Transport across Membranes

Question 1

Role of Cell Membranes

Answer 1

1. Control the entry and exit of materials
2. Separate organelles from cytoplasm so specific reactions can occur in them
3. Provide an internal transport system e.g. ER, RER
4. Isolate enzymes that might damage the cell e.g. Lysosomes
5. Provide surfaces for reactions to occur e.g. Protein Synthesis

Question 2

Structural features of Cell Membranes

Answer 2

1. Phospholipids (Hydrophillic heads, Hydrophobic tails)
2. Proteins (Surfaces, and throughout the whole thing)
3. Cholesterol
4. Glycolipids
5. Glycoproteins

Question 3

Roles of Phospholipids

Answer 3

Hydrophillic Heads

• Point to the OUTSIDE of the membrane
• Attracted to water
• Repel fats

Hydrophobic Heads

• Point to the INSIDE of the membrane
• Repel water
• Attracted to fats

Functions:

• Allow lipid-soluble substances to enter and leave the cell
• Prevent water-soluble substances entering and leaving the cell
• Make the membrane flexible and self-sealing

Question 4

Role of Proteins

Answer 4

• Some proteins are in the surface of the bilayer
• Other proteins span the complete bilayer
• Protein channels - allowing water-soluble ions to diffuse across
• Carrier proteins - bind to ions then change shape to allow them through.

Functions:

• Provide structural support
• Act as receptors
• Help cells adhere together
• Form cell-surface receptors for identifying cells

Question 5

Role of Cholesterol

Answer 5

Cholesterol is very hydrophobic and play an important role in preventing water loss.

Functions:

• Reduce lateral movement of other molecules including phospholipids
• Make the membrane less fluid at high tempteratures
• Prevent leakage of water from the cell

Question 6

Role of Glycolipids

Answer 6

Made of carbohydrate covalently bonded with a lipid.

The carbohydrate portion extends from the bilayer into the watery environment where it acts as a cell-surface receptor.

Functions:

• Act as recognition sites
• Help maintain the stability of the membrane
• Help cells to attach to one another and form tissues

Question 7

Role of Glycoproteins

Answer 7

Carbohydrates are attached to proteins on the outer surface of the membrane.

Functions:

• Act as recognition sites
• Help cells attach to one another and form tissues
  Allow cells to recognise one another for example lymphocytes can recognise an organism’s own cells

Question 8

Factors affecting the permeability of cell surface membranes

Answer 8

Most molecules cannot flow freely through the membrane because:

• Not soluble in lipds and cannot pass through the bilayer
• Too large to pass through the channels in the membrane
• Of the same charge as the protein channels so they are repelled
• Polar so cannot move through the non-polar hydrophobic tails in the bilayer

Question 9

Fluid-Mosaic model

Answer 9

Fluid - The individual phospholipid molecules can move relative to one another. The membrane is flexible

Mosaic - The proteins embedded in the bilayer vary in shape, size and pattern.