Transport across membranes - key points

Question 1

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

Answer 1

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

Question 2

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

Answer 2

• Bilayer OR Water is present inside and outside a cell;
• 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 marks) (7)

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

• Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
• Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
• Carrier proteins allow active transport;
• Channel/carrier proteins allow facilitated diffusion/co-transport;
• Shape/charge of channel / carrier determines which substances move;
• Number of channels/carriers determines how much movement;
• Membrane surface area determines how much diffusion/movement;
• Cholesterol affects fluidity/rigidity/permeability;

Question 5

Explain why phospholipids form a bilayer in plasma membranes (4 marks)

Answer 5

1. Phospholipids have a charged phosphate group which is part of the hydrophilic polar head
2. Fatty acid tails are not charged and are hydrophobic
3. Both tissue fluid and cytoplasm are aqueous solution
4. Phospholipids form two layers called a bilayer, with the hydrophobic fatty acid tails facing inwards towards each other
5. hydrophilic phosphate groups facing outward interacting with the aqueous environment