Topic 6 organisms respond to changes in their internal and external environment

Question 1

What are taxes?

Answer 1

A directional response to a stimuli

Question 2

What is kinesics?

Answer 2

Non-directional movement from an unfavourable area to a more favourable area e.g. in response to humidity
or temperature.

Question 3

What is a tropism?

Answer 3

Directional growth in response to a stimuli.

Question 4

What is the function of indolacetic acid (IAA)?

Answer 4

• Controls plant cell elongation → increases cell wall plasticity.
• High concentration of IAA increases cell elongation in shoots (shoots grow towards light → positive
  phototropism).
• High concentration of IAA inhibits cell elongation in roots (roots bend away from light → negative
  phototropism).
• Only young cells are able to elongate, mature cells are more rigid and so unable to respond.
• Transport of IAA is in one direction → away from tip of root or shoot.
• If root/shoot tips are removed → no response is seen, as source of IAA has been removed.

Question 5

Explain phototropism in flowering plants

Answer 5

1. Cells in tip of shoot produce IAA, which is evenly
   transported down the shoot.
2. Light causes movement of IAA from light side to the
   shaded side of the shoot.
3. Greater concentration of IAA on shaded shade than light
   side.
4. Shaded side of shoot elongates faster than light side →
   shoot tip bends towards light.

Question 6

Explain gravitropism in flowering plants

Answer 6

1. Cells in tip of root produce IAA, which is evenly transported to
   all sides of root.
2. Gravity causes movement of IAA from upper side to the lower
   side of the root.
3. Results in greater concentration of IAA on lower side of root
   than the upper side.
4. Cells on the lower side elongate less than cells on the upper
   side, causing the root to bend downwards towards the force of
   gravity (roots display positive gravitropism).

Question 7

Reflex arc

Answer 7

1. A RECEPTOR detects a STIMULUS.
2. This generates a nerve impulse in a SENSORY NEURONE
   → passes nerve impulses to spinal cord.
3. A RELAY NEURONE links the sensory neurone to the
   motor neurone in the spinal cord.
4. A MOTOR NEURONE carries nerve impulses to an
   EFFECTOR.
5. The effector (muscle or gland) brings about a RESPONSE.

Question 8

Structure of the Pacinian corpuscle

Answer 8

• Pacinian corpuscles contain a sensory nerve ending which is wrapped in layers of connective tissue called
  lamellae.

Question 9

How is a generator potential created when a Pacinian corpuscle is stimulated?

Answer 9

1. Pressure causes the lamellae to become deformed.
2. Causes the sensory neurone’s cell membrane to stretch, which deforms the stretch-mediated sodium
   ion channels.
3. The sodium ion channels open and sodium ions diffuse into sensory neurone cell.
4. The influx of sodium ions depolarises the membrane, creating a generator potential (if generator
   potential reaches the threshold → triggers action potential).

Question 10

How do photoreceptors convert light into an electrical impulse?

Answer 10

1. Light enters eye and hits photoreceptors (of which there are two types, rods & cones).
2. Light is absorbed by photosensitive optical pigments:

• The pigment in rod cells is RHODOPSIN, which must be broken down to create a generator potential → low-
  intensity light transfers enough energy to break down rhodopsin.
• The pigment in cone cells is IODOPSIN → requires higher intensity of light to breakdown and create a
  generator potential.
  3. The breakdown of the pigments, causes a change in the membrane permeability to sodium ions.
  4. A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
  5. Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.

Question 11

Differences between rods and cones

Answer 11

• Rods are found mainly in peripheral parts of retina, cones are concentrated at the fovea (receives the highest
  intensity of light).
• Rods give poor visual acuity, cones give good visual acuity.
• Rods are sensitive to low-intensity lights, cones are not.
• Only one type of rod, but three types of cone (red, green and blue) → each responds to different wavelengths of
  light. Rod cells produce black and white vision, whereas stimulation of cone cells results in colour vision → colour
  that is seen is dependent on the proportion of each type of cone that is stimulated.

Question 12

Difference in SENSITIVITY between rods and cones

Answer 12

• RODS are MORE SENSITIVE to light than cones.
• Rods fire action potentials in dim light because several rods are connected to a single neurone (retinal convergence)
• So enough neurotransmitter is released to reach the threshold (many weak generator potentials can combine to reach
  the threshold and trigger an action potential) (= spatial summation).
• Advantage? Still able to see in black and white in low light intensities → increase chance of survival.
• Cones only fire action potentials in bright light because one cone connects to a single neurone, so more light is required
  to reach the threshold and trigger an action potential.

Question 13

Difference in VISUAL ACUITY between rods and cones

Answer 13

• CONES give HIGH VISUAL ACUITY whereas rods give low visual acuity.
• Cones give high visual acuity because one cone connects to a single neurone, so cones send separate impulses to the
  brain.
• Rods give low visual acuity because several rods connect to the same neurone → light from two points close together can’t
  be distinguished as separate.

Question 14

How is heart rate controlled?

Answer 14

1. Cardiac muscle is myogenic (= can contract and relax without receiving signals from nerves).
2. Heart beat is initiated by the sinoatrial node (SAN) [located in the wall of the right atrium].
3. SAN sends out waves of electrical activity to the atrial walls which cause the right and left atria to contract at
   the same time.
4. The waves of electrical activity are passed from the SAN to the AVN (atrioventricular node).
5. AVN passes the electrical activity on to the bundle of His [slight delay before the AVN reacts to ensure the atria
   have emptied before the ventricles contract].
6. Bundle of His splits into Purkyne tissue, which carries the waves of electrical activity in the walls of the right and
   left ventricles, causing them to contract simultaneously, from the apex up.

Question 15

High blood pressure

Answer 15

• Baroreceptors detect high blood pressure.
• Impulses sent to medulla, which sends impulses along PARASYMPATHETIC neurones.
• Which secrete ACETYLCHOLINE.
• Binds to specific receptors on SAN.
• SAN reduces frequency that waves of electrical activity are sent out.
• Heart rate decreases and blood pressure reduces back to normal.

Question 16

Low blood pressure

Answer 16

• Baroreceptors detect low pressure.
• Impulses sent to medulla, which sends impulses along SYMPATHETIC neurones.
• Which secrete NORADRENALINE.
• Which binds to specific receptors on the SAN.
• SAN increases frequency that waves of electrical activity are sent out.
• Heart rate increases to increase blood pressure back to normal.

Question 17

• How does exercise affect heart rate?

Answer 17

• Exercise involves muscle contraction, which requires respiration to produce ATP, therefore, CO2 is released into
  blood.
• This lowers pH of blood.
• This is detected by chemoreceptors.
• Which send impulses to the medulla.
• Medulla sends impulses to SAN via sympathetic nerves.
• Sympathetic neurones secrete noradrenaline, which increases the frequency at which SAN sends out waves of
  electrical activity.
• Causes heart rate to increase.
• Advantage of this = increased blood flow to lungs to remove CO2 and take in O2.

Question 18

What is the resting potential?

Answer 18

• The membrane potential of a neurone when it is not being stimulated (approximately -70mV).
• Results from there being more positively charged ions outside the neurone compared to inside.

Question 19

How is the resting membrane potential created / re-established?

Answer 19

• Na+/K+ pump actively transports 3 Na ions out of the axon and 2 K ions into the axon
• K+ leak channels allow K+ ions to diffuse out (membrane more permeable to potassium ions)
• Inside of axon is negatively charged compared to the outside

Question 20

Explain the sequence of events that occur during an action potential

Answer 20

1. Sodium ion channels open in response to a stimulus - so sodium ions diffuse INTO the axon. Inside of the
   axon is DEPOLARISED / becomes LESS NEGATIVE.
2. If the potential difference reaches the THRESHOLD POTENTIAL (around -55mV), MORE sodium ion channels
   open and MORE sodium ions diffuse rapidly into the axon [example of POSITIVE FEEDBACK].
3. At approx +40mV, sodium ion channels close and potassium ion channels open, so potassium ions diffuse
   OUT OF the axon down the potassium
   concentration gradient. This begins to
   REPOLARISE the membrane.
4. Potassium ions channels are slow to
   close, so the membrane becomes
   HYPERPOLARISED because too many
   potassium ions diffuse out of the axon, so
   the potential difference becomes more
   negative than the resting potential.
5. The sodium-potassium pump returns
   the membrane to its resting potential

Question 21

What is the refractory period and why is it necessary?

Answer 21

• During the refractory period, ion channels are RECOVERING and so CANNOT BE OPENED.
• This important because it:
  1. Ensures action potentials pass along neurones as DISCRETE IMPULSES (don’t overlap).
  2. Ensures action potentials are UNIDIRECTIONAL.
  (The refractory period also LIMITS the FREQUENCY at which nerves impulses can be transmitted).

Question 22

How does an action potential move along a neurone?

Answer 22

• As a WAVE OF DEPOLARISATION.
• Some of the sodium ions that diffuse into the neurone, diffuse sideways resulting in sodium ion channels in the
  next section of membrane to open and more sodium ions to diffuse into the axon.

Question 23

How does the size of stimuli affect a nerve impulse?

Answer 23

• Does not affect size of action potential → larger stimuli increase the FREQUENCY of action potentials.

Question 24

What affects speed of nerve impulse?

Answer 24

• TEMPERATURE: as temperature increases, the rate of diffusion of ions increases - faster nerve impulse [BUT
  above 40oC, proteins begin to denature and so speed decreases].
• AXON DIAMETER: wider diameter, less resistance to the flow of ions → so faster nerve impulse.
• MYELINATION:
• Schwann cells wrap around axon forming the myelin sheath → acts as an electrical insulator → prevents
  the movement of ions into or out of the axon.
• Between Schwann cells are patches of bare
  membrane called nodes of Ranvier.
• In myelinated neurones, sodium ion
  channels are concentrated at the nodes
  so depolarisation can only happen at the
  nodes → action potentials jump from
  node to node → which speeds up
  transmission of nerve impulses.
• This process is called saltatory conduction.
• [action potentials pass along myelinated axon faster than along unmyelinated axons of the same diameter].

Question 25

What does it mean by synpases are unidirectional?

Answer 25

• Unidirectional = action potential travels in one direction, from pre-synaptic membrane to post-synaptic membrane
  (WHY? Because neurotransmitter only released from pre-synaptic membrane and only post-synaptic
  membrane has the receptors for neurotransmitter).

Question 26

Describe how acetylcholine transmits a nerve impulse across a cholinergic synapse:

Answer 26

1. Action potential arrives at the synaptic knob of the pre-synaptic neurone.
2. Depolarisation of presynaptic membrane stimulates voltage-gated calcium ion channels in the pre-synaptic
   membrane to open.
3. Calcium ions diffuse into synaptic knob (via facilitated diffusion).
4. Influx of calcium ions causes synaptic vesicles containing acetylcholine to move to the presynaptic membrane,
   where they fuse with the membrane and release acetylcholine into the synaptic cleft by exocytosis.
5. Acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors (which are
   complementary to acetylcholine) on the post-synaptic membrane.
6. Causes sodium ion channels to open in the post-synaptic membrane.
7. Influx of sodium ions causes depolarisation of the post-synaptic membrane and an action potential is
   generated if the threshold is reached.

Question 27

To return to rest: specific enzyme breaks down the neurotransmitter

Answer 27

• e.g. acetylcholinesterase breaks down acetylcholine and the products are reabsorbed back into presynaptic neurone
  (ATP used to reform acetylcholine into vesicles and to actively transport Ca2+ ions out).
• Acetylcholine must be removed from synaptic cleft to prevent continuous action potentials being fired → this enables
  discrete transfer of information across the synapse.

Question 28

Temporaral summation

Answer 28

EMPORAL SUMMATION = when two or more nerve impulses arrive in quick succession from the
same presynaptic neurone, together resulting in sufficient neurotransmitter to be released into the
synaptic cleft to reach the threshold and trigger an action potential.

Question 29

Spatial summation

Answer 29

SPATIAL SUMMATION = many pre-synaptic neurones connect to one post-synaptic neurone,
altogether the neurotransmitter released from each neurone is enough to reach the threshold and trigger
an action potential.

Question 30

What is the difference between excitatory and inhibitory synapses?

Answer 30

• Excitatory synapses cause action potentials, inhibitory synapses prevent action potentials from occurring by
  making the postsynaptic neurone hyperpolarised.
• Inhibitory neurotransmitters = neurotransmitters which result in hyperpolarisation of the post-synaptic
  membrane e.g.

Question 31

Differences between cholinergic synapse and neuromuscular junctions?

Answer 31

• Cholinergic synapse a synapse between two neurones, neuromuscular junction is a synapse between a motor
  neurone and a muscle cell.
• Post synaptic membrane of a neuromuscular junction has lots of folds that form clefts. The clefts store
  acetylcholinesterase.
• Post synaptic membrane of neuromuscular junctions has more receptors than other synapses.
• Acetylcholine is always excitatory at a neuromuscular junction (whereas, acetylcholine can be inhibitory at e.g.
  cholinergic synapses in the heart).
• When a motor neurone fires an action potential, it normally triggers a response in the muscle cell - this isn’t always
  the case for a synapse between 2 neurones.

Question 32

Why would a disease that destroys receptors at neuromuscular junctions lead to a weaker the normal muscular
response?

Answer 32

1. Fewer receptors for acetylcholine to bind to.
2. Fewer sodium channels will open in the post-synaptic membrane.
3. Making it less likely that the threshold potential will be reached and so less likely that an action
   potential will be generated in the muscle cell.

Question 33

Gross and microscopic structure of a muscle fibre:

Answer 33

• Each muscle is made of large bundles of long cells → muscle fibres → each fibre made up of a bundle of myofibrils.
• Sarcolemma = cell membrane of a muscle fibre.
• Sarcoplasm = a muscle cell’s cytoplasm.
• Sarcoplasmic reticulum runs through the sarcoplasm and stores calcium ions for muscle contraction.
• T-tubules = folds of the sarcolemma which stick into the sarcoplasm → help to spread electrical impulses through
  the sarcoplasm so they reach all parts of the muscle fibre.
• Muscle fibres have lots of mitochondria (to provide the ATP for muscle contraction) and are multinucleate
  (contain lots of nuclei).

Question 34

Structure of a myofibril

Answer 34

• Myofibrils contain bundles of thick and thin
  myofilaments.
• Thick myofilaments are made of myosin.
• Thin myofilaments are made of actin.
• A-band: dark bands which contain myosin and some
  overlapping actin filaments.
• I bands: light bands which contain ONLY actin
  filaments.
• H-zone: contains only myosin filaments.
• Myofibrils are made up of many sarcomeres, the ends
  of each sarcomere are marked with a Z-line.

Question 35

How do the lengths of the different bands in a myofibril change during muscle contraction?

Answer 35

• A bands stay the same length.
• I band gets shorter.
• H zones get shorter.
• Z lines move closer together.

Question 36

Sliding filament theory / how an impulse in a motor neurone stimulates contraction of a myofibril:

Answer 36

• Acetylcholine is released from the presynaptic membrane and diffuses across the synapse.
• Acetylcholine binds to specific receptors on the sarcolemma (post-synaptic membrane).
• The muscle sarcolemma is depolarised (due to influx of sodium ions).
• Depolarisation spreads along the muscle fibre.
• Causes calcium ions to be released from the sarcoplasmic reticulum into the sarcoplasm.
• Calcium ions displace tropomyosin, and so uncover the myosin binding sites on the actin filaments, which allows
  the myosin heads to bind.
• This bond is called an actin-myosin cross bridge.
• Myosin heads bend (releasing a molecule of ADP which was attached to the myosin head) and pull the actin
  filament along, which shortens the sarcomere and contracts the muscle.
• An ATP molecule attaches to each myosin head, causing it to become detached from the actin filament.
• Calcium ions activate ATP hydrolase providing the energy for myosin heads to return to their original position.
• Myosin heads reattach further along the actin filament.
• The cycle will continue as long as calcium ions are present.

Question 37

Describe the role of Ca2+ ions and ATP in muscle contraction:

Answer 37

• Ca2+ ions cause the tropomyosin to move, exposing myosin-binding sites on actin.
• Ca2+ ions stimulate ATP hydrolase.
• ATP causes myosin head to bend / detach.
• ATP actively transports Ca2+ ions back into sarcoplasmic reticulum when the muscle is relaxed.

Question 38

What happens when the muscle stops being stimulated?

Answer 38

• Calcium ions leave their binding sites and are actively transported back into the sarcoplasmic reticulum.
• Tropomyosin moves back and blocks the actin-myosin binding sites.
• Actin filaments slide back to their relaxed position → lengthens the sarcomere.

Question 39

What causes cross-bridges to remain bound in rigor mortis?

Answer 39

• Respiration stops, so no ATP production.
• ATP is required for separation of the actin and myosin cross bridge.

Question 40

Role of ATP and phosphocreatine in muscle contraction:

Answer 40

• ATP is made by phosphorylating ADP using a phosphate group taken from PCr.
• Advantages =
• PCr can be stored inside cells (unlike ATP)
• ATP-PCr system can generate ATP very quickly.
• ATP-PCr system is anaerobic (doesn’t need oxygen).
• ATP-PCr system is alactate (doesn’t produce lactate).
• Disadvantages = PCr runs out after a few seconds so it can only be used during short bursts of very vigorous
  exercise.

Question 41

Slow twitch muscles

Answer 41

• Contract slowly
• Muscles used for posture
• Good for endurance activities
• Can work for a long time without getting tired
• Lots of mitochondria and blood vessels for energy to be released slowly through aerobic respiration
• Reddish colour due to myoglobin - a protein that stores oxygen

Question 42

Fast twitch muscle fibres

Answer 42

• Contract quickly
• Muscles moved for fast movement
• Good for short bursts of speed
• Get tired quickly
• Few mitochondria or blood vessels so energy is released quickly through anaerobic respiration
• Whittish in colour don’t have much myoglobin therefore can’t store much oxygen

Question 43

What is homeostasis?

Answer 43

Homeostasis = maintaining a constant internal environment in response to changes in the internal and
external environment.

Question 44

• Why is it important to maintain a stable blood glucose concentration?

Answer 44

• If blood glucose concentration is too high:
• Water potential of the blood is reduced.
• Water moves from cells into blood by osmosis.
• Cells shrivel and die.
• If blood glucose concentration is too low:
• Water potential of the blood is increased.
• Water moves into cells by osmosis.
• Cells swell and possibly burst.
• Also, a constant blood glucose level ensures a reliable source of respiratory substrate.

Question 45

What type of cells secrete glucagaon?

Answer 45

Alpha cells secrete glucagon into blood

Question 46

What type of cells secrete insullin?

Answer 46

Beta cells secrete insullin into blood

Question 47

Glycogenesis

Answer 47

Process by which glucose is converted to glycogen

Question 48

Glycogenolysis

Answer 48

Process by which glycogen is converted to glucose

Question 49

Gluconeogenesis

Answer 49

Process by which glucose is fomed from non-carbohydrates

Question 50

What happens when blood glucose levels are TOO HIGH?

Answer 50

• Islets of Langerhans detect change in blood glucose level.
• Insulin is released by beta cells and binds to specific receptors on muscle cells and liver cells
• Causes increase in the permeability of membranes to glucose by increasing the number of channel proteins in
  cell surface membranes of target cells.
• Activates enzymes which convert glucose into glycogen for storage (glycogenesis).
• Increases rate of respiration of glucose.
• Blood glucose concentration decreases → causes the beta cells to reduce secretion of insulin (= negative
  feedback).

Question 51

What happens when blood glucose levels are TOO LOW e.g. after exercise?

Answer 51

• Islets of Langerhans detect change in blood glucose level.
• Glucagon is released by alpha cells and binds to specific receptors on liver cells (only).
• This activates enzymes which convert glycogen into glucose (glycogenolysis) AND activates enzymes which
  convert glycerol and amino acids into glucose (gluconeogenesis)
• Rate of respiration of glucose in cells is decreased.
• Blood glucose concentration increases → causes alpha cells to reduce the secretion of glucagon (= negative
  feedback).

Question 52

Adrenaline

Answer 52

• Secreted from adrenal glands when blood glucose concentration is low / due to stress / during exercise.
• Binds to receptors on the cell membrane of liver cells - activates glycogenolysis and inhibits glycogenesis.
• Activates glucagon secretion and inhibits insulin secretion → increases glucose concentration.
• Makes more glucose available for muscles to respire → gets the body ready for action.

Question 53

How do glucagon and adrenaline activate glycogenolysis inside the cell, when they bind to receptors on the outside of the cell (Second Messenger Model)?

Answer 53

• Adrenaline and glucagon bind to complementary receptors on cell surface membrane of target cell.
• Activates adenylate cyclase.
• Activated adenylate cyclase converts ATP into cAMP, which is a second messenger.
• cAMP activates protein kinase A.
• Protein kinase A activates a cascade of reactions that results in glycogenolysis.

Question 54

Type 1 diabetes

Answer 54

Type 1 usually diagnosed at young age → beta cells destroyed by an autoimmune disorder so person doesn’t
make insulin (treatment = insulin injections, carbohydrate controlled diet).

Question 55

Type 2 diabetes

Answer 55

• Type 2 normally presents > age 40 → person makes insulin but cells are less sensitive to insulin, caused by
  obesity and poor diet (treatment = carbohydrate controlled diet, exercise, drugs).

Question 56

Structure of the nephron

Answer 56

• Each nephron is made up of a Bowman’s capsule which contains a knot of blood vessels called the glomerulus.
• Bowman’s capsule is attached to the proximal convoluted tubule (PCT),
• Which leads to the loop of Henle,
• Which leads onto the distal
  convoluted tubule (DCT),
• Which leads to the collecting
  duct.

Question 57

Formation of Glomerular Filtrate

Answer 57

1. Produced by ultrafiltration [filtration assisted by blood pressure].
2. Blood in the glomerulus is under high hydrostatic pressure because afferent arteriole is wider than the efferent
   arteriole.
3. This forces liquid and small molecules [such as glucose/ions/urea] in the blood out of pores in the capillary
   endothelium and through the basement membrane into the Bowman’s capsule.
4. Large proteins and blood cells can’t pass through because they are too large, so they remain in the blood.

Question 58

Site of selective reabsorption

Answer 58

• Site of selective reabsorption = PCT (around 85% of filtrate is reabsorbed in the PCT).
• When the filtrate reaches the PCT, useful substances [such as glucose, amino acids, vitamins and ions] are
  reabsorbed by facilitated diffusion and active transport into the blood.

Question 59

• How are the epithelial cells lining the PCT adapted to reabsorb substances?

Answer 59

• Microvilli to provide a large surface area for the reabsorption of substances from the filtrate.
• Infoldings at their bases to give a large surface area to transfer reabsorbed substances into the capillaries.
• Many mitochondria to provide ATP for active transport.
• Many carrier proteins/channel proteins for active transport/co-transport.

Question 60

How are useful substances reabsorbed?

Answer 60

• Sodium ions are actively transported from cells lining the PCT into the blood.
• This lowers the sodium ion concentration in the cells → sodium ions diffuse from the lumen of the PCT into the
  cells, taking glucose (for example) with them, via co-transport.
• The glucose concentration increases inside the cell and glucose moves via facilitated diffusion into the blood.
• The movement of salt/glucose/amino acids into the blood, lowers the water potential of the blood, so water
  follows by osmosis.

Question 61

Why can a high blood glucose concentration cause glucose to be present in the urine of a diabetic person?

Answer 61

The high concentration of glucose in the filtrate results in the glucose protein carriers becoming saturated,
therefore not all of the glucose is selectively reabsorbed from the filtrate and some glucose ends up in the urine.

Question 62

Role of loop of Henle in the reabsorption of water:

Answer 62

1. Loop of Henle acts as a counter-current multiplier.
2. Sodium (and chloride) ions are actively transported out of the ascending limb of the loop of Henle into the
   surrounding medulla (ascending limb is impermeable to water, so water remains in the tubule).
3. This lowers the water potential of the medulla.
4. Water moves out of the filtrate in the descending limb by osmosis (because the descending limb is permeable
   to water), into the medulla.
5. This water is reabsorbed into the blood via capillaries.

Question 63

Different animals have different length loops of Henle

Answer 63

• Animals living in dry environments have longer loops of Henle, to produce a larger countercurrent
  multiplier.
• Therefore more sodium (and chloride) ions are actively transported out of the filtrate into the medulla.
• This results in an even more negative water potential in the medulla.
• Therefore more water is absorbed by osmosis into the blood.
• Only a small volume of very concentrated urine is produced.

Question 64

If blood water level becomes LOW

Answer 64

• Water potential of blood decreases [hypertonic].
• Osmoreceptors in hypothalamus shrink.
• Stimulates the release of ADH from the posterior pituitary gland.
• ADH stimulates the cells lining the DCT and collecting duct to increase the number of aquaporins (water
  channels), making them more permeable to water.
• So more water is reabsorbed by osmosis from the DCT/collecting duct into blood.
• Less water is lost in the urine (smaller volume of more concentrated urine produced).

Question 65

If blood water levels become HIGH

Answer 65

• Water potential of blood increases [hypotonic].
• Osmoreceptors in hypothalamus swell.
• Stimulates the posterior pituitary gland to secrete less ADH.
• DCT and collecting duct become less permeable to water (due to presence of fewer aquaporins).
• So less water is reabsorbed by osmosis from collecting duct into blood.
• More water lost in urine (larger volume of more dilute urine produced)

Question 66

What substances are present in the urine?

Answer 66

Water, excess ions, urea, hormones, excess vitamins etc.

Question 67

What should NOT be found in the urine under normal circumstances?

Answer 67

1. Proteins and blood cells (too large to be filtered out of the blood)
   * If these are present in blood, suggests damage to basement membrane (caused by e.g. chronic high blood
   pressure)
2. Glucose (should be actively reabsorbed back into the blood)
   * If present in urine, suggest diabetes (carrier proteins saturated due to high blood glucose levels).

Question 68

Explain how resting potential is maintained?

Answer 68

Sodium-potassium pump actively transports 3 sodium ions out of axon and 2 potassium ions into the axon the membrane is more permeable to K ions so they diffuse out of axon

Question 69

What substances are selectively reabsorbed in the PCT?

Answer 69

Water and glucose