Plant and animal responses — OCR A-Level Biology

Plant responses are the ways plants adjust growth or movement to stimuli, and OCR expects you to recognise the main types. Tropisms are directional growth responses: phototropism to light, geotropism (gravitropism) to gravity, hydrotropism to water and thigmotropism to touch. Shoots are positively phototropic and negatively geotropic; roots are the reverse. Nastic responses are non-directional movements, such as the folding of Mimosa leaves when touched, and are driven by turgor changes rather than growth direction. Taxis responses involve whole-organism movement, but these are not typical of rooted plants. Learn to classify an unfamiliar example by asking whether the response is directional and whether it is growth-based or turgor-based.

Practical work on tropisms means designing and running an investigation in which a plant organ's directional growth is measured against a controlled stimulus. For phototropism, use seedlings such as cress or coleoptiles grown in darkness, then expose them to light from one side only, with a matched dark control and a group lit evenly from all sides. Record the direction of shoot curvature and, if required, measure the angle of bending against a vertical reference. For geotropism, place seedlings in a horizontal position so the root and shoot are at right angles to gravity, and compare with a vertically oriented control. Keep temperature, moisture and handling constant, use several replicates, and record quantitative data at fixed time intervals. Analyse mean angles or lengths with a suitable test and link curvature to unequal auxin distribution.

Plant hormones are chemical messengers that coordinate growth and responses to the environment. Auxin, produced mainly in shoot tips, moves to regions of elongation and causes cell elongation; an unequal auxin distribution produces the differential growth seen in phototropism and geotropism. Gibberellin promotes stem elongation and can break seed dormancy, triggering germination. Abscisic acid acts as a stress hormone, promoting stomatal closure and maintaining seed dormancy. Ethene (ethylene) is a gaseous hormone that promotes fruit ripening and leaf abscission. Cytokinins promote cell division and delay leaf ageing. A multiple-choice question may ask which hormone matches a stated role, or which effect follows from a described change in hormone concentration, so link each named hormone to its specific function and site of action rather than a general idea of 'growth'.

Apical dominance is the suppression of lateral bud growth by the shoot apex. The evidence rests on experiments that remove or apply auxin and observe the effect on side shoots. Decapitation (removing the shoot tip) releases lateral buds, which then grow; applying auxin (IAA) in a paste to the cut stump restores suppression, so the buds stay dormant. This shows the apex produces a substance that inhibits bud outgrowth, and that the substance is auxin. Further support comes from auxin-transport experiments: auxin moves down the stem in a polar direction, and blocking that transport reduces the dominance. The evidence is correlational plus manipulative, so a good answer distinguishes removing the source from replacing the chemical and explains why the replacement step is the key control.

Gibberellins promote stem elongation and seed germination, and the evidence comes from mutants and application experiments. Dwarf varieties (for example short-stemmed mutants) have low gibberellin or reduced sensitivity; applying gibberellin to dwarf plants restores normal elongation, showing the hormone drives stem growth. For germination, gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, including in some light-requiring seeds where it substitutes for the light signal. The evidence combines naturally occurring mutants (loss of function) with replacement by applied hormone, so a strong answer separates the observation from the causal test and notes that sensitivity, not only hormone level, can matter.

Practical work on plant hormones means designing and running an investigation in which you apply a hormone treatment to plant material and measure a growth response against a control. A typical method uses wheat or cress seedlings grown in petri dishes on moist filter paper; you apply a range of concentrations of gibberellin solution to separate groups, keep one group untreated as a control, and measure coleoptile or shoot length with a ruler after a set number of days. You must identify the independent variable (hormone concentration), the dependent variable (growth measured as length in mm), and control the variables that could confound the result: volume applied, temperature, light, water availability, number of seeds per dish and time of measurement. Replicates allow a mean to be calculated and reduce the effect of individual variation.

Plant hormones are used commercially to control plant growth in agriculture and horticulture. Gibberellins can be applied to promote stem elongation and to break seed dormancy, for example in malting barley to increase germination and in grapes to increase berry size. Auxins are used as selective weedkillers because broad-leaved plants are affected more than narrow-leaved cereals, and as rooting powders to encourage cuttings to form roots. Ethene is used to ripen fruit such as bananas and tomatoes during transport and storage. Cytokinins can be used to delay leaf senescence, keeping cut foliage and produce fresh for longer. In each case you should link the hormone to the specific commercial outcome and the reason it is useful, such as improving yield, uniformity or shelf life.

The mammalian nervous system is organised into the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system (PNS), which links the CNS to the rest of the body. The PNS divides into the somatic nervous system, controlling voluntary skeletal muscle, and the autonomic nervous system, controlling involuntary effectors such as smooth muscle, cardiac muscle and glands. The autonomic system splits into the sympathetic division, which prepares the body for action (raising heart rate, dilating pupils), and the parasympathetic division, which restores resting conditions (slowing heart rate, constricting pupils). These two divisions act antagonistically. Sensory neurones carry impulses towards the CNS; motor neurones carry impulses away to effectors. Relay neurones connect the two within the CNS.

The human brain has several regions with distinct functions. The cerebrum, the largest part, is divided into two hemispheres and its outer cerebral cortex handles conscious thought, memory, reasoning and voluntary movement. The cerebellum, below and behind the cerebrum, coordinates muscular activity, balance and posture. The medulla oblongata, at the base of the brain, controls involuntary homeostatic processes such as heart rate, breathing rate and blood pressure. The hypothalamus regulates body temperature, water balance and hunger, and links the nervous and endocrine systems via the pituitary gland. The pituitary gland secretes hormones controlling many body functions. The brain is protected by the skull, meninges and cerebrospinal fluid.

A reflex action is a rapid, involuntary and stereotyped response to a stimulus, coordinated by a reflex arc so that the brain is not required to make the decision. The simplest arc runs from receptor to sensory neurone, into the spinal cord or brain, then to a motor neurone and effector; the knee-jerk (patellar) reflex is monosynaptic, with the sensory neurone synapsing directly onto the motor neurone, so no relay neurone is involved. Most protective reflexes, such as the blinking reflex and the withdrawal reflex, are polysynaptic and do include relay neurones, which allows rapid protective responses and some modification by the brain.

The nervous and endocrine systems coordinate responses by different means. The nervous system uses electrical impulses along neurones and chemical neurotransmitters across synapses; transmission is fast and the response is short-lived and precisely targeted at specific effectors. The endocrine system uses hormones secreted by glands into the blood; hormones travel to target cells with complementary receptors, so transmission is slower but the effect is longer-lasting and can reach many cells throughout the body. The two systems work together: for example, the hypothalamus links them by releasing hormones that control the pituitary gland, and the adrenal glands release adrenaline under nervous stimulation to raise heart rate and blood glucose during the fight-or-flight response.

Heart rate is set by the sinoatrial node (SAN) in the right atrium, which acts as the pacemaker and fires without nervous input. The autonomic nervous system modulates this intrinsic rhythm: sympathetic nerves release noradrenaline to speed the SAN and increase heart rate, while parasympathetic nerves (vagus) release acetylcholine to slow it. Adrenaline from the adrenal medulla raises heart rate during fight-or-flight. Chemoreceptors detect changes in blood CO₂ and pH, baroreceptors detect blood pressure changes, and the medulla oblongata integrates these inputs and adjusts autonomic outflow. A worked example: during exercise, CO₂ rises, chemoreceptors signal the medulla, sympathetic outflow increases and heart rate rises; at rest, vagal tone dominates and heart rate falls.

Mammalian skeletal muscle is made of bundles of fibres containing myofibrils. Each myofibril shows repeating sarcomeres bounded by Z-lines, with thick myosin filaments and thin actin filaments. The sliding filament model explains contraction: a nerve impulse releases calcium ions from the sarcoplasmic reticulum; Ca²⁺ binds troponin, moving tropomyosin to expose actin binding sites; myosin heads bind actin, form cross-bridges and pivot, pulling actin filaments towards the centre of the sarcomere. ATP binding detaches myosin heads and ATP hydrolysis re-cocks them for the next cycle. As actin slides over myosin, the sarcomere shortens and the muscle contracts, while the filaments themselves do not change length.

This clause asks you to examine skeletal muscle using stained sections or photomicrographs, so you must recognise tissue features from images rather than from a description. In a stained section, the stain makes structures visible: skeletal muscle fibres appear as long, parallel, unbranched cylindrical cells lying side by side in bundles. Each fibre is multinucleate, with several flattened nuclei pushed to the periphery just under the sarcolemma, unlike cardiac muscle (branched, centrally placed nuclei, intercalated discs) and smooth muscle (spindle-shaped, single central nucleus, no striations). Under higher magnification, alternating light and dark bands give the striped appearance, reflecting the arrangement of actin and myosin myofilaments into sarcomeres.