structure of cell membranes

Question 1

what is the plasma membrane

Answer 1

the cell surface membrane which separates the cell from its external environment

Question 2

what are membranes formed from

Answer 2

a phospholipid bilayer

Question 3

describe the phospholipid bilayer

Answer 3

the hydrophillic phosphate heads of the phospholipids from both the inner and outer surface of a membrane sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.

Question 4

where do cells normally exist

Answer 4

in aqueous environments. the insides of cells and organelles are also usually aqueous environments

Question 5

why are phospholipid bilayers perfectly suited as membranes

Answer 5

because the outer surfaces of the hydrophilic head interact with water

Question 6

what do plasma membranes contain

Answer 6

various proteins and lipids - the type and number are particular to each cell type.

Question 7

why do the components of the plasma membrane play an important membrane

Answer 7

they play in the functions of the membrane and the cell or organelle they are part of

Question 8

why are the two types of membrane proteins

Answer 8

intrinsic and extrinsic

Question 9

what are intrinsic proteins

Answer 9

transmembrane proteins that are embedded through both layers of a membrane. they have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane keeping them in place.

Question 10

what are the two types of intrinsic proteins

Answer 10

carrier and channel proteins

Question 11

what are channel proteins

Answer 11

they provide a hydrophillic channel that allow the passive movement of polar molecules and ions down a concentration gradient through membranes. they are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R groups on the outside of the proteins.

Question 12

what are carrier proteins

Answer 12

have an important role in passive and active transport. involve the shape of the protein changing.

Question 13

what are glycoproteins

Answer 13

a type of intrinsic protein. they are a protein with a chain of carbohydrate molecules attached.
they are embedded in the cell surface membrane with attached carbohydrate chains of varying shapes and lengths

Question 14

where are glycoproteins found

Answer 14

they are embedded in the cell surface membrane with attached carbohydrate chains of varying shapes and lengths

Question 15

what are the roles of glycoproteins

Answer 15

play a role in cell adhesion and as receptors for chemical signals.

Question 16

what is cell adhesion

Answer 16

when cells join together to form tight junctions in certain tissues

Question 17

what is the process of cell signalling/cell communication

Answer 17

when a certain chemical binds to the receptor it elecits a response from the cell. this may cause a direct response or set off a cascade of events inside the cell.

Question 18

what is an example of cell signalling

Answer 18

receptors for neurotransmitters at nerve cel synapses. the binding of the neurotransmitters triggers or prevents an impulse in the next neurone.
another example is receptors for peptide hormones, including glucagon and insulin which affect the uptake and storage of glucose by cells. some drugs act by binding to cell receptors.

Question 19

what are beta blockers used for

Answer 19

to reduce the response of the heart to stress

Question 20

what are glycolipids

Answer 20

they are similar to glycoproteins. they are lipids with attached carbohydrate chains. these are cell markers or antigens and can be recognised by the cells of the immune system as self or non self.

Question 21

what are extrinsic proteins

Answer 21

present in one side of the bilayer. normally have hydrophillic R groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins. they can be present in either layer and some move between layers.

Question 22

what is cholesterol

Answer 22

a lipid with a hydrophillic end and a hydrophobic end like a phospholipid. it regulates the fluidity of membranes.

Question 23

where are cholesterol molecules positioned

Answer 23

between phospholipids in a membrane bilayer with the hydrophillic end interacting with the heads and the hydrophobic end interacting with the tails pulling them together. in this way, cholesterol adds stability to membranes without making them too rigid. they prevent the molecules from becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.

Question 24

why do proteins in the membranes have to be in particular positions

Answer 24

for chemical reactions to take place

Question 25

why dont all cell membranes have the same composition

Answer 25

cells become differentiated and specialised so their membranes have particular distributions of proteins in order to enable them to carry out their specific functions

Question 26

what do membranes do in neurones

Answer 26

the protein channels and carriers in the plasma membrane covering the long axon allow entry and exit of ions to bring about the conduction of electrical impulses along their length

Question 27

what does the myelin sheath do on neurones

Answer 27

the myelin sheath, made of flattened cells, wraps around them several times giving several layers of cell membrane

Question 28

what % of protein and lipid forms the myelin sheath membrane

Answer 28

20% protein and 76% lipid

Question 29

what do the plasma membranes of white blood cells have

Answer 29

they contain special protein receptors that enable them to recognise the antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplants

Question 30

what do root hair cells have a lot of

Answer 30

many carrier proteins to actively transport nitrate ions from the soil into the cells

Question 31

what % of lipid and protein are in mitochondria membranes

Answer 31

76% protein
24% lipid
their inner membranes contain many electric carriers that are made of protein and hydrogen ion channels associated with ATP synthase enzymes

Question 32

why do cell membranes need to be partially permeable

Answer 32

because they form a barrier and separate the cell contents from the cell’s exterior environment separate organelles from the cytoplasm, they need to allow some molecules through into or out of the cell. some organelles also have membrane within them and these form barriers too

Question 33

what is permeability

Answer 33

the ability to let substances pass through

Question 34

how do small molecules pass through the membrane

Answer 34

some very small molecules simply diffuse through the cell membrane in between its structural molecules

Question 35

how do other substances pass through the membrane

Answer 35

some substances dissolve in the lipid layer and pass through
other substances pass through special protein channels or are carried by carrier proteins