B3.3 Muscle and motility HL only

1. Barnacles are not an exception: the adult uses muscles to beat its feathery cirri to capture food and to close its shell plates when disturbed. — Adaptations for movement are universal. A sessile adult barnacle does not locomote, but it has muscles that move its cirri rhythmically to feed and that withdraw them and close the shell when it is disturbed. Movement of body parts is movement.
2. The student is right: once cemented to the rock the adult barnacle does not move at all, so it has no adaptations for movement. — A student who equates sessile with motionless picks this. Sessile means the adult does not move from place to place; the barnacle still moves its cirri and shell plates by muscle contraction, so it has adaptations for movement.
3. The student is right: the adult barnacle is a passive filter feeder relying on the water current, so movement plays no part in its life. — A student who pictures a filter feeder as a sieve held in a current picks this. The barnacle actively extends and beats its cirri to sweep food from the water; the current does not do the work for it.
4. The student is right: a barnacle has hard shell plates rather than a muscular system, and without muscle there can be no adaptation for movement. — A student who equates movement with muscle picks this. The barnacle in fact has muscles that beat its cirri and close its shell plates; and even organisms without muscle, such as plants and ciliates, show movement by other mechanisms.

Syllabus statement B3.3.1 · Read this in Learn

1. ATP is hydrolysed to power the pull of the head on actin; when ATP is used up the heads let go and the muscle relaxes. — A student who thinks ATP is only 'the energy for the pull' picks this. It is ATP binding that detaches the head from actin, so when ATP runs out the heads stay attached and the muscle locks, as in rigor mortis.
2. One ATP is hydrolysed at the start of a contraction; the heads then ratchet along the actin without ever detaching. — A student who pictures the heads walking along actin without letting go picks this. Each head must detach, re-cock and reattach many times during a contraction, and every cycle uses one ATP.
3. Binding of ATP releases the myosin head from actin, and its hydrolysis re-cocks the head for the next cycle. — A myosin head bound to actin is released when a new ATP molecule binds to it. Hydrolysis of that ATP to ADP and phosphate returns the head to its cocked position, ready to attach further along the actin filament and carry out another power stroke.
4. ATP is not used by the myosin head; the power stroke is driven by calcium ions binding directly to it. — A student who attaches the trigger to the mover picks this. Calcium ions bind to troponin on the thin filament, uncovering binding sites on actin; the energy for the cycle comes from ATP hydrolysed by the myosin head.

Syllabus statement B3.3.2 · Read this in Learn

1. It acts as an elastic spring that returns a stretched sarcomere to its resting length and stiffens to stop it being overstretched. — Titin runs from the Z line to the centre of the sarcomere. When the sarcomere is stretched, titin is extended and stores potential energy, then recoils; its resistance rises steeply near the limit of extension, so it also prevents overstretching.
2. It forms a third set of filaments that shorten actively to help pull the Z lines together during contraction. — A student who assumes every sarcomere protein is contractile picks this. The force of contraction comes from myosin heads pulling on actin; titin is a passive elastic protein whose roles are in recoil and in limiting stretch.
3. It pulls a contracted sarcomere back out to its resting length, so that an antagonistic muscle is not required. — A student who takes recoil to be the opposite of contraction picks this. Titin only recoils after it has been stretched; a sarcomere that has shortened by contraction can be lengthened only by an external force, such as an antagonistic muscle.
4. It is a rigid, inextensible cable from Z line to M line that stops the sarcomere from stretching at all. — A student who hears 'prevents overstretching' as 'cannot stretch' picks this. Titin is elastic and extends readily over the normal range of sarcomere lengths; it only becomes very stiff near the limit of that range.

Syllabus statement B3.3.3 · Read this in Learn

1. One motor neuron and the single muscle fibre it contacts at one neuromuscular junction. — A student who has generalized from a diagram showing one axon terminal on one fibre picks this. A motor neuron forms junctions with many fibres, from a few to over a thousand, and all of them belong to its motor unit.
2. A whole muscle together with the single motor neuron that stimulates it to contract. — A student who remembers the single motor neuron of a reflex arc picks this. A muscle contains many motor units, each with its own motor neuron, and the number recruited sets the force of contraction.
3. One motor neuron and all of the muscle fibres that its branching axon stimulates. — The axon of a motor neuron branches to form a neuromuscular junction with each of many muscle fibres. The neuron and every fibre it supplies form one motor unit, and all of those fibres contract together when the neuron fires.
4. A muscle fibre fused to its motor neuron so that impulses pass straight into it. — A student who imagines an electrical wiring picks this. The neuron and fibre are separate cells; the junction between them is a chemical synapse at which acetylcholine is released.

Syllabus statement B3.3.4 · Read this in Learn

1. In both, muscles anchored to a rigid skeleton pull across a joint; the grasshopper's attach inside its exoskeleton, the human's to the outside of bones. — Exoskeleton and endoskeleton both provide anchorage for muscles and act as levers. Arthropod muscles attach to the inner surface of the cuticle on either side of a joint; vertebrate muscles attach by tendons to the outer surface of bones. In each case the joint is the fulcrum and the muscle pull is the effort.
2. In the grasshopper the muscles lie on the outside of the exoskeleton, the reverse of the human arrangement of muscles around bones. — A student who extends 'exo' to everything picks this. The exoskeleton is outside the body, but the muscles are inside it, attached to its inner surface; the soft tissues are enclosed and protected by the cuticle.
3. Only the human leg is a true lever, because the grasshopper has no bones and its muscles move the leg without being anchored. — A student who thinks muscles move limbs without anchorage picks this. A muscle must pull against a rigid part to produce movement; the grasshopper's cuticle plates are rigid and its leg is a lever, just as the human's is.
4. In both legs the contracting muscle acts as the fulcrum of the lever, while the joint supplies the effort that moves the load. — A student who has mapped the parts of a physics lever onto the wrong structures picks this. The joint is the fulcrum, the pull of the contracting muscle is the effort, and the weight of the leg and body is the load.

Syllabus statement B3.3.5 · Read this in Learn

1. Tendons join the pelvis to the femur and hold the joint together, and ligaments attach the muscles that move it to the bones. — A student who has swapped the two kinds of connective tissue picks this. Ligaments connect bone to bone and stabilize the joint; tendons connect muscle to bone.
2. Ligaments join the pelvis to the femur and stabilize the joint, and tendons attach the muscles that move it to the bones. — Ligaments are bone-to-bone connective tissue: they hold the head of the femur in the socket of the pelvis and limit movement to a safe range. Tendons are muscle-to-bone: they transmit the force of contraction to the femur and pelvis.
3. Cartilage covering the head of the femur secretes the synovial fluid that lubricates the joint and reduces friction. — A student who assumes one friction-reducing structure makes the other picks this. Synovial fluid is secreted by the synovial membrane lining the joint capsule; the cartilage is a smooth covering that reduces friction and absorbs shock.
4. The head of the femur rocks against the pelvis in a single plane, so the hip is a hinge joint like the knee. — A student whose model of every joint is the knee or elbow picks this. The hip is a ball-and-socket joint: the rounded head of the femur turns within the socket of the pelvis, allowing movement in three dimensions.

Syllabus statement B3.3.6 · Read this in Learn

1. 105° — A student who applies 'largest minus smallest' without picturing the movement picks this. The two extremes are on opposite sides of 0°, so the angles are added: 125° + 20° = 145°, not 125° − 20°.
2. 125° — A student who takes a single goniometer reading as the range picks this. 125° is the flexion angle only; the range of motion in this dimension includes the 20° of extension on the other side of the neutral position.
3. 145° — Flexion and extension lie on opposite sides of the neutral position, so the thigh sweeps through 125° in front of the trunk plus 20° behind it: 125° + 20° = 145°.
4. 250° — A student who assumes the hip moves equally far on both sides of neutral doubles the flexion reading. Extension was measured at only 20°, so the range is 125° + 20° = 145°.

Syllabus statement B3.3.7 · Read this in Learn

1. They are stretched, because their fibres run in the opposite direction, and the stretch stores potential energy in their titin. — The two layers are antagonistic. Because the fibres of the internal layer run in the opposite direction, the upward and outward movement of the ribs lengthens them; the extended titin in their sarcomeres stores potential energy that is released as they recoil when the external layer relaxes.
2. They contract as well, because the internal intercostal muscles are the layer that moves the ribcage during inhalation. — A student who links 'internal' with 'inhale' picks this. The external intercostal muscles raise the ribcage for inhalation; the internal layer pulls it down and inwards during forced exhalation, and is stretched while the external layer contracts.
3. They actively lengthen by pushing, so that the ribs are free to move; a muscle can exert force while lengthening. — A student who thinks muscles can push picks this. Muscle tissue exerts force only when it contracts; the internal layer is lengthened passively by the movement produced by its antagonist.
4. They are stretched, and the energy of the stretch is stored as extra ATP in their fibres for the following exhalation. — A student for whom energy in muscle always means ATP picks this. Stretching does not synthesize ATP; the energy is stored as elastic potential energy in the extended titin molecules and released when they recoil.

Syllabus statement B3.3.8 · Read this in Learn

1. A young spider being carried away from its hatching site on a long thread of silk: migration. — A student who takes any movement away from home to be migration picks this. Ballooning spiderlings disperse in one direction and do not return; migration is a regular, usually seasonal, journey between regions and back.
2. A male emperor moth flying upwind along the trail of pheromone released by a female: searching for a mate. — Male emperor moths detect the female's pheromone with their feathery antennae and fly upwind along the scent to reach her. The locomotion serves reproduction: it is a search for a mate.
3. A humpback whale swimming from polar feeding grounds to tropical waters where it does not feed: foraging for food. — A student who assumes every journey is a search for food picks this. The whales leave the feeding grounds and do not feed in the breeding waters; the journey is migration, made for breeding and calving.
4. A wildebeest herd crossing the Serengeti as seasonal rains and fresh grazing move: escaping from danger. — A student who sees herds as always fleeing predators picks this. The herd follows rainfall and grass growth in an annual circuit: this is migration. Escape from danger is a short, rapid response, such as a gazelle sprinting from a cheetah.

Syllabus statement B3.3.9 · Read this in Learn

1. Both tails beat from side to side; the dolphin's fluke is horizontal so that it can also act as a hydroplane. — A student who transfers the fish tail stroke to all swimmers picks this. A horizontal blade cannot push water backwards by moving sideways; the fluke beats up and down, which is the up-and-down movement named in the guide.
2. The dolphin rows itself along with its flippers, while the tuna's thrust comes from its tail fin. — A student who thinks of the flippers as oars picks this. In dolphins and whales the flippers steer and stabilize; the thrust comes from the fluke, which is why the tail muscles are so large.
3. The dolphin's horizontal fluke beats up and down; the tuna's vertical tail fin beats from side to side. — The fluke of a dolphin lies horizontally and is driven up and down by muscles that flex the spine vertically, as in a running land mammal. A fish's tail fin is vertical and is swept from side to side by muscles that flex the spine sideways.
4. The dolphin's flippers and fluke are fins of the same kind as the tuna's, so the two swim in the same way. — A student who judges by outward shape picks this. The flipper is a modified forelimb with the bones of a mammalian limb, and the fluke is a boneless horizontal blade; the two animals move their tails in different planes.

Syllabus statement B3.3.10 · Read this in Learn

1. The sarcomere is shorter; the A band, the I band and the H zone are all narrower, each in the same proportion. — A student who thinks the filaments themselves shrink picks this. The myosin filaments keep their length, so the A band, which they define, keeps its width; only the regions of non-overlap narrow.
2. The sarcomere is shorter; the A band keeps its width while the I band and the H zone are both narrower. — The filaments slide rather than shorten. The A band is the full length of the myosin filaments, so it is unchanged; the I band (actin only) and the H zone (myosin only) both narrow as the actin filaments slide further in over the myosin.
3. The sarcomere is shorter; the A band and the H zone keep their width and only the I band is narrower. — A student who applies 'the A band stays the same' to everything inside it picks this. The H zone is the part of the A band not overlapped by actin, so as overlap increases it narrows and can disappear.
4. The sarcomere is shorter; the I band keeps its width while the A band and H zone are both narrower. — A student who has swapped actin and myosin expects the band containing the thick filaments to keep its width but places those filaments in the I band. The thick myosin filaments define the A band, which is the one that keeps its width.

Syllabus statement B3.3.2 · Read this in Learn

1. Myosin heads attach to actin but cannot pivot, because calcium ions are needed for the power stroke. — A student who thinks calcium acts on the myosin head picks this. Calcium ions act on troponin, controlling access to actin; the power stroke is driven by the myosin head using ATP, and it cannot happen because the heads never attach.
2. The fibre contracts normally, because the action potential in its membrane makes the filaments slide directly. — A student who treats calcium ions as a detail rather than a requirement picks this. The action potential does not act on the filaments; its only effect on contraction is to trigger release of calcium ions from the sarcoplasmic reticulum, and the drug blocks that step, so the binding sites on actin stay covered.
3. The fibre contracts normally, because calcium ions enter from the synaptic cleft, not from the reticulum. — A student who merges the two roles of calcium picks this. Calcium entering the neuron terminal triggers release of acetylcholine; the calcium that triggers sliding in a skeletal muscle fibre comes from the sarcoplasmic reticulum inside the fibre.
4. No cross-bridges form, because tropomyosin still covers the myosin-binding sites on actin. — Without calcium ions, troponin does not change shape, tropomyosin is not moved and the myosin-binding sites on actin remain blocked. Myosin heads cannot attach, so there is no sliding and the fibre does not contract, even though it has been stimulated.

Syllabus statement B3.3.2 · Read this in Learn

1. The biceps lengthens itself by reversing its cross-bridge cycle, so that the myosin heads push the actin filaments back outwards. — A student who thinks muscles can push as well as pull picks this. Cross-bridges only pull actin towards the centre of the sarcomere; there is no reverse power stroke, so a muscle cannot actively lengthen.
2. Contraction of the triceps moves the forearm, and this pulls the relaxed biceps longer, since a muscle cannot lengthen itself. — Muscle tissue exerts force only when it contracts. The triceps, the antagonist of the biceps, contracts to extend the elbow; the movement of the forearm stretches the relaxed biceps back to its resting length.
3. Titin in the biceps recoils and pulls its shortened sarcomeres back out, so the triceps is not needed for the movement. — A student who takes titin recoil to be the general mechanism of relaxation picks this. Titin in a shortened sarcomere is slack, not stretched; it recoils only after the sarcomere has been pulled beyond its resting length.
4. The cross-bridges in the biceps release as its ATP is used up, and the limp muscle then falls back to its resting length on its own. — A student who thinks relaxation is what happens when ATP runs out picks this. It is ATP binding that releases the heads, and a fibre without ATP locks rather than going limp; and even after the heads release, a shortened muscle stays short until an external force, the contracting triceps, pulls it longer.

Syllabus statement B3.3.3 · Read this in Learn

1. Q can be controlled more finely than P, because each of its motor neurons commands a far larger number of fibres. — A student who equates bigger with better picks this. Large motor units give large increments of force, which suits a powerful calf muscle but not fine control; the small units of P allow smaller steps.
2. P produces the more forceful contractions, because its greater number of neurons can send larger impulses. — A student who thinks impulses vary in size picks this. Action potentials are all-or-nothing; force depends on the number of fibres recruited, and Q, with fifty times more fibres, can generate far more force.
3. Most of the fibres in Q cannot be stimulated, because a motor neuron can only form one neuromuscular junction. — A student who believes one neuron supplies one fibre picks this. The axon of a motor neuron branches to form a junction with every fibre in its unit, so 800 neurons can supply a million fibres.
4. Force can be increased in smaller steps in P than in Q, because its motor units contain fewer fibres. — Mean motor unit size is 20 000 / 2000 = 10 fibres in P and 1 000 000 / 800 = 1250 fibres in Q. Recruiting one more unit adds about 10 fibres of force in P but about 1250 in Q, so P can be controlled in much finer steps.

Syllabus statement B3.3.4 · Read this in Learn

1. The knee is the more mobile of the two joints, because its 135° flexion exceeds any single range at the hip. — A student who compares the single largest number picks this. Mobility of a joint is judged across dimensions: the knee moves in essentially one plane, the hip in three, so the hip is the more mobile joint.
2. Both joints are hinge joints, because in each the largest range measured lies in the flexion–extension dimension. — A student who thinks every joint is a hinge picks this. A hinge joint moves in one plane only; the hip's 75° of abduction–adduction and 85° of rotation show that it is a ball-and-socket joint.
3. The range of motion of the hip in the flexion–extension dimension is 100°, the difference between 120° and 20°. — A student who subtracts the two readings picks this. Flexion and extension are measured on opposite sides of the neutral position, so the range in this dimension is 120° + 20° = 140°.
4. The hip moves through useful ranges in three dimensions, whereas the knee moves mainly in one. — The hip has substantial ranges in flexion–extension (140° in total), abduction–adduction (75°) and rotation (85°). The knee has a large range in flexion–extension only, with almost no movement in the other two dimensions, which is what a comparison across dimensions reveals.

Syllabus statement B3.3.7 · Read this in Learn

1. A blowhole on top of the head through which sea water taken in with food is forced out before each dive. — A student who thinks the spout is water from the mouth picks this. The blowhole is an airway, separate from the digestive system; the visible spout is exhaled air, moist and warm, condensing in cooler air.
2. An airway that opens into the mouth as well as the blowhole, so that air can be taken in while the whale feeds. — A student who assumes whales breathe as humans do picks this. In whales the airway is separated from the mouth and oesophagus; air enters only through the blowhole, which keeps water out of the lungs during feeding.
3. Lungs that fill with sea water during the dive, so that dissolved oxygen is absorbed until the whale surfaces to empty them. — A student who thinks a marine mammal can take oxygen from water while submerged picks this. Whales have no gills and their lungs hold only air; a long dive is possible because much of the lung air is exchanged in each rapid breath at the surface and oxygen is stored in blood and muscle.
4. A blowhole on top of the head that is sealed shut while submerged and opened briefly at the surface. — The nostrils have moved to the top of the head to form the blowhole, so the whale can breathe with only a small part of its body above water. Muscles keep the blowhole sealed during the dive and open it briefly at the surface for each rapid exhalation and inhalation.

Syllabus statement B3.3.10 · Read this in Learn

1. A spindle-shaped body with the hind limbs lost, no external ears, almost no hair and an outline smoothed by a layer of blubber under the skin. — Streamlining is a fusiform body that lets water flow past with minimal turbulence. Loss of projecting structures (hind limbs, external ears, fur) and a layer of blubber that fills in the hollows of the body give the smooth, tapered outline that reduces drag.
2. A thick layer of blubber that insulates the body; it makes the outline bulkier, but the extra drag is overcome by the powerful fluke. — A student who knows blubber only as insulation picks this. Blubber fills in hollows and smooths the body outline, so it contributes to streamlining rather than adding drag.
3. A body made less dense by blubber and large lungs, so that the dolphin floats and does not have to push against the water as it swims. — A student who confuses drag with buoyancy picks this. Streamlining reduces the resistance of water to a moving body and is a matter of shape, not density; a floating body still has to push water aside to move.
4. Flippers and a fluke that are fins of the same kind as a fish's, replacing the limbs of a land mammal so that they cut through the water. — A student who judges by outward shape picks this. The flippers are modified forelimbs containing mammalian limb bones and are used for steering, and the fluke provides thrust; drag is reduced by the shape of the whole body, not by fish-type fins.

Syllabus statement B3.3.10 · Read this in Learn