S2: Membrane Structure, Synthesis and Function I Flashcards
List some common features that membranes have
- Two molecules thick and form closed boundaries of cell
- Contain lipids and proteins to mediate distinctive cell functions
- Phospholipid bilayer
- Non covalent arrangement and are asymmetric (fluid-mosaic model)
- Membranes are electrically polarised which plays a key role in transport
Molecular composition of lipids in the membrane
In general, all consist of a hydrophilic head and hydrophobic tail
- Main lipids are phospholipids with the phosphate head
- Cholesterol with OH head
- Glycolipids with a large carbohydrate head on the outside and lipid group in the membrane
What does cholesterol do in the membrane?
It regulates membrane fluidity
What does amphipathic mean?
They possess both hydrophilic/water loving/polar and lipophilic/fat loving properties.
e.g. phospholipids and glycolipids
What do phospholipids and glycolipids form in aqueous media?
They both readily form bimolecular sheets (bilayer) in aqueous media and they ability to form liposomes might have clinical uses such as the delivery of drugs or DNA.
Describe membrane fluidity
Membranes are not static, rigid structures rather they are fluid. Lipids can move across the surface (lateral movement/diffusion) rapidly. However swapping from one side to the other (transverse movement/diffusion) is slow, rarer and requires the action of the enzyme flippase. This is because it takes a lot of energy to get the hydrophilic head through the fatty membrane.
What enzyme is needed for the transverse movement of phospholipids in the membrane?
Flippase
Factors affecting the fluidity
- Temperature
- Fatty acid composition
- Chain length
- Degree and extent of saturation
- Cholesterol content
What features in a fatty acid increases rigidity of membrane?
- Saturated fatty acyl chains increase rigidity as they pack closer together meaning stronger interactions
- Increased chain length increases rigidity as there are more interactions between chains
How does temperature affect motility of membrane?
When temperature is low, the energy of molecules is low, so the motility of the membrane decreases (i.e. molecules closer together). At higher temperatures the energy associated with molecules is greater so the spaces between molecules increases and fluidity of the membrane increases.
Describe how cholesterol in the membrane acts at different temperatures
Cholesterol tries to get membrane back into normal composition during changes in temperature
- At low temperatures, cholesterol (in small numbers) interferes with the interaction between phospholipids and increases membrane fluidity.
- Conversely at high temperatures, cholesterol (in greater numbers) works to stabilize the membrane and reduce membrane fluidity by bringing the phospholipids closer together.
So cholesterol has a different role at different temperatures.
What is Spur cell anaemia?
In people who have spur cell anaemia their cholesterol content is increased. This high amount of cholesterol eventually decreases membrane fluidity. This leads to spikey RBC they are fragile and obstructive.
Describe membrane synthesis
- New membrane is synthesised from pre-existing membrane in ER
- As it moves to golgi body it undergoes modifications
- The enzymes responsible for making the membrane are present on the inside of the ER and enzymes are only on the inside of the ER (cytosolic)
The cell will insert new phospholipid into a intracellular membranous structure. As new lipids are inserted, the bilayer starts to bow as one side gets more enlarged. Flippase then flips lipids from one side to the other so that both sides get enlarged and the membrane is no longer bowed but straight.
What is apoptosis?
Cells that want to undergo programmed cell death display eat me signals for macrophages on their plasma membrane cell surface.
What is the marker for an ‘eat me signal’ during apoptosis?
Phosphatidylserine is this marker.
Usually it will be on the inside of the membrane but will undergo the transverse diffusion when needed and exposed on the surface.