How do plants respond to their environment with hormones, and how does the mammalian nervous system control responses and muscle contraction?
5.1.5 Plant and animal responses: tropisms and the role of auxin (IAA) in phototropism; the structure and function of the mammalian nervous system (central and peripheral, voluntary and autonomic), the reflex arc and the fight-or-flight response; and the structure and the sliding filament mechanism of skeletal muscle contraction.
A focused answer to the OCR H420 5.1.5 dot point on plant and animal responses. Covers tropisms and the role of auxin in phototropism, the organisation of the mammalian nervous system, the reflex arc and fight-or-flight, and the sliding filament mechanism of muscle contraction.
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What this dot point is asking
OCR wants you to explain tropisms and the role of auxin in phototropism, describe the organisation of the mammalian nervous system and the reflex arc and fight-or-flight response, and describe the structure of skeletal muscle and the sliding filament mechanism of contraction.
The answer
Plant responses: tropisms and auxin
A tropism is a directional growth response to a stimulus: positive towards the stimulus, negative away. Plant responses are coordinated by plant hormones (growth regulators), especially auxin (IAA).
In phototropism, auxin is made at the shoot tip and moves down. When light comes from one side, auxin is redistributed to the shaded side. Auxin promotes cell elongation (by loosening cell walls), so the shaded side elongates more and the shoot bends towards the light (positive phototropism), maximising light capture. In roots, high auxin concentrations inhibit elongation, so roots show positive gravitropism (growing downwards).
The mammalian nervous system
The nervous system is organised into:
- the central nervous system (CNS): the brain and spinal cord;
- the peripheral nervous system (PNS): sensory and motor neurones connecting the CNS to the body.
The motor (output) division splits into the somatic (voluntary) nervous system (conscious control of skeletal muscle) and the autonomic (involuntary) nervous system, which controls internal organs and is itself divided into the sympathetic ("fight or flight", generally stimulatory) and parasympathetic ("rest and digest", generally calming) systems.
The reflex arc and fight-or-flight
A reflex is a rapid, automatic, protective response that does not need conscious thought. The reflex arc is: stimulus, receptor, sensory neurone, (relay neurone in the CNS), motor neurone, effector, response. Because it bypasses the conscious brain, it is fast and protects the body (for example the withdrawal reflex from a hot object).
In the fight-or-flight response, a threat triggers the sympathetic nervous system and the release of adrenaline, raising heart and breathing rate, dilating pupils, diverting blood to muscles and raising blood glucose, preparing the body for action.
Muscle structure and the sliding filament mechanism
Skeletal muscle is made of myofibrils divided into sarcomeres, the repeating units between Z lines. Each sarcomere contains thin actin and thick myosin filaments. Contraction follows the sliding filament mechanism:
- A nerve impulse releases calcium ions from the sarcoplasmic reticulum.
- Calcium binds troponin, moving tropomyosin to expose the myosin-binding sites on actin.
- Myosin heads attach to actin (forming cross-bridges) and flex (the power stroke), pulling the actin filaments past the myosin, so the sarcomere shortens.
- ATP binds the myosin head, detaching it; the head is re-cocked and the cycle repeats, sliding the filaments further.
The filaments themselves do not shorten; they slide over each other, shortening the sarcomere and the whole muscle.
Examples in context
Example 1. Houseplants on a windowsill. A plant left by a window grows towards the light because auxin redistributes to the shaded side, causing it to elongate more, a familiar demonstration of positive phototropism.
Example 2. The knee-jerk reflex. Tapping the patellar tendon stretches the muscle, triggering a spinal reflex arc that contracts the muscle and extends the leg without conscious thought, a classic reflex arc with only a sensory and motor neurone.
Try this
Q1. Explain why a shoot bends towards a light source. [3 marks]
- Cue. Auxin made at the tip moves to the shaded side; auxin promotes cell elongation, so the shaded side elongates more than the lit side and the shoot bends towards the light.
Q2. State the order of components in a reflex arc. [2 marks]
- Cue. Receptor, sensory neurone, relay neurone (in the CNS), motor neurone, effector.
Q3. Name the ion that binds to troponin to start muscle contraction. [1 mark]
- Cue. Calcium ions.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20195 marksExplain how auxin causes a plant shoot to bend towards a light source (positive phototropism).Show worked answer →
Link uneven auxin distribution to uneven growth.
Auxin (IAA) is produced at the shoot tip and is transported down the shoot. When light shines from one side, auxin moves to the shaded side of the shoot, so auxin becomes more concentrated on the side away from the light.
Auxin causes cell elongation (it increases the stretchiness of the cell walls). The shaded side, with more auxin, elongates more than the lit side, so the shoot bends towards the light.
This increases the leaves' exposure to light for photosynthesis. Markers reward auxin made at the tip, moving to the shaded side, causing greater cell elongation there, so the shoot bends towards the light.
OCR H420/01 20214 marksDescribe the sliding filament mechanism by which a skeletal muscle contracts when stimulated.Show worked answer →
Sequence the cross-bridge cycle and the role of calcium.
A nerve impulse causes calcium ions to be released from the sarcoplasmic reticulum. Calcium binds to troponin, moving tropomyosin to expose the myosin-binding sites on the actin (thin) filaments.
Myosin heads attach to actin, forming cross-bridges, and flex (the power stroke), pulling the actin filaments past the myosin so the sarcomere shortens. ATP then binds the myosin head, detaching it; the head is re-cocked and the cycle repeats, so the filaments slide further over each other.
Markers reward calcium binding troponin to expose binding sites, cross-bridge formation, the power stroke pulling actin in, and ATP detaching the head for the cycle to repeat.
Related dot points
- 5.1.3 Neuronal communication: the structure of a neurone; the establishment of the resting potential by the sodium-potassium pump; the generation of an action potential by voltage-gated channels (depolarisation and repolarisation); the all-or-nothing principle and the refractory period; saltatory conduction in myelinated neurones; and synaptic transmission by acetylcholine at a cholinergic synapse.
A focused answer to the OCR H420 5.1.3 dot point on neuronal communication. Covers neurone structure, the resting potential, the action potential and its ionic basis, the all-or-nothing principle and refractory period, saltatory conduction, and synaptic transmission by acetylcholine.
- 5.1.4 Hormonal communication: the principles of hormonal coordination and the contrast with nervous coordination; the structure and function of the adrenal glands and pancreas; the control of blood glucose concentration by insulin and glucagon (glycogenesis, glycogenolysis and gluconeogenesis); the second messenger model of adrenaline and glucagon; and the causes of type 1 and type 2 diabetes.
A focused answer to the OCR H420 5.1.4 dot point on hormonal communication. Covers hormonal versus nervous coordination, the adrenal glands and pancreas, the control of blood glucose by insulin and glucagon, the second messenger model, and the causes of type 1 and type 2 diabetes.
- 5.1.2 Homeostasis and excretion: the principles of homeostasis and negative feedback; the role of the liver in deamination and detoxification; the structure of the nephron and the processes of ultrafiltration and selective reabsorption; and osmoregulation by ADH acting on the collecting duct.
A focused answer to the OCR H420 5.1.2 dot point on homeostasis and the kidney. Covers negative feedback, the liver's role in deamination and detoxification, the nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH acting on the collecting duct.
- 5.2.2 Respiration: the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation); the role of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase; the synthesis of ATP and the role of oxygen as the final electron acceptor; and anaerobic respiration in animals (lactate) and in yeast (ethanol).
A focused answer to the OCR H420 5.2.2 dot point on respiration. Covers the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), chemiosmosis and ATP synthase, the role of oxygen, and anaerobic respiration producing lactate or ethanol.
- 5.2.1 Photosynthesis: the structure of the chloroplast; the light-dependent stage (photolysis of water, photophosphorylation and the reduction of NADP); the light-independent stage (the Calvin cycle, fixing carbon dioxide using RuBP, forming GP and TP and regenerating RuBP); and the effect of limiting factors (light intensity, carbon dioxide concentration and temperature).
A focused answer to the OCR H420 5.2.1 dot point on photosynthesis. Covers chloroplast structure, the light-dependent stage (photolysis, photophosphorylation and reduced NADP), the light-independent stage (the Calvin cycle with RuBP, GP and TP), and the effect of limiting factors.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)