plasma membranes

Question 1

what does the cell membrane do?

Answer 1

controls the movement of substances in and out of the cell

Question 2

plasma membranes are…

Answer 2

semi-permeable which means that only certain substances through the membrane

Question 3

describe the phospholipid bilayer in plasma cell membranes?

Answer 3

-hydrophilic heads of both phospholipid layers point to the outside of the membrane attracted by water on both sides
-hydrophobic tails of both phospholipid layers point to the centre of the membrane, repelled by the water on both sides

Question 4

what are the functions of phospholipids in the membrane?

Answer 4

1. allow lipid-soluble substances to enter and leave the cell
2. prevent water-soluble substances entering and leaving the cell
3. make the membrane flexible and self-sealing

Question 5

what are intrinsic proteins?

Answer 5

proteins that are firmly embedded within the phospholipid bilayer

Question 6

what are extrinsic proteins?

Answer 6

proteins that occur in the surface of the bilayer and never extend completely across it

Question 7

give some examples of intrinsic proteins?

Answer 7

-some are enzymes fixed in the membrane
-carrier proteins that have receptor sites complementary to substances carried across membrane
-channel proteins which substances pass through

Question 8

what do extrinsic proteins act as?

Answer 8

receptor molecules (in conjunction with glycolipids) ;
chemicals such as hormones bind to them and this stimulates events within a cell

mechanical support

Question 9

what are the functions of proteins in the membrane?

Answer 9

-structural support
-acts as channels transporting water-soluble substances across the membrane
-allow active transport across the membrane through carrier proteins
-form cell-surface receptors for identifying cells
-helps cells adhere together
-acts as receptors eg for hormones

Question 10

what is cholestrol?

Answer 10

lipid molecules( non-polar) that occur within the phospholipid bilayer

Question 11

where does cholesterol fit?

Answer 11

fits between phospholipids and binds to the hydrophobic tails, causing them to pack more closely together- making the membrane more rigid

Question 12

what are the functions of cholesterol in the membrane?

Answer 12

-reduce lateral movement of other molecules including phospholipids
-make the membrane less fluid at high temps
-prevent leakages of water and dissolved ions from the cell

Question 13

what are glycolipids?

Answer 13

carbohydrate bonded to lipid.
the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell surface receptor for specific chemicals

Question 14

what are the functions of gylcolipids?

Answer 14

-acts as recognition sites
-help maintain the stability of membrane
-help cells to attach to one another so form tissues

Question 15

what are glycoproteins?

Answer 15

carbohydrate chains attached to many extrinsic proteins on the outer surface of the cell membrane
also act as cell-surface receptors more specifically for hormones and neurotransmitters

Question 16

what are the functions of glycoproteins?

Answer 16

-act as recognition sites
-helps cells attach to one another and so form tissues
-allows cells to recognise one another

Question 17

why can a lot of molecules not diffuse across the membrane?

Answer 17

-if they are not lipid soluble they cannot pass directly through the bilayer
-if they are too large to pass through channel proteins
-if they are of the same charge as the charge on the protein channels so are repelled
- if they are polar so have difficulty passing through non-polar tails

Question 18

what is the name for the model of the plasma membrane?

Answer 18

fluid-mosaic model