Transport across cell membranes Flashcards
What is a cell surface membrane?
The plasma membrane that surrounds cells and forms a boundary between the cytoplasm and the environment.
What does the cell surface membrane allow for?
It allows different conditions to be established inside and outside of the cell.
What does the cell surface membrane control?
It controls the movement of substances in and out of cells.
What makes up a cell membrane?
A phospholipid bilayer
Why is the phospholipid bilayer an important component of cell membranes?
Because: - the hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides and - the hydrophobic tails of both layers point into the centre of the cell membrane, repelled by water on both sides. Therefore, the membrane has a non-polar core which prevents the passage of polar molecules across it, e.g. water, and allows relatively free diffusion of non-polar molecules.
How do lipid-soluble material move through the membrane?
Via the phospholipid portion.
Name 3 functions of phospholipids in the membrane
- Allow lipid-soluble substances to enter and leave the cell. - Prevent water-soluble substance entering and leaving the cell. - Make the membrane flexible and self-sealing.
How are proteins embedded in the cell-surface membrane?
Some proteins occur only in the surface of the bilayer whereas others completely span the phospholipid bilayer from one side to the other.
What is the function of proteins which only occur in the surface of the phospholipid bilayer?
The act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules e.g. hormones.
Give 2 examples of proteins that completely span the phospholipid bilayer.
Carrier proteins, channel proteins.
What do carrier proteins do?
They bind to ions or molecules like glucose or amino acids, then change shape in order to move these molecules across the membrane.
What do protein channels / channel proteins do?
They form water-filled tubes to allow water-soluble ions to diffuse across the membrane.
Name 6 functions of the proteins in membranes.
- Provide structural support. - Act as channels transporting water-soluble substances across the membrane. - Allow active transport across the membrane through carrier proteins. - Form cell-surface receptors across the membrane for identifying cells. - Act as receptors e.g. for hormones. - Help cells stick/adhere together.
Where do cholesterol molecules occur?
Within the phospholipid bilayer.
What do cholesterol molecules add to the membrane?
Strength.