Topic 6: Environmental Changes Flashcards
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define stimulus
- a detectable change in the environment
- which can be detected by receptors
- Organisms increase their chance of survival by responding to stimmuli via different response mechanisms.
give examples of simple responses to stimuli
- taxes
- kineses
What are taxis?
give examples
- a directional response to a stimulus
- the whole organism moves directly away from or toward the stimulus
- move towards: positive taxis
- moves away: negative taxis
worms: show negative phototaxis to prevent dehydration and predators.
bacteria: show positive chemotaxis
what are kineses?
example
- non directional response to a stimulus
- when an organism** changes the speed of movement** and the rate it changes direction
- the rate of movement is impacted by the intensity of the stimulus
- ## If an organism moves from an area where there are benefical stimuli to an area with harmful sitmuli , its kinesis response will be to increase the rate it changes direction** to return to the favourable** conditions quickly.
woodlice respond to water and must be in damp areas to prevent water loss so in a dry area they would turn rapidly to increase the probabilit that it will end up back in the damp area
Investigating taxes and kineses
- Choice chambers and mazes are common pieces of apparatus that are used
define tropism
- term given to when plants respond, via growth, to stimuli.
- it can be positive or negative. Responding to light, gravity and wate.
- Phototropism - growth response to light
- Gravitropism - growth response to gravity
Give one similarity and one difference between a taxis and a tropism.
(2)
-
Similarity − directional response (to a stimulus) / movement towards / away from a stimulus;
2.** Difference** − taxis (whole) organism moves and tropism a growth (response).
- Taxis occurs in animals / motile organisms and tropism occurs in plants
What is the role of growth factors in plants?
- e.g. indoleacetic acid (IAA)
- Growth factors in plants control and regulate growth responses to environmental stimuli.
- in roots high conc of IAA inhibits cell elongation
- in shoots, high conc of IAA stimulates cell elongation
Describe Indoleacetic acid’s role as a growth factor.
- IAA is synthesised at the tips of roots and shoots i.e. in the meristems and it affects the elongation of cells in a plant.
- it binds to the protein receptors on the cell membranes
- lowers the pH causing cell wall to loosen and and allows the cells to be more easily stretched when the turgor of cell increases.
Describe how IAA causes elongation of cells
- bind to a receptor protein on the cell surface membrane
- stimulates ATPase to pump hydrogen ions from the cytoplasm into the cell wall
- acidifies the cell wall (lowers the pH of the cell wall)
- This activates expansins, which loosen the bonds between cellulose microfibrils
- At the same time, potassium ion channels are stimulated to open
- This leads to an increase in K+ concentration in the cytoplasm, decreasing the water potential
- This causes the cell to absorb water by osmosis (water enters the cell through aquaporins) which is then stored in the vacuole
- This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
- The cell elongates
Describe phototropism in a plant
in shoots and in roots
- cells in the tip of shoot to produce IAA.
- IAA diffuses down shoot/root (evenly intially)
- IAA moves to the shaded side of shoot/root
- In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation/
- so shoots bend towards light whereas roots bend towards light
root atipical meristem and shoot
Explain gravitropism in flowering plants (4)
- Cells in tip of shoot/ root produce IAA.
- IAA diffuses down shoot/root evenly initally
- IAA moves to lower side of shoot/root (so conc increases)
- In shoots , this stimulates cell elongation whereas in roots this inhibits cell elongation.
- So shoots bend away from gravity whereas roots bend towards gravity
Use your knowledge of indoleacetic acid (IAA) to explain the growth curvature shown in Figure 1. (3)
- Tip produces IAA;
-
IAA diffuses (into shoot);
Accept auxin for IAA.
Accept IAA diffuses down. - (More) elongation of cells on one side (than other);
Accept (more) elongation of cells on left side.
Reject any reference to shaded/dark side or away
from light
Using the procedure in Figure 2 and the calibration curve in Figure 3, describe how you could compare the IAA concentration in shoot tips from
two different plant species.
In your answer you should refer to all the variables that should be controlled to produce a valid comparison (5)
- Size of shoot/tip;
- Number of shoot tips;
- Size/type of agar (block);**
Accept ‘amount of agar’. - (Shoots) at same stage of growth/development;
Accept (Shoots/plants) are same age. - Time (period) tips kept on agar
OR
Time (period shoots) kept in dark; - Temperature;
- (Repeat several times and) calculate a mean;
- Compare/read degree of curvature (on calibration curve) to determine (IAA) concentration
OR
Higher the degree of curvature the higher the IAA concentration;
5 max
Mark points 1 to 6 = max 3.
Ignore pH, species, carbon dioxide, humidity,
nutrients, water and light.
Explain the protective effecct of a simple reflex (3)
- rapid as only 3 neurones and few synapses
- Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt.
- protects from harmful stimuli e.g. escape prefators/prevents damage to body tissues
Suggest 2 advantages of simple reflexes (2)
- Rapid;
- Protect against damage to body tissues;
- Do not have to be learnt;
- Help escape from predators;
- Enable homeostatic control.
max 2 points
what are three main types of neurones and their roles
-
Sensory neurones carry impulses from receptors to the Central Nervous System (CNS - the brain or spinal cord)
2. Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones - Motor neurones carry impulses from the CNS to effectors (muscles or glands)
What is a reflex arc?
- A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
Describe a reflex response
example of a pin
- A pin (the stimulus) is detected by a pain receptor in the skin.
- The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
- Electrical impulses are passed on to relay neurone in the spinal cord
- The relay neurone connects to the motor neurone and passes the impulses on
- The motor neurone carries the impulses to the muscle in the leg (the effector)
- The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)
describe the structure of the pacinian corpuscle
draw it
include:
- lamellae
- stretch mediated NA+ ion channel (closed)
- Gel
- Sensory neurone axon
- myelin sheath
- sensory neuron ending.
What does the Pacinian Corpuscle do and where is it found?
- Pacinian corpuscles are a type of receptor found deep in the skin
- They are present in the skin of fingers, soles of the feet as well as in joints, tendons and ligaments.
- They respond to changes in pressure
- When these receptors are stimulated by pressure on the skin it leads to the establishment of a generator potential
Describe how a generator potential is established in a Pacinian corpuscle (4)
- Mechanical stimulus e.g. pressure deforms lamellae and stretch mediated channels (Na+) channels
- So Na+ channels in membrane open and Na+ diffuse into sensory neurone. Greater pressure causes more NA+ channels to open and more Na+ to enter.
- This causes depolarisation, leading to a generator potential.
- If generator potential reaches threshold it triggers an action potential.
what is the eye and the main receptor cells inside of it?
- The eye **is a sense organ **containing receptors sensitive to light intensity and colour.
- contains:
- Rod cells which are sensitive to light intensity
- Cone cells which are sensitive to different wavelengths of visible light (colour)
what is the structure of the eye?
label it:
- cornea
- iris
- lens
- retina
- optic nerve
- pupil
Name and describe the function of strcutures in the eye
cornea: transparent lens that refracts light as it enters the eye
iris: controls how much light enters the pupil
lens: transparent disc that can change shape to focus light onto the retina
retina: contains light receptor cells - rods which detect light intensity and cones that detect colour.
optic nerve: sensory neurone that carries impulses between the eye and the brain.
pupil: hole that allows light to enter the eye
what is the role of optical pigments in the eye
- Rod cells contain rhodopsin
- Cone 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
- Iodopsin within cones breaks down in bright light only
Explain the difference in sensitivity to light for rods and cones in the retina
- Rod cells are more sensitive to light intensity and are primarily responsible for vision in low-light conditions.
- cone cells are sensitive to different wavelengths of visible light and enable color vision.
- Rods allow humans to distinguish between light and dark, while cones allow the perception of color in brighter light.
define visual acuity
Explain how the structure of rod and cone cells contributes to differences in visual acuity.
- Rods give lower visual acuity. This is because several rods are connected to a single neurone. So several rods send a single set of impulses, to the brain.
- Cone cells give a higher visual acuity, each cone is connected to a single neurone. Cones send seperate sets of impulses to brain, so can distinguish between 2 seperate sources of light.
Why do rod cells enable better vision in dim light compared to cone cells?
- Rod cells contain the pigment rhodopsin, which breaks down in dim light, making them highly sensitive to low light levels.
- Cone cells contain iodopsin, which only breaks down in bright light, making them less effective in dim conditions.
Explain the difference in sensitivity to colour for rods and cones in the retina.
- rods allow monochromatic vision: 1 type of rod/1 pigment.
- Cone cells contain three different types of iodopsin pigments that are sensitive to specific wavelengths of light—red, blue, and green. These pigments allow the brain to interpret colors.
What is summation, and how does it enhance vision in low-light conditions?
- a single stimulated rod is unlikely to produce a large enough generator potential to stimulate the bipolar cell for the conduction of nerve impulses.
- Thus when a group of rods are stimulated at the same time, the combined generator potentials ( spatial summation) are sufficient to reach the threshold.
- To generate an action potential.
Suggest and explain how the interaction between the muscles labelled in the diagram above could cause the pupil to constrict (narrow). (2)
muscles labelled: radial , pupil, iris , circular muscle
- Circular muscle contracts;
- Radial muscle relaxes;
The fovea of the eye of an eagle has a high density of cones. An eagle focuses the image
of its prey onto the fovea.
Explain how the fovea enables an eagle to see its prey in detail. (3)
- High (visual) acuity;
- (Each) cone is connected to a single neurone;
Accept no retinal convergence.
Accept ‘bipolar/nerve cell’ for neurone. - (Cones send) separate (sets of) impulses to brain;
The retina of an owl has a high density of rod cells.
Explain how this enables an owl to hunt its prey at night. (3)
Do not refer to rhodopsin in your answer
- High (visual) sensitivity;
Accept retinal convergence - Several rods connected to a single neurone;
Accept ‘bipolar/nerve cell’ for neurone - Enough (neuro)transmitter to reach/overcome threshold
OR
Spatial summation to reach/overcome threshold; more for ‘several’
Explain what causes vision using the fovea.
(i) to be in colour;
(1)
(ii) to have high visual acuity
(i) (Three) different types of (cone) cells / types 6 and 7 sensitive to different wavelengths / different frequencies / different colours;
(ii) Impulses along separate neurone from each receptor cell / each receptor cell connects to separate neurone;
define myogenic and its relation to the heart
- it can contract and relax without recieiving electrical impulses from nerves.
Explain the role of the sinoatrial node in a heartbeat.
- The sinoatrial node (SAN) is a group of cells in the wall of the right atrium. The SAN initiates a wave of depolarisation that causes the atria to contract
Explain the role of the non-conducting tissue Annulus fibrosus in a heartbeat.
- prevents the excitation from spreading to the ventricles and so this ensures that atria and ventricles don’t contract at the same time.
Explain the role of the atrioventricular node in a heartbeat
- The Atrioventricular node then sends the wave of excitation to the ventricles after a short delay of (around 0.1 - 0.2 seconds), ensuring that the atria have time to empty their blood into the ventricles.
Explain the role of the Purkyne fibres in a hearbeat ( check model)
- conduct the excitation down the septum of the heart and to the apex, before the excitation is carried upwards in the walls of the ventricles.
- the blood contracts from its base and blood is pushed upwards and outwards.
Describe the myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity
- SAN acts as a pacemaker - senes regular waves of electrical activity across atria. Causing atria to contract simultaneously.
- Non-conducting tissue between atria/ventricles prevents imupulse passing directly to ventricles. Preventing immediate contraction of ventricles. This delay allows blood movement.
- Waves of electircala ctivity reach atrioventricular which delays impulse. Allowing atria to fully contract and empty before ventricles contract.
- AVN sends waves of electrical activity down bundle of His, conducting wave between ventricles to apex when it branches into Purkyne tissue.
- Causing ventricles to contract simultaneously from the base up.
what is the medulla responsible for ?
- the medulla is locted at the base of the brain near the top of the spinal cord.
- it is the cardioregulatory centre in the brain regulates heart rate)
- it is made up of two distinct parts ; the accelatory centre ( speeds up the heart), the inhibitory centre. slows it down).
- These are connected to the SAN via nerves. They make up the autonomic nervous system.
What are autonomic nervous system?
- system that controls involuntary actions of glands and muscles.
- 2 subdivisions: sympathetic + parasympathetic.
- Sympathetic: responsible for fight or flight. Parasympathetic: rest and digest.
Name the receptors involved in changing heart rate and state their location.
- chemoreceptors and pressure receptors are located in the aorta and carotid arteries.
- baroreceptors: detect changes in blood pressure ; carotid body.
- chemoreceptors: detect changes in ph ( due to increase in CO2 concentration. : Carotid body and aortic body.
How does the body respond to an increase in blood pressure?
- Baroreceptors detect rise in blood pressure.
- Send impulses to the cardioinhibitory centre in the medulla oblongata
- which send more frequent impulses to SAN down vagus nerve via the parasympathetic nervous system
- This stimulates the release of acetylcholine so cardiac muscle contracts frequently.
- So heart rate decreases /increases
How does the body respond to a decrease in blood pressure?
- Baroreceptors detect a decrease in blood pressure.
- Then sends more impulses to cardioacceleratory centre in the medulla oblongata.
- More impulses to SAN via sympathetic nervous system
- Stimulates release of noradrenaline which increases heart rate and strength of contraction.
Describe how the body respond to an increase in CO2 concentration?
- Chemoreceptors detect pH decrease
- It sends more impulses to cardioacceleratory centre of medulla oblongata.
- Which send more frequent impulses to SAN along sympathetic neurones.
- So more frequent imulses sent from SAN to and from AVN.
- So cardiac mucle contracts more frequently.
- So heart rate increases.
How does the body respond to a decrease in CO2 concentration?
- Chemoreceptors detect rise in blood pH.
- Send impulses to medulla / cardiac control centre.
- Which send more frequent impulses to SAN along parasympathetic neurones.
- So less frequent impulses sent from SAN and to / from AVN.
- So cardiac muscle contracts less frequently.
- So heart rate decreases
Exercise causes an increase in heart rate. Describe the role of receptors and of the nervous system in this process. (4)
- Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid / fall in pH
OR
Baro / pressure receptors detect rise in blood pressure; - Send impulses to cardiac centre / medulla;
- More impulses to SAN;
- By sympathetic (nervous system for chemoreceptors / CO2)
OR
By parasympathetic (nervous system for baro / pressure receptors / blood pressure);
Reject: ‘messages’, ‘signals’, ‘an impulse’ or an ‘action potential’.
When the heart beats, both ventricles contract at the same time.
Explain how this is coordinated in the heart after initiation of the heartbeat by the SAN. (2)
- Electrical activity only through Bundle of His / AVN;
- Wave of electrical activity passes over / through both ventricles at the same
Describe how a heartbeat is initiated and coordinated. (5)
- SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
Accept excitation - Non-conducting tissue prevents immediate contraction of ventricles / prevents impulses reaching the ventricles;
- AVN delays (impulse) whilst blood leaves atria / ventricles fill;
- (AVN) sends wave of electrical activity / impulses down Bundle of His; Allow Purkyne fibres / tissue
- Causing ventricles to contract from base up;
Explain how the heart muscle and the heart valves maintain a one-way flow of blood from the left atrium to the aorta. (5)
- Atrium has higher pressure than ventricle (due to filling / contraction) causing atrioventricular valves to open;
- Ventricle has higher pressure than atrium (due to filling / contraction) causing atrioventricular valves to close;
- Ventricle has higher pressure than aorta causing semilunar valve to open;
- Higher pressure in aorta than ventricle (as heart relaxes) causing semilunar valve to close;
- (Muscle / atrial / ventricular) contraction causes increase in pressure;
Points 1, 2 and 3 must be comparative: eg higher 3. Allow aortic valve
Increased intensity of exercise leads to an increased heart rate. Explain how. (3)
- (Oxygen / carbon dioxide) detected by chemoreceptors / (pressure) detected by baroreceptors;
- Medulla / cardiac centre involved;
- More impulses to SAN / along sympathetic nerve;
When a wave of electrical activity reaches the AVN, there is a short delay before a new wave leaves the AVN. Explain the importance of this short delay.
Allow atria to empty / contract / ventricles to fill;
Before ventricles contract
Explain how nervous control in a human can cause increased cardiac output during exercise (4)
- Coordination via medulla (of brain) / cardiac centre;
- (Increased) impulses along sympathetic ( / cardiac accelerator) nerve
- To S.A. node / pacemaker;
- More impulses sent from / increased rate of discharge of S.A. node / pacemaker;
Some drugs inhibit the transmission of nerve impulses to the heart. Explain how these drugs reduce high blood pressure.(2)
- inhibit impulses in sympathetic nerves / from cardio-acceleratory centre;
- SAN not stimulated / noradrenaline not released so heart rate lowers / does not increase;
describe the structure of a myelinated motor neurone
direction of nerve impulse –>
Describe resting potential
- inside of axon has a negative relative to outside as more positive ions outside compared to inside.
Explain how a resting potential is established across the axon membrane in a neurone
- Na+/K+ pump actively transports
- 3NA+ out and 2K+ into the axon
- creating an electrochemical gradient with a higher potassium conc and higher sodium conc outside.
- causing a differential membrane permeability: more permeable to K+ and less permeable to NA+ ( closed channels)
Describe how an action potential is generated
- Na + channels open ( membrane permeability to NA+ increases)
- Na+ diffuse into axon down electrochemical gradient causing depolarisation
- If threshold potential reached, an action potential is generated
- As more voltage-gated Na+ channels open ( positive feedback effect)
- So more Na+ diffuse in rapidly
- Voltage-gated Na+ channels close and Voltage gated K+ channels open, K+ diffuse out of axon.
- K+ channels slow to close so there’s a slight overshoot , too many K+ diffuse out
- Resting potential is restored by Na+/K+ pump
Draw and label a graph showing an action potential
Describe the all or nothing principle
- for an action potential to be produced , depolarisation must exceed threshold potential,
- action potentials produced are always same magnitude / size / peak at same potential
- bigger stimuli increase frequency of action potentials
Describe the passage of an action potential along a non-myelinated axon
- action potential passes as a wave of depolarisation
- influx of NA+ in one region to Na+ by causing voltage-gated Na+ channels to open
- causing the adjoining region depolarises
Describe the passage of an action potential along myelinated axons results in nerve impulses
- myelination provides electrical insulation
- depolarisation of axon at nodes of Ranvier only
- resulting in saltatory conduction ( local currents circuits)
- so there is no need for depolarisation along whole length of axon
Suggest how damage to the myelin sheath can lead to slow responses and/or jerky movement
- less/no saltatory conduction : depolarisation occurs along whole length of axon:
- so nerve impulses take longer to reach neuromuscular junction , delay in muscle contraction
- Ions / depolarisation may pass / leak to other neurones
- causing wrong muscle fibres to contract
Describe the nature of the refractory period
- time taken to restore axon to resting potential when no further action potential can be generated.
- As Na+ channels are closed/ inactive/ will not open
Explain the importance of the refractory period
- ensures discrete impulses are produced ( action potentials don’t overlap)
- limits frequency of impulses transmission at a certain intensity ( prevents over reaction to stimulus)
- Higher intensity stimulus causes higher frequency of action potentials
- but only up to certain intensity
- Also ensures action potentials travel in one direction - can’t be propagated in a refractory region
Describe how myelination affects the speed of conductance
- depolarisation at Nodes of Ranvier only: salatatory conduction.
- impulse doesn’t travel / depolarise whole length of axon
Describe how axon diameter affects the speed of conductance
- bigger diameter means less resistance to flow of ions in cytoplasm
Describe how temperature affects the speed of conductance
- increases rate of diffusion of Na+ and K+ as more kinetic energy
- But proteins / enzymes could denatureat a certain temperature.
Describe the structure of a synapse
Wht are cholinergic synapses?
- synapses that use the neurotransmittter acetylcholine (ACh)
Describe transmission across a cholinergic synapse
- Depolarisation of presynaptic membrane causes opening of voltage gated CA2+ channels. Ca2+ diffuse into presynaptic neurone/knob.
- Causing vesicles containing ACh to move and fuse with pre-synaptic membrane. Releasing ACh into the synaptic cleft ( by exocytosis)
- ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane.
- Causing Na+ channels to open. Na+ diffuse into post-synaptic knob causing depolarisation.
- If threshold is met an action potential is initiated.
Explain what happens to acetylcholine after synaptic transmission
- it is hydrolysed to acetylcholinesterase
- products are reasbsorbed by the presynaptic neurone
- to stop overstimulation ( if not removed it would keep binding to recepors , causing depolarisation)
Explain how synapses result in unidirectional nerve impulses
- neurotransmitter only made in/released from pre-synaptic neurone
- receptors only on post-synaptic membrane
Explain summation by synapses
- addition of a number of impulses converging on a single post-synaptic neurone
- causing rapid builduop of neurotransmitter (NT)
- so threshold more likely to be reached to generate an action potentia
Describe spaial summation
- Many presynaptic neurones share one synaptic cleft/ post-synaptic neurone.
- Collectively release sufficient NT to reach threshold to trigger an action potential.
Describe temporal summation
- One pre-synaptic neurone releases neurotransmitter many times over a short time.
- Sufficient NT to reach threshold to trigger an action potential.
Describe inhibition by inhibitory synapses
- inhibitory neurotransmitters hyperpolarise postsynaptic membrane as
- Cl- channels open - Cl- diffuse in
- K+ channels open - K+ diffuse out - More Na+ required for depolarisation
- Reduces likelihood of threshold being met/action potential formation at post-synaptic membrane
Describe the structure of a neuromuscular junction
similar to synapse
- receptors are on muscle fibre instead of postsynaptic membrane and there are more
- muscle fibre forms clefts to store enzymes e.g. acetylcholinesterase to break down neurotransmitter.
Compare transmission across cholinergic synapses and neuromuscular junctions
in both transmission is unidirectional
cholinergic synapse:
- neurone to neurone ( or effectors, glands)
- neurotransmitters can be excitatory or inhibitory
- action potential may be initiated in post synaptic neurone
neuromuscular junction:
- motor neurone to muscle
- always excitatory
- Action potential propogates along sarcolemma down T tubules
Explain the effect of inhibitory drugs on a synapse
- inhibit release of neurotransmitter e.g. prevent opening of calcium ion channels
- block receptors by mimicking shape of neurotransmiter
- leads to fewer action potentials
Explain the effect of stimulatant drugs on a synapse
- similar shape to neurotransmitter
- stimulate release of more neurotransmitter
- inhibit enzyme that breakes down neurotransmitter - Na+ continues to enter
- leading to more action potentials
Describe how mucles work
- work in antagonistic pairs - pull in opposite directions
- one muscle contracts and one muscle relaxes
- skeleton is incompressible so muscle can transmit force to bone
Describe the gross and microscopic structure of skeletal muscle
- made of many bundles of muscle fibre (cells) packaged together
- attached to bones by tendons
- muscle fibres contain:
- sarcolemma ( cell membrane) which folds inwards ( invaginates) to form T tubules
- multiple nuclei
- many myofibrils
- many mitochondria
- sarcoplasm
Describe the ultrastructure of a myofibril
- made of two types of long protein filaments , arranged in parallel:
- myosin - thick filament
- actin - thin filament
- arranged in functional units called sarcomeres
Explain the banding patter to be seen in myofibrils
- I bands: light + thin actin filaments
- A-bands: dark+ thick myosin filaments
- H zone contains only myosin
- Darkest region contains overlapping actin + myosin
Describe muscle contraction with reference to banding patterns
- sarcomeres contract
- H zones get shorter
- I band get shorter
- A band stays the same
- Z lines gert closer
Describe the roles of actin , myosin , calcium ion , and ATP in myofibril contraction
- Depolarisaton spreads down sarcolemma via T tubules causing Ca+ release from sarcoplasmic reticulum , which diffuse to myofibrils.
- Calcium ions bind to tropomyosin , causing it to move - exposing binding sites on actin.
- Allowing myosin head , with ADP attached, to bind to binding sites on actin - forming an actinomyosin crossbridge.
- Myosin heads change angle, pulling actin along myosin , ADP released, using energy from ATP hydrolysis
- New ATP binds to myosin head causing it to detach from binding site
- Hydrolysis of ATP by ( hydrol)ase activated by Ca2+ releases energy for myosin heads to return to original position.
- Myosin reattaches to a different binding site durther along actin. Process is repeated as long as as calcium ion concentration is high
Describe muscle relaxtion
- Ca 2+ actively transported back into the endoplasmic reticulum using energy from ATP.
- Tropomyosin moves back to block myosin binding site on actin again - no actinomyosin cross bridges
Describe the role of phosphocreatine in muscle contraction
- a source of inorganic phosphate - rapidly phosphorylated ADP to regenerate ATP. ADP + phosphocreatine - ATP + creatine
- Runs out after a few seconds - used in short bursts of vigorous exercise
- Anaerobic and alactic
Compare the structure of slow and fast skeletal muscle fibres
slow twitch:
- high concentration of myoglobin - stores oxygen for aerobic respiration
- many mitochondria: high rate of aerobic respiration.
- many capillaries - supply high concentration of oxygen/glucose for aerobic respiration and to prevent build up of lactic acid causing muscle fatigue.
fast twitch:
- low levels of myoglobin
- lots of glycogen - hydrolysed to provide glucose for glycolysis / anaerobic respiration which is inefficient so large quantities of glucose required.
- High concentration of enzymes involved in anaerobic respiration in cytoplasm
- store phosphocreatine
Dopamine is a neurotransmitter released in some synapses in the brain. The transmission of dopamine is similar to that of acetylcholine.
Dopamine stimulates the production of nerve impulses in postsynaptic neurones.
Describe how. (3)
*Do not include in your answer the events leading to the release of dopamine and the events following production of nerve impulses at postsynaptic neurones. *
1. (Dopamine) diffuses across (synapse);
2. Attaches to receptors on postsynaptic membrane;
3. Stimulates entry of sodium ions and depolarisation/action potential;
Accept Na+ for sodium ions
Accept generator potential for action potentia
The release of dopamine can be stimulated by chemicals called endorphins produced in the brain. Endorphins attach to opioid receptors on presynaptic neurones that release dopamine.
Morphine is a drug that has a similar structure to endorphins and can provide pain relief.
Explain how. (2)
- Morphine attaches to opioid receptors;
Reject reference to active site
2. (More) dopamine released (to provide pain relief);
GABA is a neurotransmitter released in some inhibitory synapses in the brain. GABA causes negatively charged chloride ions to enter postsynaptic neurones.
Explain how this inhibits postsynaptic neurones. (3)
- (Inside of postsynaptic) neurone becomes more negative/hyperpolarisation/inhibitory postsynaptic potential;
Ignore K+
Accept -75mV or any value below this as equivalent to more negative /Accept ‘decrease in charge’
2. More sodium ions required (to reach threshold)
OR
Not enough sodium ions enter (to reach threshold);
Accept Na+ for sodium ions
3. For depolarisation/action potential;
Context must convey idea that depolarisation / action potential is less likely
When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob.
Describe how. (3)
- (Nerve impulse / depolarisation of membrane) causes Ca 2+ channel (proteins) to open;
2. Ca 2+ enter by (facilitated) diffusion;
3. Causes (synaptic) vesicles to fuse with (presynaptic) membrane;
*1. Reject carrier proteins
3. Reject ref to release of vesicles
3. Ignore vesicles bind to membrane (but accept merge with) *
This movement of mitochondria happens when nerve impulses arrive at the synapse. Suggest and explain one advantage of the movement of mitochondria towards the presynaptic membrane when nerve impulses arrive at the synapse. (2)
- (Mitochondria) supply (additional) ATP / energy;
2. To move vesicles / for active transport of ions / for myosin to move past actin
OR
Re-synthesis / reabsorption of neurotransmitter / named neurotransmitter;
*1. Reject produces energy
2. Ignore ref. to ATP for opening calcium ion channels/making vesicles fuse with membrane *
(a) Describe the roles of calcium ions and ATP in the contraction of a myofibril. (5)
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).
Explain how the resting potential of –70 mV is maintained in the sensory neurone when no pressure is applied. (2)
1. Membrane more permeable to potassium ions and less permeable to sodium ions;
2. Sodium ions actively transported / pumped out and potassium ions in.
The blink reflex involves synapses. Channel proteins on presynaptic neurones are involved in reflex responses.
Explain how. (3)
1. Allows calcium ions in;
2. At end of presynaptic neurone;
3. Causing release of neurotransmitter;
*1. Accept Ca2+/Ca ions but not Ca/Ca+
2. The idea of the end of the presynaptic neurone must be given e.g. presynaptic knob *
(i) Suggest why the drug used to treat schizophrenia is able to bind to the same receptor as dopamine. (1)
(ii) Suggest why binding of the drug does not lead to production of an action potential. (2)
i) Has similar shape/structure to dopamine
OR Complementary (to binding site on receptor);
ii) 1. (Binding) does not lead to opening of sodium ion channels;
2. (So) no depolarisation / threshold not reached / sodium ions do not diffuse in;
OR
3. Opens chloride ion channels;
4. Causing hyperpolarisation / preventing depolarisation
(a) Damage to the myelin sheaths of neurones can lead to problems controlling the contraction of muscles.
Suggest one reason why. (2)
One suitable suggestion; explained;
E.g.
1. Action potentials travel more slowly / don’t travel;
Accept: fewer / no saltatory movement of potentials
2. So delay in muscle contraction / muscles don’t contract / muscles contract slow(er);
OR
3. Action potentials / depolarisation ‘leaks’ to adjacent neurones;
Accept: neurones not insulated
4. So wrong muscle (fibres) contract.
Cannabinoid receptors are found in the pre-synaptic membrane of neuromuscular junctions. When a cannabinoid binds to its receptor, it closes calcium ion channels.
Suggest how cannabinoids could prevent muscle contraction. (4)
- Prevents influx of calcium ions (into pre-synaptic membrane);
Need idea of moving into pre-synaptic membrane / synaptic knob Accept Ca++ / Ca2+ - (Synaptic) vesicles don’t fuse with membrane / vesicles don’t release neurotransmitter;
Accept vesicles don’t release acetylcholine - Neurotransmitter does not diffuse across synapse / does not bind to receptors (on post-synaptic membrane);
Accept: sarcolemma / muscle membrane for post-synaptic membrane - No action potential / depolarisation (of post-synaptic membrane) / sodium (ion) channels do not open / prevents influx of sodium ions.
Accept Na+
(a) Synapses are unidirectional. Explain how acetylcholine contributes to a synapse being unidirectional. (2)
1. (Acetylcholine) released from / in presynaptic side;
2. Receptors in postsynaptic (side) / binds on postsynaptic (side);
*2. Mark for diffusion only awarded in context of unidirectional movement. *
(c) After exercise, some ATP is used to re-establish the resting potential in axons. Explain how the resting potential is re-established. (2)
- Pump / active transport / transport against concentration gradient;
- Of sodium from axon / sodium out / of potassium in;
Explain how the release of acetylcholine at an excitatory synapse reduces the membrane potential of the postsynaptic membrane. (2)
- Binds to receptor / proteins; and opens Na+ channels;
- Na+ enter and make membrane potential less negative / depolarised
What is the role of ATP in myofibril contraction? (2)
5a
1. (Reaction with ATP) breaks/allows binding of myosin to actin/ actinomyosin bridge;
2. Provides energy to move myosin head;
If myosin molecules are unable to bind to other myosin molecules, this prevents muscle contraction.
Use the diagram and your knowledge of how muscles contract to suggest why. (3)
6b
- Can’t form myosin / thick filaments;
- Can’t pull / can’t move actin / slide actin past / (myosin) have to be joined / fixed to pull actin;
Accept: myosin can’t pull on each other - Myosin moves / if attached doesn’t move;
- Can’t move actin towards each other / middle of sarcomere / between myosin / can’t shorten sarcomere / can’t pull Z lines together.
7(a) Both slow and fast muscle fibres contain ATPase. Explain why. (2)
1. Splitting / breakdown / hydrolysis of ATP;
2. (Muscle) contraction requires energy / ATP;
Accept ‘uses energy’. Reject idea of ‘movement’ of muscles requiring energy.
3. Use of ATP by myosin.
Reject suggestion that ‘energy is produced’.
10 (a) What is the role of phosphocreatine (PC) in providing energy during muscle contraction? (2)
1. (Phosphocreatine) provides phosphate / phosphorylates;
Accept Pi or P in circle
2. To make ATP;
Accept:
ADP + CP → ATP + C
Reject phosphorus
11(a) Describe the part played by each of the following in myofibril contraction.
(i) Tropomyosin (2)
(ii) Myosin (2)
1. Moves out of the way when calcium ions bind;
Accept shape change with Ca2+ / Don’t accept just “calcium”
2. Allowing myosin to bind (to actin) / crossbridge formation;
Accept presence of calcium ions leads to movement instead of binds.
Accept references to troponin
Use information from the table to suggest and explain one advantage of:
(i) the high glycogen content of fast muscle fibres (2)
(ii) the number of capillaries supplying slow muscle fibres. (2)
1. (Glycogen broken down) gives (lots of) glucose for glycolysis / anaerobic respiration;
2. Glycolysis / anaerobic respiration not very efficient / only yields 2 ATP per glucose;
Accept very little ATP
(ii) 1. (Many capillaries) give high concentration / lots of oxygen / shorter diffusion pathway for oxygen / large surface area for oxygen exchange / diffusion / good glucose supply with little glycogen present;
2. Allows high rate of / more aerobic respiration OR prevents build-up of lactic acid / (muscle) fatigue;
- Accept idea of aerobic respiration during endurance events / long periods of exercise
18a) Describe two features, visible in the diagram, which show that the myofibril is contracted. (2)
- H band not visible / reduced / little / no thick filament / myosin only region / ends of thin filaments / actin close together;
- I band not visible / reduced / little / no thin filament / actin only region;
- A band occupies nearly all sarcomere / thick filament / myosin close to Z line;
- Large zone of thick-thin overlap;
(i) An action potential is generated at the cell body of the motor neurone. Explain how this action potential passes along the motor neurone to the neuromuscular junction. (3)
- Depolarisation of axon membrane / influx of Na+ establishes local currents;
- Change permeability to Na+ / open Na+ gates of adjoining region;
- Adjoining region depolarises / influx of Na+;
When the action potential arrives at the neuromuscular junction, it results in the secretion of acetylcholine into the synaptic cleft. Explain how. (3)
- Depolarisation of (presynaptic) membrane;
- Ca2+ channels open / increased permeability to Ca2+ causing influx of Ca2+ ;
- Vesicles move towards / fuse with presynaptic membrane;
Describe homeostasis in mammals
- maintenance of a stable internal environment within restricted limits.
- by physiological control systems ( normally involve negative feedback)
Explain the importance of maintaing stable core temperature
- if temperature is too high:
- hydrogen bonds in tertiary structure of enzymes break
- enzymes denature; active sites change shape and substrates can’t bind
- so fewer enzyme-substrate complexes
If temperature is too low: - not enough kinetic energy so fewer enzyme-substrate complexes.
Explain the importance of maintaing stable blood pH
- above or below optimal pH , ionic , hydrogen bonds in tertiary structure break
- enzymes denature, active sites change shape and substrate can’t bind
- so fewer enzyme substrate complexes
Explain the importance of maintaing stable blood glucose concentration
too low vs too high
too low:
- not enough glucose ( respiratory substrate) for respiration
- so less ATP produced
- Active Transport etc can’t happen -> cell death
too high:
- water potential of blood decreases
- water lost from tissue to blood via osmosis
- kidneys cannot absorb all glucose -> more water lost in urine causing dehydration
Describe the role of negative feedback in homeostasis
- Receptoors detect change from optimum
- Effectors respond to counteract change
- Returning levels to optimum/normal
E.G: control of blood glucose concentration , blood ph , core temperature and blood water potential
Explain the importance of conditions being controlled by seperate mechanisms involving negative feedback
- departures in different directions from the original state can be all controlled / reversed
- giving a greater degree of control ( over changes in internal environment)
Describe positive feedback
- Receptors detect change from normal
- Effectors respond to amplify change
- Producing a greater deviation from normal
e.g. onset of contractions in childbirth , blood clotting
The secretion of osteocalcin (in an inactive form) by osteoblasts is controlled by positive feedback.
Use information from the diagram to explain why this is positive feedback. (2)
Describe the factors that influence blood glucose concentrations
- consumption of carbohydrates: glucose absorbed into blood
- rate of respiration of glucose e.g increases during exercise due to muscle contraction
Describe glycogenesis, glycogenolysis, gluconeogenesis
glycogenesis: converts glucose —> glycogen
glycogenolysis: converts glycogen -> glucose
gluconeogenesis: converts amino acid acids and/or glycerol -> glucose
Explain the action of insulin in decreasing blood glucose concentration
• Attaches to specific receptors on cell surface membranes of target cells eg. liver / muscles
- This causes more glucose channel proteins to join cell surface membrane
Increasing permeability to glucose
So more glucose can enter cell by facilitated diffusion
- This also activates enzymes involved in conversion of glucose to glycogen (glycogenesis)
- Lowering glucose concentration in cells, creating a concentration gradient
• So glucose enters cell by facilitated diffusion
where is blood concetration detected
- Beta cells in islets of Langerhans in pancreas detect blood glucose concentration is too high → secrete insulin:
Explain the action of glucagon in increasing blood glucose concentration
- alpha cells in islets of Langerhans in pancreas detect blood concentration is too low. : secrete glucagon.
- attaches to specific receptors on cell surface membrane of target cells e.g liver
- activates enzymes involved in hydrolysis of glycogen to glucose
- activated enzymes involved in conversion of glycerol / amino acids to glucose.
- this establishes a concentration gradient - glucose enters blood by facilitated diffusion
Explain the role of adrenaline in increasing blood glucose concentration
- secreted by adrenal gland , during fear / stress / exercise.
- attaches to specific receptors on cell surface membranes of target cells e.g liver.
- activates enzymes involved in hydrolysis of glycogen to glucose.
- this establishes a concentration gradient. - glucose enters blood by facilitated diffusion
Describe the second messenger model of adrenaline and glucagon action
- adrenaline/glucagon ( ‘first messengers’) attach to specific receptors on cell membrane which
- activates enzyme adenylate cyclade ( change shape)
- which converts many ATP to many cyclic AMP ( cAMP)
- cAMP acts as the second messenger - activates protein kinase enzymes.
- protein kinases activate enzymes to break down glyvogen to glucose
Suggest an advantage of the second messenger model
- amplifies signal from hormone
- as each hormone can stimulate production of many molecules of seconde messenger ( cAMP )
- which can in turn activate many enzymes for rapid increase in glucose
Describe the cause of type I diabetes
- B cells in islets of Langerhans in pancreas produce insufficient insulin
- normally develops in childhood due to an autoimmune response destroying B cells of Islets of Langerhans
Describe the cause of type II diabetes
- receptor is faulty loses responsiveness/sensitivity to insulin ( but insulin still produced)
- so fewer glucose transport proteins - less uptake of glucose - less conversion of glucose to glycogen
- risk factor : obesity
Describe how Type 1 diabetes can be controlled
- injections of insulin
- blood glucose concentration monitored with biosensors, dose of insulin matched to glucose intake
- eat regularly and control carbohydrate intake e.g. those that are broken down / absorbed slower
- to avoid sudden rise in glucose
Suggest why insulin can’t be taken as a tablet by mouth
- Insulin is a protein
- would be hydrolysed by endopeptidases / exopeptidases
Describe how of type II diabetes can be controlled
- not normally treated with insulin injections but may use drugs which target insulin receptors to increase their sensitivity
- to increase glucose uptake by cells / tissues
- reduce sugar intake ( carbs)/ low glycacemic index - less absorbed
- reduce fat intake - less glycerol converted to glucose
- more regular exercise - uses glucose / fats by increasing respiration
- lose weight- increased sensitivity of receptors to insulin
Describe the structure of a nephron
- nephron: basic structural and functional unit of the kidney
- associated with each nephron are a network of blood vessels
Summarise the role of different parts of the nephron
- Bowman’s Capsule : formation of glomeluar filtrate
- Proximal Convoluted tubule: Réabsorption of water and glucose
- Loop of henle: maintenance of a gradient of sodium ions in the medulla
- Distal Convoluted tubule + Collecting duct: réabsorption of water (permeability controlled by ADH)
Describe the formation of glomerular filtrate
- High Hydrostatic pressure in glomerulus as diameter of afférent artériole is wider than the efferent artériole
- Small substances e.g. water , glucose uptake, ions , urea forced into glomeular filtrat, filtered by
a) pores/ fenestrations between capillary endothelial cells
b) capillary basement membranes
c) podocytes - Large Proteins/blood cells remain in blood
Describe the réabsorption of glucose by the proximal convoluted tubule
- Na+ actively transported out of epithelial cells to capillary.
- Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient
- Glucose moves into capillary by facilitated diffusion down its concentration gradient
Describe the reabsorption of of water by the proximal convoluted tubule
- glucose etc. In capillaries lower water potential
- water moves by osmosis down a water potential gradient
Describe and explain how features of the cells in the PCT allow the rapid réabsorption of glucose into the blood
- microvilli/folded cell-surface membrane : provides a large surface area
- many channel/carrier proteins for facilitated disgusting
