paper 1: Transport Across Membranes

Question 1

what is the cell surface membrane sometimes known as

Answer 1

plasma membrane

Question 2

what are the functions of the cell membrane

Answer 2

• it controls the substances that enter and leave the cells
• it holds the contents of the cell/ organelle in
• it allows different conditions to be established between the inside and outside of the cell/organelle
• the cell surface membrane only allows certain molecules through it, for this reason it is known as partially permeable

Question 3

describe and explain how phospholipids are arranged in a cell membrane

Answer 3

the hydrophilic phosphate heads face inside and outside of the cell to stay in contact with water
the hydrophobic fatty acid tails in the middle of the membrane away from the water

Question 4

what properties do phospholipids give a cell membrane

Answer 4

• make the membrane flexible
• allow lipid soluble substances to cross the membane
• prevent water soluble substances from crossing the membrane

Question 5

why is the plasma membrane described as the fluid-mosaic model

Answer 5

fluid: molecules can move around within the layers of the membrane
mosaic: due to proteins embedded throughout the membrane

Question 6

how does cholesterol effect a cell membrane

Answer 6

• it prevents the membrane being too fluid by restricting movement of other molecules
• this increases the rigidity of plasma membranes

Question 7

what are the functions of proteins in cell surface membranes

Answer 7

• transport proteins
• enzymes
• antigens
• receptors

Question 8

what do channel proteins do

Answer 8

form a channel to allow ions and polar molecules and large molecules to pass through the bilayer

Question 9

what is the role of carrier proteins

Answer 9

• molecules bind specifically to carrier protein binding site
• protein changes shape to allow molecule through
• returns to normal once molecule has passed through
• normally needs energy from ATP to change shape

Question 10

what kind of binding site do receptors have

Answer 10

specific and complimentary to the substance they respond to

Question 11

describe antigens

Answer 11

• proteins which stick out of the cell membrane
• if they are proteins that your body does not make then they can be recognised as non self
• this triggers an immune response

Question 12

what is the difference between a glycolipid and a glycoprotein

Answer 12

glycoprotein: carbohydrate bound to a protein
glycolipid: carbohydrate bound to a phospholipid

Question 13

what are glycoproteins and glycolipids used for

Answer 13

receptors used in cell recognition

Question 14

how does temperature effect cell membranes

Answer 14

• low temperatures can make membranes less fluid so membrane permeability becomes lower
• high temperatures make lipids more fluid so increase the membrane permeability
• the heat makes the molecules of the membrane vibrate as they have more kinetic energy. the higher the temp. the higher the vibration
• if the temperature is too high it causes too many vibrations which causes the membrane to rupture

Question 15

define diffusion

Answer 15

the passive net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient

Question 16

what is passive transport

Answer 16

a transport process that does not require any extra energy from the hydrolysis of ATP

Question 17

which molecules can diffuse across the phospholipid bilayer and how

Answer 17

• small molecules as they can pass between phospholipids
• lipid soluble (non-polar) substances because they can dissolve in the phospholipid bilayer

Question 18

which molecules can’t diffuse directly across the phospholipid bilayer and why

Answer 18

• large molecules because they cannot pass between the phospholipids
• water soluble (polar) substances because they are repelled by the hydrophobic fatty acid tails in the phospholipid bilayer

Question 19

define facilitated diffusion

Answer 19

the passive movement of particles down a concentration gradient, using channel or carrier proteins

Question 20

how do channel proteins allow molecules through the phospholipid bilayer

Answer 20

pores in the membrane for molecules to diffuse through

Question 21

draw a graph to show what happens to rate of entry as the external concentration of substance increases during simple diffusion

Answer 21

Question 22

sketch a graph to show what happens to rate of entry as the external concentration of substance increases during facilitated diffusion

Answer 22

Question 23

define active transport

Answer 23

the movement of particles from a region pf lower concentration to a region of higher concentration using carrier proteins and energy from the hydrolysis of ATP

Question 24

describe the stages of active transport

Answer 24

• the molecule and ATP binds to the specific carrier protein
• the carrier protein changes shape and transports the molecule across the membrane against its concentration gradient. this requires energy from the hydrolysis of TAP
• as the molecule is released the carrier goes back to its original shape

Question 25

what factors effect the rate of diffusion

Answer 25

- size of concentration gradient - surface area the diffusion is occurring across - the thickness of the diffusion pathway - the number of carrier/ channel proteins for facilitated diffusion

Question 26

define osmosis

Answer 26

the passive net movement of water molecules from a region of higher water potential to a region of lower water potential down a water potential gradient across an partially permeable membrane

Question 27

what is water potential measured in

Answer 27

KPa

Question 28

what is a hypertonic solution

Answer 28

the solution has a lower water potential than inside the cell so H2O moves out of the cell

Question 29

what is a hypotonic solution

Answer 29

he solution has a higher water potential than inside the cell so H2O moves into the cell

Question 30

what is an isotonic solution

Answer 30

the solution has the same water potential as inside the cell so there is no net movement of H2O

Question 31

what happens to an animal cell if it is placed in a hypotonic solution

Answer 31

- too much water had transported into the cell by osmosis because the solution has a higher water potential than inside the cell. - the cell cannot hold anymore water so the cell membrane bursts (osmotic lysis)

Question 32

what happens to an animal cell if it is in an isotonic solution

Answer 32

the water potential of the cell and the solution are the same so there is no net movement of water and the rate of osmosis has reached an equilibrium so the cell remains normal

Question 33

what happens to an animal cell is it is placed in a hypertonic solution

Answer 33

- the water potential outside the cell is lower than inside the cell - so water transports out of the cell by osmosis - if too much water leaves the cell it becomes shrivelled (crenated)

Question 34

what happens to a plant cell if it is in a hypotonic solution

Answer 34

- higher water potential outside than inside the cell - water moves into cell via osmosis - cell becomes turgid due to the cell wall

Question 35

what happens to a plant cell if it is in an isotonic solution

Answer 35

the water potential is the same inside and outside the cell so there is no net movement of water by osmosis the cell stays the same

Question 36

what happens if a plant cell is placed in a hypertonic solution

Answer 36

- lower water potential outside cell than inside - water moves out of cell by osmosis - cell becomes flaccid and plasmolyses

Question 37

describe the process of co transport of glucose

Answer 37

- sodium ions are actively transported from the epithelial cells into the blood. this maintains the sodium concentration gradient between the lumen of the small intestine and the epithelial cell - glucose and sodium ions are co-transported from the lumen of the small intestine, into epithelial cell via a carrier protein, this is facilitated diffusion, because sodium ions move down their concentration gradient - the glucose molecules move into the blood via facilitated diffusion down the concentration gradient

Question 38

why do people with lactose intolerance get diarrhoea when they consume lactose

Answer 38

- some people do not produce the enzyme lactase - this means they cannot hydrolase lactase into glucose and galactose - lactose remains in the lumen of the small intestine - the lactose lowers the water potential in the lumen of the small intestine - water would move into the lumen of the small intestine by osmosis - this causes diarrhoea

Question 39

what is co transport

Answer 39

a form of facilitated diffusion where the concentration gradient of one molecule is sued to move the other molecule against its own concentration gradient

Question 40

how does alcohol affect the permeability of membranes

Answer 40

- makes membrane more permeable | - dissolves the phospholipid bilayer

Question 41

how does acid affect the permeability of a membrane

Answer 41

- makes it more permeable - protein channels/ carrier proteins denature so are non- selective allowing any molecule through

Question 42

how to increase the rate of diffusion across a membrane

Answer 42

- higher temp: increases kinetic energy so faster diffusion - larger surface area: more phospholipids for simple diffusion and more carrier/channel proteins for facilitated diffusion - high conc. gradient: as concentration increases so does rate of diffusion - more carrier/ channel proteins: more proteins for molecules to pass through