Membrane Structure and Function I Flashcards
What are some common features that membranes have, despite their diversity?
- they’re two molecules thick, and form the closed boundaries of the cell
- they consist of lipids and proteins, and contain specific proteins to mediate distinctive cell functions
- they have a non-covalent arrangement and are asymmetric (aka, the fluid mosaic model)
- they’re electrically polarised, which plays a key role in transport
What are the three main lipids, and what do they consist of?
- PHOSPHOLIPIDS, with the phosphate head
- CHOLESTEROL, with the -OH head
- GLYCOLIPIDS, with the large carbohydrate head and lipid group
What two of the lipids are amphipathic, and what does that mean?
PHOSPHOLIPIDS and GLYCOLIPIDS are amphipathic (meaning they possess both hydrophilic/water-loving/polar and lipophilic/fat-loving properties).
Describe the movement of lipids within the membrane surface.
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.
List some factors that will alter the fluidity of the membrane.
- temperature
- fatty acid composition
- chain length
- degree, and extent, of saturation
- cholesterol content
The closer the fatty acid chains pack together, the stronger the interactions between them. This increases the rigidity of the membrane.
Describe how cholesterol and temperature interact to affect the membrane fluidity.
When the temperature is low, the energy of molecules is low, so the motility of the membrane decreases (i.e. molecules get closer together). At higher temperatures, the energy associated with molecules is greater so the spaces between molecules increases and fluidity of the membrane increases.
However, cholesterol acts as the maintenance of the membrane and tries to get it back to the normal composition during the 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.
How do cholesterol levels differ in people with Spur Cell Anaemia?
In people who have Spur Cell Anaemia, their cholesterol content is increased by 25-65%. This very high amount of cholesterol eventually decreases membrane fluidity. This leads to spikey RBCs, which are fragile and obstructive.
Describe membrane synthesis.
New membrane is synthesised in the ER. As it moves to the Golgi, it undergoes modifications. The enzymes responsible for making the membrane are only present on the inside of the ER.
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.
How does the membrane signal apoptosis?
Cells that want to undergo programmed cell death display ‘eat me’ signals for macrophages on their plasma membrane cell surface. Phosphatidylserine is this marker; usually, it will be on the inside of the membrane but will undergo transverse diffusion when needed to be exposed on the surface.
Describe Integral Membrane Proteins.
Integral membrane proteins span the entire membrane and it is very difficult to remove them from the membrane. Structurally, they tend to be similar: usually in the transmembrane domain being alpha helical in shape, with the R groups in the being (hydrophobic in nature) facing outwards in the membrane. The proteins can have a single transmembrane domain or multiple. They have strong, non-covalent bonds. IMPs interact extensively with the lipid bilayer.
Describe Peripheral Membrane Proteins.
These are located on the extracellular or cytosolic membrane surface and are loosely associated with it, but not embedded into it. They are simply bound to a phospholipid polar head group or integral membrane protein.
They are associated, by non-covalent bonds, to the surface, and as the association is not that strong they associate transiently with the membrane.
Describe Lipid Anchored Membrane Proteins.
These are proteins that are covalently attached to lipids embedded within the cell membrane, such as glycerol-phosphatidylinositol.
Describe Membrane Carbohydrates.
Carbohydrates are associated with both membrane lipids and proteins. They form 2-10% of the membrane weight (in RBCs, 8%).
Carbohydrates on all membranes face the outside (EC domain), away from the cytosol.
