Lecture 35: Biological Membranes and Lipids Flashcards
What is the function of membranes?
Semipermeable barrier
Detects and interprets changes in extracellular environment
Provide anchorage sites for extracellular proteins and cytoskeleton
Provides an alternative environment to the cytoplasm
What does the fluid mosaic model say about membranes?
That they are semi-fluid, not static
Do membranes have a cytoplasmic and exoplasmic side?
Yes
The lumen (inside) of the ER corresponds to the exoplasmic side of the plasma membrane.
Modifications done in the ER (e.g., glycosylation) appear only on the exoplasmic side after vesicle fusion.
How many lipids does the plasma membrane of a typical cell contain?
The plasma membrane of a ‘typical’ cell contains ~10^9 lipids
What are the different types of lipids that can be found in a plasma membrane?
- Phospholipids
- Glycolipids
- Cholesterol
is it true that membranes typically don’t contain that many carbohydrates?
Yes
Is Myelin an insulator for nerve cells?
Yes
What do phospholipids consist of?
Phosphoglycerides and Sphingolipids
What do signalling molecules consist of?
- Steroid hormones e.g., sex hormones and cortisol
- Eicosanoids: short range signalling molecules involved in pain, inflammation, etc.
Which vitamins are important for membrane function
Vitamins: A, D, E and K
Are tryglycerides fuel for metabolism?
Yes
Because membrane lipids are very diverse in structure, what are they defined by?
Defined by their hydrophobicity rather than structure
Are lipids soluble in water?
No, they are only soluble in organic solvents such as chloroform
Glycerol would be hydrophilic if not for…
The fatty acid tails it has, which makes it hydrophobic
Describe the structure of tryglycerides
- One glycerol molecule attached to 3 fatty acid chains
Describe the structure of phosphoglycerides (type of phospholipid)
- One glycerol molecule(backbone) attached to 2 fatty acid chains, and a phosphate group
Describe the structure of sphingolipids
- 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.
Where are sphingolipids often found?
in the myelin sheath of nerve cells.
What type of phospholipid makes up the bulk of our biological membranes?
Phosphoglycerides
What is meant by the fact that phospholipids are amphiphilic/amphipathic?
They are both hydrophilic and hydrophobic
What part of phospholipids is hydrophobic and what part is hydrophilic?
Fatty acid chains are hydrophobic
Polar head is hydrophilic
Describe the key properties of lipid bilayers
- 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.
Describe fatty acids
- 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)
What are the 3 systems for numbering the carbons in fatty acids?
1,2,3…… Chemical numbering system
α, β, 𝛾…… Biochemical still used in metabolism
ω, ω-1, ω-2….. Used in nutrition
Describe unsaturated fatty acids
- 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
Where must essential fatty acids be obtained from?
Our diet
What is the importance of essential fatty acids?
- Important function in cell membrane in cell signalling
- Serve as a precursor for other signalling molecules, such as arachidonic acid
Key essential fatty acids
Linoleic acid: 18:2 C18 Δ^9,12
(ω 6 fatty acid
)
Linolenic acid: 18:3 C18^ Δ9,12,15
(ω 3 fatty acid
)
Describe Arachidonic acid
- 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
What are Eicosanoids important for?
Pain and inflammation (they allow you to feel pain)
What are Eicosanoids synthesised from?
Arachidonic acid
Is it true that the Hydroxy groups (or O-)
can be further modified in phospholipids?
Yes, by the addition of a head group, such as choline (which will be cleaved off when signals come in)
Is it true that phospholipids can contain either saturated or unsaturated fatty acids?
Yes
In sphingolipids (the second type of phoshpholipid) is one of the fatty acid chains inbuilt?
Yes
What band atatches a choline molecule to a phosphate group?
An ester bond
Are head groups like choline and inositol typically good signalling molecuels?
Yes
What amino alcohol are Sphingolipids based on?
sphingosine
Describe glycolipids
- 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
Is cholsesterol only present in animal membranes?
Yes
Describe cholesterol
- 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
Key messages
- 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
