1.3 MEMBRANE STRUCTURE

Question 1

Hydrophilic

Answer 1

substances are attracted to water

Question 2

Hydrophobic

Answer 2

substances not attracted to water

Question 3

Amphipathic

Answer 3

When part of a molecule is hydrophilic and part is hydrophobic

Question 4

The hydrophilic part of a phospholipid

Answer 4

the phosphate group (head)

Question 5

The hydrophobic part of a phospholipid

Answer 5

two hydrocarbon chains (tail)

Question 6

Properties of phospholipids bilayers

Answer 6

1. The bilayer is held together by weak hydrophobic interactions between the tails
2. Hydrophilic/hydrophobic layers restrict the passage of many substances
3. Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
4. This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis/exocytosis)

Question 7

How is the bilayer formed?

Answer 7

When mixed with water the phosphate heads are attracted to water but the hydrocarbon
tails are attracted to each other, but not to water.

Because of this the phospholipids become arranged into double layers (bilayers), with the hydrophobic
hydrocarbon tails facing inwards towards each other and the hydrophilic heads facing the water on either side.

Question 8

Integral proteins

Answer 8

proteins permanently attached to the membrane and are typically transmembrane (they span across the bilayer)

Question 9

Peripheral proteins

Answer 9

proteins temporarily attached by non-covalent interactions and associate with one surface of the membrane

Question 10

The amino acids of a membrane protein are localised according to polarity:

Answer 10

1. Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer
2. Polar (hydrophilic) amino acids are located internally and face aqueous solutions

Question 11

Function of a membrane protein

Answer 11

Junctions – Serve to connect and join two cells together

Enzymes – Fixing to membranes localises metabolic pathways

Transport – Responsible for facilitated diffusion and active transport

Recognition – May function as markers for cellular identification

Anchorage – attaching to cytoskeleton struture to help with cell shape

Question 12

Cholesterol

Answer 12

An amphipathic molecule, component of animal cell membranes ONLY

Question 13

Cholesterol’s hydroxyl (-OH) group

Answer 13

is hydrophilic and aligns towards the phosphate heads of phospholipids

The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails

Question 14

Cholesterol interacts with the fatty acid tails of phospholipids to:

Answer 14

1. Immobilise the outer surface of the membrane, reducing fluidity
2. Less permeable to very small water-soluble molecules that would freely cross
3. Separates phospholipid tails and so prevent crystallisation of the membrane

Question 15

Fluid mosaic model

Answer 15

Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position

Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

Question 16

Davson Daniellli model

Answer 16

1st model (1935)
trilaminar appeareance → 3 layer (2 dark out layers & 1 lighther inner layer)

Question 17

What did Davson-Danielly assumed wrongly?

Answer 17

1. All membranes were of a uniform thickness
2. All membranes would have a constant lipid-protein ratio
3. All membranes would have symmetrical internal and external surfaces

Question 18

how did they discovered the Davson-Danielli model was wrong?

Answer 18

They used freeze fracturing to split open the membranes and revealed irregular rough surfaces within the membrane → transmembrane proteins