Membrane structure and Synthesis Flashcards
Examples of membranes
Plama membrane, Nuclear membrane, Mitochondrial membrane
Why do biological membranes form bilayers?
Because this arrangement is energetically favourable as the hydrophobic heads will face the aqueous environments at each surface of the bilayer while the hydrophobic tails will be shielded from these aqueous environments
Why are biological membranes stabalised by non-covalent forces?
It makes the membrane very dynamic as non-covalent forces are quite weak
What are the 3 main components of a biological membrane?
- Phospholipids
- Cholesterol
- Glycolipids
Basic structure of a phospholipid
Hydrophobic tail - 2 Fatty acids
Glycerol molecule
Hydrophilic head - Phosphate group attached to other groups e.g. Choline, Serine
What is the only part of the cholesterol molecule is present within the hydrophilic part of the membrane?
OH group of cholesterol molecules
What is the reason for the composition of the inside and outside of a cell being different?
Because the phospholipids (as well as other components within the membrane) aren’t distributed evenly within it - makes composition of the membrane asymmetrical
What are the factors that affect membrane fluidity?
- Temperature
- Fatty acid composition - saturated vs unsaturated
- Chain length
- Degree and extent of saturation
- Cholesterol content
How does fatty acid composition affect membrane fluidity?
If membrane contains mainly saturated fatty acids then molecules can pack closely together thus decreasing membrane fluidity whereas if membrane made up of saturated and unsaturated fatty acids membrane fluidity increases as molecules can’t pack as closely
How does Cholesterol affect membrane fluidity?
At low temperatures cholesterol interacts with the molecules of the phospholipids and prevents them from forming a tight association and crystallising
This causes an increase in membrane fluidity
At high temperatures cholesterol reduces fluidity of membrane by stabilising the membrane and increasing its melting point
In spur cell anaemia by what percentage does the cholesterol content of the red blood cell membrane increase by?
25-65%
What are the consequences of the increase in cholesterol within the red blood cell membrane?
Leads to decreased membrane fluidity meaning that the membrane becomes rigid. This results in certain molecules not being able to diffuse through the cell membrane of the red blood cell
What are the different ways that a molecule can move through a membrane?
Lateral movement - sideways movement
Transverse movement - movement across a membrane
Is lateral movement of phospholipids slow or rapid?
Lateral movement of lipids in the membrane is rapid
Is transverse movement of phospholipids slow or rapid?
Transverse movement is slow and therefore requires the action of three enzymes
What are the 3 enzymes involved in facilitating transverse movement of phospholipids?
Floppase - moves phospholipids from the cytosolic side to the extracellular side (requires ATP)
Flippase (flipase) – moves phospholipids from the extracellular side to the cytosolic side (requires ATP)
Scramblase – bidirectional movement (requires calcium but doesn’t require ATP)
What is the main process that these enzymes are involved in? why is this?
Synthesis of new lipids
Enzymes that synthesise new membrane are present on cytosolic surface of the Golgi apparatus
This means that new membrane synthesis only takes place on cytosolic side of the membrane
To synthesise new membrane that is structurally correct however, there needs to be transverse movement of phospholipids from cytosolic side to luminal side
This movement is facilitated by the ABC family of enzymes
Why is the movement of phospholipids and other molecules across a membrane so important?
It maintains the asymmetry of the membrane
What is Apoptosis?
programmed cell death
During Apoptosis how does the distribution of Phosphotidylserine change?
Phosphatidylserine mainly deposited on inner surface of the plasma membrane
During process of Apoptosis distribution of Phosphatidylserine changes significantly as it then expressed on the outer surface of the plasma membrane
This is caused by the inhibition of the enzyme aminophospholipid translocase which transports Phosphatidylserine rom the luminal side to the cytosolic side of the membrane
What causes the distribution of phosphotidylserine to change?
This is caused by the inhibition of the enzyme aminophospholipid translocase which transports Phosphatidylserine from the luminal side to the cytosolic side of the membrane
What occurs as a result of the change in distribution of Phosphotidylserine?
Movement recognised as a signal for a cell undergoing apoptosis by macrophages which will then ‘consume” the cell via phagocytosis
What is the advantage of cells being destroyed by apoptosis?
Apoptosis allows cells to be turned over without having to induce an inflammatory response
What are the 3 types of protein that can be associated within a membrane?
- Integral membrane proteins
- Peripheral membrane proteins
- Lipid anchored membrane proteins
Features of integral membrane proteins
Strongly associated with phospholipids within the membrane
Categorized as single or multi pass
Contain hydrophobic and hydrophilic regions
Hydrophobic region usually associated with an alpha-helix
Hydrophillic region usually within the extracellular and cytosolic part of the protein
Features of peripheral membrane proteins
More weakly associated with the phospholipids than the integral membrane proteins
Located on the extracellullar or cytosolic membrane
Associated by non-covalent bonds
Can be associated with integral membrane proteins or be associated directly with the phospholipids
Features of lipid anchored membrane proteins
Covalently linked to a lipid molecule
Includes proteins with lipid component embedded directly into membrane or proteins that are associated with a glycolipid or proteins
Features of membrane carbohydrates (Glycolipids)
Carbohydrates are associated with both membrane lipids and proteins
Carbohydrate on all membranes faces away from the cytosol
They are often involved in cell-cell interactions or cellular recognition
