Biological Membranes

Question 1

Membranes

Answer 1

barrier surrounding cells and membranous organelles - but allow selective transport between compartments.
Receive extracellular signals which influence gene expression.
Anchorage points for structures, both inside the cell (cytoskeleton) and outside (extracellular matrix; other cells). (Lecture 6&7)
site for specialised enzyme function (e.g. mitochondrion or chloroplast).

Question 2

Functions

Answer 2

Boundary and Permeability
Organisation and localisation
Transport processes
Nutrients
Signal
Cell-cell processing

Question 3

Phospholipid bilayer

Answer 3

Permeable to hydrophobic molecules
Permeable to small, uncharged molecules
Very low permeability/not permeable to large, uncharged molecules
Not permeable to charged ions

Question 4

Components

Answer 4

The lipid content is largely made up of phospholipids which are Amphipathic
This means molecules with a polar (hydrophilic) head and a non-polar (hydrophobic) tail

Question 5

The chemical properties of water impact molecular interactions in cells

Answer 5

Hydrophilic head group
backbone (e.g. glycerol)
Hydrophobic carbon tail
Molecule is always amphipathic

Question 6

Phospholipid backbone

Answer 6

Phospholipids constructed from a glycerol backbone are called phosphoglycerates
Examples: phosphatidylcholine (most common), phosphatidylserine, phosphatidylethanolamine….
The head group.

glycerol

Question 7

Sphingosine

Answer 7

backbone in sphingomyelin – rich in myelin

Question 8

Most phospholipids have the glycerol backbone

Answer 8

Sphingomyelin is the exception.

Question 9

What happens if you mix water with uncharged (hydrophobic) molecules?

Answer 9

favorable interactions – a cage-like structure forms around it

Question 10

Why do phospholipid bilayers form?

Answer 10

Bilayer formation is energetically favorable.
Hydrophilic head interact with water.
Hydrophobic tails are in contact with each other

Question 11

Factors

Answer 11

Temperature
The nature of the phospholipids (Phosphatidylcholine, Phosphatidylinositol etc.)
The amounts of saturated and unsaturated fatty acids.
The presence of cholesterol or other sterols

Question 12

Excitable Membranes

Answer 12

Excitable Membranes

Question 13

The outer Carbohydrate layer

Answer 13

Glycoproteins - short oligosaccharide chain (3-10)
Proteoglycans – one/more polysaccharide chain (>10)
Cells have an outer sugar coating – ‘carbohydrate layer’
Protective – sugar attracts water – slimy layer that protects and helps motion
Cell-cell recognition

Question 14

Sugars=Simple Carbs

Answer 14

Composed entirely of carbon, hydrogen and oxygen: CnH2nO
Form rings or linear strands
Monosaccharides,
disaccharides,
oligo-, poly-saccharides

Question 15

Membranes also have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules

Answer 15

Glycolipids—carbohydrate + lipid

Glycoproteins—carbohydrate + protein

Adding carbohydrate moieties (Glyco…

Question 16

Membrane Asymmetry

Answer 16

The two leaflets of a membrane are not identical. Each may contain different types of phospholipid and a different arrangement of proteins and carbohydrates.
The two sides of the membrane may have different functions

Question 17

Double bonds make a kink – influence fluidity

Answer 17

Double bond in unsaturated tail makes kinks in the tail
Gaps caused by kinks in plasma membrane filled with cholesterol
High cholesterol levels makes membranes more rigid.

Question 18

Peripheral

Answer 18

Can be attached to an integral membrane protein
Attachments are temporarily:
Hydrophobic interaction
Electrostatic interaction
Non-covalent interactions
Easily removed from the cell membrane

Question 19

Integrtal

Answer 19

Transmembrane proteins extend all the way through the phospholipid bilayer.
Permanently embedded – can only remove with detergent
They have one or more transmembrane domains:
Transmembrane domains are usually composed of a-helices
Hydrophilic amino acids are internalised
Hydrophobic amino acids are on the outside in contact with the lipid bilayer

Question 20

Lipid anchored

Answer 20

covalently attached to lipid bilayer and are referred to as anchored membrane proteins
These lipid anchors include long-chain acyl or prenyl groups, glycosyl-phosphatidylinositol (GPI) and cholesterol.
The lipid association with proteins can be either reversible or irreversible and an individual protein may have more than one lipid anchor attached.