1.3 (Membrane Structure)

Question 1

Phospholipid molecules.

Answer 1

Have a polar (charged, hydrophilic) phosphate head and long non-polar (hydrophobic) lipid tails

Question 2

Phospholipids emergent property.

Answer 2

Will self-organise to keep their heads ‘wet’ and their tails ‘dry’

Question 3

What are integral proteins?

Answer 3

Integral proteins are permanently embedded, many go all the way through and are polytopic (poly = many, topic = surface), integral proteins penetrating just one surface are monotopic.

Question 4

What are peripheral proteins?

Answer 4

Usually have a temporary association with the membrane, they can be monotopic or attach to the surface

Question 5

What are glycoproteins?

Answer 5

• Proteins with an oligosaccaride (oligo = few, saccharide = sugar) chain attached.
• Important for cell recognition by the immune system and as hormone receptors

Question 6

TRACIE.

Answer 6

• Transport: Protein channels (facilitated) and protein pumps (active)
• Receptors: Peptide-based hormones (insulin, glucagon, etc.)
• Anchorage: Cytoskeleton attachments and extracellular matrix
• Cell recognition: MHC proteins and antigens
• Intercellular joinings: Tight junctions and plasmodesmata
• Enzymatic activity: Metabolic pathways (e.g. electron transport chain)

Question 7

What is cholesterol?

Answer 7

Makes phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane.

Question 8

Explain membrane fluidity.

Answer 8

• Hydrophobic hydrocarbon tails usually behave as a liquid. Hydrophilic phosphate heads act more like a solid.
• Difficult to determine whether the membrane is truly either a solid or liquid it can definitely be said to be fluid

Question 9

Why is it important to regulate the degree of fluidity?

Answer 9

• Need to be fluid enough that the cell can move
• Need to be fluid enough that the required substances can move across the membrane
• If too fluid membrane could not effectively restrict the movement of substances across itself

Question 10

Outline cholesterol’s role in fluidity.

Answer 10

1. Presence of cholesterol in the membrane restricts movement of phospholipids and other molecules – reduces membrane fluidity
2. Presence of cholesterol disrupts regular packing of hydrocarbon tails of phospholipid molecules - increases flexibility as it prevents tails from crystallising and behaving like a solid
3. Reduces permeability to hydrophilic molecules and ions such as sodium and hydrogen

Question 11

Evidence for fluid mosaic model.

Answer 11

Biochemical techniques:
- Membrane proteins found to be very varied in size and globular in shape
- Such proteins would be unable to form continuous layers on the periphery of the membrane
- Membrane proteins had hydrophobic regions and therefore would embed in the membrane not layer the outside
Fluorescent antiobody tagging:
- red or green fluorescent markers attached to antibodies which would bind to membrane proteins
- Red or green fluorescent markers attached to antibodies which would bind to membrane proteins
- Membrane proteins of some cells were tagged with red markers and other cells with green markers
- Cells were fused
- Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell
- Showed membrane proteins are free to move within the membrane, not fixed in a peripheral layer

Question 12

Davson- Danielli model evidence.

Answer 12

Evidence:
- In high magnification electron micrographs membranes appeared as two dark parallel lines with a lighter coloured region in between
- Proteins appear dark in electron micrographs and phospholipids appear light - possibly indicating proteins layers either side of a phospholipid core
Model:
- Protein-lipid sandwich
- Lipid bilayer composed of phospholipids (hydrophobic tails inside, hydrophilic heads outside)
- Proteins coat outer surface
- Proteins do not permeate the lipid bilayer
Explains:
Despite being very thin membranes are an effective barrier to the movement of certain substances

Question 13

Falsification of Davson-Danielli model.

Answer 13

Interpreting image:
- Fracture occurs along lines of weakness, including the centre of membranes
- Fracture reveals an irregular rough surface inside the phospholipid bilayer
- Globular structures were interpreted as trans-membrane proteins
Conclusion:
Contradicts Davson-Danielli model which only involves proteins coating the surface of the membrane