Plant & Animal Responses (OCR A Level Biology): Flashcards

The leaflets of the touch-sensitive leaves fold rapidly when touched.

Local bioelectrical signals.

The environmental stimuli to which plants respond can be either abiotic or biotic.

It decreases the formation of ice crystals, which can destroy plant cells if allowed to form within them.

It may be an adaptation to protect the leaflets from herbivorous insects, or it could reduce transpiration when the leaves are no longer photosynthesising.

False — herbivory is a biotic stress factor.

True

A protective sheath that surrounds the young growing shoot of grass plants, often used in historical phototropism experiments.

Growth of a plant organ towards a light stimulus. Plant shoots are positively phototropic, which maximises the light they can absorb for photosynthesis.

Removing the tip of a coleoptile, or covering it with an opaque cap, stopped the phototropic response to unidirectional light.

This showed that the tip of the coleoptile is responsible for detecting light.

• A gelatin block placed between the cut tip and coleoptile restored the response, so the growth stimulus is a chemical (hormone) that can diffuse through it

• An impermeable mica barrier stopped the response only when placed on the shaded side

• This showed the hormone is made at the tip, travels down the shaded side, and causes growth there

A cut coleoptile tip replaced off-centre in the dark caused the coleoptile to curve, with more growth on the side under the tip.

This showed the phototropic response is caused by a hormone diffusing through the tissue and stimulating growth.

Went let the tip's hormones diffuse into a gelatin block, then placed the block off-centre on a coleoptile in the dark, causing it to curve.

The greater the hormone concentration in the block, the more the coleoptile curved.

A specific plant growth factor and type of auxin, synthesised in the growing tips (meristems) of roots and shoots, that coordinates tropisms by controlling growth by elongation.

IAA activates proteins in the cell wall called expansins.

Expansins loosen the bonds between cellulose microfibrils, making the cell wall more flexible so the cell can elongate.

• IAA moves from the illuminated side of the shoot to the shaded side

• In shoots, a higher IAA concentration causes a greater rate of cell elongation

• The shaded side therefore elongates faster than the illuminated side, bending the shoot towards the light

In shoots, a higher concentration of IAA results in a greater rate of cell elongation.

Gravity causes IAA to accumulate on the lower side of the shoot.

In shoots, IAA increases the rate of cell elongation, so the lower side grows faster and the shoot bends upwards.

Gravity causes IAA to accumulate on the lower side of the root.

In roots, a high IAA concentration inhibits cell elongation, so the lower side grows slower than the upper side, bending the root downwards.

In roots, higher concentrations of IAA result in a lower rate of cell elongation.

IAA is synthesised in the root tips, so removing them means no IAA is produced.

With no IAA there is no inhibition of cell elongation, so the roots grow evenly on both sides and longer than tips that still produce IAA.

False — in roots a high IAA concentration inhibits cell elongation.

True

A layer of parenchyma cells with thin walls that develops at the base of the leaf stalk. The weak, easily broken walls allow the leaf to drop off during leaf loss.

Leaf loss reduces water loss.

Shortening day length in the autumn.

Ethene stimulates the breakdown of cell walls in the abscission layer, causing the leaf to drop off.

Auxins are produced in young leaves and normally inhibit leaf loss by making the leaf stalks insensitive to ethene.

During times of water stress, the hormone abscisic acid (ABA) is produced to stimulate the closing of stomata.

• ABA binds to receptors on the guard cell surface membranes\n\n- This inhibits the proton pumps, stopping the active transport of H⁺ ions out of the guard cells\n\n- ABA also causes Ca²⁺ ions to move into the cytoplasm of the guard cells

• They open channel proteins that let negatively charged ions leave the guard cells\n\n- They open further channel proteins that let K⁺ ions leave the guard cells\n\n- They close channel proteins that would allow K⁺ ions to enter the guard cells

• The loss of ions increases the water potential of the guard cells\n\n- Water leaves the guard cells by osmosis\n\n- The guard cells become flaccid, causing the stomata to close

Seed germination and stem elongation.

• Embryo – grows into the new plant when the seed germinates\n\n- Endosperm – a starch-containing energy store surrounding the embryo\n\n- Aleurone layer – a protein-rich layer on the outer edge of the endosperm

• The seed absorbs water, stimulating the embryo to produce gibberellins\n\n- Gibberellins diffuse into the aleurone layer and stimulate the cells to synthesise amylase (by increasing transcription of mRNA coding for amylase)\n\n- Amylase hydrolyses starch in the endosperm into soluble maltose\n\n- Maltose is converted to glucose and transported to the embryo\n\n- The embryo respires the glucose, breaking dormancy and providing energy for growth

• Abscisic acid has the opposite effect to gibberellins, maintaining dormancy by inhibiting amylase production\n\n- The start of germination is therefore determined by the balance of abscisic acid and gibberellins present in the seed

A state in which a shed seed contains very little water and is metabolically inactive, allowing it to survive harsh conditions until the conditions are right for successful germination.

Water absorption is difficult due to frozen soils, and photosynthesis is limited by low temperatures and reduced light.

As the leaf ages, the concentration of auxins decreases, until leaf loss can occur in response to ethene.

False — auxins make leaf stalks insensitive to ethene, so they normally inhibit leaf loss.

True

A plant hormone (growth regulator) produced at the growing tip at the apex of a plant stem that promotes upward stem growth and inhibits the growth of lateral (side) buds.

The inhibition of lateral (side) bud growth by auxins produced at the apex (growing tip) of a plant stem, ensuring that growth is preferentially upwards.

At the growing tip, at the apex (the top) of the plant stem.

• They cause the stem to grow upwards

• They stop lateral (side) buds from growing

Growing upwards towards the light ensures the plant has access to more energy for photosynthesis.

Sideways growth is usually not as useful, so apical dominance ensures growth is preferentially upwards.

The lateral buds begin to grow, because the source of auxins has been removed and there is no longer apical dominance.

With time, the lateral shoots that grow from the lateral buds curl up towards the light, meaning the plant continues to grow in an upwards direction.

If the growing tip at the apex of a plant is removed, lateral buds grow because there is no longer apical dominance.

• The apical bud of the first test plant is removed (decapitated) — this allows the lateral buds to grow

• A second, genetically identical plant is decapitated, but the cut tip is immediately replaced with an agar block containing auxin

• This restores the inhibition of lateral bud growth, so no lateral buds grow

The auxin in the agar block replaces the auxin normally supplied by the apex, restoring apical dominance so that no lateral buds grow — showing it is the auxin, not the physical tip, that inhibits lateral bud growth.

Auxins produced at the apex of a stem stop lateral (side) buds from growing.

True — auxins are produced at the growing tip at the apex of the stem.

False — auxins from the apex inhibit lateral bud growth, causing apical dominance.

A group of plant hormones that help plants grow by stimulating cell division and elongation in the stem, and that stimulate seed germination.

By stimulating cell division and cell elongation in the stem.

Dwarf plant varieties have very low levels of gibberellins.

Because of a mutation in a gene involved in the synthesis of gibberellins.

They grow to the same height as normal varieties.\n\nThis provides experimental evidence for the role of gibberellin in stem elongation.

To stimulate their growth.

Gibberellins stimulate germination and the breaking of dormancy in seeds.

Seeds of mutant Arabidopsis varieties that do not produce gibberellins can be induced to germinate if gibberellins are applied.

Certain lettuce varieties that require light to germinate can be made to germinate in the dark if gibberellins are applied.

False — dwarf varieties have very low levels of gibberellins, often due to a mutation in a gibberellin-synthesis gene.

An auxin (a type of plant hormone) that is often used as a rooting powder to promote root growth.

To investigate the effect of different concentrations of the plant hormone IBA (an auxin) on root growth.

Because IBA is insoluble in water, so it must be dissolved in alcohol instead.

Each successive dilution is 10-fold, made in alcohol:\n\n- Solution B (0.5%): add 1 cm³ of Solution A to 9 cm³ of alcohol\n\n- Solution C (0.05%): add 1 cm³ of Solution B to 9 cm³ of alcohol\n\n- Solution D (0.005%): add 1 cm³ of Solution C to 9 cm³ of alcohol\n\n- Solution E (0.0005%): add 1 cm³ of Solution D to 9 cm³ of alcohol

A serial dilution is made by transferring 1 cm³ of solution into 9 cm³ of alcohol to achieve each 10-fold dilution.

The dipped-end down method is used:\n\n- Dip the ends of 10 cuttings into a solution\n\n- Place these cuttings into soil or compost, dipped-end down\n\n- Repeat with each IBA solution (A to E), using a fresh set of 10 cuttings each time

The final 10 cuttings are dipped in the alcohol used to make the IBA solutions (containing no IBA), then placed into soil, dipped-end down.

The cuttings in the control group are dipped in alcohol only, to check the effect is due to the hormone and not the solvent.

Carefully remove all the cuttings, then for each set of 10 cuttings remove all the roots and weigh them using a digital balance to find the mass of roots grown.

• Concentration of auxin (IBA) on the x-axis\n\n- Mass of roots grown on the y-axis\n\nThis shows the effect of different hormone concentrations on root growth.

• Volume of water provided to each set of cuttings\n\n- Temperature\n\n- Humidity\n\n- Light intensity\n\nThese are controlled so that the concentration of plant hormone is the only variable affecting root growth.

So that all the cuttings respond to the hormone in the same way. Cuttings from genetically dissimilar plants may respond differently, introducing another variable.

It ensures any effect on root growth is due to the hormone, not the alcohol it was dissolved in.

True

False — IBA is insoluble in water, so it is dissolved in alcohol.

Because the growth and development of plants is controlled by hormones, so these hormones can be used to benefit commercial plant growing in a variety of ways.

• As selective weed killers

• As rooting powders

• To control ripening

Although auxins are normally growth-promoting, at high concentrations they cause such rapid growth that plant tissues (e.g. the roots) become distorted and damaged.

This damage allows pathogens to enter the plant, killing the weed.

Grasses are significantly less sensitive to these selective weed killers than broadleaved weeds.

The broadleaved weeds die while the grasses (cereal crops or lawn) remain behind.

Synthetic auxins used as selective weed killers are applied to plants in concentrations 100 times greater than the natural hormones found in plants.

At low doses, auxins stimulate cuttings to grow new roots.

The lower end of the cutting is dipped in the powder before being planted in compost; with the correct environmental conditions, roots begin to grow shortly afterwards.

A commercial product containing auxins (at the correct low dosage) into which the lower end of a cutting is dipped to stimulate the growth of new roots.

Ethene is used to stimulate fruit to ripen.

Delicate fruits are soft when ripe and are easily damaged during transport.

Instead, they are harvested when unripe (and therefore harder), then transported.

They are ripened artificially using ethene during transport or once they reach their destination.

Auxins and gibberellins can make unpollinated flowers develop fruit (which in nature normally only occurs after pollination and fertilisation).

A fruit formed without fertilisation, e.g. seedless grapes produced using auxins and gibberellins.

Auxins can be used to stop trees from dropping their fruit before it has been harvested.

This is useful because dropped fruit becomes bruised and soon rots on the ground, making it no longer usable.

It allows the production of seedless fruits (e.g. seedless grapes).

False — grasses are much less sensitive than broadleaved weeds, so the weeds die while the grass survives.

True

• Central nervous system (CNS) – the brain and the spinal cord

• Peripheral nervous system (PNS) – all of the nerves in the body

The part of the nervous system consisting of the brain and the spinal cord, which acts as the decision-making centre.

The part of the nervous system made up of all of the nerves in the body outside the brain and spinal cord.

To allow us to make sense of our surroundings and respond to them, and to coordinate and regulate body functions.

A neurone is a nerve cell that carries nerve impulses.

A nerve is a bundle of neurones.

Information is sent through the nervous system as nerve impulses, which are electrical signals that pass along neurones.

• Sensory receptors (e.g. those in the eye)

• Decision-making centres in the central nervous system

• Effectors, such as muscles and glands

• The somatic nervous system

• The autonomic nervous system

It is required for the voluntary control of body movements.

• Sensory nerves – consist of sensory neurones; carry impulses from sense organs to the CNS

• Motor nerves – consist of motor neurones; carry impulses from the CNS to muscles and glands

• Spinal nerves – mixed nerves in the spinal cord that consist of both sensory and motor neurones

It is a self-controlling system required for involuntary actions and functions.

• The sympathetic nervous system

• The parasympathetic nervous system

It controls the release of adrenaline (a hormone produced by the adrenal glands).

Adrenaline causes the heart rate to increase, raising blood supply to respiring muscles.

This gives the muscles more oxygen and glucose for respiration, enabling an immediate high-intensity response such as running away.

A hormone produced by the adrenal glands, released under control of the sympathetic nervous system during a fight-or-flight response; it increases heart rate.

It controls actions such as heart rate, regulation of blood vessel diameter, and peristalsis in the gut.

• The sympathetic nervous system controls 'fight-or-flight' responses

• The parasympathetic nervous system controls the 'rest and digest' system

False — a nerve is a bundle of neurones (nerve cells).

True

The brain and the spinal cord.

• The cerebrum

• The hypothalamus

• The pituitary gland

• The cerebellum

• The medulla oblongata

The cerebrum is the largest part of the brain and controls conscious activities.

The band of nerve fibres that joins the two cerebral hemispheres together.

The right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body.

The thin, highly folded outer layer of the cerebrum, consisting of the cell bodies of neurones.

Folding increases the surface area of the cortex, allowing it to contain a greater number of neurones.

More neurones allow more connections to be made between them.

This increases the brain's ability to carry out more complex behaviours.

Beneath the grey matter of the cerebrum is the white matter, which consists of the myelinated axons of neurones.

• Regulating body temperature (monitoring blood temperature)

• Osmoregulation (monitoring blood concentration; stimulating ADH release)

• Regulating digestive activity (controlling gut enzyme secretion and peristalsis; generating hunger)

• Controlling endocrine functions (stimulating the pituitary gland to release hormones)

It releases hormones involved in homeostasis itself, or stimulates the pituitary gland to release certain hormones.

Anterior pituitary — produces and releases certain hormones.

Posterior pituitary — stores and releases hormones produced by the hypothalamus (e.g. ADH and oxytocin).

The cerebellum controls motor coordination, including balance.

• Cardiac centre — controls heart rate

• Vasomotor centre — controls blood pressure

• Respiratory centre — controls breathing rate

The medulla oblongata is found at the very base of the brain, where it joins the spinal cord.

Conscious activities, such as vision, hearing, speech, thinking and memory.

It functions only subconsciously — all the actions it controls are involuntary.

True

False — the cerebellum functions only subconsciously, controlling involuntary actions.

An involuntary (automatic) response to a stimulus that is very fast and usually has a protective purpose or survival value.

• Yawning

• Saliva production

• Swallowing

• Pulling a body part away from a source of pain (the withdrawal reflex)

• Blinking

• Constricting the iris muscles in response to bright light (the pupil reflex)

They can also be carried out on purpose (voluntarily), so in those cases they are not reflex actions.

Some actions, such as the pupil reflex, are always automatic (involuntary).

The sequence of components in a reflex action is: Stimulus → Receptor → Coordinator → Effector → Response

To detect a stimulus.

Some receptor cells produce electrical activity in nerve cells in response to stimuli, while others secrete substances in response to stimuli.

The spinal cord.

Some reflexes may instead involve unconscious parts of the brain, e.g. the medulla in the brainstem.

The impulse follows a fixed nervous pathway from receptor to a specific effector, so the same effector always produces the same appropriate response.

To assess whether a patient's nervous system is working properly.

• Stimulus – stretching of the quadriceps muscle caused by pressure on the ligament (created by the hammer)

• Receptor – stretch receptors in the quadriceps muscle

• Coordinator – the spinal cord

• Effector – the quadriceps muscle

• Response – contraction of the quadriceps muscle, causing the leg to straighten

The stretch receptors send impulses down a sensory neurone, which connects directly, via a single synapse, with a motor neurone in the spinal cord.

There is no relay neurone in this pathway.

The motor neurone carries the impulses to the effector (the quadriceps muscle), which contracts.

Nerve impulses are delayed by synapses. In this reflex the signal crosses only a single synapse, allowing a very rapid response.

Information still travels to the brain, but by the time the brain processes it the response has already occurred, so the brain has no chance to make a decision.

• Something travelling towards the eye at high speed

• Something contacting the cornea

• Drying of the cornea

Irritation or drying of the cornea sends impulses down the trigeminal sensory nerve to the medulla of the brain, where it connects with other neurones to transmit the signal to the effector muscles.

Relay neurones are involved in transmitting the impulses to the effectors.

The pathway goes via the brain but not via any decision-making areas, and the number of synapses is still minimal.

• Superior levator palpebrae muscle – lowers the upper eyelid

• Orbicularis oculi muscle – pulls the eyelids inwards and helps to close them

A specialised hammer is used to hit the tendon between the knee cap and the tibia, and the leg involuntarily straightens in a small kicking motion.

False — blinking can also be done on purpose (voluntarily), so it is not always a reflex action.

False — the knee-jerk reflex has no relay neurone; the sensory neurone connects directly to the motor neurone via a single synapse.

A reflex action in which a body part is pulled away from a source of pain.

To survive. Responding to change allows organisms to:

• Find favourable conditions for living

• Find food

• Avoid being eaten

If these requirements are not met, a species will die out or go extinct.

• Stimulus – a detectable change in the environment

• Receptor – detects the stimulus

• Coordinator – receives and processes the impulse/hormone

• Effector – brings about the response

• Response – the appropriate action produced

• Some receptor cells produce electrical activity in nerve cells

• Other receptor cells secrete messenger chemicals, such as hormones

A structure (such as a muscle or gland) that brings about the appropriate response to a stimulus, following signals received from a coordinator.

It is produced in situations of a high level of stress, fear or aggression induced by environmental stimuli.

They work in a complementary manner to coordinate this fast response:

• The initial part of the response is controlled by the nervous system (the sympathetic nervous system coordinates many responses to danger)

• The response is continued by the endocrine system, via the hormones adrenaline and cortisol (secreted from the adrenal glands)

• Sensory neurones detect the stimulus and send impulses to the brain

• The amygdala sends impulses to other parts of the brain, including the hypothalamus

• The hypothalamus sends impulses via sympathetic nerves to the adrenal glands

• This causes the adrenal medulla to secrete adrenaline, which increases sensory awareness and alertness

• The hypothalamus releases a peptide hormone

• This stimulates the anterior pituitary gland to release ACTH (adrenocorticotropic hormone)

• ACTH is transported to the adrenal glands via the bloodstream

• This causes the adrenal cortex to secrete cortisol

• Increases blood pressure

• Increases blood glucose, ensuring tissues have sufficient glucose and oxygen for a rapid response

• Suppresses the immune system

Any of the following:

• Dilates the pupils (contraction of iris muscles)

• Dilates the bronchioles (relaxing smooth muscle) to increase airflow to the alveoli

• Causes vasoconstriction of blood vessels to the gut and skin, raising blood pressure

• Causes vasodilation of blood vessels to the brain and muscles

• Increases heart rate and stroke volume

• Stimulates the breakdown of glycogen to glucose in liver cells, raising blood glucose concentration

• Adrenaline is secreted by the adrenal medulla

• Cortisol is secreted by the adrenal cortex

(Both the adrenal medulla and adrenal cortex are regions of the adrenal glands.)

• Adrenaline binds to a specific receptor on the liver cell membrane

• This activates the enzyme adenylyl cyclase (it changes shape)

• Active adenylyl cyclase catalyses the conversion of ATP to the second messenger cyclic AMP (cAMP)

• cAMP activates protein kinase A enzymes

• Protein kinase A activates phosphorylase kinase (by adding phosphate groups)

• Phosphorylase kinase activates glycogen phosphorylase, which catalyses the breakdown of glycogen to glucose (glycogenolysis)

It amplifies the original signal from adrenaline, so that each adrenaline molecule leads to the release of a large amount of extra glucose by the liver, quickly raising blood glucose concentration.

The breakdown of glycogen to glucose, stimulated by adrenaline in liver cells, is known as glycogenolysis.

The response is rapid and can be crucial for preserving life.

True

False — adrenaline is secreted by the adrenal medulla; the adrenal cortex secretes cortisol.

Describes muscle that initiates its own contraction, without requiring any external (nervous) stimulus. Cardiac muscle is myogenic, allowing the heart to beat at its own regular intervals.

• The nervous system (autonomic nervous system)

• The endocrine system (hormones)

The region at the base of the brain, near the top of the spinal cord, that controls heart rate. It contains the acceleratory centre and the inhibitory centre, both connected to the sinoatrial node (SAN) by nerves of the autonomic nervous system.

• The acceleratory centre, which causes the heart to speed up

• The inhibitory centre, which causes the heart to slow down

Impulses are sent along sympathetic neurones to the SAN.

Noradrenaline is secreted at the synapse with the SAN.

This increases the frequency of the electrical waves the SAN produces, raising the heart rate.

Impulses are sent along parasympathetic neurones to the SAN.

Acetylcholine is secreted at the synapse with the SAN.

This reduces the frequency of the electrical waves the SAN produces, lowering the heart rate towards the resting rate.

The acceleratory centre sends impulses along sympathetic neurones, where the neurotransmitter noradrenaline is secreted at the synapse with the SAN.

The inhibitory centre sends impulses along parasympathetic neurones, where the neurotransmitter acetylcholine is secreted at the synapse with the SAN.

• Blood carbon dioxide concentration increases

• An initial fall in blood pressure, caused by dilation of the muscle arterioles

• Chemoreceptors, which detect blood carbon dioxide concentration

• Pressure receptors, which detect blood pressure

• Higher frequency impulses activate the acceleratory centre to speed up the heart rate

• Lower frequency impulses activate the inhibitory centre to slow down the heart rate

• Adrenaline and noradrenaline — secreted by the adrenal glands

• Thyroxine — secreted by the thyroid gland

It allows a rapid increase in blood supply to respiring muscles, delivering more oxygen and glucose for respiration.

This enables high-intensity activities, such as running from a predator, to happen as an immediate response.

• In the aorta, close to the heart

• In the carotid arteries, which supply the head with oxygenated blood

True

False — the acceleratory centre uses sympathetic neurones; parasympathetic neurones carry impulses from the inhibitory centre.

Any of the following:

• Drugs

• Caffeine

• Alcohol

• Sex

• Weight

• Height

• Temperature

• Diet

• Dehydration

The other variables must be controlled (kept constant) as far as possible, so that any change in heart rate can be attributed to the single factor being investigated.

To ensure that no test subjects are harmed. The experiment must be designed so that the safety of the subjects is never at risk (e.g. subjects should not be pushed to exercise so intensely that it could cause a heart attack).

• Use a heart rate monitor to record each individual's resting heart rate (to give a baseline mean and range)

• Give each individual a measured volume (e.g. 200 ml) of caffeine solution of known concentration to drink

• Wait 15 minutes, then record each individual's resting heart rate again

• Repeat the measurements every 15 minutes for 2 hours

• Present the results in a table or graph

To show the duration for which caffeine has an effect on the heart rate.

So that movement and activity level are controlled. Otherwise these variables could alter the heart rate and confound the effect of the caffeine.

Because too much caffeine can be dangerous to the subjects, while too little may show no measurable effect on heart rate.

It is very difficult to ensure that individuals in a sample are sufficiently similar. Using the same group for both measurements means individual differences (sex, age, weight, height, etc.) are largely accounted for, reducing their impact as a variable.

Care would need to be taken to select individuals with a similar life history (e.g. same sex, age, weight and height). Otherwise differences between the groups, rather than the caffeine, could account for any difference in heart rate.

The range of resting heart rates for the group, which shows the spread/variation in heart rate across the individuals (before and after caffeine consumption).

When investigating the effect of one factor on heart rate, all other variables should be controlled so that the results are valid.

The moral requirements that must be met when using human subjects, including obtaining informed consent and ensuring the experiment is designed so that no subject's safety or wellbeing is ever put at risk.

False — the other variables must be controlled so that the results are valid.

True

The difference between the highest and lowest heart rate readings in a group, showing the spread of values.

• Skeletal muscle (striated / voluntary)

• Smooth muscle (involuntary)

• Cardiac muscle

A highly specialised, cell-like unit that makes up skeletal (striated) muscle. It contains contractile proteins in its cytoplasm, is surrounded by a cell surface membrane, and contains many nuclei.

Because each muscle fibre contains many nuclei, rather than a single nucleus like a typical cell.

The cell surface membrane of a muscle fibre.

The cytoplasm of a muscle fibre. It contains mitochondria and myofibrils.

The muscle-fibre equivalent of the endoplasmic reticulum. Its membranes contain protein pumps that transport calcium ions into its lumen.

T-tubules (transverse system tubules) are deep, tube-like projections that fold inwards from the outer surface of the sarcolemma.

They carry out aerobic respiration to generate the ATP required for muscle contraction.

Bundles of two types of protein filament that slide past each other during contraction:

• Thick filaments made of myosin

• Thin filaments made of actin

The membranes of the SR contain protein pumps that transport calcium ions into the lumen of the SR.

• Contains both actin and myosin filaments, but has no banding or striation

• Made of small, elongated, spindle-shaped cells/fibres

• Each cell contains a single nucleus

It can contract without external stimulation by nerves or hormones, allowing the heart to beat at its own regular intervals (which can still be regulated by the nervous and endocrine systems).

• It is myogenic (contracts without external stimulation)

• It does not tire or fatigue, so it can contract continuously throughout life

• Fibres form a network joined by intercalated discs, with many mitochondria to supply ATP

• It is easier to find the specimen in the field of view

• It helps prevent damage to the lens or coverslip if the stage has been raised too high

A graticule has no fixed units, so it is calibrated against a stage micrometer (a scale engraved on a microscope slide).

By lining up the two scales, you work out how many micrometres each graticule unit represents for the objective lens in use.

They run close to the sarcoplasmic reticulum (SR).

True — it contracts without external stimulation by nerves or hormones.

False — smooth muscle contains actin and myosin but has no banding or striation.

The point where a motor neurone meets a muscle cell. It works in a very similar way to a synapse, transmitting an impulse to trigger contraction of striated muscle.

The surface (cell surface) membrane of a muscle fibre cell.

An enzyme present in the synaptic cleft that breaks down acetylcholine (ACh), preventing the muscle from being continually stimulated by a single impulse.

Between a motor neurone and a muscle cell.

The action potential causes calcium ions to diffuse into the neurone.\n\nThis stimulates vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh.

ACh diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (the surface membrane of the muscle fibre).

• Ion channels in the sarcolemma open, allowing sodium ions to diffuse in\n- This depolarises the sarcolemma, generating an action potential

It passes down the T-tubules, which carry the action potential towards the centre of the muscle fibre.

They cause voltage-gated calcium ion channels in the membranes of the sarcoplasmic reticulum (SR) to open.\n\nCalcium ions then diffuse out of the SR and into the sarcoplasm surrounding the myofibrils.

• Calcium ions bind to troponin, causing it to change shape\n- This moves troponin and tropomyosin on the thin (actin) filaments\n- The myosin-binding sites on actin are exposed, so contraction (the sliding filament model) can begin

Calcium ions bind to troponin molecules, stimulating them to change shape and move tropomyosin to expose the myosin-binding sites.

The neurotransmitter released at the neuromuscular junction is acetylcholine (ACh).

• Acetylcholinesterase in the synaptic cleft breaks down acetylcholine, preventing continual stimulation\n- Calcium ions are pumped back into the sarcoplasmic reticulum once the sarcolemma, T-tubules and SR are no longer polarised\n- The removal of calcium ions terminates muscle contraction

True

False — sodium ions diffuse in, which depolarises the sarcolemma.

Thick filaments are made of myosin molecules.

Thin filaments are made of actin molecules (globular proteins).

The functional contractile unit of a myofibril; it shortens during muscle contraction as the actin and myosin filaments slide past each other.

A fibrous protein twisted around the two actin chains of a thin filament that, in a resting muscle, helps block the myosin-binding sites on actin.

A protein attached to the actin chains at regular intervals. When calcium ions bind to it, it changes shape, moving troponin and tropomyosin to expose the myosin-binding sites on actin.

Sarcomeres within the myofibrils shorten as the actin and myosin filaments slide past each other.

The filaments themselves do not shorten; they move relative to one another towards the centre of the sarcomere. This is the sliding filament model of muscle contraction.

• Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm by diffusion

• Calcium ions bind to troponin, stimulating it to change shape

• Troponin and tropomyosin change position on the actin filaments

• The myosin-binding sites on actin are now exposed

Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm by diffusion.

The globular myosin heads bind to the exposed binding sites on actin, forming cross-bridges between the two filaments.

• When the myosin head bends (power stroke), it releases a molecule of ADP

• ATP binds to the myosin head, allowing it to detach from actin

• The myosin head acts as an ATPase, hydrolysing ATP into ADP and Pi; the energy released returns the myosin head to its original position

• The myosin head can then bind to a new binding site further along the actin, repeating the cycle

Calcium ions are returned to the sarcoplasmic reticulum by active transport.

A molecule stored by muscles that allows the rapid production of ATP. A phosphate ion is transferred from phosphocreatine to ADP:

ADP + phosphocreatine → ATP + creatine

It lets muscles keep contracting for a short time until the mitochondria can supply enough ATP.

Resting muscles store only a small amount of ATP, enough for roughly 3-4 seconds of intense exercise.

Aerobic respiration produces ATP but is slow, and anaerobic respiration takes around 10 seconds before it begins producing ATP.

Phosphocreatine provides rapid ATP to keep the muscle contracting until respiration can meet demand (e.g. in a 100 m sprinter).

Myosin is a fibrous protein with a globular head.

The fibrous part anchors the molecule into the thick filament, with many myosin molecules lying side by side and their globular heads all pointing away from the M line.

Many actin molecules link to form a chain, and two actin chains twist together to form one thin filament.

• Tropomyosin (a fibrous protein) is twisted around the two actin chains

• Troponin is attached to the actin chains at regular intervals

The myosin heads bend, pulling the actin filaments towards the centre of the sarcomere (the power stroke).

This causes the muscle to contract a very small distance.

The cross-bridge cycle repeats and the muscle contracts further as long as:

• troponin and tropomyosin are not blocking the myosin-binding sites, and

• the muscle has a supply of ATP

False — the filaments do not shorten; they slide past each other so the sarcomere shortens.

True