active transport/co-transport

Question 9

sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. some of these cells have membranes with a carrier protein called NHE3.

NHE3 actively transports one sodium ion into the cell in exchange for one proton (hydrogen ion) out of the cell.

use your knowledge of transport across cell membranes to suggest how NHE3 does this. (3)

Answer 9

1. co-transport
2. uses (hydrolysis of) ATP
3. sodium ion and proton bind to the protein
4. protein changes shape (to move sodium ion and/or proton across the membrane)

Question 10

compare and contrasts the processes by which water and inorganic ions enter cells (3)

Answer 10

comparisons:
1. both move down concentration gradient
2. both move through (protein) channels in membrane

contrast:
3. ions can move against a concentration gradient by active transport

Question 11

contrast the processes of facilitated diffusion and active transport (3)

Answer 11

1. facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins
2. facilitated diffusion does not use ATP/is passive whereas active transport uses ATP
3. facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient

Question 12

putting bee honey on a cut kills bacteria. honey contains a high concentration of sugar.

use your knowledge of water potential to suggest how putting honey on a cut kills bacteria (3)

Answer 12

1. water potential in (bacteria) cells is higher (than in honey)/water potential in honey is lower than bacterial cells
2. water leaves bacteria by osmosis
3. loss of water stops metabolic reactions

Question 13

E. coli has no cholesterol in its cell-surface membrane. despite this, the cell maintains a constant shape. explain why. (2)

Answer 13

1. cell unable to change shape
2. because the cell has a cell wall
3. wall is rigid, because it is made of murein/peptidoglycan

Question 14

explain the arrangement of phospholipids in a cell-surface membrane (2)

Answer 14

1. bilayer
2. hydrophobic fatty acid tails point away/repel from water OR hydrophilic phosphate heads face toward water/attracted to water

Question 15

describe how substances move across the cell-membrane by facilitated diffusion (3)

Answer 15

1. facilitated diffusion uses channel and carrier proteins
2. proteins are complementary to substance
3. the substance moves down a gradient

Question 16

explain why the structure of a membrane is described as fluid-mosaic (2)

Answer 16

1. it is fluid because phospholipids are constantly moving
2. it is mosaic because phospholipids are arranged like a mosaic

Question 17

explain how sodium chloride increases uptake of glucose in the small intestine (4)

Answer 17

1. sodium ions and glucose absorbed by co-transport
2. co-transport via carrier/channel proteins
3. sodium ions removed by active transport into the blood
4. maintains low concentration of sodium ions
5. sodium ions enter epithelial cells by facilitated diffusion taking glucose with them from the ileum
6. glucose moved by facilitated diffusion into the blood

Question 18

give two reasons why glucose uptake by muscle cells increases significantly during exercise. explain your answers (4)

Answer 18

1. increased respiration
2. to provide ATP for muscle contraction
3. more glucose being used so concentration gradient is generated
4. glucose enters by facilitated diffusion