Membranes Flashcards
Describe the structure of a cell membrane?
Phospholipid bilayer embedded with proteins (intrinsic - run all way through inside) and extrinsic (attached to the outside).
These extrinsic proteins can also be added to carbohydrate to form glycoproteins. Carbohydrate and lipids can also form glycolipids.
Cytoskeleton plays a role in keeping the cell shape.
Cholesterol - helps increase the rigidity.
What is the job of the cell membrane?
To maintain a constant internal environment by controlling what comes in and out of the cell. It is therefore selectively permeable and uses transport proteins to allow substances in which can’t diffuse freely through the membrane.
What are the different types of membrane proteins?
Channels and carrier proteins.
Channels - are selective for size and shape, are passive and are usually used for ions or water (aquaporins). The substances are also moved down the concentration gradient.
Carriers - have a specific binding site and usually require energy to transport a substance often against the concentration gradient.
What are the different types of carrier proteins you can get?
Uniport - single substance
Symport - 2 substances same direction
Antiport - 2 substances opposite direction
What are the different driving forces for the movement of a substance in and out of a cell?
Chemical - based on concentration gradient (high to low conc) - force directionally proportional to gradient.
Electrical - also known as membrane potential!
It is based on the distribution of charges across a membrane.
Forced dependent on membrane potential.
E.g. more negative outside charge positive molecules (K+) will have a driving force helping them move.
Electrochemical - combination of both chemical and electrical
Net direction equal to the sum of both.
Again only charges substances can move with this force.
Passive transport of a substance is…
Is movement of a substance down a concentration gradient using no energy. Examples of passive transport includes simple and facilitated diffusion.
What is active transport?
Is the movement of a substance against a concentration gradient using ATP and carrier proteins.
There are 2 types.
What is type 1 active transport?
Directly uses ATP and moves the substance against the concentration gradient.
What is type 2 active transport?
It is transport of a substance down a concentration gradient and transport of a substance against it concentration gradient.
E.g cotransport in the ileum.
What is a ligand gated ion channel?
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What is a G protein receptor?
How does cholera affect it?
Cholera produces toxins which cross the cell membrane and modifies the G protein. this leaders to more cAMP produced which leads to an increased secretion of ions and hence water into the gut — dehydration and diarrhoea.
What is endocytosis?
What is exocytosis?
Endocytosis - Large molecules that require a different method of transport into a cell.
E.g. phagocytosis — pathogen absorbed into cell in a vesicle…
Exocytosis - large molecules are transported out of the cell
E.g. acetyl choline at synapses.
Note drugs can be used to target membrane transporters and hence inhibit or increase certain processes.
If these transporters they can causes disease due to consequences of not carrying put their function.
E.g. cystic fibrosis — mutation in CFTR protein - affects Cl- channels and leads to sticky mucus — Cystic fibrosis.
GLUT1 Deficiency syndrome — where transporters for glucose in the brain don’t function properly — seizures
What is the job of cell surface receptors?
Receptors on cell surface membranes can be active by hormones or other signalling molecules leading to a response.
See the membranes structure lecture for the structure of a phospholipid.
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