topic 1.4- membrane transport

Question 1

state the 4 ways in which particles move across membranes

Answer 1

• simple diffusion
• facilitated diffusion
• osmosis
• active transport

Question 2

define diffusion

Answer 2

the net, passive movement of molecules from a region of high concentration to a region of low concentration

Question 3

explain the different abilities of:
- small non-polar molecules
- small polar molecules
- large polar molecules
to diffuse across the phospholipid bilayer

Answer 3

easiest->hardest
1. small non-polar molecules
2. small polar molecules
3. large polar molecules
polar/charged molecules repel hydrophobic tails of phospholipids

Question 4

give 3 examples of molecules that will move easily by diffusion

Answer 4

oxygen, CO2, glycerol

Question 5

give a real-life application of simple diffusion

Answer 5

the cornea has no blood supply so its cells obtain oxygen by simple diffusion from the air (it passes through the fluid/tears and into the cornea)

Question 6

define facilitated diffusion

Answer 6

the passive movement of molecules across the cell membrane via the aid of a membrane protein

Question 7

what types of molecules use facilitated diffusion?

Answer 7

molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)

Question 8

what two types of protein mediate facilitated diffusion?

Answer 8

channel proteins and carrier proteins

Question 9

what are carrier proteins?

Answer 9

Integral glycoproteins

Question 10

how do carrier proteins function?

Answer 10

bind a solute and undergo a conformational change to translocate the solute across the membrane

Question 11

can carrier proteins bind to any molecules?

Answer 11

no; they will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction

Question 12

can carrier proteins bind to any molecules?

Answer 12

no; they will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction

Question 13

can carrier proteins move molecules against the concentration gradient?

Answer 13

may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)

Question 14

are carrier proteins faster or slower than channel proteins?

Answer 14

Carrier proteins have a much slower rate of transport than channel proteins

Question 15

what are channel proteins?

Answer 15

Integral lipoproteins

Question 16

how do channel proteins work?

Answer 16

they contain a pore via which ions may cross from one side of the membrane to the other

Question 17

do channel proteins let any molecules through?

Answer 17

no; they are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli

Question 18

can channel proteins move molecules against the concentration gradient?

Answer 18

no; they can only move molecules along a concentration gradient (i.e. are not used in active transport)

Question 19

what is an axon? and what is its function?

Answer 19

• a part of a neuron which consists of a tubular membrane with cytoplasm inside
• used to convey messages rapidly in the form of an electrical impulse

Question 20

what does a nerve impulse consist of?

Answer 20

rapid movements of Na ions into and K ions out of the axon membrane via facilitated diffusion (through Na and K channels)

Question 21

potassium channels in axons are —-

Answer 21

voltage gated

Question 22

what causes voltage across membranes?

Answer 22

• an imbalance of positive and negative charges across the membrane
• more +ve outside axon than inside= K channel closed
• more +ve inside axon than outside = K channel open, so ions diffuse through

Question 23

describe and draw the 3 steps of facilitated diffusion of potassium in axons

Answer 23

1. channel closed; there is a net -ve charge inside the axon and a net +ve charge outside
2. channel briefly opens; net +ve charge inside and net -ve charge inside; K ions rush outside by translocating ions to create a voltage difference across the membrane
3. channel closed by ‘ball and chain’ within milliseconds

Question 24

how is the necessary concentration gradient restored for nerve impulses?

Answer 24

active transport, done by a sodium-potassium pump protein

Question 25

describe and draw the first three steps of the sodium-potassium pump cycle

Answer 25

1. the interior of the pump is open to the inside of the axon; three sodium ions enter the pump and attach to their binding sites
2. ATP transfers a phosphate group from itself to the pump; this causes the pump to change shape and the interior is then closed
3. the interior of the pump opens to the outside of the axon and the 3 Na ions are released

Question 26

describe and draw the final three steps of the sodium-potassium pump cycle

Answer 26

4. 2 K ions from outside can then enter and attach to their binding sites
5. binding of K causes release of the phosphate group; this causes the pump to change shape again
6. the interior of the pump opens to the inside of the axon and the 2 K ions are released

Question 27

define active transport

Answer 27

the use of energy to move molecules against a concentration gradient

Question 28

how is energy for active transport generated?

Answer 28

• The direct hydrolysis of ATP (primary active transport)
• Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Question 29

what proteins does active transport involve?

Answer 29

carrier proteins (called protein pumps due to their use of energy)

Question 30

describe active transport

Answer 30

• A specific solute binds to the protein pump on one side of the membrane
• The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
• The solute molecule is consequently translocated across the membrane (against the gradient) and released

Question 31

define osmosis

Answer 31

the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration/high water potential to a region of high solute concentration/low water potential

Question 32

why can osmosis happen in all cells?

Answer 32

water molecules, despite being hydrophilic, are small enough to pass through the phospholipid bilayer

Question 33

some cells have water channels called ——-

Answer 33

aquaporins

Question 34

describe the function of aquaporins and give examples

Answer 34

they greatly increase membrane permeability to water- eg kidney cells that reabsorb water and root hair cells in plants

Question 35

what does the fluidity of membranes allow?

Answer 35

materials to be taken into cells by endocytosis or released by exocytosis

Question 36

define endocytosis

Answer 36

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

Question 37

describe endocytosis

Answer 37

• An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
• The invagination is then sealed off to form an intracellular vesicle containing the material

Question 38

is endocytosis a passive process?

Answer 38

no, the proteins in the membrane that carry out this process requires energy from ATP

Question 39

give two main examples of endocytosis

Answer 39

• phagocytosis to ingest/kill pathogens
• in the placenta, proteins from the mother’s blood, including antibodies are absorbed into the foetus

Question 40

describe the movement of vesicles within a cell

Answer 40

1. endocytosis; vesicles can then move through the cytoplasm
2. proteins are synthesised by ribosomes
3. vesicles enter the rough endoplasmic reticulum and accumulate there
4. vesicles bud off from the rER
5. the Golgi apparatus modifies the proteins
6. vesicles bud off from the Golgi and carry the modified proteins to the plasma membrane
7. endocytosis

Question 41

describe exocytosis

Answer 41

• vesicles fuse with the plasma membrane
• the contents of the vesicle are expelled and the membrane flattens out again

Question 42

give 2 examples of exocytosis

Answer 42

Digestive enzymes released from gland cells by exocytosis (secretion)
expulsion of waste products or unwanted materials (eg water in unicellular organisms through contractile vacuoles)

Question 43

define a isotonic solution

Answer 43

a solution that has the same osmolarity as a tissue

Question 44

define a hypertonic solution

Answer 44

a solution that has a higher osmolarity than a tissue

Question 45

define a hypotonic solution

Answer 45

a solution that has a lower osmolarity than a tissue

Question 46

describe what will happen to animal cells in hypertonic/hypotonic solutions

Answer 46

hypertonic solutions; water will leave the cell causing it to shrivel (crenation)
hypotonic solutions; water will enter the cell causing it to swell and potentially burst (lysis)

Question 47

describe what will happen to plant cells in hypertonic/hypotonic solutions

Answer 47

hypertonic solutions; the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
hypotonic solutions; the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

Question 48

what is a real life application of isotonic solutions

Answer 48

it is important for any human tissues or organs to be bathed in an isotonic solution during medical procedures. usually, NaCl solution is used (normal saline)- osmolarity= 300mOsm= cytoplasm osmolarity

Question 49

how could we determine the osmolarity of a tissue?

Answer 49

bathe it in a salt solution for a short time and measure the increase/decrease in mass- when mass change=0, that is the same conc