Lecture 35: Biological Membranes and Lipids

Question 1

What is the function of membranes?

Answer 1

• Semipermeable barrier
• Detects and interprets changes in extracellular environment
• Provide anchorage sites for extracellular proteins and cytoskeleton
• Provides an alternative environment to the cytoplasm

Question 2

What does the fluid mosaic model say about membranes?

Answer 2

That they are semi-fluid, not static

Question 3

Do membranes have a cytoplasmic and exoplasmic side?

Answer 3

Yes

Question 4

The lumen (inside) of the ER corresponds to the exoplasmic side of the plasma membrane.

Answer 4

Modifications done in the ER (e.g., glycosylation) appear only on the exoplasmic side after vesicle fusion.

Question 5

How many lipids does the plasma membrane of a typical cell contain?

Answer 5

The plasma membrane of a ‘typical’ cell contains ~10^9 lipids

Question 6

What are the different types of lipids that can be found in a plasma membrane?

Answer 6

• Phospholipids (Phosphoglycerides and sphingolipids)
• Glycolipids
• Cholesterol

Question 7

Sphingophospholipids are phospholipids that have a sphingosine backbone instead of a glycerol backbone.

Answer 7

Sphingophospholipids are phospholipids that have a sphingosine backbone instead of a glycerol backbone.

Question 8

is it true that membranes typically don’t contain that many carbohydrates?

Answer 8

Yes

Question 9

Is Myelin an insulator for nerve cells?

Answer 9

Yes, which is why the lipid composition of the membrane of myelin is very high (lipids are very good insulators).

Question 10

What do phospholipids consist of?

Answer 10

Phosphoglycerides and Sphingolipids

Question 11

What do membrane signalling molecules consist of?

Answer 11

• Steroid hormones e.g., sex hormones and cortisol
• Eicosanoids: short range signalling molecules involved in pain, inflammation, etc.

Question 12

Which vitamins are important for membrane function?

Answer 12

Vitamins: A, D, E and K

Question 13

Are tryglycerides fuel for metabolism?

Answer 13

Yes

Question 14

Because membrane lipids are very diverse in structure, what are they defined by?

Answer 14

Defined by their hydrophobicity rather than structure

Question 15

Are lipids soluble in water?

Answer 15

No, they are only soluble in organic solvents such as chloroform

Question 16

Glycerol would be hydrophilic if not for…

Answer 16

The fatty acid tails it has, which makes it hydrophobic

Question 17

Describe the structure of tryglycerides

Answer 17

• One glycerol molecule attached to 3 fatty acid chains

Question 18

Describe the structure of phosphoglycerides (type of phospholipid)

Answer 18

• One glycerol molecule(backbone) attached to 2 fatty acid chains, and a phosphate group

Question 19

Describe the structure of sphingolipids

Answer 19

• One Sphingosine molecule (backbone) attached to: One fatty acid tail attached via an amide bond (hydrophobic); A phosphate group (hydrophilic) , and a polar head group, often choline.

Question 20

Where are sphingolipids often found?

Answer 20

in the myelin sheath of nerve cells.

Question 21

What type of phospholipid makes up the bulk of our biological membranes?

Answer 21

Phosphoglycerides

Question 22

What is meant by the fact that phospholipids are amphiphilic/amphipathic?

Answer 22

They are both hydrophilic and hydrophobic

Question 23

What part of phospholipids is hydrophobic and what part is hydrophilic?

Answer 23

Fatty acid chains are hydrophobic

Polar head is hydrophilic

Question 24

Describe the key properties of lipid bilayers

Answer 24

• Extensive: They can grow and spread.
• Self-sealing: No edges are exposed to water due to the exclusion of hydrophobic regions.
• Amphipathic: Lipids have both hydrophobic tails and hydrophilic heads, promoting bilayer formation.

Question 25

Describe fatty acids

Answer 25

- They are terminated with a carboxylic acid group - They form the 'tails' of phospholipids and triglycerides - They are hydrophobic - They can be saturated or unsaturated (have a double bond)

Question 26

What are the 3 systems for numbering the carbons in fatty acids?

Answer 26

1,2,3…… Chemical numbering system α, β, 𝛾…… Biochemical still used in metabolism ω, ω-1, ω-2….. Used in nutrition

Question 27

Describe unsaturated fatty acids

Answer 27

- Unsaturated fatty acids have C=C double bonds - Double bonds in fatty acids are usually cis, but can be trans as well - Trans is almost straight, but cis introduces a kink - Melting point decreases with increasing double bonds - Unsaturated fatty acids can't be packed as closely together, this irregularity of packing resulting in more fluid membranes

Question 28

Where must essential fatty acids be obtained from?

Answer 28

Our diet

Question 29

What is the importance of essential fatty acids?

Answer 29

- Important function in cell membrane in cell signalling - Serve as a precursor for other signalling molecules, such as arachidonic acid

Question 30

Key essential fatty acids

Answer 30

Linoleic acid: 18:2 C18 Δ^9,12 (ω 6 fatty acid ) Linolenic acid: 18:3 C18^ Δ9,12,15 (ω 3 fatty acid )

Question 31

Describe Arachidonic acid

Answer 31

- Can be synthesised, or obtained from our diet -20:4 C20 Δ5,8,11,14 - Synthesised from linoleic acid - Precursor for eicosanoids - Also, important functions as part of phospholipids in membrane - Plays important role in inflammation

Question 32

What are Eicosanoids important for?

Answer 32

Pain and inflammation (they allow you to feel pain)

Question 33

What are Eicosanoids synthesised from?

Answer 33

Arachidonic acid

Question 34

Is it true that the Hydroxy groups (or O-) can be further modified in phospholipids?

Answer 34

Yes, by the addition of a head group, such as choline (which will be cleaved off when signals come in)

Question 35

Is it true that phospholipids can contain either saturated or unsaturated fatty acids?

Answer 35

Yes

Question 37

In sphingolipids (the second type of phoshpholipid) is one of the fatty acid chains inbuilt?

Answer 37

Yes

Question 38

What bond atatches a choline molecule to a phosphate group?

Answer 38

An ester bond

Question 39

Are head groups like choline and inositol typically good signalling molecules?

Answer 39

Yes

Question 40

What amino alcohol are Sphingolipids based on?

Answer 40

sphingosine

Question 41

Describe glycolipids

Answer 41

- Sugar containing lipids - Sugar instead of phosphate group - Can be more than one sugar unit - In animal cells they are derived from sphingosine not glycerol - Sugar always on the outside of the cell - Functions include: Immune responses, cell cell recognition and attachment

Question 42

Is cholesterol only present in animal membranes?

Answer 42

Yes

Question 43

Describe cholesterol

Answer 43

- Sterol – modified steroid - Structure of 4 hydrocarbon rings (all steroids have this structure) - Rings are planar, but not completely flat - Present in only mammalian cell membranes - Has a hydrophobic tail - Cholesterol has important effects on membrane fluidity

Question 44

Key messages

Answer 44

- Amphipathic lipids generating the membrane bilayer, which is semi fluid - There are three main types of lipids in membranes Phospholipids (Phosphoglycerides and sphingolipids) Glycolipids Cholesterol - Membrane composition differ in terms of which lipids it contains and which and how many proteins it contain reflecting the function - Lipids are not just structural but have functions in signalling as well