Lecture 1.2 - Membrane lipids

Question 1

The definition of a lipid

Answer 1

Any of the large group of fats and fat-like compounds which occur in living organisms and are characteristically soluble in certain organic solvents but only sparingly soluble in water

Question 2

Four lipid families

Answer 2

• Triacylglycerols
• Glycerophospholipids
• Sphingolipids
• Isoprenoids

Question 3

Triacylglycerols

Answer 3

Glycerol backbone with three fatty acid (acyl) tails

Hydrophobic

Question 4

(Glycero)phospholipids

Answer 4

Glycerol backbone, phosphate moieties, with a polar head group (can include glycoconjugates)

Amphipathic

Question 5

Sphingolipids

Answer 5

Built on a sphingosine backbone, often glycoconjugates

Amphipathic

Question 6

Isoprenoids

Answer 6

Include steroids, lipids, vitamins, and hormone

Largely hydrophobic - variable polar group content

Question 7

Membrane lipids: what are the main ones and how do they typically vary?

Answer 7

Membrane lipids are mainly phospholipids, glycolipids and cholesterol

They vary mainly by the head group for PL and GL and by the cholestene fused ring of sterols

Question 8

What determines what group a lipid belongs in?

Answer 8

The head group of the lipid:

Choline, Ethanolamine, and Polyalcohols such as glycerol, inositol and serine are three groups

Question 9

Acyl chains: what is determined by the carbon length, how many carbons are there on average, saturation, and sphingolipid acyl chains?

Answer 9

Shorter - more fluid

18 on average

• Saturated - no free rotation (less fluid)
• Unsaturated - kink at each bond (more fluid)

Generally unsaturated and longer chains (22/24)

Question 10

Natural lipid acyl chains: structure and use in thermoregulation

Answer 10

Contain phospholipids where one chain is saturated and one chain is unsaturated with a cis bond with around 18C’s in the chain

Organisms that survive in different temperatures have different lipid membrane composition in order to thrive in certain conditions

Question 11

Sphingomyelins: the types

Answer 11

Cerebrosides, gangliosides, ceramide, sphingomyelins, gangliosides, and sphingolipids

Question 12

Cerebrosides

Answer 12

Have a ceramide unit linked by glycosidic bond at carbon 1 of long-chain base to glucose or galactose

Question 13

Gangliosides

Answer 13

Have sialic acid components - they have larger more complex head groups and are numbered according to how many sialic acids they have

Sphingomyelins and gangliosides have been found in lipid rafts - they associate with cholesterol and regulate the cell surface location of membrane proteins

Question 14

Ceramide

Answer 14

Known to function as intercellular signalling molecules and can affect cell growth, differentiation and programmed cell death

Question 15

Sphingomyelins

Answer 15

Sphingomyelins and gangliosides have been found in lipid rafts - they associate with cholesterol and regulate the cell surface location of membrane proteins

Question 16

Sphingolipids

Answer 16

Sphingolipids that form rafts tend to have more saturated and longer side chains which makes them less fluid with a higher melting temperature

Question 17

Cholesterol: what does it do and how much of it is present in the membrane?

Answer 17

A cyclical structure that increases bulkiness and affects membrane fluidity (stiffens it)

20-40%

Question 18

Question: How does the saturation of the fatty acid chains influence membrane structure?

Answer 18

Unsaturated fatty acids have more gaps within their chains allowing for more fluid, permeable, and thicker membranes which have a lower melting point

This is because of the decreased intermolecular forces and the higher flexibility due to the gaps within the chains

Question 19

Question: Describe how phosphatidylinositol differs in head group biochemical structure compared to cholesterol.

Answer 19

Phosphatidylinositol has an inositol-based head group with various phosphorylation sites, making it a key player in cell signalling and membrane dynamics

On the other hand, cholesterol has a steroidal head group, which is primarily involved in regulating membrane fluidity and stability. These differences in head group structure reflect these lipids’ distinct roles in cellular membranes and physiology.

Question 20

Question: Explain how the fatty acid chains differ between the glycerophospholipids, sphingomyelin and cholesterol.

Answer 20

The key differences in fatty acid chains are their structures and roles. Glycerophospholipids have two fatty acid chains attached to glycerol, sphingomyelin has a single fatty acid chain attached to sphingosine, and cholesterol lacks fatty acid chains but has a steroidal structure

These differences contribute to the diversity of lipids in cell membranes and their roles in maintaining membrane integrity and function.

Question 21

Where are lipids typically synthesised?

Answer 21

The ER

Question 22

Where are sphingolipids and sterols synthesised?

Answer 22

Sphingolipid synthesis begins in the cystolic ER and finishes in the Golgi apparatus

Sterols are synthesised in the lumenal leaflet of the ER membrane (a complex pathway)

Question 23

How are glycerol-backed phospholipids synthesised?

Answer 23

The synthesis of membrane phospholipids begins with the phospholipid PA, which is produced from glycerol-3-phosphate or dihydroxyacetone phosphate after fatty acyl coenzyme A (CoA)-dependent reactions that are catalyzed by the SCT1- and GPT2-encoded glycerol-3-phosphate acyltransferases and the SLC1- and ALE1-encoded …

(google)

Question 24

What are the three ways fatty acids differ?

Answer 24

• Length of the hydrocarbon tails
• Degree of unsaturation (double bond)
• Position of the double bonds in the chain