Function of the Plasma Membrane?
- Partially permeable barrier that seperates the cell’s components from the external environment - i.e. compartmentalisation.
- Regulates the transport of materials in and out of the cell.
- May contain enzymes involved in specific metabolic pathways/reactions
- Has antigens for cell recognition
- Has receptors for cell signalling/communication - hormones, drugs and other chemical signals can bind to membrane bound receptors (it can also release chemical signals).
- Site of chemical reactions
State the permeability of plasma membranes?
- Plasma membranes are partially permeable.
- Partially permeable means that it allows some specific molecules to pass through but not all.
How do things move across the membranes?
- Some very small molecules diffuse through the plasma membrane between structural molecules.
- Some dissolve in the lipid layer and then pass through.
- Some pass through channel proteins or are carried through by carrier proteins.
What is the permeability (of a membrane) determined by?
Permeability is determined by the components that make up the plasma membrane.
As discussed metabolic processes can occur on membranes and these are usually mebranes within a cell. Give examples?
- Cristae (in mitochondria) have a large SA to catalyse the reactions involved in ATP production (aerobic respiration) + to localise enzymes needed.
- Thylakoid membranes (in chloroplast) house chloropyll + allow some photosynthesis reactions to take place on it.
- Digestive enzymes can be found on the plasma membranes of epithelial cells that line the small intestines - they can catalyse the final stages of sugar breakdown.
What is the main component of plasma membranes and explain how it forms the plasma membrane?
The main component of plasma membranes are phospholipids. They consist of a phosphate head which is hydrophilc/polar and two lipid (fatty acid) tails which are hydrophobic/non-polar.
- When in water, the phospholipids automatically form a bilayer made up of two layers of phospholipids. They are organized in such a way that the lipid tails (of the phospholipids in the bilayer) face inwards towards each other, creating a hydrophobic region, while their phosphate heads face outwards, creating a hydrophilic region.
- This phospholipid bilayer arrangement means that the polar phosphate heads are in contact with the internal and external aqueous environment while the non-polar lipid tails are hidden away inside the bilayer, not in contact with water (otherwise the lipids would prevent polar molecules from interacting with the membrane).
SO.. what is the arrangement of biological plasma membranes?
A phospholipid bilayer.
Plasma membranes contains many different components - what do the different proportions depend on?
The different types of components and the proportion of each component that make up a plasma membrane depends on the function of the membrane.
Plasma membranes contain 2 main types of proteins. What are they?
Intrinsic/Integral Proteins
Extrinsic/Peripheral Proteins
What are Intrinsic/Integral proteins?
Intrinsic proteins are proteins that span both sides of the phospholipid bilayer/plasma membrane.`
What are Extrinsic/Peripheral proteins?
Extrinsic proteins are proteins that span only onse side of the phospholipid bilayer/plasma membrane.
Examples of intrinsic proteins (2 types)? Give examples.
Channel Proteins e.g. aquaporin
Carrier Proteins e.g. sodium-potassium pump
What are channel proteins?
1) Channel proteins are intrinsic proteins which allow the passive movement of small polar molecules and ions like water (and water-soluble substances) from one side of the membrane to the other by facillitated diffusion
2) This movement occurs down a concentration gradient (high conc to low conc) via a hydrophilic channel.
Why is the hydrophilic channel hydrophilic?
This is because it is lined with hydrophilic amino acids.
What are carrier proteins?
1) Carrier proteins are intrinsic proteins which actively move polar (and non-polar) molecules and ions across the plasma membrane via active transport using energy in the form of ATP
2) This movement occurs when molecules are moved against the concentration gradient from low conc to high conc.
3) These proteins have an ATP binding site which uses the ATP, produced in aerobic respiration, to trigger conformational change in its shape/structure, thus allowing it to accept, transport and release molecules from one side of the mebrane to the other.
4) Carrier proteins can also be used for facilitated diffusion, a form of passive transport, in which the molecules use their inbuilt kinetic energy to move and bind with the carrier protein triggering conformational change, moving it to the other side. No ATP required.
5) This occurs when molecules are moved down the concentration gradient from high conc to low conc.
6) In both situations, with carrier proteins, the molecules BINDS TO THE SPECIFIC COMPLEMENTARY SITE ON THE PROTEIN.
What kind(s) of transport occur across channel proteins?
- Facillitated diffusion (passive movement).
What kind(s) of transport occur across carrier proteins?
- Active transport (active movement when moving molecules against the concentration gradient).
- Facillitated diffusion (passive movement when moving molecules down the concentration gradient).
Can two or more proteins be tranported simultaneously across channel or carrier proteins?
In channel proteins, molecules can move acroos the membrane in both directions at the same time.
Whereas in carrier proteins, molecules can only move in one direction at any given time.
(facillitated diffusion and active transport cannot occur at the same time)
How can water move across the plasma membrane?
Sometimes, due to the very small size of water molecules, they can pass through the hydrophobic region of the membrane, between structural components.
However, when there is an osmotic potential difference between the two areas seperated by the membrane, a gradient is created in which water diffuses down it, through a channel protein.
Examples of molecules that move through a channel proteins?
Water molecules and any molecule of the appropriate size and charge
Examples of molecules that move through a carrier proteins?
- Sugars e.g. glucose (too large for channel proteins)
- Amino acids
- Nucleosides
Example of an extrinsic/peripheral protein?
*Extrinisc proteins are usually on the extracellular side
G-protein receptors.
What do extrinsic proteins do?
They accept hormones, drugs and minerals causing them to shift, triggering a cascade of enzyme activity.
Examples of chemical signalling molecules?
Drugs and hormones
