Mod. 2 Membranes Flashcards
What molecules are able to diffuse directly through a phospholipid bilayer?
O2, fat soluble hormones (e.g. oestrogen), CO2
why are lipids insoluble in water?
There is no uneven distribution of molecules, so there is no charge for water molecules to be attracted to.
Describe a channel protein.
Channel proteins are proteins that form molecule-specific channels in the phospholipid bilayer, allowing molecules that would not be able to diffuse directly though the membrane (i.e. hydrophilic, positive ions, etc) to diffuse through.
They are often gated so that levels can be controlled. They work on a diffusion gradient, the potential difference allows the molecules to diffuse through the channel into the cell without use of ATP, it is FACILITATED DIFFUSION.
Describe sodium potassium pumps.
3 sodium ions are moving from the inside of the cell to the outside of the cell up the concentration gradient. ATP causes the protein to change shape and release the sodium ions on the outside of the cell, K ions diffuse into protein. The release of the Na+ ions causes the protein to revert back to original shape, releasing K+ into the cell. The process then starts again.
This process is particularly important in the nervous system and muscle function, and requires 1/3 of resting potential energy.
Describe a glucose transporter protein.
The glucose molecule binding to the protein causes the protein to change shape and move glucose across the membrane in a passive action.
Describe a co-transporter protein.
Transports glucose across a phosopholipid bilayer from a high concentration to a low concentration. The glucose is pulled through the membrane protein by the diffusing Na+
What structures can be found in the cell membrane?
- proteins that hold the cytoskeleton in place
- cholesterol which regulates the fluidity of the membrane
- membrane bound enzymes (e.g. in gut)
- glycoproteins that are modified proteins with carbohydrate receptors to detect hormones
- glycoproteins for cell-cell attachment.
- channel proteins
How do large molecules enter a cell?
endocytosis. the membrand folds and surrounds the molecule, forming a bubble around it. The bubble is then pinched off, forming a vesicle. In reverse this is exocytosis.
How is water potential regulated?
aquaporins (channel proteins for water) can be inserted into the cell membrane, or gated, allowing water to diffuse down the water gradient from a high Ψ to a low Ψ. Adding solutes to a cell (e.g. vacuole) lowers the Ψ. The amount that it is lowered by forms Ψs, therefore Ψ=Ψs
