Transport across cell membranes

Question 1

Describe the structure of Phospholipids in membrane (2)

Answer 1

1. Hydrophilic heads point outside of cell membrane attracted by water
2. Hydrophobic tails point into centre of cell membrane, repelled by water

Question 2

What is the function of Phospholipids in the membrane? (3)

Answer 2

1. Allows lipid-soluble substances to enter and leave cell
2. Prevents water-soluble substances entering and leaving cell
3. Makes membrane flexible and self-sealing

Question 3

Describe the types of proteins found in the cell membrane (2)

Answer 3

1. Peripheral proteins (extrinsic) occur in surface of bilayer and never extend completely across it
2. Integral (intrinsic) proteins completely span phospholipid bilayer (such as channel proteins and carrier proteins)

Question 4

Explain the functions of proteins in the membrane (5)

Answer 4

1. Provide structural support
2. Act as protein channels for water-soluble substances
3. Allow active transport via carrier proteins
4. Form cell-surface receptors for identifying cells (or receptors such as for hormones)
5. Helps cells adhere to each other

Question 5

Explain the function of cholesterol in the membrane (3)

Answer 5

1. Reduce lateral movement of molecules in membrane by pulling together fatty acid tails
2. Make membrane less fluid at high temperatures and more fluid at low temperatures (acts as buffer)
3. Hydrophobic and so prevents leakage of water and dissolved ions from cell

Question 6

Explain the functions of glycolipids and glycoproteins in the membrane (3)

Answer 6

1. Act as recognition sites
2. Helps cells to attach to each other
3. Maintains stability of membrane (glycolipid)

Question 7

Explain what types of molecules do not diffuse freely through the cell membrane (4)

Answer 7

1. Not soluble in lipids and so cannot pass phospholipid layer
2. Too large to pass through protein channels
3. Of same charge as protein channels and so repel
4. Electrically charged (polar) so have trouble passing non-polar fatty acid tails

Question 8

Explain the factors that affect membrane fluidity (3)

Answer 8

1. Increase in temperature increases membrane fluidity as molecules have more kinetic energy
2. Cholesterol acts as buffer so at high temperatures decreases membrane fluidity and at low temperatures increases membrane fluidity
3. Saturated tails are straight so pack closer decreasing membrane fluidity & Unsaturated tails are bent (where C=C present) creating spaces increasing fluidity

Question 9

Explain the functions of cell membrane (5)

Answer 9

1. Controls entry and exit of materials in the cell
2. Separate organelles from cytoplasm so that metabolic reactions can take place within them
3. Provides internal transport system (pathway ER)
4. Isolate enzymes that may damage cell (lysosomes)
5. Provide surface on which reactions can occur (rough ER)

Question 10

Define Diffusion (1)

Answer 10

1. The net movement of particles from areas of high concentration to areas of low concentration along a concentration gradient until equilibrium is reached

Question 11

Explain how protein channels work (3)

Answer 11

1. Channel proteins form water-filled hydrophilic channels allowing passage of water soluble ions
2. Channels selectively open in presence of specific ion - ion binds to protein causing a change in shape
3. Facilitated diffusion occurs

Question 12

Explain how carrier proteins work in facilitated diffusion (3)

Answer 12

1. Specific molecule binds with protein
2. This causes a change in shape so that molecule is released through the membrane
3. No external energy is needed and molecules move down a concentration gradient

Question 13

Define Osmosis (1)

Answer 13

1. The net movement of water from an area of high water potential to an area of low water potential through a partially permeable membrane until equilibrium is reached

Question 14

State the effects of water potential on plant cells (3)

Answer 14

1. Higher water potential of solution (hypotonic) - `Plant cell turgid
2. Equal water potential of solution (isotonic) - plant cell flaccid or incipient plasmolysis
3. Lower water potential of solution (hypertonic) - plant cell plasmolysed

Question 15

Define active transport (1)

Answer 15

1. The movement of particles from an area of low concentration to an area of high concentration against a concentration gradient using ATP and carrier proteins

Question 16

Explain how active transport occurs (5)

Answer 16

1. Particle binds to carrier protein on outside of membrane
2. ATP binds to protein on inside of membrane splitting into ADP and a Phosphate molecule
3. The protein changes shape releasing particle through membrane
4. Phosphate molecule released from protein causes protein to revert back to original shape.
5. Process repeats and phosphate molecule recombines with ADP during respiration

Question 17

Explain how co-transport works in the Ileum (4)

Answer 17

1. Sodium ions actively transported out of epithelial cells into blood (by sodium-potassium pump using a specific carrier protein)
2. This maintains a higher concentration of sodium ions in the lumen of intestine than inside epithelial cell
3. Therefore, Sodium ions diffuse into epithelial cells via facilitated diffusion of a different carrier protein, carrying amino acid or glucose molecules
4. Glucose/amino acids pass into blood via facilitated diffusion using another carrier protein