Lipids in Cell Membranes Lecture

Question 1

What is a lipid?

Answer 1

A lipid is an organic molecule categorised by its hydrophobic or amphipathic properties. They have low solubility in water. Some examples are phospholipids, fats or sterols.

Question 2

What do lipids do?

Answer 2

They are sources of energy, responsible for forming membranes and can participate in cell signalling.

Question 3

Outline the structure of fatty acids

Answer 3

They have a carboxyl group attached to a long hydrocarbon chain. Natural fatty acids have an even number of carbons, and they are rarely free - almost always attached to, or part of lipids.

Question 4

What is the function of fatty acids?

Answer 4

Storing energy.

Question 5

How does the length of fatty acid chains affect their outcome?

Answer 5

Short and medium fatty acid chains can be absorbed into the bloodstream.

Longer chains cannot and need to be synthesised.

Question 6

How are fatty acids named? (nomenclature)

Answer 6

Carbon atoms:double bonds ratio

e.g. 16:0 (saturated), 18:1 (unsaturated), 20:4 (polyunsaturated)

Double bonds numbered as Δ(delta; carboxyl end)

Question 7

What are sterols vital for?

Answer 7

Cholesterol is a main sterol, and is a precursor for the synthesising of vital molecules like bile acids, steroid hormones, vitamins and cell membranes.

Question 8

What are the characteristics of inherited disorders in lipid pathways?

Answer 8

They are usually recessive, and often based around enzyme defects in lipid metabolism.
They are associated with poor development and low life expectancy.

Question 9

What are the type of lipids?

Answer 9

Phospholipids → glycerophospholipids

Glycolipids → sphingolipids → 1 Fatty acid tail, 1 Sphingosine

Sterols → Cholesterol

Question 10

Key molecules to do with phospholipids

Answer 10

Choline: Essential for cell membrane integrity (as phosphatidylcholine), neurotransmitter synthesis (as a precursor to acetylcholine), and liver function (lipid transport and prevention of fatty liver).

Ethanolamine: Used to form phosphatidylethanolamine, a phospholipid that contributes to cell membrane structure and function. It also plays a role in lipid signaling and metabolic pathways.

Serine: A non-essential amino acid used in protein synthesis and forms phosphatidylserine, crucial for cell membrane structure and signaling, especially in brain function.

Inositol: A carbohydrate that forms phosphatidylinositol and derivatives, important for cell membrane integrity and involved in cellular signaling pathways, including those that regulate cell growth and metabolism.

Question 11

Why does the lipid bilayer arrange itself in such a structure?

Answer 11

It’s more energetically favourable for the hydrophilic phosphate heads to be exposed to water.

Question 12

Why are membranes important?

Answer 12

Compartmentalisation, organelles, selective barriers, have sensors to respond to internal and external conditions.

Question 13

What is the fluid-mosaic model?

Answer 13

The phospholipid bilayer is a fluid matrix.

Contain 2 sorts of proteins: peripheral (extrinsic) and integral (intrinsic) proteins.

Question 14

How is the fluidity of the membrane controlled?

Answer 14

More unsaturated → more kinks → more fluidity

Question 15

What is the function of cholesterol?

Answer 15

Cholesterol maintains cell membrane structure and fluidity, acts as a precursor for synthesising steroid hormones.

Question 16

How does membrane synthesis occur?

Answer 16

Occurs in the endoplasmic reticulum (ER), new membrane is then transported to other parts of the cell which occurs through through a series of vesicle budding and fusion. Bits of membrane pinch off from ER to form vesicles and then fuse with other membranes.

Question 17

What are flippases?

Answer 17

Enzymes that facilitate the movement of specific phospholipids from the outer leaflet to the inner leaflet of the bilayer, known as ‘flip-flop’

Question 18

What is membrane asymmetry?

Answer 18

A process ensured by flippases - By moving certain lipids to the inner leaflet, flippases help maintain the distinct composition between the inner and outer leaflets of the membrane which is crucial for proper membrane function and signalling.

Question 19

What is exocytosis?

Answer 19

The movement of vesicles to the outside of the membrane. For example, delivery of lipids to the membrane.

Question 20

What is endocytosis?

Answer 20

Movement of vesicles into cells. Mediated by specific receptors - this process is important in the immune response.

Question 21

What are the different types of membrane proteins?

Answer 21

Transmembrane - span across the bilayer, surrounded by a hydrophobic amino acid side chain.

Membrane associated proteins - contain domains that selectively bind to phospholipids.

Lipid linked proteins