response to stimuli and muscles

Question 1

autonomic nervous system

Answer 1

• autonomic means self-governing. Autonomic nervous system controls the involuntary/ self-governing activities of internal muscles and glands
  Has two divisions:
• sympathetic nervous system. Stimulates effectors and so speeds up any activity. Controls effectors when we exercise strenuously or experience powerful emotions. Prepares for flight or fight. Sympathetic neurones secrete noradrenalin- another type of neurotransmitter that increases heart rate
• parasympathetic nervous system. Inhibits effectors and slows down any activity. Controls activities under normal resting conditions and is concerned with conserving energy and replenishing the bodys reserves. Parasympathetic neruones secrete acetylcholine- a type of neurotransmitter that decreases heart rate

Question 2

how does the heart ensure complete ventricular filling

Answer 2

• important as the ventricle needs to be completely full to ensure maximum blood volume is pumped to the lungs/ the body
• occurs during atrial systole
• atrial systole must be complete before ventricular systole can begin
• the SA node (sino-atrial node)/ pacemaker is found in the right atrium. The sino-atrial node sends electrical impulses/ a wave of depolarisation across to the left atrium. Means left and right atrium essentially contract at the same time
• However, the electrical depolarisation moves across the heart walls at a high speed of 100m/s. This suggests the ventricles should start to contract before atrial systole is complete
• the wave of depolarisation however cannot go into the ventricles, as there is a layer underneath the atrium where electrical impulses cannot pass. This is a layer of non-conductive tissue (atrioventricular septum)
  -The electrical impulses are instead passed to the atrial-ventricular node (AVN). This holds the electrical impulses, causing a delay, allowing the atrium to fully contract before ventricle systole occurs.

Question 3

the heart muscle

Answer 3

• heart muscle is known as cardiac muscle
• myogenic, so its contraction is initiated from within the muscle itself, rather than by nervous impulses from outside (neurogenic)
• SA node within the upper lateral wall of the right atrium. It is from here that the initial stimulus for contraction originates. The SA node has a basic rhythm of stimulation that determines the beat of the heart

Question 4

How does the heart achieve complete ventricular emptying

Answer 4

• there is a problem as the wave of depolarisation coming from the AVN is at the top of the ventricle. Blood needs to be squeezed from the bottom of the ventricle to the top/ out of the artery’s to allow for complete ventricular emptying
• bundle of His conducts impulses to the base of the heart. The bundle branches into smaller fibres of Purkyne tissue which conveys and then releases the electrical impulses, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards
• wave of electrical impulses passes over both ventricles at the same time, through the Purkyne fibres

Question 5

modifying the resting heart rate

Answer 5

• resting heart rate of a typical human adult around 70 beats per minute. Essential that this rate can be altered to meet varying demands and needs for oxygen.
• changes to heart rate are controlled by a region of the brain called the medulla oblongata, which contains the cardiovascular centre. Has two centres concerned with heart rate:
• centre that increases the heart rate- linked to the sinoatrial node by the sympathetic nervous system
• centre that decreases the heart rate, linked to the sinoatrial node by the parasympathetic nervous system
• baro receptors (monitor blood pressure) and chemo receptors (monitor blood pH/02 conc/ CO2 conc) are both found in the carotid bodies (in the carotid artery) and in the aortic arch

Question 6

control by chemoreceptors

Answer 6

found in walls of carotid artery/ in the carotid body or in the aortic arch. Sensitive to changes in pH of blood/ O2 conc/ CO2 conc
-if a person begins to exercise, more CO2 will be produced, reducing pH and therefore increasing H+ conc, causing more H2C03
- chemoreceptors detect this change in pH
- this creates an increase in the frequency of action potentials which travel along the sensory neurone to the cardiovascular centre in the medulla oblongata
- this then increases the frequency of action potentials sent along the sympathetic nerve which are stimulated in the cardiovascular system. The frequency of action potentials along the vagus nerve therefore decreases, as the vagus and sympathetic neve are antagonistic pairs
- this causes the SAN to cause the heart rate to increase, as electrical impulses sent along the heart have increased
- the increased blood flow that this causes leads to more carbon dioxide being removed by the lungs and so the carbon dioxide concentration of the blood returns to normal
- as a consequence the pH of the blood rises to normal and the chemoreceptors in the carotid body and aortic arch reduces the frequency of action potentials to the medulla oblongata
- the medulla oblongata then reduces the frequency of impulses to the SAN, which therefore leads to a reduction in the heart rate

Question 7

control by pressure receptors

Answer 7

Baro-receptors are also found in the aortic arch and carotid bodies
- baro-receptors will detect an increase in blood pressure.
-This increases the frequency of action potentials sent along the sensory neurone to the cardiovascular centre in the brain, which is the medullar oblongata
- stimulates an increase in the frequency of action potentials sent along the vagus nerve via the parasympathetic nervous system. This may decrease the frequency of action potentials along the sympathetic nerve
- causes the SAN in the heart to cause the heart rate to decrease and therefore cause less electrical impulses to be sent along the heart

If baro receptors detect a decrease in blood pressure- pressure receptors transmit more nervous impulses along sensory neurones to the medulla oblongata. This increases the frequency of impulses sent down the sympathetic nerves to the SAN, which increases the rate at which the heart beats

Question 8

features of sensory reception as illustrated by Pacinian corpuscle

Answer 8

Pacinian corpiuscles respond to changes in mechanical pressure. As with all sensory receptors, a Pacinian corpuscle:
- is specific to a single type of stimulus. in this case responds only to mechanical pressure and wont respond to other stimuli eg heat, light or sound
- produces a generator potential by acting as a transducer. All stimuli involve a change in some form of energy. Role of the transducer to convert the change in form of energy by the stimulis into a form, eg nervous impulses, that can be understoood by the body, so it can convert/ transduce one form of energy into another.
- receptors in the nervous system convert the energy of the stimulus into a nervous impulse known as a generator potential

How do receptors work?
- detect a stimulus and produce a generator potential that usually leads to a depolarisation. The size of the generator potential is related to stimulus intensity. When threshold reached, an action potential will occur in the neurone

Question 9

structure and function of a Pacinian corpuscle

Answer 9

• responds to mechanical stimuli eg pressure. Occur deep in the skin and are most abundant on fingers, soles of feet and external genitalia, also occur in ligaments and tendons.
• the single sensory neurone of a Pacinian corpuscle is at the centre of layers of tissue, each seperated by a gel. This sensory neurone has a special type of sodium channel in its plasma membrane called stretch-mediated sodium chanells

How does it work?
1. In its normal(resting) state the strech-mediated sodium channels of the membrane around the neurone of a Pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the neurone of the Pacinian corpuscle has a resting potential
2. When pressure is applied to the Pacinian corpuscle, it is deformed and the membrane around its neurones becomes stretched. This widens the stretch-mediated sodium channels in the membrane and sodium ions diffuse into the neurone
3. The influx of sodium ions changes the potential of the membrane, becomes depolarised, thereby producing a generator potential
4. The generator potential in turn created an action potential that passes along the neurone, then via other neurones to the CNS

The greater the pressure, the more sodium channels open

Question 10

structure of the eye

Answer 10

1. Cornea. Transparent to admit light to anterior chamber. Most light refraction occurs at the cornea
2. Iris. Pigmented muscular structure that controls the entry of light via the pupil. When pupil constricts- radial muscles relax and circular muscles contract
3. Pupil. Circular opening that admits light into the lens
4. Lens. Allows light to enter the eye and is responsible for the refraction necessary to complete the fine focusing of an image on the retina
5. Optic nerve. Sensory neurone that carries impulses between the eye and the brain
6. Retina. Contains light-sensitive cells, rods and cones, and a series of neurones which enhance image formation and transmit action potentials to the optic nerve
7. Blind spot. This region contains no light-sensitive cells and thus an image falling on this area cannot be perceived.
8. Fovea/ yellow spot. Region in which only cones are found so the area with the greatest visual acuity

Question 11

photo-receptors

Answer 11

Photo receptors found in the eye are rods and cones.
Rods- periphery of the retina (covers all the retina other than the blind spot and fovea). These are sensitive to light intensity
cones- found in the fovea. Sensitive to different wavelengths of light (colour)

• light is focused by then lens on the part of the retina opposite the pupil called the fovea. This fovea receives the highest intensity of light so this is where cones, but not rod cells are found. The concentration of cones diminishes further away from the fovea. At the peripheries of the retina, where light intensity is at its lowest, only rod cells are found

Question 12

convergence

Answer 12

• Rods display convergence, as multiple rods share synaptic connections with one bipolar cell. Cones don’t display this as each cone has an individual synaptic connection with an individual bipolar cell, leading to one sensory neurone per cone

Question 13

characteristics of rods and cones

Answer 13

• rod cells contain rhodopsin
• cones cells contain iodopsin
  The breakdown of optical pigments results in a generator potential being produced. The pigments within the receptors are broken down by different conditions
• Rhodopsin within rods breaks down in dim light, into opsin and retinal, after a photon of light hits it,. The fact it is split leads to the generator potential. When there is no light, rhodopsin reforms from these products.
• Iodopsin breaks down in bright light only

Cones: located in fovea, high resolution image, high acuity image, coloured image, trichromatic vison- three different types of cones where each one is sensitive to each primary colour- red sensitive cones, green-sensitive cones and blue-sensitive cones,
,low sensitivity to light

Rods: located in periphery, low resolution, low acuity image, monochromatic vision- only one type of rod that sees in black and white- also a small amount of cones spread throughout rest of the retina, high sensitivity to light- can produce an image at low light intensities, more rods than cones in the retina

Sensitivity to light refers to the amount of light required to stimulate the receptor
- the combined effect of all three pigments allows humans to observe all the other colours that are visible on the spectrum

Question 14

visual acuity

Answer 14

• acuity is the ability to distinguish two separate points, otherwise known as ‘visual clarity’
• receptors that are hit by light rays become stimulated and those that are not hit by light rays remain unstimulated
• once a receptor is stimulated it can send impulses to the brain
• the brain is able to interpret the pattern of impulses to form an image
  -There is no direct connection between rods and cones and the central nervous system
  -There are synapses connecting the rods and cones to bipolar neurones
  -The bipolar neurones connect to ganglion cells via synapses
  -The ganglion cells have axons that extend to the optic nerve which is directly connected to the brain
  -Due to the high number of receptors on the retina, it is not possible for there to be individual connections between each receptor and the brain
  -The way that rods and cones are connected to the optic nerve affects visual acuity
  -Visual acuity is essentially the resolution or amount of detail that is perceived in an image
  -It is measured by how far apart two spots of light need to be in order to be seen separately

Question 15

rod cells and visual acuity

Answer 15

• multiple rod cells synapse with a single bipolar cell. This is called convergence. Multiple bipolar cells synapse with a single ganglion cell
• the brain isnt able to interpret which impulses are sent by specific rods
• if multiple rods are connected to the same bipolar cell , only one impulse from the bipolar cell is sent
• therefore the brain receives general, NOT SPECIFIC, understanding of the fields of vison that are light or dark

Question 16

cones cells and visual acuity

Answer 16

• cones provide higher visual acuity as a single cone cell synapses with a single bipolar cell
• a single 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
• these leads to accurate depiction of the object in front of them
• as cone cells detect only one of three colours, 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 17

cones and sensitivity to light

Answer 17

• low intensity stimulus means low generator potential
• isnt enough to reach threshold so action potential wont be stimulated along the bipolar cells to the visual cortex and no image will form

Question 18

rods and sensitivity

Answer 18

• low light stimulus means low generator potential. Due to convergence, the individual generator potentials add up. This causes spacial summation to occur in the bipolar cells, which means there is enough generator potential to reach threshold and so an action potential can be stimulated. This results in a blurred, low resolution image

Question 19

desensitising cones

Answer 19

• staring at one colour can desensitise cones. This means when you then stare at white light, the object appears but in a different colour. This effect will not last very long
• eg if staring at yellow object for long enough to desensitise the cones, object will appear blue. As red and green light-sensitive cones are desensitised

Question 20

skeletal muscles

Answer 20

• muscles are effector organs that respond to nervous stimulation by contracting and so bring about movement. Types of muscle- cardiac, smooth muscle and skeletal muscle.
  Skeletal muscle acts under voluntary, conscious control
• muscles act in antagonistic pairs against an incompressible skeleton to create movement. This can be automatic as part of a reflex response or controlled by conscious thought

Question 21

structure of skeletal muscles

Answer 21

-bundles of muscle fibres are found within the muscle (called fascicle)
- these single muscle fibres (fascicle) are made up of many myofibrils. There are millions of myofibrils (muscle fibres) within the muscle
- if the cells of muscles were joined together from the end of one cell to another, the point between cells would be a point of weakness
- so instead the separate cells have become fused together into muscle fibres. These muscle fibres share nuclei and also cytoplasm, called sarcoplasm, which is mostly found around the circumference of the fibre.
- within the sarcoplasm is a large concentration of mitochondria, to generate ATP, and endoplasmic reticulum
- the membrane of these bundles of myofibrils is called the sarcolemma
- and the endoplasmic reticulum is called the sarcoplasmic reticulum (SR). This acts as an intra-cellular store of Ca2+. Membranes of the SR contain protein pumps that transport calcium ions into the lumen of the sarcoplasmic reticulum
- the sarcolemma has many deep tube-like projections that fold in from its outer surface. These are known as T-tubules and run close to the sarcoplasmic reticulum
- membranes of the SR contain protein pumps that transport calcium ions into the lumen of the sarcoplasmic reticulum

Question 22

myofibrils

Answer 22

• myofibrils are located in the sarcoplasm. Myofibrils contain lots of sarcomeres
  Each myofibril is made up of two types of protein filament:
• myosin- thick filaments
• actin- thin filaments

Myofibrils appear striped due to their alternating light-coloured and dark-coloured bands.
-The light bands are called I bands. They appear lighter because the thick and thin filaments don’t overlap in this region, so only the thin filaments are present.
-The dark bands are called A bands. They appear darker because they contain the thick filaments myosin, and the overlap of actin and myosin
- at the centre of each A band is the H zone, which is slightly lighter than regions contain both actin and myosin, as the H zone contains only actin.
- M line is the middle point of the myosin. It provides attachment for myosin filaments
- the Z lines indicate the parameters of ONE sarcomere. Provides attachment for actin filaments

Question 23

slow twitch muscle fibres

Answer 23

• Contract more slowly than fast-twitch fibres and provide less powerful contractions but over a longer period.
• They are therefore adapted to endurance work, eg running a marathon. as they fatigue less quickly due to reduced lactate formation
• mainly aerobic respiration, in order to avoid a build-up of lactic acid which would cause them to function less effectively and prevent long-duration contraction
• large store of myoglobin- stores oxygen- accounts for the red colour of slow-twitch fibres
• rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration (highly vascularised)
• numerous mitochondria to produce ATP
• small amounts of glycogen present
• many found in calf muscles

Question 24

fast-twitch muscle fibres

Answer 24

• contract rapidly. The myosin heads bind and unbind from the actin-binding sites five times faster than slow muscle fibres. Their rapid contraction-relaxation cycle means they need large amounts of Ca2+ to stimulate contraction
• rely on anaerobic respiration for ATP supply, as produces ATP rapidly. So have higher concentration of enzymes involved in anaerobic respiration
• suited to short bursts of high-intensity activity as they fatigue quickly due to the lactate produced from anaerobic respiration. So short acting but more powerful contractions
• less vascularised, so fewer capillaries. This means they have quite a slow supply of oxygen and glucose for aerobic respiration
• low amounts of myoglobin are present. This is because respiration anaerobic so dont need oxygen store
• higher levels of phosphocreatine present- a molecule that can rapidly regenerate ATP from ADP in anaerobic conditions so can provide energy for muscle contraction
• higher concentration of glycogen- as need to quickly convert glucose into energy
• many found in biceps

Question 25

neuromuscular junction

Answer 25

- point where a motor neurone meets a skeletal muscle fibre. - as rapid and coordinated muscle contraction is frequently essential for survival there are many neuromuscular junctions spread throughout the muscle. This ensures that contraction of a muscle is rapid and powerful when it is simultaneously stimulated by action potentials. All muscle fibres supplied by a single motor neurone act together as a single functional unit and are known as a motor unit - this arrangement gives control over the force that the muscle exerts. If only slight force is needed, only a few units are stimulated but if a greater force is required, a larger number of units are stimulated - when a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine. The acetylcholine diffuses to the postsynaptic membrane (which is the sarcolemma), across the neuromuscular junction - it binds to receptors on the sarcolemma, stimulating Na+ channels to open, allowing sodium ions to diffuse in. This depolarises the sarcolemma, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre - these action potentials cause voltage-gated calcium ion channels proteins in the membranes of the SR to open. - this causes calcium ions to diffuse out of the sarcoplasmic reticulum into the sarcoplasm surrounding the myofibrils - calcium ions then bind to troponin molecules, stimulating them to change shape - this stimulates the process of muscle contraction - the acetylcholine is broken down by acetylcholinesterase to ensure the muscle isn't over-stimulated. This results in choline and ethanoic acid diffusing back into the neurone, where they are recombined to form acetylcholine using energy provided by the mitochondria found there - are different to a cholinergic synapses as can only be excitatory, not inhibitory, and are found between motor neurones and muscles, not between two neurones. Also only involves motor neurones but motor, sensory and intermediate neurones may be involved in a cholinergic synapse. In a neuromuscular junction the action potential ends here, but in a cholinergic synapse a new action potential may be passed along the post-synaptic neurone Both use acetylcholine as a neurotransmitter, and both are stimulated by action potential travelling along the pre-synaptic neurone

Question 26

structure of myosin

Answer 26

- fibrous protein molecules with a globular head - the fibrous part of the myosin molecule 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 - the globular heads have an ATP binding sight and an actin binding site - a myosin filament contains many myosin molecules with 3D head groups pointing out. This allows for each myosin filament to make multiple connections to multiple actin molecules

Question 27

actin structure

Answer 27

- these are 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. This is to cover the myosin binding sites at rest, so prevents the contraction of muscles occurring - troponin is also attached to the actin chains at regular intervals. This contains a Ca2+ binding site, for when action potentials have been fired

Question 28

muscle contraction

Answer 28

- muscles cause movement by contracting - muscle contraction is explained by the sliding filament theory. This is where myosin and actin filaments slide over one another to make the sarcomeres contract- as the myofilaments themselves don't contract - simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract. The sarcomeres then return to their original length as the muscle relaxes - when contracting the actin slides over the myosin. This causes the I band to decrease, as the region of actin only reduces. This causes the H zone to decrease, as the area with myosin only decreases. The A band stays the same, as although actin is moving, the region with only myosin stays the same, as the myosin doesnt move/slide - the sarcomeres within myofibrils shorten, as the Z discs are pulled closer together

Question 29

how does the sliding filament theory work

Answer 29

- an action potential arrives at the neuromuscular junction - stimulates Ca2+ to be released from the SR - calcium ions bind to troponin molecules, stimulating them to change shape - this causes troponin and tropomyosin proteins to change shape on the actin filaments, so the myosin binding sites are now exposed, as the tropomyosin is pulled away/ **MOVES** from the myosin binding sites - this causes the globular heads of the myosin molecules to bind with these sites, forming cross-bridges between the two types of filament (actin-myosin cross bridge). This occurs whilst ADP and Pi are still bound to the myosin heads - the formation of the **cross-bridges** causes the myosin heads to spontaneously bend, due to the tension created by the angle from this cross-bridge. (releasing ADP and inorganic phosphate, from the myosin head), pulling the actin filaments towards the centre of the sarcomere and causing 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 - the enzyme ATP hydrolase, hydrolyses ATP into ADP and inorganic phosphate which causes the myosin heads to move back to their original positions (known as the recovery stroke). ADP and Pi remain bound to myosin head until next power stroke - the myosin heads are then able to bind to new binding sites on the actin filaments, closer to the Z disc - the myosin heads move again, pulling the actin filaments even closer to the centre of the sarcomere, causing the sarcomere to shorten once more and pulling the Z-discs closer together - ATP binds to the myosin heads one more in order for them to detach again - as long as troponin and tropomyosin aren't blocking the myosin-binding sites and the muscle has a supply of ATP, this process repeats until the muscle is fully contracted (explains why rigor mortis occurs, as muscles remain contracted as no ATP being produced to release the myosin head from the actin)

Question 30

the role of ATP and phosphocreatine

Answer 30

- a supply of ATP is required for muscle contraction. Energy is needed for the return movement of myosin heads that causes the actin filaments to slide. The return of calcium ions back into the sarcoplasmic reticulum also occurs via active transport - resting muscles have a small amount of ATP stored that will only last for 3/ 4 seconds of intense exercise - the 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 still takes 10 seconds before any ATP begins to be produced - 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, to form 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 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 31

muscle relaxation

Answer 31

- when nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP - this reabsorption of the calcium ions allows tropomyosin to block the actin filament again - myosin heads are now unable to bind to actin filaments and contraction ceases, that is, the muscle relaxes - in this state force from antagonistic muscles can pull actin filaments out from between myosin

Question 32

stimulus and response

Answer 32

- a stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism. The ability to respond to stimuli is a characteristic of life and increases the chance of survival of an organism - there is always selection pressure favouring organisms with more appropriate responses - stimuli are detected by receptors, and these are SPECIFIC to one type of stimulus. A coordinator formulates a suitable response to a stimulus, and the effector produces the response

Question 33

Taxis

Answer 33

A taxis is a simple response whose direction is determined by the direction of the stimulus. As a result, a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one - taxes are classified according to whether the movement is away from the stimulus (negative taxis) or towards the stimulus (positive taxis) and also by the nature of the stimulus - eg a single-celled algae will move towards light, positive phototaxis. This increases their chances of survival, since being photosynthetic they require light to manufacture their food - some bacterial will move towards a region where glucose is more highly concentrated (positive chemotaxis)

Question 34

kinesis

Answer 34

A kinesis is a form of response in which the organism doesnt move towards or away from the stimulus. Instead it changes the speed at which it moves and the rate at which it changes direction. If an organism crosses a sharp dividing line between a favourable and unfavourable environment, its rate of turning increases. This raises the chances of a quick return to its favourable environment. However, if it moves a considerable distance into an unfavourable environment its rate of turning may slowly decrease so that is moves in a long straight line, before it very sharply terns. This type of response teds to bring the organism into a new region with favourable conditions. It is important when the stimulus is less directional, eg humidity and temperature dont always produce a clear gradient from one extreme to another - an example of kinesis occurs in woodlice. Woodlice lose water from their bodies in dry conditions. When they move from a damp area into a dry one, they move more rapidly and change direction more often. This increases their chance of moving back into the damp area. Once back in the damp area, they slow down and change direction ,less often so they are more likely to stat in the damp area

Question 35

tropisms

Answer 35

A tropism is the growth of part of a plant in response to a directional stimulus. - eg plant shoots grow towards the light (positive phototropism) - plant roots grow away from the light (negative phototropism) and towards gravity (positive gravitropism). This increases the probability that roots will grow into the soil, where they are able to better absorb water and minerals

Question 36

plant growth factors

Answer 36

Plants respond to: light, gravity and water - plant responses to external stimuli involve plant growth factors. - they exert their influence by affecting growth and may be made by cells located throughout the plant, rather than in particular organs - unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a distant target organ - they are produced in small quantities. eg indoleacetic acid (IAA), which belongs to a group of substances called auxins. IAA controls plant cell elongation

Question 37

control of tropisms by IAA

Answer 37

phototropism in flowering plants: 1. Cells in the tip of the shoot produce IAA, which is then transported down the shoot 2. The IAA is initally transported evenly throughout all regions as it begins to move down the shoot 3. Light causes the movment of IAA from the light side to the shaded side of the shoot 4. A greater concentration of IAA builds up on the shaded side of the shoot than on the light side 5. As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more 6. The shaded side of the shoot elongaates faster than the light side, causing the shoot tip to bend towards the light IAA also controls the bending of roots in response to light. Where IAA increases cell elongation in shoots, it **inhibits cell elongation in roots.** As a result, in roots the elongation of cells is greater on the light side than on the shaded side so roots bend away from the light, so are negatively phototrophic

Question 38

gravitropism in flowering plants

Answer 38

- in many plants, gravity leads to a change in the distribution of the carrier proteins that export IAA from cells 1. Cells in the tip of the root produce IAA, which is then transported along the root 2. The IAA is initially transported to all sides of the root 3. Gravity influences the movement of IAA from the upper side to the lower side of the root 4. A greater concentration of IAA builds up on the lower side of the root than on the upper side 5. As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side 6. The realtively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity In shoots, the greater the concentration of IAA on the lower side increases cell elongation and causes this side to elongate more than the upper side. As a result, shoots grow upwards away from the force of gravity

Question 39

Role of IAA in elongation growth

Answer 39

The transport of IAA is in one direction, namly away from the tip of shoots and roots where it is produced. IAA has a number of effects on plant cells including increasing the plasticity (ability to stretch) of their cell walls. The response only occurs on young cell walls where cells are able to elongate. As the cells mature they develop greater rigidity- therfore older parts of the shoot/root will not be able to respond. The proposed explination of how IAA increases the plasticity of cells is called the acid growth hypothesis. It involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall, causing the cell wall to become more plastic allowing the cell to elongate by expansion - The elongation of cells on one side only of a stem or root can lead to them bending. This is the means by which plants respond relatively quickly to environmental stimuli like light and gravity. These responses can be explained in terms of the stimuli causing uneven distribution of IAA, as it moves away from the tip of the stem or root