Cell membranes

Question 1

What are cell membranes?

Answer 1

Protects the inside from the outside.
Composed of lipids and proteins.

Question 2

What is the lipid bilayer?

Answer 2

Each lipid has a hydrophobic tail and hydrophilic head which forms a bilayer.
It is dynamic and selectively permeable.

Question 3

What is the fluid mosaic model?

Answer 3

The lipid bilayer contains membrane proteins which enable signalling, communication and selective permeability.

Question 4

What are phospholipids?

Answer 4

The predominant lipids in eukaryotic cells.
Hydrophobic tails face each other.
Hydrophilic heads face out towards fluids.
Fatty acid chains determine the fluidity of the membrane.

Question 5

What are the fatty acid chains?

Answer 5

Each phospholipid has two acyl chains.
These are 14-24 carbons long.
If there is at least 1 double bond it is unsaturated - this produces a kink and affects compacting in the membrane.

Question 6

What are the major phospholipids?

Answer 6

Phoshoglycerides (derived from glycerol):
Phosphatidylethanolamine
Phosphatidylserine - negatively charged
Phosphatidyl
Sphingolipid (derived from sphingosine) - sphingomyelin

Question 7

Why do lipid bilayers spontaneously seal?

Answer 7

A planar phospholipid bilayer has hydrophobic edges exposed to water, which is energetically unfavourable.
It spontaneously closes to form sealed compartments - which are energetically favourable.

Question 8

What are the benefits of membranes being fluid structures?

Answer 8

Allows signalling lipids and membrane proteins to rapidly diffuse.
Ensures membranes are equally shared between daughter cells after cell division.
Allows cells to change shape.
Allows membranes to fuse with other membranes - exocytosis.

Question 9

How does the composition of membranes determine its fluidity?

Answer 9

In colder environments bacteria and yeasts synthesise:
Shorter fatty acid chains.
Chains with increased saturations.
This decreases the interactions between the chains and means the membrane remains fluid at lower temperatures.

Question 10

How does cholesterol modulate lipid bilayers?

Answer 10

Cholesterol inserts between membrane phospholipids.
This tightens packing in the bilayer and decreases membrane permeability to small molecules.

Question 11

What are the intracellular signal transduction lipids?

Answer 11

Phosphatidylinositol
Diacylglycerol
Ceramide
Sphinosine-1-phosphate
Derived from lipids residing in the plasma membrane.

Question 12

What are features of intracellular signal transduction lipids?

Answer 12

Rapidly generated and destroyed by enzymes in repsonse to a specific signal.
Spatially and temporally generated = highly specific signal.
Bind specifically to conserved regions in proteins and induce conformational and or localisation and activity changes in the proteins.

Question 13

What is the structure of phosphatidylinositol?

Answer 13

Important substrate for signalling proteins

Question 14

How are lipid bilayers synthesised?

Answer 14

Fatty acids synthesised in cytosol are transported to the smooth Endoplasmic Reticulum by Fatty Acid Binding Protein.
Fatty acids are embedded in the membrane, and glycerol, phosphate and choline are added.

Question 15

What are glycolipids?

Answer 15

Contain sugar, made from sphingosine.
Glycosylation occurs in the lumen of golgi.
Founds exclusively on non-cytosolic monolayer of plasma membrane.

Question 16

How is the asymmetric bilayer created?

Answer 16

Phospholipid synthesised in outer, cytosolic leaflet of ER membrane.
Enzyme scramblase catalyses trans-bilayer movement of phospholipids from the outer to the inner leafers of ER so they distribute evenly.
New membrane is transported to the plasma membrane in vesicles.
Plasma membrane enzyme - flippase (requires ATP) flips phospholipids from extracellular leaflet to cytosolic leaflet.

Question 17

Where does asymmetric distribution of glycolipids occur?

Answer 17

lumen of the ER/Golgi

Question 18

What are the types of membrane proteins?

Answer 18

Single pass
Multipass transmembrane protein - these two are hydrophobic amino acids with side chains interacting with lipid monolayer.
B-barrel
Lipid linked
Peripheral membrane protein

Question 19

What are the functions of membrane proteins?

Answer 19

Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix.

Question 20

How are secreted proteins translated?

Answer 20

Soluble proteins contain hydrophobic signal peptide at N-terminal.
This directs the polypeptide chain to a translocator in the ER membrane.
Growing polypeptide chain is threaded through the membrane.
Signal peptide is cleaved and newly synthesised protein is released into ER lumen.

Question 21

What is the glycocalyx?

Answer 21

Carbohydrate-rich layer surrounding cells.
Composed on glycolipids and glycoproteins.
Protects cells against chemical and mechanical damage.

Question 22

Why is the glycocalyx important?

Answer 22

It traps water molecules and provides protection.
In the vascular system it prevents red blood cells sticking to the wall.

Question 23

What is the nuclear envelope?

Answer 23

Consists of outer and inner membrane.
Continuous with ER membrane.
Penetrated by nuclear pore complexes - allows bidirectional exchange between nucleoplasmic and cytoplasmic compartments.
3000-4000 pores per nucleus.

Question 24

How can you test how fluid a membrane is?

Answer 24

Fluorescence
Recovery
After
Photobleaching

Question 25

Why is lipid asymmetry important?

Answer 25

Inner and outer monolayers have different composition.
Phosphatidylserine is concentrated on cystolic layer (inner):
Important for protein kinase C activity in cytosol - converts extracellular signals to intracellular.
Translocates to outer membrane during apoptosis and signals neighbouring cells to phagocytose cell.