2: CELLS - TRANSPORT

Question 1

What is diffusion?

Answer 1

the net movement of particles from an area of high conc to an area of low conc/down the conc gradient

Question 2

Is diffusion an active/passive process?

Answer 2

passive

Question 3

What are the factors affecting rate of diffusion?

Answer 3

• conc gradient (the higher the conc gradient the higher the rate of diffusion)
• thickness of exchange surface (the thinner the exchange surface the faster the rate of diffusion)
• surface area (the larger the surface area the faster the rate of diffusion)

Question 4

Why does diffusion slow down over time?

Answer 4

as diffusion takes place the difference in conc between the 2 sides of the membrane decreases until it reaches equilibrium therefore diffusion slows down over time

Question 5

What is facilitated diffusion?

Answer 5

diffusion where transport proteins are needed for large/polar molecules to diffuse across a membrane

Question 6

What are the 2 types of transport proteins present in cell membranes?

Answer 6

• channel proteins
• carrier proteins

Question 7

What type of molecules are transported across a membrane by carrier proteins?

Answer 7

large molecules

Question 8

How do carrier proteins facilitate the diffusion of large molecules?

Answer 8

• a large molecule attaches to a carrier protein in the membrane
• the protein changes shape which releases the molecule on the other side of the membrane
• different carrier proteins facilitate the diffusion of different charged particles

Question 9

What type of molecules are transported across a membrane by channel proteins?

Answer 9

polar molecules/ions

Question 10

How do channel proteins facilitate the diffusion of ions/polar molecules?

Answer 10

• form pores in the membrane for charged particles to diffuse through (some are gated meaning that they open and close in response to stimuli)
• different channel proteins facilitate the diffusion of different charged/polar molecules

Question 11

What is Fick’s law?

Answer 11

rate of diffusion is directly proportional to the surface area x the conc difference / the thickness of the membrane

Question 12

What is osmosis?

Answer 12

the net movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane

Question 13

What are the factors affecting rate of osmosis?

Answer 13

• water potential gradient (the higher the water potential gradient the higher the rate of osmosis)
• thickness of exchange surface (the thinner the exchange surface the faster the rate of osmosis)
• surface area (the larger the surface area the faster the rate of osmosis)

Question 14

What is water potential?

Answer 14

the tendency of water to move from 1 place to another by osmosis

Question 15

What is water potential measured in?

Answer 15

kPa

Question 16

What is the water potential of distilled/pure water?

Answer 16

0kPa (highest water potential)

Question 17

Why does pure/distilled water have a water potential of 0kPa?

Answer 17

highest conc of freely moving water particles

Question 18

Why do all solutions have a -ve water potential?

Answer 18

lower conc of freely moving water molecules

Question 19

How does conc of solutes affect water potential of a solution?

Answer 19

the higher the solute conc, the more -ve the water potential

Question 20

What are aquaporins?

Answer 20

channel proteins that are specific to water and facilitate osmosis

Question 21

Explain what you would expect to happen when visking tubing containing a concentrated sugar solution is placed in distilled water (4 marks)?

Answer 21

• water moves by osmosis into the visking tubing causing the visking tubing to swell
• water moves from a high water potential (0kPa) into the concentrated solution with a lower water potential across the visking tubing which acts as a partially permeable membrane

Question 22

What is an isotonic solution?

Answer 22

a solution with the same water potential as the cell

Question 23

What would happen to a cell in an isotonic solution?

Answer 23

won’t lose/gain any water as there is no difference in water potential

Question 24

What is a a hypotonic solution?

Answer 24

a solution with a higher water potential than the cell

Question 25

What would happen to an animal cell in a hypotonic solution?

Answer 25

• water moves into cell
• swells
• may undergo lysis (burst)

Question 26

Why do plant cells not burst in hypotonic solutions?

Answer 26

cell wall strengthens cell

Question 27

What is a hypertonic solution?

Answer 27

a solution with a lower water potential than the cell

Question 28

What would happen to an animal cell in a hypertonic solution?

Answer 28

• water moves out of cell
• cell shrinks

Question 29

What would happen to an plant cell in a hypertonic solution?

Answer 29

plasmolysis (water loss causes cell membrane to move away from cell wall)

Question 30

What is active transport?

Answer 30

the movement of molecules from an area of lower conc to an area of higher conc against the conc gradient, requiring ATP from respiration (also involves use of transport proteins)

Question 31

What factors affect the rate of active transport?

Answer 31

• the speed of carrier proteins
• the no. of carrier proteins
• faster respiration/availability of ATP
• temp

Question 32

How do carrier proteins work?

Answer 32

• molecule binds to carrier protein
• carrier protein changes shape (hydrolysis of ATP (forming ADP and Pi) releases energy to change the shape of the carrier protein)
• molecule released on the other side of the membrane

Question 33

How does the sodium-potassium pump work?

Answer 33

• cytoplasmic Na^+ binds to the sodium-potassium pump
• Na^+ binding stimulates phosphorylation by ATP
• phosphorylation causes protein to change its conformation, expelling Na^+ to the outside
• extracellular K^+ binds to the protein, triggering release of the phosphate group
• loss of the phosphate restores the protein’s original conformation
• K^+ is released, Na^+ sites are receptive again and the cycle repeats

Question 34

Why do we need the sodium-potassium pump?

Answer 34

• to maintain electrochemical gradient across cell membranes (resting membrane potential)
• maintains osmotic pressure

Question 35

What are co-transporters?

Answer 35

a type of carrier protein that can transport 2 molecules at a time

Question 36

How do co-transporter proteins work?

Answer 36

conc gradient of 1 of the molecules is used to move the other molecule against its conc gradient

Question 37

Describe the co-transport of glucose across the ileum epithelium (6 marks)

Answer 37

• Na^+ ions pumped out of epithelial cell by active transport/sodium-potassium pump into blood
• maintaining Na^+ ion conc gradient between lumen of ileum and epithelial cell
• glucose moves in with Na^+ ion into epithelial cell via co-transporter protein
• Na^+ ion moves down conc gradient whilst glucose moves against conc gradient
• glucose moves into blood by facilitated diffusion via carrier protein

Question 38

Why does glucose have to move into the blood via co-transport instead of (facilitated) diffusion?

Answer 38

in the ileum, the conc of glucose is too low for it to just diffuse out into the blood so glucose is absorbed from the ileum of the blood via co-transport