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.
Are cholesterol molecules very hydrophobic or hydrophilic? What does this mean for the cell?
Hydrophobic. Therefore plays an important role in preventing loss of water and dissolved ions from the cell
What do cholesterol molecules pull together? What does this achieve?
They pull together the fatty acid / hydrophobic tails of the phospholipid molecules. This limits their movement and that of other molecules but doesn’t make the membrane too rigid.
Name the 3 overarching functions of cholesterol.
- Reduce lateral movement of other molecules including phospholipids. - Make the membranes less fluid at high temperatures. - Prevent leakage of water and dissolved ions from the cell.
What are glycolipids made up of?
A carbohydrate covalently bonded with a lipid
What does the carbohydrate portion of a glycolipid act as?
It acts as a cell-surface receptor for specific chemicals.
Name 3 functions of glycolipids in the membrane.
- Acts as recognition sites. - Helps maintain the stability of the membrane. - Helps cells to attach to one another and therefore form tissues.
Where are glycoproteins found?
On the outer surface of the cell membrane.
What are glycoproteins made up of?
Carbohydrate chains attached to many extrinsic proteins.
What are the functions of glycoproteins in the membrane?
- Act as recognition sites (allows cells to recognise one another - lymphocytes can recognise an organism’s own cells). - Help cells attach to one another and form tissues. - Act as cell-surface receptors, more specifically for hormones and neurotransmitters
What controls the movement of substances in and out of cells?
The cell surface membrane.
Why does a molecule not being soluble in lipids affect its movement across a membrane?
Because if it is not soluble in lipids, it can’t pass through the phospholipid bilayer.
Why does a molecule being large affect its movement across a membrane?
Because it is too large to pass through protein channels in the membrane.
Why does a molecule having the same charge as the protein channel affect its movement across the membrane?
Because like charges repel one another.
Why do polar molecules, e.g. water, have difficulty passing through the membrane?
Because the membrane has a non-polar core (from the hydrophobic tails) which prevents the passage of polar molecules across it.
Why is the model of a cell surface membrane described as fluid?
Because the individual phospholipid molecules can move relative to one another. This gives the molecule a flexible structure that is constantly changing in shape.
Why is the model of a cell surface membrane described as a mosaic?
Because the proteins embedded in the bilayer vary in size, shape and pattern.
