Chapter 5 - Plasma Membrane Flashcards
What is compartmentalisation and why is it necessary?
The formation of separate membrane bound areas in a cell.
Vital because metabolism has different and incompatible reactions so separate parts allow specific conditions to be maintained.
What is the plasma membrane and outline its structure.
Plasma membrane= the cell surface membrane that separates a cell from its external environment.
It is formed from a phospholipid bilayer:
- hydrophobic tails form a hydrophobic core inside the membrane.
- hydrophilic phosphate heads form the inner and outer surface of the membrane. Can interact with water.
What is the fluid mosaic model and why is it given this name?
It is a model of the structure of a plasma membrane.
Called fluid mosaic because:
- phospholipids are free to move within the layer (are fluid), giving the membrane flexibility.
-proteins embedded in the bilayer vary in shape/size (like tiles of a mosaic).
What are intrinsic proteins?
They are transmembrane proteins embedded through both layers of a membrane.
- contain amino acids with hydrophobic R groups on their external surfaces which interact with the hydrophobic core and keep them in place.
What are channel and carrier proteins?
Intrinsic proteins involved in transport.
- Channel proteins:
- provide a hydrophilic channel that allows the passive movement (diffusion) of polar molecules and ions down a concentration gradient through membranes. - Carrier proteins:
- have an important role in both passive and active transport into cells. This often involves the shape of proteins changing.
What are glycoproteins?
(Intrinsic)
-proteins with a carbohydrate attached.
- they are embedded in the cell surface membrane.
- play a role in cell adhesion and as receptors for chemical signals.
The chemical binds to the receptor, it elicits a response which may causes a direct response inside the cell. = called cell signalling.
Eg. They act as receptors for neurotransmitters and hormones (insulin)
What are glycolipids?
Lipids with attached carbohydrate chains.
- act as cell recognition sites (detect pathogens).
- are cell markers/ antigens. Recognised by immune system as self/ non-self.
What are extrinsic proteins?
- present in one side of the bilayer.
- have hydrophilic R groups on outer surfaces and interact with polar heads of phospholipids/ intrinsic proteins.
- can be present in either layer, some move between layers.
What is cholesterol?
- a lipid which regulates fluidity of membranes. Has a hydrophilic end and hydrophobic end.
- positioned between phospholipids in a bilayer.
- binds to the hydrophobic tails of lipids which causes them to pack closely= Therefore adds stability and some rigidity.
- stops membranes being solid by preventing phospholipid molecules getting close and crystallising.
How does temperature affect membrane structure?
- An increase in temp=
- phospholipids have more kinetic energy and move more.
- membrane becomes more fluid and loses its structure.
- cell eventually breaks down completely.
- loss of structure= increases permeability so more particles cross.
- carrier/ channel proteins denature= membrane permeability is affected.
How do solvents affect membrane structure?
- water is essential for the formation of the bilayer because phosphate heads interact with it to keep the bilayer in tact.
-organic solvents dissolve membranes= disrupts cells. (alcohols used in antiseptic as they dissolve bacteria membranes and reduce infection). - non-polar alcohol molecules enter the cell membrane and disrupt it:
When the membrane is disrupted it becomes more fluid/permeable.
This makes specific functions hard such as transmission of nerve impulses.
Explain diffusion.
The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.
- passive process which continues until there is a concentration equilibrium.
- particles move at high speeds and constantly collide which slows down the overall movement.
Diffusion rate slows down as diffusion distance increases (more collisions happen).
What factors affect the rate of diffusion?
- Temperature:
- higher the temp, higher the rate of diffusion as particles have more kinetic energy (move faster). - Concentration difference:
- greater the difference in conc, faster the rate of diffusion. Overall movement from hgh to low is larger.
Explain diffusion across membranes.
Involves particles passing through the phospholipid bilayer.
-only happens if the membrane is permeable to the particles. Non-polar molecules diffuse freely.
-hydrophobic interior repels ions so they can’t pass through easily.
-polar molecules diffuse at very slow rates due to partial charges.
-small polar molecules diffuse more easily.
Therefore membranes = partially permeable.
What factors affect the rate of diffusion across membranes?
- Surface area:
- larger the area of an exchange surface, higher the rate of diffusion. - Thickness of membrane:
- the thinner the exchange surface, higher the rate of diffusion.
