4.1 Structure of the cell-surface membrane

Question 1

Describe the structure of the phospholipid bilayer

Answer 1

-Hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.
-The hydrophilic tails of both phospholipid layers point into the surface of the cell surface membrane repelled by water.

Question 2

Describe the functions of the phospholipids in the cell-surface membrane

Answer 2

-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 3

Describe the function of membranes within cells

Answer 3

• Control the entry and exit of material in discrete organelles
  -Separate organelles from cytoplasm so that specific metabolic reactions can take place within them
  -Provide and internal transport system
  -Isolate enzymes that might damage the cell
  -Provide surfaces on which reactions can occur

Question 4

What are intrinsic proteins?

Answer 4

-Occur in the surface of the bilayer and never extend completely across it
-Act to give mechanical support to the membrane or act as cell-surface receptors

Question 5

What are extrinsic proteins?

Answer 5

• Completely span the phospholipid bilayer
  -Protein channels or carrier proteins

Question 6

Describe the function of proteins in the membrane

Answer 6

• provide structural support
• act as channels to transport water soluble substances across the membrane
  -allow active transport through carrier proteins
• form cell-surface receptors for identifying cells
• help cells adhere together

Question 7

Describe the function of cholesterol in the membrane

Answer 7

• Hydrophobic- prevent the leakage of water and dissolved ions from the cell
• Pull together fatty acid tails of phospholipids- reduce lateral movement of phospholipids and other molecules
• Makes the membrane less fluid at high temperatures

Question 8

Describe the function of glycolipids in the membrane

Answer 8

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

Question 9

Describe the function of glycoproteins in the membrane

Answer 9

-Act as recognition sites
- Help cells attach to one another to form tissues
-Allow cells to recognise one another, for example lymphocytes can recognise and organisms own cells

Question 10

Why can’t most molecules freely diffuse across the cell-surface membrane?

Answer 10

• Not solulable in lipids so can’t pass through the bilayer
• Too large to pass through protein channels
  -Polar-can’t pass through non-polar tails

Question 11

Describe the fluid-mosaic model

Answer 11

Fluid- individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure
Mosaic- proteins that are embedded the membrane are all different shapes and sizes