Cell Membranes & Transport

Question 1

What is the permeability of the membrane affected by?

Answer 1

• Temperature
• Organic solvents eg ethanol

Question 2

How does temperature affect membrane permeability?

Answer 2

Increased temperature increases membrane permeability as the molecules moving through the membrane have more kinetic energy and will diffuse more quickly. If temp gets too high, proteins may denature.

Question 3

How do organic solvents affect membrane permeability?

Answer 3

Ethanol dissolves/emulsifies the phospholipids and may denature the proteins, resulting in gaps or holes within the cell membrane. The cell membrane becomes more porous.

Question 4

What are the 6 methods of transport across the membrane?

Answer 4

1. Diffusion
2. Facilitated diffusion
3. Co-transport
4. Active transport
5. Bulk transport
6. Osmosis

Question 5

Which methods of transport require ATP?

Answer 5

Active transport and bulk transport (endo/exocytosis)

Question 6

What is diffusion?

Answer 6

The passive movement of a molecule or ion down a concentration gradient, from a region of higher concentration to a region of lower concentration

Question 7

What are the 6 factors affecting the rate of diffusion?

Answer 7

• Concentration gradient
• Thickness of the surface
• Surface area of the membrane
• Size of the diffusing molecule
• Nature of the diffusing molecule (polar/non-polar)
• Temperature

Question 8

What is the equation for rate of diffusion?

Answer 8

Rate of diffusion = surface area x difference in concentration / length of diffusion path

Question 9

What is facilitated diffusion?

Answer 9

The passive transfer of polar molecules or charged ions (insoluble in phospholipids) down a concentration gradient, across a membrane, by channel or carrier proteins in the membrane

Question 10

What is the rate of facilitated diffusion through a membrane affected by?

Answer 10

1. The number of carrier/channel proteins present in the membrane - once they are all filled the rate will plateau
2. The steepness of the concentration gradient

Question 11

What are channel proteins and what are their features?

Answer 11

Protein molecules filled with hydrophilic pores
- Ions (water soluble) pass through as the pores are hydrophilic
- Each channel is specific for one type of ion
- Channels open and close depending on needs of the cell

Question 12

What do carrier proteins do and how?

Answer 12

They allow for diffusion of larger polar molecules eg glucose
- Polar molecule attaches to a binding site on the carrier protein, causing the protein to change shape releasing the molecule through to the other side of the membrane

Question 13

What is co-transport?

Answer 13

Type of facilitated diffusion that brings molecules and ions into cells together on the same protein transport molecule. PASSIVE

Question 14

Describe sodium-glucose co-transport

Answer 14

1. There’s a concentration gradient for sodium ions to move into the cell
2. Sodium ions and glucose bind to the carrier protein
3. The carrier protein changes shape, and the sodium ions and glucose are transported to the other side of the membrane

Question 15

What is active transport?

Answer 15

Transport of ions and molecules against the concentration gradient. Uses energy from the hydrolysis of ATP, made by the cell in respiration. If respiration is inhibited, eg by cyanide poison, then active transport is too

Question 16

What kind of particles are actively transported?

Answer 16

Only charged particles, eg ions and polar molecule eg glucose that are insoluble in lipid.

Question 17

Describe active transport

Answer 17

1. The molecule or ion binds to the carrier protein on outside of cell membrane
2. ATP transfers a phosphate group to the carrier protein
3. The carrier protein changes shape and carries the ion or molecule across the membrane
4. The molecule or ion is released into the cytoplasm of the cell
5. The carrier protein returns to its original shape

Question 18

What are the 2 types of bulk transport?

Answer 18

Endocytosis
Exocytosis

Question 19

What is exocytosis?

Answer 19

The process by which substances may leave the cell having been transported through the cytoplasm in a vesicle, which fuses with the cell membrane and secretes its contents to the outside of the cell

Question 20

What is endocytosis?

Answer 20

The cell membrane folds around the particle. The folding closes off the link to the outside of the cell so now the particle is fully trapped inside the cell in a vesicle or vacuole.

Question 21

What are the 2 types of endocytosis?

Answer 21

1. Phagocytosis (uptake of solids)
2. Phinocytosis (uptake of liquids)

Question 22

What is osmosis?

Answer 22

Osmosis is the diffusion of water from an area of higher water potential to an area of lower water potential across a selectively permeable membrane. It’s a passive process so doesn’t require ATP.

Question 23

What is water potential?

Answer 23

The tendency for water to leave a solution or cell by osmosis. Measured in kPa

Question 24

Water potential equation

Answer 24

Water potential = solute potential + pressure potential

Question 25

What happens if a plant cell is placed in a hypotonic solution?

Answer 25

Water will move into the cell by osmosis causing the plant to become TURGID

Question 26

What happens if a plant cell is placed in a hypertonic solution?

Answer 26

Water will move out of the cell and into the surrounding solution by osmosis. The vacuole shrinks and the cytoplasm and cell membrane will withdraw from the cell wall causing the cell to become plasmolysed.

Question 27

What is incipient plasmolysis?

Answer 27

Theoretically: the point at which the cell membrane is just about to come away from the cell wall.
Experimentally: the point where 50% of the cells in a sample are plasmolysed

Question 28

At incipient plasmolysis, what would the pressure potential be?

Answer 28

Pressure potential = 0
So water potential = solute potential

Question 29

What do animal cells become when water moves out of them?

Answer 29

Crenated