cell membrane structure and function

Question 1

what is a plasma membrane?

Answer 1

all membranes around and within cells (including those surrounding organelles)

Question 2

what do we call the plasma membrane that surrounds cells?

Answer 2

cell-surface membrane

Question 3

what is the function of a cell surface membrane?

Answer 3

1. to separate the internal and external environments of the cell.
2. to control the movement of substances in and out of the cell.
3. to allow cellular compartments to have different conditions.

Question 4

name the components of a cell surface membrane

Answer 4

Phospholipids
Proteins (intrinsic and extrinsic)
Glycoproteins
Glycolipids
Cholesterol

Question 5

what is a polar molecule?

Answer 5

a molecule that does not have an evendistribution of electrons making some regions more negative than others.

Question 6

what is a non-polar molecule?

Answer 6

a molecule that has an even distribution of electrons, meaning it does not have areas that are more negative than other areas.

Question 7

phospholipids are made form which 3 molecules?

Answer 7

phosphate

glycerol

fatty acid ‘tails’x 2

Question 8

why do phospholipids form a bilayer?

Answer 8

1. phospholipids are amphipathic:
2. polar phosphate head - hydrophilic - will orientate towards aqueous environments.
3. non-polar fatty acid ‘tails’ - hydrophobic - will orientate away from aqueous environments.
4. as both inside and outside the cell is aqueous, phospholipids form two layers
5. bilayer with hydrophobic tails facing inwards and hydrophilic heads facing outwards (interacting with aqueous environment).

Question 9

what is the function of the phospholipid membrane?

Answer 9

1. allows lipid soluble (non-polar) molecules to enter and leave.
2. barrier to water soluble (polar) molecules - prevents them from entering and leaving.
3. makes the membrane flexible and self-healing

Question 10

substances able to cross plasma membrane?

Answer 10

lipid soluble/ small non-polar molecules e.g. carbon dioxide, oxygen and lipid based hormones e.g. steroids.

Question 11

substances NOT able to cross plasma membrane?

Answer 11

water soluble/large polar molecules e.g. glucose, ions, proteins, urea, amino acids of the same charge as the charge on the protein channels - repelled.

Question 12

proteins in a cell membrane can either be __________ or _________.

Answer 12

Intrinsic or extrinsic

Question 13

what’s an intrinsic protein?

Answer 13

span the full membrane. Could be protein channel or protein carrier.

Question 14

what’s a protein channel?

Answer 14

water filled tubes to diffuse/transport water soluble molecules/proteins/ions across membrane.

Question 15

what’s a protein carrier?

Answer 15

bind to large/charged water soluble molecules/proteins/ions (glucose/amino acids) and change shape to actively transport/diffuse molecules across membrane.

Question 16

what’s an extrinsic protein?

Answer 16

located on the surface and partially embedded. Don’t fully span across membrane and serve in transport of molecules and as receptors.

Question 17

what is facilitated diffusion?

Answer 17

the passive movement of particles from an area of high concentration to an area of low concentration via transmembrane transport proteins until there is no net movement and equilibrium has been reached.

Question 18

what are the functions of protein?

Answer 18

1. provide structural support.
2. act as channels and allow the facilitated water-soluble/ large polar substances across the membrane.
3. allow active transport across the membrane through carrier proteins.
4. form cell surface receptors for identifying cells so immune system doesn’t attack.
5. help cells adhere together.
6. act as receptors for example hormones.

Question 19

explain the fluid mosaic model

Answer 19

1. phospholipid molecules can move laterally/freely and makes the membrane fluid/flexible.
2. proteins are distributed throughout the membrane unevenly and vary is shape, size and pattern - like a mosaic.
3. agreed structure based upon experimental + chemical evidence and so is called a model.

Question 20

cholesterol molecules occur within the…

Answer 20

phospholipid bilayer of the cell-surface membrane.

Question 21

cholesterol molecules are very…

Answer 21

hydrophobic - preventing loss of water and dissolved ions from the cell. Also, pull together fatty acid ‘tails’ , limiting their own and other molecules movement without making membrane too rigid.

Question 22

what are the functions of cholesterol?

Answer 22

1. reduce lateral movement of other molecules including phospholipids.
2. makes the membrane less fluid at high temperatures.
3. prevent leakage of water + dissolved ions from the cell.

Question 23

what are glycolipids?

Answer 23

carbohydrates covalently bonded to lipids.

Question 24

the carbohydrate portion extends from the phospholipid bilayer into the…

Answer 24

aqueous environment outside of cell where it acts as a self surface receptor for specific chemicals.

Question 25

what are the functions of glycolipids?

Answer 25

1. act as recognition sites.
2. help maintain the stability of the membrane.
3. help cells attach to one another and to form tissues.

Question 26

carbohydrate chains are attached to many…

Answer 26

extrinsic proteins on the outer surface of the cell membrane Glycoproteins also act as cell surface receptors more specifically for hormones and neurotransmitters.

Question 27

what are the functions of glycoproteins?

Answer 27

1. act as recognition sites.
2. help cells to attach to one another and form tissues.
3. allow cells to recognise one another.

Question 28

In an investigation about diffusion and plasma membranes (beetroot discs) why would you ensure the temperature remained constant?

Answer 28

so that rate of diffusion remains constant

no change in fluidity of phospholipids

no denaturation of membrane proteins

Question 29

In an investigation about diffusion and plasma membranes (beetroot discs) why would you ensure the the beetroot discs where shaken?

Answer 29

To ensure all the discs exposed to water/ all discs fully submerged

maintain concentration/diffusion gradient

Question 30

name the type of molecules that can diffuse through the membrane

Answer 30

lipid soluble

small

non-polar

Question 31

during cell fractionation a detergent can be used - why?

Answer 31

cell membranes made of phospholipids

detergent dissolves membranes

releasing cell contents

Question 32

How does ultracentrifugation work to separate cell organelles?

Answer 32

spin supernatant at low speed initially

then gradually increase the speed

separating organelles out based on their size/density

Question 33

non-polar =

Answer 33

hydrophobic

Question 34

polar =

Answer 34

hydrophilic

Question 35

lipids are…

Answer 35

insoluble in water