membranes

Question 1

how many membranes do eukaryotes have?

Answer 1

many - they have organelles

Question 2

how many membranes do prokaryotes have?

Answer 2

one - they have no organelles

Question 3

describe a membrane

Answer 3

partially permeable (selective) barrier that separate the contents of cells from their environment and different areas within cells (organelles)

Question 4

describe compartmentalisation

Answer 4

formation of separate membrane-bound areas in a cell

vital to a cell because metabolism includes many different (sometimes incompatible) reactions

containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions

Question 5

how are membranes formed?

Answer 5

phospholipid bilayer

hydrophilic phosphate heads form the inner and outer surface of a membrane

hydrophobic core of fatty acid (tails)

Question 6

where do cells usually exist?

Answer 6

in aqueous environments

Question 7

describe the fluid mosaic model

Answer 7

phospholipids are free to move within the layer relative to each other (fluid)

which gives the membrane flexibility

proteins are embedded in the bilayer and vary in size, shape and position (mosaic)

Question 8

describe intrinsic proteins (integral proteins)

Answer 8

transmembrane proteins embedded through both layers of membrane

have amino acids with hydrophobic r-groups on external surfaces

which interact with the hydrophobic core of a membrane, keeping them in place

Question 9

how do channel proteins carry out their function?

Answer 9

intrinsic protein

provide a hydrophilic channel

allowing passive movement of polar molecules & ions down a concentration gradient through membrane

Question 10

what is the function of a carrier protein?

Answer 10

have a role in passive transport (down conc. gradient)
and active transport (against the gradient) into cells

involves the shape of a protein changing

Question 11

describe glycoproteins

Answer 11

instrinsic proteins

embedded in cell-surface membrane

carb chains (glycocalyx) attached to it with varying shape

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

Question 12

how does chemical signalling work in glycoproteins

Answer 12

when a chemical binds to the receptor it elicits a response from the cell

may cause a direct response or set off a cascade of events inside the cell (cell communication/cell signalling)

Question 13

describe glycolipids

Answer 13

lipids with attached carb chains

cell markers or antigens and are recognised by cells of the immune system as self (of the organism) or non-self (cells belong to another organism)

Question 14

describe extrinsic proteins (peripheral proteins)

Answer 14

present in one side of the bilayer

usually have hydrophilic r-groups on their outer surfaces and interact with polar heads of phospholipids or with intrinsic proteins

some move between layers

Question 15

describe and outline the function of cholesterol in the membrane

Answer 15

lipid

has hydrophilic end and hydrophobic end

regulates fluidity of membrane

positioned between phospholipids with hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails pulling them together

which adds stability to membranes without making them too rigid

cholesterol prevents membranes becoming too solid by stopping the phospholipids from grouping too closely and crystallising

Question 16

what do enzymes have to do in membranes forming organelles for chemical reactions to take place

Answer 16

have to be in particular positions

Question 17

what kind of molecules are able to rapidly diffuse into the membrane, diffuse slower, or are unlikely to diffuse across the membrane?

Answer 17

small, non-polar - diffuse quickly

small, polar - diffuse slowly

charged particles - unlikely to diffuse

Question 18

how is membrane stability ensured by phospholipids

Answer 18

phospholipids are amphipathic

interaction between hydrophobic and hydrophilic ends helps give membrane stability

Question 19

what kind of molecules are able to diffuse through the membrane

Answer 19

lipid soluble (hydrophobic) molecules

hydrophilic molecules cannot diffuse through

Question 20

what gives the membrane fluidity

Answer 20

lipids move laterally within membrane

exchange position with each other

Question 21

what is the degree of cell membrane fluidity determined by

Answer 21

length of fatty acid side chains
(longer chains, lower fluidity)

proportion of fatty acids which are saturated
(higher the % of fatty saturated fats, lower fluidity)

steroid content (cholesterol)
(higher the steroid content, lower the fluidity)

Question 22

what are the four main functions of intrinsic proteins?

Answer 22

act as channels

transport

receptors

enzymes

Question 23

what do intrinsic proteins and the lipids have in common?

Answer 23

have hydrophilic and hydrophobic regions which interact

confers stability on the membrane

Question 24

describe the method to which glucose is transported across the cell surface membrane

Answer 24

facilitated diffusion

transport protein binds to glucose
(transport protein is specific to a molecule)

protein changes shape

glucose transported to the other side of the membrane

glucose detaches from transporter protein

protein reverts back to original shape

glucose is phosphorylated
(maintains diffusion/conc gradient)

Question 25

what affects the rate of facilitated diffusion?

Answer 25

temperature

concentration gradient

membrane surface area

★ number of channel proteins present

Question 26

describe the sodium/potassium ion pump

Answer 26

on the inside of the cell
sodium ions bind to the membrane

triggers breakdown of ATP -> ADP -> E released

E is used to phosphorylate protein
which forms the pump

protein changes shape
sodium transported out the cell

changed shape of the cell allows potassium ions on the outside to bind to the protein

triggers dephosphorylation of the protein (phosphate group removed

leads to change of protein shape
leads to potassium being transported to the inside of the cell

tendency for potassium ions to diffuse back out and sodium ions to diffuse back in

since the membrane is more permeable to potassium than sodium, more ions leave the cell than enter

reducing the tendency for water to enter the cell by osmosis (cell volume controlled)

Question 27

describe how glucose is transported through the cell membrane by active transport

Answer 27

sodium ions pumped out the cell against electrochemical gradient

glucose molecules and sodium ions bind to transport proteins which span the membrane

sodium ions diffuse back across the membrane (into the cell) carrying glucose with them

glucose and sodium ions separate once they’re in the cell

Question 28

what are the 2 forms of endocytosis

Answer 28

phagocytosis

pinocytosis

Question 29

describe phagocytosis

Answer 29

cell surface membrane produces 2 extensions of pseudopodia

surrounds solid particle to be taken into the cell

pseudopodia fuse and create a vacuole around particle

Question 30

describe pinocytosis

Answer 30

cell surface membrane invaginates

allowing fluid to flow inwards

liquid is then enclosed within a vesicle

Question 31

how is endocytosis possible

Answer 31

only possible as a result of the fluidity of a membrane

determined by the lipids of a membrane