Cell membranes and transport

Question 1

why is the fluid mosaic model used to describe a phospholipid bilayer?

Answer 1

fluid - means the phospholipid molecules can rotate more relative to one another
mosaic - means that the protein is embedded in the bilayer and they vary in size and shape and they are arranged in a random pattern

Question 2

explain how phospholipid molecules interact with water that is both in the cytoplasm and outside the cell.

Answer 2

phospholipid heads are hydrophilic so they are attracted to water
the fatty acid tails are hydrophobic so they are parted inwards and face each other. the heads form a layer facing the water and cytoplasm fluid.

Question 3

what is a glycoprotein?

Answer 3

proteins with connected carbohydrates pointing out of the outside of the cell membrane

Question 4

state 2 functions of membrane glycoproteins

Answer 4

cell to cell recognition
act as receptor molecules

Question 5

suggest the function if channel proteins

Answer 5

allow passage of charged/polar molecules such as glucose

Question 6

which soluble substances move more easily? lipid or water-soluble substances

Answer 6

lipid-soluble substances

Question 7

what three substances, that are lipid soluble, pass through the membrane?

Answer 7

small uncharged molecules, gases, small polar molecules, and hydrophobic molecules.

Question 8

how do these lipid-soluble substances diffuse across the cell membrane?

Answer 8

they dissolve in the hydrophobic tails of the phospholipids and diffuse across the cell membrane

Question 9

what substances are water-soluble substances?

Answer 9

large polar molecules and charged ions

Question 10

how do water-soluble substances diffuse across the cell membrane?

Answer 10

They can’t easily pass through the phospholipid molecules so they pass through the intrinsic protein molecules (channel/carrier proteins)

Question 11

what two things affect the permeability of the membrane?

Answer 11

temperature
organic solvents

Question 12

how does temperature affect the permeability of the membrane?

Answer 12

increased temp. increases permeability since the molecules moving through the membrane will have more kinetic energy so will diffuse more quickly

Question 13

how does organic solvents such as ethanol affect permeability?

Answer 13

ethanol dissolves/emulsifies the phospholipids and may denature the proteins resulting in gaps or holes within the cell membrane. acetone has a similar effect.

Question 14

what are the 6 methods of transport across the membrane?

Answer 14

diffusion
facilitated diffusion
co-transport
active transport
bulk transport
osmosis

Question 15

define diffusion

Answer 15

the passive movement of a molecule or ion down a concentration gradient from a region of high concentration to a region of low concentration

Question 16

give the 6 factors that affect diffusion

Answer 16

the concentration gradient
the thickness of the surface
the surface area of the membrane
the size of the diffusing molecule
the nature of the diffusing molecule
temperature

Question 17

how will the rate of diffusion change if the concentration gradient changes?

Answer 17

the greater the difference in conc. of molecules in two areas, the more molecules will diffuse in a given time

Question 18

how will the rate of diffusion change if the thickness of the surface changes?

Answer 18

the shorter the difference over which diffusion takes place, the more molecules will diffuse in a given time

Question 19

how will the rate of diffusion change if the surface area of the membrane changes? And give Fick’s Law

Answer 19

the larger the area, the higher the number of molecules that will diffuse in a given time

rate of diffusion = surface area x difference in concentration/thickness of the diffusion path

Question 20

how will the rate of diffusion change if the size of the diffusing molecule changes?

Answer 20

smaller molecules diffuse faster than larger molecules because they have more kinetic energy

Question 21

how will the rate of diffusion change if the nature of the diffusing molecule changes?

Answer 21

molecules that are soluble in phospholipids, diffuse faster than water-soluble molecules because they can diffuse anywhere through the membrane

Question 22

how will the rate of diffusion change if the temperature changes?

Answer 22

increased temperature means the rate of diffusion increases since the molecules/ions have more kinetic energy

Question 23

how does the solubility in a lipid affect the rate of diffusion through a membrane? (graph including large and small molecules)

Answer 23

the solubility of lipids affects the rate of diffusion through a membrane because as solubility increases so does the rate of diffusion
the more lipid-soluble, the less they will be repelled by water
the size affects the rate of diffusion because as solubility in lipids increases, the smaller molecules’ rate of diffusion is steeper and higher than the large molecules so they travel more quickly
smaller molecules have more kinetic energy so they diffuse faster and more energy is needed to move larger molecules.

Question 24

define facilitated diffusion

Answer 24

it is the passive transfer of polar molecules or charged ions down a concentration gradient, across a membrane, by channel or carrier proteins in the membrane

Question 25

what are the 3 factors that affect facilitated diffusion?

Answer 25

number of channel/carrier proteins present in the membrane, once they are all filled, the rate will plateau the steepness of the concentration gradient also affected by the same things as diffusion

Question 26

what are the two types of transport proteins?

Answer 26

channel and carrier

Question 27

give 4 features of channel proteins

Answer 27

they have water-filled hydrophilic pores ions can pass through the pores because they are hydrophilic each channel is specific for one type of ion channels open and close depending on the needs of the cell

Question 28

give 2 features of carrier proteins

Answer 28

they allow the diffusion of larger polar molecules such as glucose and amino acids a polar molecule attaches to a binding site on the carrier protein which causes the protein to change shape releasing the molecule through to the other side of the membrane.

Question 29

give 2 types of facilitated diffusion

Answer 29

gated protein channels co-transport

Question 30

describe gated protein channels

Answer 30

they can be opened and closed to allow ions through, for e.g, Na+ ion gated channels that open during the initiation if the nerve impulse

Question 31

describe co-transport

Answer 31

they bring molecules and ions into cells together on the same protein transport molecule

Question 32

how does co-transport work? use sodium-glucose

Answer 32

there is a conc. gradient for Na+ ions to move into a cell Na+ ions and glucose bind to the carrier protein the carrier protein changes shape and the Na+ ions and glucose are transported to the other side of the membrane this process can move glucose against its conc. gradient without the use of ATP - known as secondary active transport

Question 33

give 2 examples of processes that involve the uptake of sodium-glucose molecules by co-transport

Answer 33

kidneys - reabsorbs maximum glucose from urine before excretion small intestine - absorbs maximum glucose from food before egestion

Question 34

describe active transport

Answer 34

the transport of charged ions and polar molecules against the concentration gradient uses energy from hydrolysis of ATP, made by the cell in respiration if respiration is inhibited, for e.g, by cyanide poison, active transport is too

Question 35

what does active transport require?

Answer 35

an intrinsic carrier protein - the carrier acts as a pump

Question 36

give the 5 steps of how active transport works

Answer 36

the molecule/ion binds to the carrier protein on the outside of the cell membrane ATP transfers a phosphate group to the carrier protein the carrier protein changes shape and carries the ion or molecule across the membrane the molecule/ion is released into the cytoplasm of the cell the carrier protein returns to it's original shape

Question 37

give some examples of processes that involve active transport

Answer 37

muscle contraction nerve impulse transmission reabsorption of glucose in the kidney mineral ion uptake in plant roots

Question 38

describe what a graph for active transport will look like when a respiratory pigment is added and what impact does the rate of uptake have as a result of the concentration difference across the membrane

Answer 38

A: rate of uptake is a proportional increase until all carrier proteins are in full use, which is when the line starts to level off B: rate of uptake stops once respiratory inhibitor is added because active transport needs ATP for the process, and there is no longer any ATP being produced.

Question 39

describe what a graph for simple diffusion will look like when a respiratory pigment is added and what impact does the rate of uptake have as a result of the concentration difference across the membrane

Answer 39

The line across the graph does not curve and level off when a respiratory pigment is added since diffusion does not require ATP and transport proteins aren't being used. The rate is not limited by the number of carrier proteins in the membrane since the rate of uptake still increases due to the conc. difference.

Question 40

describe what a graph for facilitated diffusion will look like when a respiratory pigment is added and what impact does the rate of uptake have as a result of the concentration difference across the membrane

Answer 40

rate of uptake is proportional until the curve levels off, however the line doesn't go down since the rate isn't affected by the respiratory inhibitor and facilitated diffusion doesn't require ATP. However it is affected by the number of carrier proteins in the membrane so rate is limited as they are in full use.

Question 41

what are the two processes where the cell transports materials in bulk into or out of the cell? and what is this collectively known as?

Answer 41

endocytosis and exocytosis. bulk transport

Question 42

is ATP required for bulk transport?

Answer 42

yes

Question 43

describe the process of exocytosis (secretion)

Answer 43

it is the process by which substances may leave the cell having been transported through the cytoplasm in a vesicle, which fuses with the cell membrane: 1 - a vesicle is produced in the cytoplasm (budding off at one end of the Golgi body) 2 - the vesicle migrates to the plasma membrane, fuses with it and secretes it's contents to the outside of the cell e.g secretion of the hormone insulin/digestive enzymes

Question 44

what happens to the surface area of the cell after exocytosis?

Answer 44

increases

Question 45

describe endocytosis (uptake)

Answer 45

the cell membrane folds around the particle which closes off the link to the outside of the cell , so the particle is fully trapped inside the cell in a vesicle/vacuole. a lysozyme fuses with the vesicle releasing digestive enzymes onto ingested material.

Question 46

what are the two types of endocytosis?

Answer 46

phagocytosis and pinocytosis

Question 47

what is phagocytosis the uptake of?

Answer 47

of solids e.g white blood cells engulfing bacteria

Question 48

what is the pinocytosis the uptake of?

Answer 48

of liquids e.g liquid droplets

Question 49

define osmosis

Answer 49

it is the diffusion of water, from an area of high water potential to an area of low water potential across a selectively permeable membrane. it is a passive process so does not require ATP

Question 50

what is meant by water potential and what is unit?

Answer 50

it is the tendency for water to leave a solution or cell by osmosis and it is measured by kPa

Question 51

what happens when you add solute to water?

Answer 51

the number of free water molecules is decreased, the potential energy of the water decreases and therefore water potential becomes more negative

Question 52

what water has 0kPa and the greatest potential energy

Answer 52

pure water

Question 53

what are three different solutions of water?

Answer 53

hypertonic hypotonic isotonic

Question 54

describe the movement of water in a hypotonic solution

Answer 54

if the water potential of the external solution is less negative than the solution inside the cell, water flows into the cell

Question 55

describe the movement of water in a hypertonic solution

Answer 55

if water potential of external solution more negative than solution outside the cell, water flows out of the cell

Question 56

describe the movement of water in a isotonic solution

Answer 56

if the water potential of external solution is the same as the solution inside the cell, there is no net movement of water.

Question 57

what equation is used to describe the relationship between the forces involved in water potential in plants?

Answer 57

water potential = solute potential + pressure potential

Question 58

what is meant by pressure potential?

Answer 58

the pressure exerted by the cell contents on the cell wall creating a force which increases the tendency of water to move out

Question 59

how does pressure potential work?

Answer 59

water entering a plant cell by osmosis causes the vacuole and cytoplasm to swell, making the cell turgid the cell wall of a plant is turgid so outward pressure builds up as the cytoplasm pushes against the cell wall. this is the pressure potential

Question 60

why don't pant cell burst?

Answer 60

the cell wall is inelastic so it will exert the same pressure back that the water is exerting, increasing water potential. this prevents the cell from overfilling and bursting

Question 61

what happens to a plant cell when placed in hypotonic solution?

Answer 61

water will move into the cell by osmosis and causes the plant cell to become turgid

Question 62

what happens to the plant cell when placed in hypertonic solution?

Answer 62

water will move out of the cell, vacuole shrinks and cytoplasm and cell membrane will withdraw from the cell wall causing the plant to become plasmolysed

Question 63

define incipient plasmolysis

Answer 63

for plant cells it is theoretically defined as the point at which the cell membrane is just about to come away from the cell wall. experimentally, it is the point where 50% of the cells in a sample are plasmolysed.

Question 64

what would pressure potential be at incipient plasmolysis?

Answer 64

0kPa

Question 65

what is the equation for water potential at incipient plasmolysis?

Answer 65

water potential = solute potential

Question 66

explain why animal cells burst in a hypotonic solution?

Answer 66

animal cells have no cell wall so any large movement of water into/out of the cell causes it to burst or shrivel. water moves into the cell by osmosis down a water potential gradient and the cell swells and bursts

Question 67

explain why animal cells crenate in a hypertonic solution

Answer 67

water leaves the cell by osmosis to an area of low water potential until the cells shrink and crenate