Biological membranes

Question 1

what are membranes

Answer 1

partially permeable barriers that allow cell compartmentalisation, resulting in different conditions either side.

Question 2

why are membranes defined as fluid mosaic

Answer 2

• phospholipid molecules are constantly moving
• embedded with proteins : glycoproteins (proteins that have a carbohydrate chain)
• glycolipids: lipids that have a carbohydrate chain attached
• cholesterol

Question 3

description of phospholipids in the plasma membrane

Answer 3

• forms a bilayer: hydrophilic phosphate heads oriented towards water, hydrophilic fatty acid tails oriented inwards away from water
• lipid-soluble substances can pass

Question 4

role of proteins in plasma membrane

Answer 4

act as receptors, channels, carriers. Can support membrane and cytoskeleton

Question 5

role of cholesterol in plasma membrane

Answer 5

• regulates membrane fluidity and provides mechanical stability by binding to hydrophobic tails causing them to pack closely together
• resists the effects of temperature changes on the structure of the membrane

Question 6

role of glycoproteins/glycolipids

Answer 6

• receptors for cell signalling
• antigens
• sites where drugs, hormones and antibodies can bind
• they stabilise the membrane by forming H bonds with surrounding water molecules

Question 7

effect of low temp on plasma membrane

Answer 7

• saturated fatty acids pack more closely together, reducing fluidity and increasing rigidity
• counteracted by unsaturated fatty acids with kinks pushing phospholipids apart and cholesterol

Question 8

effect of high temp on plasma membrane

Answer 8

• phospholipids gain more Ke and vibrate more, plasma membrane becomes more fluid and permeable

Question 9

role of plasma membranes

Answer 9

• barrier between cell and its environment, controlling which substances enter or leave
• allow recognition by other cells ( immune system)
• allow cell communication (cell signalling)

Question 10

role of membranes within cells

Answer 10

• membranes around organelles divide cells into compartments making different functions more efficient like substances needed for respiration being kept inside mitochondria
• formation vesicles to transport substances
• control which substances enter and leave organelle e.g. RNA leaves nucleus via nuclear membrane
• site of chemical reactions like inner membrane of mitochondrion contains enzymes for respiration membranes within organelles act as barriers (thylakoid membrane in chloroplast)

Question 11

in which membranes is cholesterol not found in

Answer 11

bacterial

Question 12

effect of high temperature on cytoskeleton

Answer 12

proteins in it denature

Question 13

how is permeability investigated

Answer 13

• investigated using beetroots and solvents
  1. make a serial dilution of solvent using stock solution
  2. add beetroot different dilutions
  3. measure pigment released into solution from cells using a colorimeter calibrated with water
  4. record data and interpret results

Question 14

effect of destroyed plasma membranes on light transmission in colorimetry

Answer 14

reduced

Question 15

define diffusion

Answer 15

the passive net movement of molecules from an area of higher conc to an area of lower conc down the conc gradient

Question 16

which type of molecules can enter the cell by simple diffusion via channel proteins

Answer 16

small and non-polar (CO2 and O2)

Question 17

which type of molecules require facilitates diffusion via carrier or channel proteins proteins and why

Answer 17

• large and polar (aa, glucose)
• polar/charged particles are water-soluble and are repelled by the hydrophobic tails of the phospholipids. Aquaporins enable transport

Question 18

what factors affect the rate of diffusion

Answer 18

1. conc gradient - the higher it is, the higher the rate of diffusion
2. thickness of exchange surface - thinner, shorter diffusion distance, faster rate
3. surface area - larger, faster rate
4. temperature - warmer, particles have more Ke so rate is faster

Question 19

define osmosis

Answer 19

the movement of water down a water potential gradient across a partially permeable membrane from an area of higher water pot to an area of lower water pot

Question 20

water potential of pure water

Answer 20

0kPa

Question 21

what happens to the water potential when solutes are added

Answer 21

it decreases

Question 22

what’s an hypotonic solution

Answer 22

solution with higher water pot than inside the cell

Question 23

what’s an isotonic solution

Answer 23

solution with same water pot as the inside of cell

Question 24

what’s a hypertonic solution

Answer 24

solution with a lower water pot than the inside of cell

Question 25

what happens when an animal cell is placed in a hypotonic solution

Answer 25

- net movement of water molecules is into the cell - cell bursts (cytolysis)

Question 26

what happens when an animal cell is placed in a isotonic solution

Answer 26

- water moves into and out of cell in equal amounts - cell stays the same

Question 27

what happens when an animal cell is placed in a hypertonic solution

Answer 27

- net movement of water is out of the cell - cell shrinks (crenation)

Question 28

what happens when a plant cell is placed in a hypotonic solution

Answer 28

- net movement of water molecules is into the cell - vacuole swells causing the vacuole and cytoplasm to push against the cell wall - cell is turgid

Question 29

what happens when a plant cell is placed in a isotonic solution

Answer 29

- water moves into and out of cell in equal amounts - cell stays the same

Question 30

what happens when an plant cell is placed in a hypertonic solution

Answer 30

- net movement of water is out of the cell - cell becomes **flaccid** and the cytoplasm + membrane pull away from the cell wall - plasmolysis

Question 31

how can osmosis be investigated

Answer 31

- by observing the size and shape of onion or cheek cells under a light microscope at different water potentials

Question 32

what can be used to demonstrate osmosis

Answer 32

- potato cores - water potential of potato cores can be calculated using % change in mass of the cores at various known water potentials and plotting a graph 1. set up test tubes of known water potentials by making serial dilutions of salt and water 2. place potato cores of known mass into each solution 3. after set time interval has passed, measure the new mass of potato core

Question 33

what does the point at which the graph meets the x axis show

Answer 33

potato had same water pot as solution

Question 34

define active transport

Answer 34

- movement of substances into across a cell membrane from a region of low concentration to a region of higher concentration **against the concentration gradient** using ATP and carrier proteins - ATP is used to directly move particles, such as in the accumulation of ions in root hair cells

Question 35

outline active transport

Answer 35

1. Particle binds to carrier protein with specific tertiary structure 2. ATP binds to protein and is hydrolysed to ADP+Pi, releasing energy 3. Protein undergoes a conformational change and carries particles to the other side of plasma membrane 4. Process can be repeated as new ATP molecules replaces ADP+Pi and protein regains original shape

Question 36

what's bulk transport

Answer 36

- use of ATP to move large amounts of particles that are too big to cross via the membrane or carrier proteins - example: exocytosis, (pino)endocytosis

Question 37

how does endocytosis occur

Answer 37

- the membrane encloses particles, the membrane pinches off to form a vesicle, such as in phagocytosis or pinocytosis (for liquids) - requires ATP

Question 38

how does exocytosis occur

Answer 38

- internal vesicles pinch off from sacs of the Golgi apparatus and move towards plasma membrane via cytoskeleton then fuse with the membrane to release particles to the extracellular fluid such as at synapses - some substances like membrane proteins aren't released but are inserted into plasma membrane - requires ATP