3.2.3 Transport across cell membranes

Question 1

what are plasma membranes?

Answer 1

all membranes in and around all cells, they have the same basic structure

Question 2

what is the cell surface membrane?

Answer 2

the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment

Question 3

what is the function of the cell surface membrane?

Answer 3

allows different conditions to be established inside and outside a cell, it controls the movement of substances in and out of the cell

Question 4

what is the structure of a phospholipid?

Answer 4

a hydrophilic (polar) phosphate head, a hydrophobic (non polar) fatty acid tails

Question 5

what are the functions of phospholipids in the membrane?

Answer 5

allow lipid soluble substances to enter and leave the cell, prevent water soluble substances entering and leaving the cell

Question 6

how do both parts of the phospholipid bilayer behave?

Answer 6

the hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted to water on both sides, the hydrophobic tails of both phospholipid tails of both phospholipid layers point into the centre of the cell membrane, repelled by water on both sides

Question 7

what are the components of the cell surface membrane?

Answer 7

phospholipids, proteins (intrinsic and extrinsic), cholesterol, glycolipids, glycoproteins.

Question 8

what is the function of extrinsic proteins?

Answer 8

occur in the surface of the bilayer and never extend completely across it, they act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones

Question 9

what is the function of intrinsic proteins?

Answer 9

proteins that completely span across the phospholipid bilayer from one side to the other, some are protein channels which form water filled tubes to allow water soluble ions to diffuse across the membrane and others are carrier proteins that bind to ions or molecules like glucose and amino acids, then change shape in order to move across the membrane.

Question 10

what are the functions of proteins in the membrane (intrinsic and extrinsic)?

Answer 10

provide structural support, act as channels transporting water soluble substances across the membrane, allow active transport across the membrane through carrier proteins, form cell surface receptors for identifying cells, help cells adhere together, act as receptors e.g. for hormones

Question 11

what is cholesterol?

Answer 11

cholesterol molecules occur in the phospholipid bilayer of cell surface membranes, they are very hydrophobic and therefore play an important role in preventing the loss of water and dissolved ions from the cell, they also pull together the fatty acid tails of the phospholipids, limiting their movement and that of other molecules but without making the membrane as a whole too rigid

Question 12

what are the functions of cholesterol in the membrane?

Answer 12

reduce lateral movement of molecules including phospholipids, make the membrane less fluid at high temperatures, prevent leakage of water and dissolved ions from the cell

Question 13

what are glycolipids?

Answer 13

made up of a carbohydrate covalently bonded with a lipid, the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell surface receptor for specific chemicals, for example the human ABO blood system operates as a result of glycolipids on the cell surface membrane

Question 14

what are the functions of glycolipids?

Answer 14

act as recognition sites, help maintain the stability of the membrane, help cells attach to one another and so form tissues

Question 15

what are glycoproteins?

Answer 15

carbohydrate chains are attached to many extrinsic proteins on th eouter surface of the cell membrane, these glycoproteins also act as cell surface receptors , more specifically for hormones and neurotransmitters

Question 16

what are the functions of glycoproteins?

Answer 16

act as recognition sites, help cells attach to one another and so form tissues, allows cells to recognise one another, e.g. lymphocytes can recognise an organism’s own cell

Question 17

why can some molecules not pass through the membrane via simple diffusion?

Answer 17

not soluble in lipids and therefore cannot pass through the phospholipid layer, too large to pass through the channels in the membrane, electrically charged (polar) and therefore have difficulty passing through the non polar hydrophobic tails in the phospholipid bilayer, water soluble molecules cannot pass the hydrophobic tails

Question 18

why is the model of the cell membrane known as a fluid mosaic?

Answer 18

fluid because the individual phospholipid molecules can move relative to each other, this gives the membrane a flexible structure that is constantly changing shape, mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones in a mosaic do

Question 19

what is diffusion an example of?

Answer 19

passive transport

Question 20

what is meant by diffusion as a passive process?

Answer 20

passive means that the energy comes from the natural, inbuilt motion of particles, rather than from some external source such as ATP, all particles are constantly in motion due to the kinetic energy they possess, this motion is random with no set pattern to the way particles move around

Question 21

what is diffusion defined as?

Answer 21

the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower, down a concentration gradient until equilibrium is met

Question 22

what factors affect the rate of diffusion?

Answer 22

temperature (more kinetic energy), surface area (how much space to diffuse across e.g. villi and micro villi in small intestine), concentration gradient (the difference between the two conditions)

Question 23

what is meant by semi permeable?

Answer 23

allows some substances through but not others

Question 24

which substances need to move in and out of our cells?

Answer 24

oxygen (for respiration), carbon dioxide (too much is toxic), glucose (need for respiration), ions (opposing charges), lipids (solubility), protein/amino acids (growth and repair), glycerol and fatty acids (membranes, make new fats), urea (toxic)

Question 25

what is fick’s law?

Answer 25

states a relationship between the rate of diffusion at a constant temperature and three variables: concentration gradient, length of diffusion pathway, surface area over which diffusion is occurring
rate of diffusion is directly proportional to surface area x concentration gradient/length of diffusion pathway

Question 26

why does an increase of concentration gradient increase the rate?

Answer 26

there are more particles to move, further away from equilibrium, more space to move into

Question 27

why does an increase of diffusion pathway decrease the rate of diffusion?

Answer 27

travelling dither takes longer

Question 28

why does increased surface area increase the rate of diffusion?

Answer 28

there is more surface for the movement of substances, more space for intrinsic proteins

Question 29

how are the lungs adapted for diffusion?

Answer 29

high concentration gradient (lots of capillaries surroundings the alveoli), short diffusion pathway (alveoli are only one cell thick), large surface area (lungs are very large, millions of alveoli with folded surfaces)

Question 30

which substances move via facilitated diffusion?

Answer 30

charged e.g. ions, polar e.g. water, too large e.g. glucose

Question 31

what is facilitated diffusion?

Answer 31

the diffusion of molecules through the membrane aided by an intrinsic protein

Question 32

how is facilitated diffusion selective?

Answer 32

tertiary structure of intrinsic proteins is specific to the molecule that’s moving into it

Question 33

what factors affect facilitated diffusion?

Answer 33

mutations in primary structure due to temperature or pH which changed the tertiary structure, the number of intrinsic proteins (reach v-max/saturation)

Question 34

what is active transport?

Answer 34

against the concentration gradient, low to high, across a partially permeable membrane, an active process which requires energy from respiration/ ATP hydrolysis, uses carrier proteins

Question 35

why is active transport vital for survival?

Answer 35

important for transporting substances into the cell that there is more of inside the cell than outside the cell e.g. ions in a plant root, glucose in the small intestine

Question 36

how do channel proteins work?

Answer 36

water filled tubes, provide hydrophilic channels, water soluble ions and polar molecules can diffuse, selective: only open for specific ions, facilitated diffusion

Question 37

how do carrier proteins work?

Answer 37

change shape to move molecules, selective (only open for specific ions), movement of larger molecules, facilitated diffusion and active transport

Question 38

describe the process of active transport

Answer 38

the solute moved into the carrier protein and binds to the binding site, the ATP is hydrolysed by ATP hydrolase to produce ADP and a phosphate ion and donates phosphate to bind to the carrier protein. The phosphate phosphorylates the carrier protein which changes the tertiary structure, this allows the the carrier protein to open by changing the shape of the binding site so the solute is released into the cell, moving against the concentration gradient

Question 39

what adaptations might cells have for active transport?

Answer 39

many mitochondria, large number of intrinsic proteins, large surface area, good blood supply

Question 40

why is having many mitochondria important for transport?

Answer 40

release energy/ATP (phosphate ions) needed for active transport

Question 41

why is a large number of intrinsic proteins important for transport?

Answer 41

allows more to move into the cell at a time through carrier proteins and channel proteins, prevents saturation, higher concentration for v max

Question 42

why is a large surface area important for transport?

Answer 42

e.g. villi and micro villi in the small intestine so there is more space to diffuse across, more space for intrinsic proteins

Question 43

why is a good blood supply/being well ventilated important for transport?

Answer 43

e.g. capillaries around the alveoli moves oxygen away from the lungs to maintain concentration gradient

Question 44

what is the ileum?

Answer 44

at the end of the small intestine, absorbs products of digestion not yet reabsorbed into the blood moving through e.g. glucose, minerals, ions so they are not excreted

Question 45

how is the ileum adapted as an exchange surface?

Answer 45

large surface area with villi and microvilli more space for diffusion to happen and for intrinsic proteins ileum wall has villi, epithelial cells have micro villi, muscle layer contracts to break down food molecules into smaller pieces increasing the concentration gradient for diffusion and large surface area to volume ratio for enzymes

Question 46

what is co transport?

Answer 46

the coupled movement of one molecule with its concentration gradient and another molecule against its concentration gradient

Question 47

how does the sodium potassium pump work?

Answer 47

3 sodium ions are actively removed from the cell/organelle while 2 potassium ions age actively taken in from the surroundings.

Question 48

why is the co transport of glucose and sodium important?

Answer 48

vital to prevent equilibrium, endures sufficient glucose for respiration

Question 49

Describe the co transport of glucose and sodium in the ileum

Answer 49

Sodium is actively transported out of the epithelial cell using the sodium potassium pump which decreases the concentration of sodium inside the cell, this means that there is a higher concentration of sodium in the lumen than in the cell which draws sodium ions into the cell through co transporter carrier proteins coupled with glucose which moves against its concentration gradient from low to high. As there is now a higher concentration of glucose in the cell than in the blood, it moves via facilitated diffusion

Question 50

why is the co transport of glucose and sodium known as indirect active transport?

Answer 50

the sodium potassium pump is an active process that requires energy

Question 51

what is osmosis?

Answer 51

the net movement of water molecules through a partially permeable membrane from a region of higher water potential to a region of lower water potential, it is a form of diffusion and requires no energy (passive process)

Question 52

what is water potential?

Answer 52

the pressure created by water molecules, the higher the amount of water molecules present the higher the pressure and therefore the higher the water potential is

Question 53

what is the biggest water potential?

Answer 53

closest to 0kPa

Question 54

what is water potential measured in?

Answer 54

kPa (kilo pascals)

Question 55

what is the water potential of pure water?

Answer 55

0kPa

Question 56

what type of water potential will all solutions have?

Answer 56

a negative water potential

Question 57

where will water move to in osmosis?

Answer 57

to the solution that has a more negative water potential

Question 58

what is a hypertonic solution?

Answer 58

concentration of solutes in the solution is higher than inside the cell (less water)

Question 59

what is a hypotonic solution?

Answer 59

concentration of diluted in the solution surrounding the cell is lower than inside the cell (more water)

Question 60

what is an isotonic solution?

Answer 60

water potential is the same inside and outside the cell, concentration of solutes in the solution surrounding the cell is the same as inside the cell

Question 61

what happens to an animal cell in a hypotonic solution?

Answer 61

water would move into the cell via osmosis as there is a higher concentration of solutes inside the cell than outside the cell so there is a higher water potential outside of the cell than inside the cell so water moves into the cell across the partially permeable membrane via osmosis so the cell becomes lysed and therefore would burst (osmotic lysis)

Question 62

what happens to an animal cell in an isotonic solution?

Answer 62

in an isotonic solution the concentration of solutes inside the cell is the same as outside the cell, water potential is the same inside and outside the cell and there is no concentration gradient so there is no net movement of water molecules

Question 63

what happens to an animal cell in a hypertonic solution?

Answer 63

in a hypertonic solution the concentration of solutes inside the cell is lower than the concentration of solutes outside the cell therefore there is a higher water potential inside the cell so water moves out of the cell via osmosis across a partially permeable membrane and the cell becomes shrivelled (shrinks)

Question 64

what happens to a plant cell in a hypertonic solution?

Answer 64

water moves out of the cell across a partially permeable membrane via osmosis vacuole shrivels, cell membrane pulls away from cell wall, the cell is plasmolysed

Question 65

what happens to a plant cell in an isotonic solution?

Answer 65

vacuole contains some water, gives cell support, cell is flaccid

Question 66

what happens to a plant cell in a hypotonic solution?

Answer 66

water moves into the cell across a partially permeable membrane via osmosis, vacuole swells, cell membrane pushes against cell wall, cell is turgid, cell wall prevents cell from bursting