Cell membranes Flashcards
What is the cell-surface membrane
The name given specifically to the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment.
Why are phospholipids important components of cell surface membranes
- the hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides
- the hydrophobic tails of both phospholipid layers point into the the centre of the cell membrane, repelled by the water on both sides.
What is the function of phospholipids in the cell membrane
Allow lipid soluble substances to enter and leave the cell.
Prevent water-soluble substances entering and leaving the cell.
Make the membrane flexible and self-sealing.
What role do proteins play in diffusion across the cell surface membrane
They allow molecules that are not lipid soluble or too large to diffuse across the phospholipid layer to diffuse into the cell.
What are the two ways in which proteins are embedded in the phospholipid bilayer
- some proteins are found in the surface of the bilayer and never extend fully across it
- some proteins completely span the phospholipid bilayer from one side to the other.
What is the role of the proteins in the cell surface membrane that are found in the surface of the phospholipid bilayer but do not extend fully across it
They act to either give mechanical support to the membrane,or, in conjunction with glycolipids, as cell receptors for molecules such as hormones
How do channel proteins help molecules to diffuse across the cell-surface membrane
They form water filled tubes that allow water-soluble ions to diffuse across the membrane.
How do carrier proteins help to move molecules across the cell surface membrane
They bind to ions or molecules like glucose and amino acids, then change shapes in order to move these molecules across the membrane.
List the role of proteins in the cell- surface membrane
1) provide structural support
2) Act as channels supporting water-soluble substances across the membrane
3) Allow active transport across the membrane through carrier proteins
4) form cell-surface receptors for identifying cells
5) help cells adhere together
6) act as receptors for example for hormones
How does cholesterol molecules being hydrophobic relate to their function in the cell surface membrane
It helps them prevent the loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules which limits their movement and that of other molecules but without making the membrane too rigid
Where are cholesterol molecules found in a cell membrane
They can be found within the phospholipid bilayer
List the functions of cholesterol molecules in the cell membrane
1) add strength
2) reduce lateral movement of other molecules including phospholipids
3) make the membrane less fluid at high temperatures
4) prevent leakage of water and dissolved ions from the cell
What are glycolipids
Molecules made up of a carbohydrate covalently bonded with a lipid.
Describe how glycolipids are found in the cell surface membrane
The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside of the cell where it acts as a cell-surface receptor for specific chemicals.
What is an example of how glycolipids are used in the body
The human ABO blood system operates as a result of glycolipids on the cell surface membrane.
List the roles of glycolipids in the cell-surface membrane
Act as recognition sites
Help maintain the stability of the membrane
Help cells to attach to one another and so form tissues
Describe how glycoproteins are found in the cell surface membrane
carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane
List the roles of glycoproteins in the cell surface membrane
- Act as cell surface receptors, more specifically for hormones and neurotransmitters
- Act as recognition sites
- help cells to attach to one another and so form tissues
- allows cells to recognise one another, for example, lymphocytes can recognise an organisms own cells.
Explain why most molecules do not freely diffuse across the cell surface membrane.
- most molecules are not lipid soluble and so cannot pass through the phospholipid layer
- many molecules are too large to pass through the channels in the membrane
- many molecules have the same charge as the charge on the protein channels and so even if they are small enough to pass through, they are repelled
- many molecules are polar and so have difficulty passing through the non polar hydrophobic tails in the phospholipid bilayer
List the functions of the membranes found within cells
- Control the entry of material in and out of discrete organelles such as chloroplasts and mitochondria.
- Separate material from the cytoplasm so that specific metabolic reactions can take place within them.
- Provide an internal transport system (eg. Endoplasmic reticulum)
- Isolate enzymes that might damage the cell (eg. Lysosomes)
- Provide surfaces on which reactions can occur (eg. Ribosomes on RER)
What is the name of the model that describes how molecules are arranged in a cell surface membrane
The fluid-mosaic model
Explain why ‘fluid’ is used to describe the cell surface membrane in the fluid mosaic model
The fluid part describes the fact that the individual phospholipid molecules can move relative to each other which gives the membrane a flexible structure that is constantly changing in shape.
Explain why ‘mosaic’ is used to describe the structure of a cell membrane in the fluid-mosaic model
The mosaic refers to the fact that the proteins that are embedded in the phospholipid bilayer vary in size, shape and pattern like mosaic tiles.
