Transport Across Membranes

Question 1

Explain why a cell membrane may be described as a fluid-mosaic?

Answer 1

1. The position of the molecules within the membrane is fluid – they are able to move around within the membrane.
2. Membrane is made up from a variety of different (protein) molecules arranged into a mosaic.

Question 2

Explain the arrangement of phospholipids in a cell-surface membrane.

Answer 2

1. Bilayer OR Water is present inside and outside a cell;
2. Hydrophobic (fatty acid) tails point away/are repelled from water OR Hydrophilic (phosphate) heads point to/are in/are attracted to water;

Question 3

Many different substances enter and leave a cell by crossing its cell surface membrane.

Describe how substances can cross a cell surface membrane. [5]

Answer 3

1 (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;

2 Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;

OR

Large / polar / water-soluble molecules go through proteins;

3 Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;

4 Active transport is movement from low to high concentration / against concentration gradient;

5 Active transport / facilitated diffusion involves proteins / carriers;

6 Active transport requires energy / ATP;

7 Ref. to Na+ / glucose co-transport;

Question 4

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

Describe how.

Answer 4

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;
3. Carrier proteins allow active transport;
4. Channel/carrier proteins allow facilitated diffusion/co-transport;
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. Cholesterol affects fluidity/rigidity/permeability;

Question 5

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

Answer 5

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;

Question 6

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

Answer 6

1. (Movement) down a gradient / from high concentration to low concentration;
2. Passive / not active processes;
   OR
3. Do not use energy from respiration / from ATP / from metabolism;
   OR
4. Use energy from the solution;