1.3 Membrane structure

Question 1

The fluid mosaic model

Answer 1

• Proposed by Singer and Nicolson in 1972
• According to this model, biological membranes consist of phospholipid bilayers with proteins embedded in the bilayer.

Question 2

Hydrophilic

Answer 2

• ‘Water loving’
• Can interact with water
• Polar (have partial or full charges), allowing them to interact with dipoles in water

Question 3

Hydrophobic

Answer 3

• Repels water
• Non-polar (does not have a charge)

Question 4

Structure of phospholipids

Answer 4

• Hydrophilic head: composed of a phosphate and glycerol molecule
• Two hydrophobic tails: fatty acid chains

Question 5

Amphipathic molecules

Answer 5

Molecules that have both a hydrophilic and a hydrophobic part, like phospholipids.

Question 6

Phospholipid arrangement in membranes

Answer 6

Phospholipids are arranged into a bilayer. The hydrophilic phosphate heads face the watery environment (cytoplasm and extracellular fluid), while the hydrophobic fatty acid chains are sandwiched in between completely isolated from the water.

Question 7

Property of phospholipid bilayer: fluidity

Answer 7

Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility. This is what allows the breaking and reforming of membranes (exocytosis/endocytosis).

Question 8

Membrane proteins: integral proteins

Answer 8

• Amphipathic
• Embedded in the membrane (in most cases they past completely through the membrane)

Question 9

Membrane proteins: peripheral proteins

Answer 9

• Hydrophilic
• Attached to the outside of the plasma membrane

Question 10

Facilitated diffusion

Answer 10

When proteins are used for diffusion for difussion of molecules with low permeability
- Anything that can dissolve in water (water soluble)
- Anything with an electrical charge
- Anything large in size

Question 11

Channel proteins

Answer 11

• Allows passive transport of substances between the inside and outside the cell
• Facilitated diffusion

Question 12

Carrier protein

Answer 12

These proteins bind to substances on one side of the membrane and change shape to transport them on the other side.
- They have a slower rate of transport that chennel proteins

They can be used for both facillitated diffusion or active transport:
- Carrier proteins that use energy to change shape are called pump proteins (against concentration gradient)
Example: sodium potassium pump

Question 13

Glycoprotein

Answer 13

Proteins with a carbohydrate attached to the surface.
They play a role in cell-to-cell communications and in transport across the membrane.

Question 14

Cholesterol

Answer 14

• Amphipathic
• Hydrophobic part: steroid ring and hydrocarbon tail
• Hydrophilic part: hydroxyl group
• The presence of cholesterol restricts the movement of phospholipids and other molecules, reducing membrane fluidity.
• This reduces membrane permeability to very small hydrophilic molecules that would otherwise freely cross (e.g. sodium and hydrogen).
• At low temperatures, it disrupts the regular packing of hydrocarbon tails (of the phospholipid molecules), preventing the solidification of the membrane.

Question 15

The Davson-Danielli membrane model

Answer 15

• Proposed in 1935 (before the Fluid Mosaic)
• Suggests that the phospholipids formed a bilayer which was covered on both sides (sandwiched between) by a layer of globular protein.

Question 16

Evidence for the Davson-Danielli model

Answer 16

Electron micrographs showed membranes have three layers. The two layers in the edges appeared dark and since proteins normally appear dark and phospholipids lighter it was (wrongly) assumed that these were proteins.

Question 17

Falsification of the Davson-Danielli model

Answer 17

• It assumed all membranes have the same structure which does not explain how different types of membranes can carry out different functions.
• Proteins are amphipathic, through largely non polar (hydrophobic), which makes it improbable that they would be found in contact with the aqueous environment on either side of the membrane.
• Freeze fracturing was used to split open the membrane and revealed irregular rough structures within the membrane. These were interpreted to be integral proteins, demonstrating that proteins as embedded in the membrane and not outside of the membrane.