3.6

Question 1

Survival and Response

Question 2

What are taxis and kinesis

Answer 2

-simple responses that enable mobile organisms to stay in a favourable environment

taxis = directional response to stimuli
-kinesis = non-directional response to stimuli

Question 3

Kinesis

Answer 3

-changes speed of movement and the frequency of changes in direction

-e.g. organism moves from an area of beneficial stimuli to an area with harmful stimuli
-increases frequency of directional changes = quickly returns to favourable conditions

-surrounded by negative stimuli = decreased frequency of turns = moves in a relatively straight line =

Question 4

Taxis

Answer 4

-moves towards favourable stimulus or away from an unfavourable stimulus

-positive taxis = moving towards stimulus

-negative taxis = moving away from stimulus

Question 5

Why do organisms respond to changes in their environment

Answer 5

Organisms must respond to changes in their environment in order to survive, so that they can:
-find favourable conditions for living
-find food
-avoid being eaten (predation)

-prevents extinction
-stimulus being detected by a receptor cell
There are different types of receptors
-some receptors produce electrical activity in nerve cells in response to stimuli
-others secrete substances in response to stimuli
-nerve impulses sent by receptor cells travel to a coordinator (brain/spinal cord)
-impulse is conducted to the specific effector to produce the appropriate response

-stimulus: sudden movement by a crow is detected by the receptors in the robin’s eye
-receptor cells send an impulse along the nerves to the brain (coordinator)
-brain sends an impulse to the wing muscles (effectors) of the red robin so it can fly away (response)

Question 6

protective effect of a simple reflex

Question 7

Define tropism and growth factors

effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants.

Answer 7

Tropism: plants respond to stimuli via growth
Growth factors:

-IAA is a type of auxin
-controls cell elongation in shoots
-inhibits growth of cells in roots
-made in tips of roots and shoots but can diffuse to other cells

Positive phototropism = plant bends towards light as it’s needed for LDR (photosynthesis)

Shoots:
-shoot tip cells produces IAA, causing cell elongation
-IAA diffuses to other cells and towards the shaded side of the shoot, to increase IAA concentration on that side

Roots:
-anchors plant deep in the soil
-high IAA concentration inhibits cell elongation
-cells elongate more on the lighter side
-root bends away from the light

Shoots:
-IAA diffuses from upper side to lower side of a shoot
-if plant is on its side, the shoot bends upwards = negative gravitropism
-vertical plant = cell elongation = plant grows upwards

Roots:
-IAA moves to the lower side of roots
-upper side elongates = root bends towards gravity = positive gravitropism

Question 8

basic structure of a Pacinian corpuscle.

Answer 8

-not a separate cell, as they are found at the ends of sensory neurone axons

-made of many layers of membrane separated by a gel

-gel between the layers contains positively charged sodium ions (Na+)

-the section of axon surrounded by layers of membrane contains stretch-mediated sodium ion channels

• these open when sufficient pressure is applied

Question 9

how does deformation of stretch-mediated sodium ion channels in a Pacinian corpuscle leads to the establishment of a generator potential.

Answer 9

-an excess of positively charged sodium ions surrounds the axon
-pressure is exerted on the Pacinian corpuscle
-the layers of membrane become distorted and the stretch-mediated sodium channels in the axon membrane open
-sodium ions enter the axon via facilitated diffusion
-changes the electrical potential difference across the membrane
-leads to depolarisation when a threshold exceeded
-establishes a generator potential
-the generator potential triggers impulses (action potentials) that travel along the sensory neurone to the central nervous system

Question 10

What are Pacinian corpuscles

Answer 10

-receptors that respond to changes in pressure
-are present in the skin of fingers, soles of the feet, joints, tendons and ligaments
-stimulating these receptors with excess pressure on skin leads to a generator potential

Question 11

Function of:
cornea
retina
iris
optic nerve
pupil
lens
fovea

Answer 11

cornea
transparent layer that retracts light as it enters eye

retina:
contains light receptors
-rods: detect light intensity
-cones: detect colour

iris:
controls how much light enters pupil

optic nerve
sensory neurone that carries impulses between the eye and brain

pupil
hole that allows light to enter the eye

lens
transparent disc that can change shape to focus light onto retina

Question 12

Rods, cones and distribution of photoreceptors

Answer 12

Rods:
-process images in black and white
-to create a generator potential, the pigment of rod cells (rhodopsin) must be broken down by light energy
-can detect light in low intensity, due to retinal convergence (many rod cells connect to one sensory neurone)
-hence low visual acuity (brain cannot distinguish between separate light sources)
-spatial summation =threshold can be met in low light if enough pigment is broken down between multiple rods

Cones:
-process images in colour
-iodopsin pigment can be red-sensitive, green-sensitive or blue-sensitive
-each absorbs different wavelengths of light
-APs can only be generated with enough light, as iodopsin only breaks down at high light intensities
-each cone cell connects to its own bipolar cell
-hence high visual acuity (brain can distinguish between separate light sources)
-no spatial summation

Distribution of rods/cones
-lens focuses light on the fovea, which receives the highest intensity of light
-most cones are located near the fovea
-rod cells are found further away

Question 13

Cone cells provide higher visual acuity

Answer 13

-one cone cell synapses with a single bipolar cell
-one bipolar cell synapses with a single ganglion cell

-if two cones are stimulated to send an impulse the brain is able to interpret these as two different spots of light

-cone cells detect only one of three colours (red, green or blue) , so the brain will receive information about the colour of light detected by the stimulated cone cell and where this light is

-this is because the brain knows which bipolar cell connects to which cone cell

Question 14

Rod cells provide lower visual acuity

Answer 14

-multiple rod cells synapse with a single bipolar cell

-multiple bipolar cells synapse with a single ganglion cell

-brain is not able to interpret which impulses are sent by specific rods

-if multiple rod cells connected to the same bipolar cell detect light, only one impulse from the bipolar cell is sent

-hence the brain receives a general, not specific, understanding of the fields of vision that are light or dark

Question 15

what is summation and the benefit of it

Answer 15

There is a benefit to how the rods are connected to the optical nerve
Each rod is very sensitive to light however a single stimulated rod is unlikely to produce a large enough generator potential to stimulate the bipolar cell for the conduction of nerve impulses
When a group of rods are stimulated at the same time the combined generator potentials are sufficient to reach the threshold and stimulate the bipolar cell for the conduction of nerve impulses onwards towards the optic nerve
This additive effect of rods is known as summation
Summation produces a less sharp image but enables organisms to see in much dimmer light than cones allow
Nocturnal animals tend to have mostly or solely rods present in their eyes

Question 16

Explain why the heart is considered myogenic

Answer 16

It contracts without any external stimulus

Question 17

Outline how heart rate is controlled and coordinated
the roles of the autonomic nervous system and effectors in controlling heart rate.

Answer 17

-rate of cardiac muscle contraction is controlled by wave of electrical activity
-SAN is located in the wall of right atrium (known as pacemaker)
-AVN is located in the atria, near the border of the RV and LV
-bundle of His: collection of conducting tissue in the septum (middle) of the heart, divides into two conducting fibres (Purkyne tissue)

Process:
-SAN initiates a wave of depolarisation that causes the atria to contract
-impulses carried to AVN
-after a slight delay, the AVN is stimulated and passes the stimulation along the bundle of His
-delay means that the ventricles contract after the atria
-the bundle of His transmits wave of excitation along Purkinje fibres
-the Purkinje fibres spread around the ventricles and initiate the depolarisation of the ventricles from the apex (bottom) of the heart
-makes the ventricles contract and forces blood out of the pulmonary artery and aorta

Control:
-medulla oblongata controls heart rate via autonomic nervous system

-baroreceptors in wall of aorta and carotid artery are stretched due to high BP
-hence detects increased pressire
-more electrical impulses sent to medulla oblongata
-more impulses sent via parasympathetic nervous system to SAN to decrease frequency of electrical impulses
-lowered heart rate

-chemoreceptors in wall of aorta and carotid artery detect decreased pH
-more electrical impulses sent to medulla oblongata
-more impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses
-increased heart rate to deliver blood to lungs rapidly to remove CO2

Question 18

How does exercise affect control of heart rate?

Answer 18

-higher demand for O2
-chemoreceptors detect increase in CO2
-baroreceptors detect decrease in blood pressure
-more impulses sent to medulla oblongata
-more impulses sent to SAN via sympathetic nerve
-heart rate increases

Question 19

Outline the role and location of chemoreceptors and baroreceptors

Answer 19

Chemoreceptors:
-detect oxygen concentration in the blood
-also sensitive to changes in pH (due to CO2 in blood reacting with water to form carbonic acid)
-indication of oxygen availability

Baroreceptors:
-detect changes in blood pressure
-both types of receptors are found in the aorta and carotid arteries

Question 20

Outline the structures of the three types of neurones

Answer 20

-Schwann cells wrap around axon to form myelin sheath (lipid)
-charged ions can’t pass through
-gaps between = nodes of Ranvier

sensory neurone has its cell body in the middle and has a dendron and axon

motor neurone has its cell body at the start and only has a long axon

Question 21

Describe the two types of motor neurones

Answer 21

Somatic supplies skeletal muscle = under conscious control

Autonomic supplies cardiac muscle, smooth muscle, glands = under unconscious control

Question 22

Define action potential

Answer 22

impulses or electrical charges that travel along the axon, due to changes in the membrane potentiakl along a neurone

Question 23

Outline what happens during an action potential

Answer 23

stimuli causes Na+ ions to enter the start of the neurone

makes membrane potential less negative

if it reaches threshold (-50mV), Na+ channels open

therefore more Na+ ions diffuse into the neurone, therefore membrane potential becomes positive (depolarised)

the membrane potential reaches +40mV

then the Na+ channels close, the K+ channels open

therefore K+ ions diffuse out, therefore membrane potential becomes negative (repolarised)

too many K+ ions move out, so the membrane potential becomes more negative than normal (hyperpolarised, known as the refractory period)

-resting potential is achieved when it slowly begins to increase again

Question 24

What affects action potentials
Properties of synapses

Answer 24

frequency of impulses
high frequency = larger stimuli
Low frequency = smaller stimuli

unidirectionality
-AP/nerve impulse travels in one direction
-from pre to post
-pre-synaptic membrane has the neurotransmitter
-post-synaptic membrane has the receptors

filters out low level stimuli
-low level stimuli do not release enough neurotransmitter
-hence not enough Na+ ion channels open
-not enough Na+ ions enter postsynaptic neurone for threshold to be reached
-no AP produced

inhibitory synapses:
-normal synapses are excitatory (cause AP)
-inhibitory prevent action potential from occurring by making postsynaptic neurone hyperpolarised

Question 25

Factors affecting speed of nervous impulses

Answer 25

Temperature
-higher temp = higher kinetic energy
-faster rate of diffusion of ions
-enzymes involved in respiration also work faster, so more ATP produce
-more active transport in the Na+/K+ pump
-faster nerve impulse

Axon diameter
-wider diameter
-neurone less leakage of ions
-APs travel faster
-faster nerve impulse

Myelination:
-Schwann cells wrap around axon
-insulates axon preventing AP
-therefore AP only occurs in gaps – called node of Ranvier
-so AP jumps from node to node = saltatory conduction
-faster nerve impulse

Question 26

What is a synapse
outline the stages of synaptic transmission

Answer 26

-connection between 2 different neurones
-sends nerve impulse across the synaptic cleft using neurotransmitters (e.g. acetylcholine)

Process:
-AP arrives in end of presynaptic neurone
-voltage-gated Ca2+ channels open
-Ca2+ ions enter presynaptic neurone
-causes vesicles containing neurotransmitter to move to presynaptic membrane
-vesicle binds to membrane releasing neurotransmitter into cleft
-neurotransmitter diffuses across cleft
-binds to complementary receptors on postsynaptic membrane
-Na+ channels open, Na+ ions enter
-if threshold is reached, AP occurs

To return to rest
-enzyme used to break down neurotransmitter
-e.g. acetylcholinesterase breaks down acetylcholine into ethanoic acid and choline
-diffuses back into presynaptic neurone
-ATP used to reform neurotransmitter into vesicle and actively transport Ca2+ ions out

Question 27

Neuromuscular junctions

Question 28

types of muscle

Answer 28

Skeletal

Smooth

Cardiac

Question 29

structure

Answer 29

-basic structure = sarcomeres
-made up of actin and myosin

Actin is thin and has tropomysosin wrapped around it
Myosin is thick and has heads

when the sarcomere contracts the whole muscle contracts by the sliding filament mechanism

many sarcomeres = myofibril
many myofibrils = muscle fibre
surrounded by a membrane called sarcolemma (contains myofibrils, fluid called sarcoplasm and tubes called sarcoplasmic reticulum)
many muscle fibres = bundle
many bundles = whole muscle

Locations in a Sarcomere?
A band = location of myosin [no change in contraction]
I band = location between the myosin [shortens in contraction]
H zone = location between the actin [shortens in contraction]
Z line = end line of sarcomere [moves closer together in contraction]

Question 30

Overall function of skeletal muscles?

Answer 30

-moves the body skeleton
-when the muscle contracts (shortens) the tendon pulls on joints causing movement

Question 31

what occurs in Sliding Filament Mechanism?

Answer 31

-sarcomere shortens
-the myosin heads pull the actin inwards
-somatic motor neurone connects to the skeletal muscle via a neuro-muscular junction
-one motor neurone connects to a few muscle fibres = motor unit
(benefit = simultaneous muscle contraction and can control strength of contraction)
-releases acetylcholine that binds to complementary receptors on the muscle fibre membrane (sarcolemma)
-Na+ channels open, Na+ ions enter the muscle fibre causing depolarisation
-wave of depolarisation travels through sarcoplasmic reticulum
-causes release of Ca2+ ions into the sarcoplasm (fluid surrounding sarcomeres/myofibril)
-this moves the tropomyosin on the actin
-exposes binding sites on the actin
-myosin heads now bind to the actin (form actin-myosin cross bridge)
-a power stroke occurs, the myosin pulling the actin inwards
-ATP attaches to myosin head so it detaches
-ATP broken down by ATPase to release energy
-causes myosin head to go back to its original position
-so it reattaches, pulling the actin further inwards

Question 32

Establishing resting potential

Answer 32

-sodium potassium pump pumps 3Na+ out and 2K+ in
-creates electrochemical gradient
-causes K+ ions to diffuse out (from high to low concentration)
-Na+ ions diffuse in (higher outside than in)

-cell membrane is more permeable to K+ ions than Na+, as there are more K+ ion channels,so more facilitated diffusions
-also some Na+ ion channels only open when the voltage is high enough, but K+ ion channels are always open

-70mV inside the neurone

Question 33

What is the all or nothing principle

Answer 33

If the depolarisation doesn’t exceed -55mV, then an action potential and impulse are not produced

Any stimulus that triggers depolarisation to-55mV will always peak at the same maximum voltage

Bigger stimuli = higher frequency of AP

Important as animals only respond to large stimuli

Question 34

Importance of refractory period

Answer 34

1. Discrete APs that don’t overlap so you can process the info in more detail
2. Ensures that it only travels in one direction
3. Limits the number of impulses transmitted, so not as overwhelming to senses

Question 35

What is summation?

Answer 35

-rapid build-up of neurotransmitters in synapse
-helps generate AP
Spatial:
-many different neurones collectively trigger new AP by combining the neurotransmitter they release to exceed the threshold value

Temporal:
-one neurone repeatedly releases neurotransmitter over short period of time to add up to enough to exceed the threshold value

Question 36

Inhibitory synapses

Answer 36

-cause chloride ions to move into the postsynaptic neurone and potassium ions to move out
-hyperpolarisation occurs as membrane potential decreases
-hence AP highly unlikely

-potsynaptic neurone becomes more negative/hyperpolarisation

-More sodium ions required (to reach threshold)
OR
Not enough sodium ions enter (to reach threshold);
- depolarisation/action potential; highly unlikely

Question 37

Neuromusclar junction vs cholinergic synapse

Answer 37

Neuromusclar junction:
-unidirectional as receptors for neurotransmitters are only on post-synaptic membrane
-only excitatory
-connects motor neurone to muscles
-end point for AP
-acetylcholine binds on receptors on muscle fibre membranes

Cholinergic synapse:
-unidirectional as receptors for neurotransmitters are only on post-synaptic membrane
-could be excitatory/inhibitory
-connects 2 neurones
-new AP is generated in the next neurone
-acetylcholine binds to receptors on post-synaptic membrane of a neurone

Question 38

Sliding Filament theory sme

Answer 38

-during muscle contraction, sarcomeres within myofibrils shorten as the Z discs are pulled closer together

Process:
-action potential arrives at the neuromuscular junction
-calcium ions are released from the sarcoplasmic reticulum
-calcium ions bind to troponin molecules, stimulating them to change shape
-causes troponin and tropomyosin proteins to change position on the actin (thin) filaments
-myosin binding sites are exposed on the actin molecules

-globular myosin heads bind with these sites, forming cross-bridges between the two types of filament
-formation of the cross-bridges causes the myosin heads to spontaneously bend (releasing ADP and inorganic phosphate), pulling the actin filaments towards the centre of the sarcomere
-causes the muscle to contract a very small distance
-ATP binds to the myosin heads producing a change in shape that causes the myosin heads to release from the actin filaments

-ATP hydrolase hydrolyses ATP into ADP and inorganic phosphate
-causes the myosin heads to move back to their original positions (known as the recovery stroke)
-myosin heads are then able to bind to new binding sites on the actin filaments, closer to the Z disc
-myosin heads move again, pulling the actin filaments even closer the centre of the sarcomere,
-causes the sarcomere to shorten once more and pulling the Z discs closer together
-ATP binds to the myosin heads once more in order for them to detach again
-as long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP
-process repeats until the muscle is fully contracted

Question 39

Outline the structure of the two types of myofibrils (sme)

Answer 39

-thick filaments within a myofibril contain myosin molecules
myosin are ffibrous protein molecules with a globular head
-fibrous part anchors the molecule into the thick filament
-in the thick filament, many myosin molecules lie next to each other with their globular heads all pointing away from the M line

-thin filaments within a myofibril contain actin molecules
-globular protein molecules
-many actin molecules link together to form a chain
-two actin chains twist together to form one thin filament
-a fibrous protein known as tropomyosin is twisted around the two actin chains
-another protein known as troponin is attached to the actin chains at regular intervals

Question 40

Outline the role of ATP and phosphocreatine in muscle contraction (sme)

Answer 40

-ATP supply required for:
-the initial movement of myosin heads (powerstroke0
-the return movement of myosin heads that causes the actin filaments to slide
-the return of calcium ions back into the sarcoplasmic reticulum occurs via active transport

-resting muscles have a small amount of ATP stored that will only last for 3/4 seconds of intense exercise
- mitochondria present in the muscles fibres are able to aerobically respire and produce ATP but this is slow and can take a considerable amount of time
Anaerobic respiration, which is faster than aerobic, still takes 10 seconds before it even begins to produce any ATP
Phosphocreatine is a molecule stored by muscles that can be used for the rapid production of ATP
A phosphate ion from phosphocreatine is transferred to ADP
ADP + phosphocreatine → ATP + creatine
Different muscle fibre types contain different limited amounts of phosphocreatine
It allows for muscles to continue contracting for a short period of time until the mitochondria are able to supply ATP
For example, it would be utilised by the muscles of a 100m sprinter as sprinting involves an intense level of muscle contraction
For prolonged activity, once the supply of phosphocreatine has been used up then the rate of muscle contraction must equal the rate of ATP production from both aerobic and anaerobic respiration

Question 41

Describe the roles of calcium ions and ATP in the contraction of a
myofibril.

Answer 41

1. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
2. (Calcium ions) cause movement of tropomyosin (on actin);
3. (This movement causes) exposure of the binding sites on the
   actin;
4. Myosin heads attach to binding sites on actin;
5. Hydrolysis of ATP (on myosin heads) causes myosin heads to
   bend;
6. (Bending) pulling actin molecules;
7. Attachment of a new ATP molecule to each myosin head
   causes myosin heads to detach (from actin sites).

Question 42

Define homeostasis

Answer 42

maintenance of a constant internal environment using physiological control systems

Question 43

factors influencing how well a cell functions

Answer 43

1) temperature
- in very low temperatures, metabolic reactions slow down
-in very high temperatures, proteins (including enzymes) get denatured

2) water potential
-when water potential increases, water may enter the cell, it swells and maybe bursts
-when water potential decreases, water leaves the cells, metabolic reactions may slow or stop

3) concentration of glucose
-lack of glucose causes respiration to slow or stop, depriving the cell of an energy source
-too much glucose may cause water to move out of the cell, disturbing the metabolism

Question 44

importance of maintaining a stable core temperature

Answer 44

Enzymes have a specific optimum temperature – the temperature at which they catalyse a reaction at the maximum rate

Lower temperatures either prevent reactions from proceeding or slow them down:

Molecules move relatively slow

Lower frequency of successful collisions between substrate molecules and active site of enzyme

Less frequent enzyme-substrate complex formation occurs

Substrate and enzyme collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)

high
Higher temperatures speed up reactions:

Molecules move more quickly

Higher frequency successful collisions between substrate molecules and active site of enzyme

More frequent enzyme-substrate complex formation

Substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

However, as temperatures continue to increase, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:

Bonds (eg. hydrogen bonds) holding the enzyme molecule in its precise shape start to break

This causes the tertiary structure of the protein (ie. the enzyme) to change

This permanently damages the active site, preventing the substrate from binding

Denaturation has occurred if the substrate can no longer bind

Very few human enzymes can function at temperatures above 50°C

This is because humans maintain a body temperature of about 37°C, therefore even temperatures exceeding 40°C will cause the denaturation of enzymes

High temperatures cause the hydrogen bonds between amino acids to break, changing the conformation of the enzyme

Question 45

importance of maintaining a stable blood pH

Answer 45

All enzymes have an optimum pH or a pH at which they operate best

Enzymes are denatured at extremes of pH

Hydrogen and ionic bonds hold the tertiary structure of the protein (ie. the enzyme) together

Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH– ions (alkaline solutions) can cause these bonds to break

This alters the shape of the active site, which means enzyme-substrate complexes form less easily

Eventually, enzyme-substrate complexes can no longer form at all

At this point, complete denaturation of the enzyme has occurred

Where an enzyme functions can be an indicator of its optimal environment:

Eg. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)

Pepsin’s optimum pH, not surprisingly, is pH 2

Question 46

Examples of some physiological factors controlled in homeostasis

Answer 46

1) core body temperature
2) concentration of metabolic waste substances (e.g., CO2, urea)
3) blood pH
4) water potential (Ψ) of blood
5) concentration of respiratory gases (CO2, O2) in the blood

Question 47

importance of maintaining a stable blood glucose concentration in terms of availability of respiratory substrate and of the water potential of blood.

Answer 47

Another key factor that must be controlled within mammals is the concentration of glucose in the blood

The amount of glucose present in the blood affects the water potential of the blood and the availability of respiratory substrate for cells

The normal glucose concentration for human blood is roughly 90mg per 100cm3

A sufficient amount of circulating glucose is essential for cellular respiration

Brain cells can become rapidly damaged or die if they do not receive a sufficient supply of glucose

Alternatively, if the blood glucose concentration is too high then it will have a dramatic effect on the water potential of the blood

Question 48

Purpose of negative feedback

Answer 48

restores systems to their original level.

Most control mechanisms in living organisms use a negative feedback control loop to maintain homeostatic balance - * a receptor senses a stimulus (can be internal or external) and sends this input through the nervous system to a control centre in the brain or spinal cord - a stimulus is any change in a factor (e.g., change in blood temperature or blood water potential) * the control centre signals an effector (muscles and glands) to respond to the stimuli * the response to the stimulus is usually an action being carried out i.e., output * these actions are sometimes called corrective actions as their effect is to correct (or reverse) the change * continuous monitoring of the factor by receptors produces a steady stream of information that makes continuous adjustments, which causes the factor to fluctuate around a set-point * this mechanism is known as negative feedback

Question 49

thermoregulation - control of body temperature

Answer 49

the hypothalamus in the brain is the central control for body temperature * this region of the brain receives a constant input of sensory information about the temperatures of the blood and surroundings * it has thermoreceptor cells that continually monitors the temperature of the blood flowing through it (the core temperature)

Responses to low temperatures
1) vasoconstriction – muscles in the walls of arterioles that supply blood to capillaries near the skin surface contract, decreasing the lumen size; blood supply is reduced so less heat is lost from blood

2) less sweat produced – reduces heat loss by evaporation from the skin surface

3) body hairs are raised – traps air, which is an insulator, although not very effective for humans

4) shivering – involuntary contraction of skeletal muscles generates heat

5) adrenaline secreted – increases rate of heat production from the liver

When the temperature of the environment decreases gradually
1) the hypothalamus secretes a hormone which stimulates the anterior pituitary gland to release thyroid stimulating hormone (TSH)

2) TSH stimulates the thyroid gland to secrete thyroxine into the blood

3) thyroxine increases the metabolic rate which increases heat production, especially in the liver When the temperature of the environment starts to increase again, the hypothalamus responds by reducing the release of TSH by the anterior pituitary gland.

Responses to high temperatures
1) vasodilation
2) increased sweating – water has high latent heat of evaporation
3) body hairs are lowered

Question 50

factors that influence blood glucose concentration.

Question 51

Define glycogenesis and outline the role the livers plays in it

Question 52

Define gluconeogenesis and outline the role the livers plays in it

Question 53

Define glycogenolysis and outline the role the livers plays in it

Question 54

The action of insulin by: * attaching to receptors on the surfaces of target cells * controlling the uptake of glucose by regulating the inclusion of channel proteins in the surface membranes of target cells * activating enzymes involved in the conversion of glucose to glycogen.

Question 55

The action of glucagon by: * attaching to receptors on the surfaces of target cells * activating enzymes involved in the conversion of glycogen to glucose * activating enzymes involved in the conversion of glycerol and amino acids into glucose.

Question 56

The role of adrenaline by: * attaching to receptors on the surfaces of target cells * activating enzymes involved in the conversion of glycogen to glucose.

Question 57

second messenger model of adrenaline and glucagon action, involving adenylate cyclase, cyclic AMP (cAMP) and protein kinase.

Question 58

causes of types I and II diabetes and their control by insulin and/or manipulation of the diet.

Question 59

Deamination and the formation of urea

Answer 59

-extra proteins cannot be stored in the body
-liver removes –NH2 along with an extra hydrogen atom
-produces a keto acid and ammonia
-NH3 is extremely toxic, so it’s converted to urea and excreted
-keto acid that remains may enter the Krebs cycle and be respired /may be converted to glucose/glycogen/fat for storage.

Nitrogenous excretory products of humans
1) urea – the main nitrogenous excretory product in humans
2) creatinine – made in the liver from certain amino acids
-creatine (made in the liver from certain amino acids) is used in the form of creatine phosphate in the muscles as an energy store
-some of this is creatine is converted to creatinine and excreted
3) uric acid – made from the breakdown of purines from nucleotides, not amino acids

Question 60

Section through a kidney

Question 61

Structure of a nephron

Question 62

process of ultrafiltration and selective reabsorption

Question 63

countercurrent mechanism in kidneys

Answer 63

-using energy to generate an osmotic gradient
-enables reabsorption of water from the tubular fluid and produces concentrated urine
-prevents too much dilute urine, hence why continually drinking water isn’t needed to stay hydrated

-kidneys contain two types of nephrons
-superficial cortical nephrons (70-80%) are found in outer cortex
-juxtamedullary nephrons (20-30%) are found near the corticomedullary border
-loop of Henle is a hairpin-like structure comprised of a thin descending limb, a thin ascending limb and a thick ascending limb
-loops of Henle of cortical nephrons penetrate only as far as the outer medulla of the kidney
-those of the juxtamedullary nephrons penetrate deeply within the inner medulla
-both cortical and juxtamedullary nephrons regulate the concentrations of solutes and water in the blood
-but countercurrent multiplication in the loops of Henle of juxtamedullary nephrons is largely responsible for developing the osmotic gradients that are needed to concentrate urine

Process:
-fluid leaving the ascending limb of the loop of Henle enters the distal convoluted tubule
-fluid composition is further adjusted, and then drains into collecting tubules
-tubules empty into collecting ducts that descend back through the medulla
-eventually connect to the ureter, which transports urine to the bladder

Although the loops of Henle are essential for concentrating urine, they do not work alone. The specialized blood capillary network (the vasa recta) that surrounds the loops are equally important. The vasa recta capillaries are long, hairpin-shaped blood vessels that run parallel to the loops of Henle. The hairpin turns slow the rate of blood flow, which helps maintain the osmotic gradient required for water reabsorption.
The three segments of the loops of Henle have different characteristics that enable countercurrent multiplication.
The thin descending limb is passively permeable to both water and small solutes such as sodium chloride and urea. As active reabsorption of solutes from the ascending limb of the loop of Henle increases the concentration of solutes within the interstitial space (space between cells), water and solutes move down their concentration gradients until their concentrations within the descending tubule and the interstitial space have equilibrated. As such, water moves out of the tubular fluid and solutes to move in. This means, the tubular fluid becomes steadily more concentrated or hyperosmotic (compared to blood) as it travels down the thin descending limb of the tubule.

-thin ascending limb is passively permeable to small solutes, but impermeable to waterr
-so water cannot escape from this part of the loop
-hence solutes move out of the tubular fluid, but water is retained
-tubular fluid becomes steadily more dilute or hyposmotic as it moves up the ascending limb of the tubule.

-thick ascending limb actively reabsorbs sodium, potassium and chloride
-impermeable to water, which again means that water cannot escape from this part of the loop. This segment is sometimes called the “diluting segment”.
Countercurrent multiplication moves sodium chloride from the tubular fluid into the interstitial space deep within the kidneys. Although in reality it is a continual process, the way the countercurrent multiplication process builds up an osmotic gradient in the interstitial fluid can be thought of in two steps:
The single effect. The single effect is driven by active transport of sodium chloride out of the tubular fluid in the thick ascending limb into the interstitial fluid, which becomes hyperosmotic. As a result, water moves passively down its concentration gradient out of the tubular fluid in the descending limb into the interstitial space, until it reaches equilibrium.
Fluid flow. As urine is continually being produced, new tubular fluid enters the descending limb, which pushes the fluid at higher osmolarity down the tube and an osmotic gradient begins to develop.
As the fluid continues to move through the loop of Henle, these two steps are repeated over and over, causing the osmotic gradient to steadily multiply until it reaches a steady state. The length of the loop of Henle determines the size of the gradient - the longer the loop, the greater the osmotic gradient.

Absorbed water is returned to the circulatory system via the vasa recta, which surrounds the tips of the loops of Henle. Because the blood flow through these capillaries is very slow, any solutes that are reabsorbed into the bloodstream have time to diffuse back into the interstitial fluid, which maintains the solute concentration gradient in the medulla. This passive process is known as countercurrent exchange.
The concentration of urine is controlled by antidiuretic hormone, which helps the kidneys to conserve water. Its main effects in the renal tubules is to increase water permeability in the late distal tubule and collecting ducts, increase active transport of sodium chloride in the thick ascending limb of the loop of Henle, and enhance countercurrent multiplication and urea recycling, all of which increase the size of the osmotic gradient.

Question 64

urea recycling

Answer 64

Urea recycling in the inner medulla also contributes to the osmotic gradient generated by the loops of Henle. Antidiuretic hormone increases water permeability, but not urea permeability in the cortical and outer medullary collecting ducts, causing urea to concentrate in the tubular fluid in this segment. In the inner medullary collecting ducts it increases both water and urea permeability, which allows urea to flow passively down its concentration gradient into the interstitial fluid. This adds to the osmotic gradient and helps drive water reabsorption.

Question 65

ultraafiltration sme

Answer 65

Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman’s capsule

The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them (which is already at high pressure because it is coming directly from the renal artery which is connected to the aorta)

This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate

The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between them:

The first cell layer is the endothelium of the capillary – each capillary endothelial cell is perforated by thousands of tiny membrane-lined circular holes

The next layer is the basement membrane – this is made up of a network of collagen and glycoproteins

The second cell layer is the epithelium of the Bowman’s capsule – these epithelial cells have many tiny finger-like projections with gaps in between them and are known as podocytes

As blood passes through the glomerular capillaries, the holes in the capillary endothelial cells and the gaps between the podocytes allows substances dissolved in the blood plasma to pass into the Bowman’s capsule

The fluid that filters through from the blood into the Bowman’s capsule is known as the glomerular filtrate

The main substances that pass out of the capillaries and form the glomerular filtrate are: amino acids, water, glucose, urea and inorganic ions (mainly Na+, K+ and Cl-)

Red and white blood cells and platelets remain in the blood as they are too large to pass through the holes in the capillary endothelial cells

The basement membrane acts as a filter as it stops large protein molecules from getting through

Ultrafiltration occurs due to the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule

Remember – water moves down a water potential gradient, from a region of higher water potential to a region of lower water potential. Water potential is increased by high pressure and decreased by the presence of solutes

Question 66

Factors Affecting Water Potential in the Glomerulus & Bowman’s Capsule Table sme

Answer 66

Pressure:
-afferent arteriole is wider than efferent arteriole
-hence BP is relatively high in glomerular capillaries
-increases WP of blood plasma in glomerular capillaries
-exceeds WP of filtrate in the Bowman’s capsule
-water moves down WP gradient from blood plasma in the glomerular capillaries to the Bowman’s capsule

Solute concentration:
-basement membrane allows most solutes in blood plasma to filter into the Bowman’s capsule
-plasma proteins too big, so remain in blood
-hence solute concentration in blood plasma in the glomerular capillaries is higher than that in the filtrate in the Bowman’s capsule
-lowers WP of blood plasma than filtrate in Bowman’s capsule
-hence water moves down WP gradient from Bowman’s capsule to blood plasma in the glomerular capillaries

Question 67

selective reabsorption sme

Answer 67

-many substances in the glomerular filtrate need to be kept by the body, so are reabsorbed into the blood as the filtrate passes along the nephron
-most of this reabsorption occurs in the proximal convoluted tubule

-the lining of the proximal convoluted tubule is composed of a single layer of epithelial cells, which are adapted to carry out reabsorption in several ways:
-microvilli
-co-transporter proteins
-a high number of mitochondria
-tightly packed cells

-water and salts are reabsorbed via the Loop of Henle and collecting duct

How selective reabsorption occurs:
-blood capillaries are located very close to the outer surface of the proximal convoluted tubule
-as the blood comes straight from the glomerulus, it has very little plasma and has lost much of its water, inorganic ions and other small solutes
-the basal membranes (of the proximal convoluted tubule epithelial cells) are the sections of the cell membrane that are closest to the blood capillaries
-Na+/K+ pumps in these basal membranes move Na+ ions out of the epithelial cells and into the blood, where they are carried away
-lowers Na+ concentration inside the epithelial cells
-causes Na+ ions in the filtrate to diffuse down concentration gradient through the luminal membranes (of the epithelial cells)
-using co-transporter proteins in the membrane

-several types of these co-transporter proteins
-each type transports a sodium ion and another solute from the filtrate (eg. glucose or a particular amino acid)

-once inside the epithelial cells these solutes diffuse down their concentration gradients
-pass through transport proteins in the basal membranes (of the epithelial cells) into the blood

Molecules reabsorbed from the proximal convoluted tubule during selective reabsorption
-all glucose in the glomerular filtrate, so none should be present in the urine
-amino acids, vitamins and inorganic ions are reabsorbed
-movement of all these solutes from the proximal convoluted tubule into the capillaries increases the water potential of the filtrate
-decreases water potential of the blood in the capillaries
-creates a steep water potential gradient and causes water to move into the blood by osmosis
-significant amount of urea is reabsorbed too
-concentration of urea in the filtrate is higher than in the capillaries
-causes urea to diffuse from the filtrate back into the blood

Reabsorption of water and salts
-as the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion
-as salts are reabsorbed back into the blood, water follows by osmosis
-water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time
-after the necessary reabsorption of amino acids, water, glucose and inorganic ions is complete, the filtrate eventually leaves the nephron
-now referred to as urine
-flows out of the kidneys, along the ureters and into the bladder, where it is temporarily stored

Question 68

Adaptations for Selective Reabsorption Table sme

Answer 68

PCT epithelial cells
-many microvilli on luminal membrane = increased surface area for reabsorption
-many co-transporter proteins in the luminal membrane = each transports specific solute across the luminal membrane
-many mitochondria = energy from ATP for Na+/K+ pump proteins in the basal membranes
-cells tightly packed together = no fluid can pass between cells = all substances reabsorbed must pass through the cells

Question 69

class ppt loop of henle notes

Answer 69

-collecting duct receives the output from many nephrons
-collecting duct and loop of Henle are reabsorb most of the remaining 30% of the filtered salt and water that the PCT can’t manage

-all other parts of the nephron are in the cortex of the kidney
-loop can extend deep into the medulla​

-not all nephrons have a long loop of Henle

process:
-Na+ ions move into the interstitial region by active transport
-increases ion concentration of this area​
-as this tissue is now more concentrated, water leaves the descending limb and enters the capillaries via osmosis
-water cannot leave the ascending limb as it is impermeable to water​
-the interstitial region becomes increasingly concentrated, so water loss continues along the entire length of the descending limb​
-filtrate becomes its most concentrated at the apex of the Loop of Henle due to water leaving​
-as the filtrate moves up the ascending limb, Na+ ions diffuse out at first before being actively transported out
-filtrate leaves the ascending limb and flows through the DCT, before entering the collecting duct​
-at the top of the collecting duct, water is able to leave and re-enter the blood as it has a higher water potential than the interstitial fluid.
-as you progress along the collecting duct, water is able to leave along it’s entire length as WP of surrounding interstitial fluid is always lower

Question 70

1. What is ADH
2. Function of ADH
3. What are aquaporins

Answer 70

-anti-diuretic hormone
-acts on the permeability to water molecules of the cell membranes lining the collecting duct and DCT

what are aquaporins:
-membrane-spanning channel proteins that allow water to pass through the membrane
-aquaporins are stored in vesicles within the cell (inserted rapidly into the cell surface membrane by fusion of these vesicles)
-fusion of vesicles is triggered by a signal transduction cascade involving cAMP (2nd messenger) in response to binding of ADH to cell surface receptors.
-binding of ADH to these receptors causes the cells in the hypothalamus to shrink
-ADH released and travels to the posterior pituitary gland​
-ADH also has a longer-term effect in increasing the expression of the aquaporin gene
-increases the total number of aquaporin proteins in the cell

Question 71

What happens when there is low wp in blood

Answer 71

-osmoreceptors in the hypothalamus detect lower water potential of the blood
-pituitary gland secretes ADH into the blood​
-ADH makes the cell surface membrane of the epithelial cells of the collecting duct more permeable to water​​
-more water is returned to the bloodstream​

Question 72

Outline the counter-current system in the loop of Henle and explain how it ensures maximum water reabsorption [8]​

Answer 72

-loop of Henle consists of descending and ascending limb.​

-differ in permeability to water; descending limb is water-permeable while ascending limb is impermeable to water.​

-active transport of Na+ and Cl- ions out of cells at top of ascending limb.​

-into interstitial fluid making it have a low water potential.​

-water leaves the descending limb down a water potential gradient.​

-fluid at tip of loop is very concentrated.​

-some ions diffuse out of ascending limb.​

-loop’s function is to create a very high concentration of sodium ions and chloride ions in the tissue fluid in the medulla.​

-allows a lot of water to be reabsorbed from the contents of the nephron as they pass through the collecting duct.​