Unit 3

Question 1

Discovery of Biological Membranes

Answer 1

1. Cells must be surrounded by lipids since lpid-soluble substances penetrated cells but polar substances did not
2. Lipids arrange themselves into a layer, called a monolayer
3. Erythrocyte plasma membrane occupies twice the expected area. Therefore, the membrane is composed of a lipid bilayer
4. Proteins must account for the selective permeability of membranes. Proposed to coat both sides as a sheet (sandwich model)
5. Some proteins are hydrophobic & must embed in the membrane. The fluid-mosaic model proposes an underlying fluid-like bilayer with a mosaic of proteins embedded within the lipids.

Question 2

Functions of biological membranes

Answer 2

1. Compartmentalisation: Membranes form continuous sheets that enclose intracellular compartments
2. Scaffold for biochemical activities: Membranes provide a framework that organises enzymes for effective interaction
3. Selectively permeable barrier: Membranes allow regulated exchange of substances between compartments
4. Transporting solutes: Membrane proteins facilitate the movement of substances between compartments
5. Responding to external signals: Membrane receptors transduce signals from outside the cell in response to specific ligands
6. Intercellular interaction: Membranes mediate recognition and interaction between adjacent cells
7. Energy transduction: Membranes transduce photosynthetic energy, convert chemical energy to ATP, & store energy

Question 3

Plasma Membrane - Specific functions

Answer 3

1. Import & export of molecules: Nutrients pass inward across the plasma membrane, waste products pass outward
2. Receiving information: Some proteins in the plasma membrane act as sensors (receptors) to enable the cell to respond to changes in its environment
3. Capacity for movement and expansion: When the cell grows or changes shape, the plasma membrane enlarges its area by addition to new membrane and it can deform without tearing

Question 4

Fatty acids

Answer 4

• Consists of a long hydrocarbon chain terminating in a carboxyl group at one end
• In saturated fatty acids all of the carbon atoms are bonded to the maximum number of hydrogen atoms (no double bonds between carbon atoms)
• Unsaturated fatty acids contain one or more double bonds between carbon atoms

Question 5

3 main types of membrane lipids

Answer 5

1. Phosphoglycerides are diaglycerides with small functional head groups linked to the glycerol backbone by phosphate ester bonds
2. Sphingolipids are ceramides formed by the attachment of sphingosine to fatty acids
3. Cholesterol is a smaller & less amphipathic lipid that is only found in animals

Question 6

Sphingolipids

Answer 6

• Derived from sphingosine
• The addition of a second fatty acid to sphingosine through the amine group results in a lipid called ceramide
• The addition of phosphorylcholine to ceramide results in a lipid called sphingomyelin
• The addition of galactose to ceramide results in a lipid called a cerebroside
• The addition of complex carbohydrates including sialic acide to ceramide results in a lipid called a ganglioside

Question 7

3 types of lipid mobility

Answer 7

1. Lateral diffusion: Lipids rapidly exchange places with their neighbors (10^7 times per second). Gives rise to rapid diffusion (diffusion coefficient of 10^-8 cm2/sec) -> A lipid can move the length of a bacterial cell in 1 second
2. Rotation: lipids can rotate around their axis at speeds as high as 500 rpm
3. Flip-flop: rarely occurs, on the time scale of hours, assisted by enzymes called flippases. The exception is cholesterol which can flip rapidly on its own

Question 8

2 main properties of lipids that contribute to fluidity

Answer 8

1. Length of hydrocarbon tail: Shorter chain legth reduces tendency of hydrocarbon tails to interact with one another & therefore increases fluidity of bilayer
2. Unsaturated hydrocarbon tails introduce kinks into the chain, making them more difficult to pack together, increasing the fluidity of the bilayer. Such membranes are less viscous

Question 9

Lipid composition influences properties of membranes:

Answer 9

• Relatively small polar head groups give the lipid a “cone” structure.
• Those with larger head groups give the lipid a cylindrical structure
• An abundance of conical lipids on the inner leaflet as opposed to the outer leaflet could allow for natural curvature of membranes

Question 10

Asymmetric distribution of lipids in the plasma membrane

Answer 10

• Glycolipids & sphingomyelin are only in the extracellular leaflet
• Both glycolipids & sphingomyelin are produced by enzymes exposed to the Golgi lumen & are not substrates for flippases
• Phosphatidylcholine is mostly found in the outer leaflet
• Phosphoglycerides with terminal primary amine groups in the polar heads are mainly found in the inner leaflet. This asymmetry is due to the action of flippases
• Phosphatidylinositols are minor constituents of the inner leaflet of the plasma membrane, with a role in cell signaling. This asymmetry is due to the action of flippases
• Cholesterol is evenly distributed between both leaflets and spontaneously shuttles between both leaflets without the need for flippases