2B - Cell Membranes

Question 1

What are the basic function of membranes

Answer 1

They control what passes through them.
All cells are surrounded by membranes; in eukaryotic cells, many organelles are too,

1. Cell surface membranes are a barrier between cell and environment.
   Controlling what substances leave and enter cell.
   - They’re partially permeable (letting some through; not others)
   - can move across by diffusion, osmosis or active transport
2. Organelle membranes divide cell into compartments
   Acting as a barrier between organelle and cytoplasm.
   - Also partially permeable me and control what enters/leaves

Question 2

What’s a fluid mosaic structure

Answer 2

The basic structure of all cell membranes are similar (no. 1)
—> composed of lipids, proteins and carbohydrates
Lipids are mainly phospholipids
The carbohydrates are attached to proteins/lipids

..

1. In 1972, The fluid mosaic model described the molecule arrangement in membrane
   —> phospholipids molecules form a continuous bilayer (double layer)
   — This bilayer is fluid as phospholipids constantly move
2. cholesterol molecules are in the bilayer
3. proteins are scattered through the bilayer like mosaic tiles
   These include channel and carrier proteins which allow big molecules/ions to go through
4. receptor proteins on cell surface membrane allow cell to detect chemicals from other cells
   — the chemicals signal to the cell to respond specifically (eg, insulin tells cells to absorb glucose)
5. Some proteins can move sideways through bilayer but others are fixed.
6. Some proteins have a polysaccharide chain attached (glycoprotein)
7. Some lipids have a polysaccharide chain attached (glycolipids)

Question 3

What does cholesterol do in membranes

Answer 3

It’s a lipid type.
—> in all cell membranes (not bacterial though)

Cholesterol molecules fit between phospholipids
They bind to the hydrophobic tails of phospholipids
Causing them to pack more closely together
—> restricting movement of phospholipids (membrane = less fluid, more rigid)

Cholesterol helps maintain shape of animal cells (no cell walls)
Particularly important if cells aren’t supported by other cells (eg, red blood, wch float in blood)

Question 4

What do intrinsic and extrinsic proteins do in cell membrane

Answer 4

Intrinsic proteins..
Spam from one side of bilayer to another
- some are enzymes
- some help move water soluble materials across (protein channels)
- others are carrier proteins and bind to glucose eg. And change shape to allow crossing

Extrinsic ..
Lie on surface of membrane
Either help to give membrane support
Or work with glycolipids as cell receptors

Question 5

What do glycolipids do

Answer 5

Are Lipids covalently bonded with carbohydrates
—> outer layer only
- acts as recognition site eg. Toxins
- Help cells join to form tissues
- gives membrane stability

Question 6

What do glycoproteins in a cell membrane do

Answer 6

Are carbohydrates combined with extrinsic proteins
Acts as a recognition site (eg, toxins)
- help cells join to form tissues
- allow cells to recognise each other (like lymphocytes can recognise an organism’s own cells)

Question 7

What can pass through a semi permeable membrane

Answer 7

• lipid soluble substances (eg, hormone)
• very small molecules (co2 eg)