1.3 Cell Membrane

Question 1

Parts of the cell membrane (fluid mosaic model)

Answer 1

Lipid bilayer, phospholipid head, phospholipid tail
Membrane channel, integral protein, peripheral protein, glycoprotein, glycolipid
Carbohydrate of glycoprotein
Cholesterol

Question 2

Function of the plasma membrane

Answer 2

1. hold the cell together
2. controls what enters and exits the cell
3. protect the cell
4. allow the cell to recognise and be recognised (cell signalling and immunity)
5. bind to other cells and molecules
6. a site for biochemical reactions (enzymes, areas for reactions)

Question 3

Amphipathic property

Answer 3

Cell membranes are composed of phospholipids that consist of a hydrophilic (attracted to water) head and a hydrophobic (repelled by water) tail

Question 4

Phospholipid bilayer

Answer 4

• Phospholipids are amphipathic - molecules that have hydrophilic and hydrophobic regions
• When phospholipids are put into water, they naturally arrange into bilayers with the heads facing out and making contact with the water and the tails facing inwards, away from the water
• The attraction between the hydrophobic tails in the the centre and the heads surrounding the water is what makes membranes very stable

Question 5

Phospholipid head

Answer 5

Hydrophilic (attracted to water)

Question 6

Phospholipid tail

Answer 6

Hydrophobic (repelled by water)

Question 7

Membrane channel

Answer 7

• Integral proteins that go through the membrane

- carry molecules through the plasma membrane

Question 8

Integral protein

Answer 8

• proteins embedded in the phospholipid layer
• enzymes - site for chemical reactions
• pumps - for active transport of molecules

Question 9

Peripheral proteins

Answer 9

• proteins attached to an outer surface of the plasma membrane
• acts as receptors and recognise other cells

Question 10

Glycoprotein

Answer 10

• proteins with a carbohydrate chain attached to them

Question 11

Glycolipid

Answer 11

• lipids with a carbohydrate chain attached to them

Question 12

Cholesterol

Answer 12

• component of animal cell membranes; most of the cholesterol molecule is hydrophobic but there is one hydrophilic end (like phospholipids); fits between phospholipids
• reduces fluidity/increases stability

Question 13

Davson-Danielli Model

Answer 13

• date of development: 1930’s

- model of the cell membrane in which the phospholipid bilayer is between two layers of protein

Question 14

Justification for the Davson-Danielli Model

Answer 14

• chemical analysis of membranes showed that they were made of phospholipids and protein
• evidence suggested that the plasma membrane of red blood cells has enough phospholipids in it to form an area twice as large as the area of the plasma membrane → suggesting a phospholipid bilayer
• membranes form a barrier to the passage of some substances, despite being very thin - layers of protein could act as the barrier

Question 15

Singer-Nicolson Model

Answer 15

• date of development: 1950’s/1960’s

- current model of membrane structure that incorporates a fluid mosaic structure in a discontinuous lipid bilayer

Question 16

Justification for the Singer-Nicolson Model

Answer 16

• freeze-fracture electron micrographs showed that globular proteins were present in the centre of the phospholipid bilayer
• analysis of membrane proteins showed that parts of their surfaces were hydrophobic → positioned in the bilayer and in some cases, would extend from one side to another
• fusion of cells with membrane proteins tagged with different coloured fluorescent markers showed that these proteins can move within the membrane as the colours became mixed within a few minutes of cell fusion

Question 17

The term “fluid mosaic” as applied to the structure of membranes

Answer 17

Fluid = always moving, not solid
Mosaic = collection of things stuck together (collection of proteins)
Model = representation