3.2.3 Transport across cell membranes

Question 1

What is the basic structure of all cell membranes including cell-surface membranes and membranes around organelles in eukaryotes?

Answer 1

All cell membranes have the same basic structure: a phospholipid bilayer with embedded proteins glycoproteins glycolipids and cholesterol as described by the fluid-mosaic model.

Question 2

How are phospholipids arranged in the fluid-mosaic model?

Answer 2

Phospholipids form a bilayer where hydrophilic phosphate heads face outward towards the aqueous environment and hydrophobic fatty acid tails face inward away from water.

Question 3

What is the role of cholesterol in the cell membrane?

Answer 3

Cholesterol restricts the movement of other molecules within the membrane maintaining stability and reducing fluidity especially at higher temperatures.

Question 4

What is simple diffusion and how is it limited by the phospholipid bilayer?

Answer 4

Simple diffusion is the passive movement of small non-polar molecules (e.g. oxygen carbon dioxide) down their concentration gradient. It is limited to substances that can pass through the hydrophobic core of the phospholipid bilayer.

Question 5

What is facilitated diffusion and what roles do carrier and channel proteins play?

Answer 5

Facilitated diffusion is the passive movement of molecules down their concentration gradient via specific carrier or channel proteins. Carrier proteins change shape to transport substances while channel proteins form hydrophilic pores for ions or polar molecules.

Question 6

What is osmosis and how is it explained in terms of water potential?

Answer 6

Osmosis is the passive diffusion of water molecules across a selectively permeable membrane from a region of higher (less negative) water potential to a region of lower (more negative) water potential.

Question 7

What is active transport and what is the role of ATP in this process?

Answer 7

Active transport is the movement of molecules or ions against their concentration gradient using carrier proteins and energy from the hydrolysis of ATP.

Question 8

What is co-transport and how is it illustrated in the absorption of sodium ions and glucose in the mammalian ileum?

Answer 8

Co-transport involves the coupled movement of sodium ions and glucose into epithelial cells of the ileum. Sodium ions move down their concentration gradient through a co-transporter protein facilitating the simultaneous uptake of glucose against its gradient.

Question 9

How are cells adapted for rapid transport across membranes?

Answer 9

Cells are adapted by increasing their surface area (e.g. microvilli) increasing the number of channel and carrier proteins and maintaining steep concentration or water potential gradients.

Question 10

How does surface area affect the rate of movement across membranes?

Answer 10

An increased surface area provides more space for diffusion facilitating faster movement of substances across membranes.

Question 11

How do the number of channel or carrier proteins affect the rate of movement across membranes?

Answer 11

A greater number of channel or carrier proteins increases the capacity for facilitated diffusion or active transport thus enhancing the rate of movement.

Question 12

How do differences in concentration or water potential gradients affect the rate of movement across membranes?

Answer 12

Steeper gradients increase the rate of diffusion or osmosis as substances move more rapidly down their respective gradients.

Question 13

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

Answer 13

1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;
2. Facilitated diffusion down a concentration gradient via protein carrier/channel;
3. Osmosis of water down a water potential gradient;
4. Active transport against a concentration gradient via protein carrier using ATP;
5. Co-transport of 2 different substances using a carrier protein;

For any answer accept a correct example
For ‘carrier protein’ accept symport OR co transport protein

Question 14

The movement of Na+ out of the cell allows the absorption of glucose into
the cell lining the ileum.

Explain how. (2)

Answer 14

1. (Maintains/generates) a concentration/diffusion gradient for Na+ (from ileum into cell);

Accept ‘(Maintains/generates) a lower concentration of Na+ inside the cell compared with outside the cell’.

2. Na+ moving (in) by facilitated diffusion, brings glucose with it
   OR
   Na+ moving (in) by co-transport, brings glucose with it;

Question 15

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

Answer 15

1. Folded membrane/microvilli so large surface area (for absorption);

Reject references to ‘villi’.
Accept ‘brush border’ for ‘microvilli’.

2. Large number of co-transport/carrier/channel proteins so
   fast rate (of absorption)

OR
Large number of co-transport/carrier proteins for active transport
OR
Large number of co-transport/carrier/channel proteins for facilitated
diffusion;

Question 16

The movement of substances across cell membranes is affected by
membrane structure.

Describe how. (5)

Answer 16

1. Phospholipid (bilayer) allows movement/diffusion of non-
   polar/lipid-soluble substances;
2. Phospholipid (bilayer) prevents movement/diffusion of polar/
   charged/lipid-insoluble substances
   OR
   (Membrane) proteins allow polar/charged substances to cross
   the membrane/bilayer;
   Accept water-soluble
3. Carrier proteins allow active transport;
4. Channel/carrier proteins allow facilitated diffusion/co-transport;

Accept aquaporins allow osmosis

5. Shape/charge of channel / carrier determines which
   substances move;
6. Number of channels/carriers determines how much movement;
7. Membrane surface area determines how much
   diffusion/movement;
8. and 7. Accept correct reference to faster/slower/rate for
   ‘how much movement’
   Accept microvilli / Golgi (apparatus) / ER / rER
   Accept surface area to volume for ‘surface area’
9. Cholesterol affects fluidity/rigidity/permeability;

Accept cholesterol affects vesicle formation/
endocytosis/exocytosis/phagocytosis;

Question 17

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

Answer 17

1. (Movement) down a gradient / from high concentration to low

concentration;
Ignore along / across gradient
Reject movement from gradient to gradient

2. Passive / not active processes;
   OR
   Do not use energy from respiration / from ATP / from metabolism;
   OR
   Use energy from the solution;

Reject do not use energy unqualified

Question 18

Compare and contrast the processes by which water and inorganic ions
enter cells. (3)

Answer 18

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

Accept aquaporins (for water) and ion channels

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

Question 19

Contrast the processes of facilitated diffusion and active transport. (3)

Answer 19

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 20

Membrane structure – Describe how proteins arrange themselves in the membrane (2)

Answer 20

1. Hydrophobic parts of helix/AP (to the outside) to sit within the (hydrophobic) fatty acid (tails) of the phospholipids;
2. Hydrophilic parts of helix/AP (to the inside) as ions are charged/polar/water soluble;

Question 21

Membrane structure – Describe the role of cholesterol (1)

Answer 21

Cholesterol stabilises the membrane OR
Cholesterol restricts the movement of molecules/phospholipids (making up the membrane);

Question 22

Req prac 3- How do we find water potential of plant tissue practically? (3)

Answer 22

1. Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis;
2. Find concentration where curve crosses the x-axis/where percentage change is zero;
3. Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis);