Plant and Animal Responses¶

Part of Module 5: Communication, homeostasis and energy.

Both plants and animals coordinate responses to their environment, but they do so through fundamentally different systems. Plants use chemical growth regulators — hormones that alter the rate and direction of cell division and elongation. Animals rely on integrated nervous and endocrine systems working together. This topic also covers the anatomy of the mammalian nervous system and brain, control of heart rate, and the structure of skeletal muscle.

Learning Objectives¶

Plant Responses: Tropisms¶

A tropism is a directional growth response of a plant to a directional stimulus. The direction of growth is either towards or away from the stimulus.

Tropisms are adaptive: shoots grow towards light (photosynthesis) and upward (support structure); roots grow downward (anchoring, water uptake) and towards water.

Plant Growth Regulators¶

Plant growth regulators (plant hormones) are signalling molecules produced in low concentrations that coordinate plant development and responses. Unlike animal hormones, they are not secreted by specialised glands; they are produced locally and transported through the plant via the phloem or by diffusion.

Auxins (e.g. IAA — Indole-3-Acetic Acid)¶

Produced in: shoot tips, developing leaves and seeds

Main effects: - Promotes cell elongation at low concentrations in shoot tissue - Inhibits cell elongation at high concentrations in root tissue (roots are much more sensitive to auxin) - Inhibits growth of lateral buds (apical dominance — see below) - Promotes rooting from cuttings

Mechanism of action (acid growth hypothesis): 1. Auxin binds to receptors in the plasma membrane or cytoplasm 2. Auxin stimulates the pumping of H⁺ ions into the cell wall (acidification of the wall) 3. Low pH in the cell wall activates expansins — enzymes that break non-covalent bonds between cellulose microfibrils and matrix polysaccharides 4. The loosened cell wall allows the cell to take in water by osmosis and expand 5. Cell elongation occurs

Phototropism mechanism: When a shoot is illuminated from one side, auxin produced at the tip is redistributed towards the shaded side of the shoot. The shaded side therefore has higher auxin concentration, its cells elongate more, and the shoot bends towards the light.

Apical Dominance¶

Apical dominance is the suppression of lateral bud growth while the apical bud (tip) is present. The mechanism involves an interaction between auxin, abscisic acid (ABA) and cytokinin:

• High auxin production at the apex maintains high ABA levels, which inhibit lateral bud outgrowth
• When the apex is removed (e.g. by pruning), auxin levels fall, ABA levels fall, and lateral buds begin to grow
• Cytokinins (produced in roots) promote lateral bud growth; when the apex is present, cytokinin is concentrated near the auxin-inhibited bud, but when the apex is removed, cytokinin spreads out and promotes branching

This is why pinching out growing tips makes plants bushier.

Leaf Abscission¶

The dropping of leaves in deciduous plants in autumn is controlled by changes in hormone balance: - Young leaves produce cytokinins (which maintain the leaf as a metabolic sink) and auxin (which prevents abscission layer cell separation) - As the leaf ages, cytokinin and auxin levels decline - Rising ethene levels promote the production of cellulase enzymes that break down cell walls in the abscission layer (a zone of cells at the base of the petiole) - Below the abscission layer, a protective layer of suberin (a waxy polymer) is deposited to seal the wound and prevent pathogen entry

Gibberellins¶

Produced in: young leaves, seeds, roots

Main effects: - Stimulates stem elongation (promotes cell division and elongation in internodes) - Breaks seed dormancy and stimulates germination (activates enzymes e.g. amylase to digest endosperm starch) - Promotes flowering in some long-day plants - Delays senescence of citrus fruits

Commercial uses: elongating grape stalks to improve bunch size; stimulating seed germination; delaying fruit senescence; brewing (gibberellins stimulate production of amylase in barley grains for malt production); increasing sugar cane yield.

Abscisic Acid (ABA)¶

Main effects: - Inhibits seed germination (maintains dormancy) - Causes stomata to close by triggering K⁺ efflux from guard cells (response to water stress/drought) - Promotes leaf abscission - Slows growth in unfavourable conditions

ABA is often described as a stress hormone — it prepares the plant to survive adverse conditions.

Ethene¶

Main effects: - Gaseous hormone; diffuses through intercellular spaces - Promotes fruit ripening (stimulates cellulase and pectinase production that soften fruit) - Promotes leaf abscission - Promotes lateral root growth

Commercial use: ethene gas (or ethylene-releasing compounds) is used to ripen fruit transported while still unripe, or to synchronise ripening for harvest.

Cytokinins¶

Produced in: roots, developing seeds

Main effects: - Promote cell division (cytokinesis) — hence the name - Promote shoot growth and delay leaf senescence (yellowing) - Interact with auxin to control apical dominance

Commercial use: prevent premature yellowing of cut lettuce leaves; used in tissue culture to stimulate shoot growth (see 6.2.1 Cloning and biotechnology).

Hormone Interactions¶

Plant hormones do not act in isolation — they interact, often in opposing ways:

• Synergistic interactions: two hormones work together to produce a greater combined response than either would alone (e.g. auxin and cytokinin in cell division during tissue culture).
• Antagonistic interactions: two hormones have opposing effects; the balance between them determines the plant's response (e.g. auxin promotes and cytokinin opposes apical dominance; ABA inhibits and gibberellin promotes seed germination).

Plant Responses to Abiotic Stress¶

Photoperiodism and Phytochrome¶

Photoperiodism is the sensitivity of plants to the relative lengths of light and dark periods in a 24-hour cycle. It aligns plant responses (dormancy, flowering, leaf fall) to the seasons.

Plants detect photoperiod using phytochrome, a light-sensitive pigment that exists in two interconvertible forms: - Pr absorbs red light (wavelength ~660 nm) → converted to Pfr - Pfr absorbs far-red light (wavelength ~730 nm) → converted back to Pr; Pfr also slowly degrades in darkness

During the day, Pr is converted to Pfr. During the night, Pfr slowly reverts to Pr. The ratio of Pfr:Pr acts as a clock signal, with the duration of darkness being the key trigger for many responses including: - Breaking dormancy in buds (spring) - Initiating tuber formation - Timing the flowering phase (critical photoperiod)

In deciduous trees, lengthening of the dark period in autumn is detected via phytochrome, triggering leaf abscission and entry into dormancy.

Etiolation¶

Etiolation is the rapid, spindly upward growth that occurs when a plant is grown in darkness. The seedling elongates quickly to reach light for photosynthesis, but lacks chlorophyll (appearing pale and elongated). Gibberellins are responsible for the extreme internode elongation during etiolation. Once the plant reaches light, gibberellin levels fall and normal growth resumes.

Plant Responses to Herbivory¶

Plants cannot move away from herbivores, so they have evolved a range of physical and chemical defences.

Physical Defences¶

Chemical Defences¶

Many plants produce toxic or unpalatable compounds:

• Tannins: bind to proteins in herbivore digestive systems, reducing digestibility
• Alkaloids: nitrogen-containing bitter compounds; examples include:
• Morphine, cocaine — act as psychoactive drugs, making the plant aversive
• Caffeine, nicotine — toxic or unpleasant to many herbivores
• Terpenoids: volatile or resinous compounds with repellent or toxic properties

These chemicals are concentrated in the most vulnerable parts of the plant (leaves, seeds).

Volatile Organic Compounds (VOCs)¶

When damaged by herbivores, many plants release volatile organic compounds (VOCs) into the air. VOCs serve multiple purposes:

1. Attracting predators of the herbivore (e.g. VOCs from caterpillar-damaged cabbage attract parasitic wasps)
2. Repelling other herbivores from feeding on the same plant
3. Signalling to neighbouring plants — nearby plants detect airborne VOCs and upregulate their own defences before being attacked

Rapid Movement Responses¶

Some plants respond rapidly to touch:

• Mimosa pudica (sensitive plant): leaves fold inwards and the plant droops within seconds of being touched. Caused by rapid changes in turgor pressure (ion redistribution) in cells at the base of leaflets. The response dislodges or deters small insect herbivores and may make the plant appear less palatable. The plant reopens as cells regain turgor.

Commercial Uses Summary¶

Organisation of the Mammalian Nervous System¶

The mammalian nervous system is divided into:

Central nervous system (CNS): - Brain and spinal cord - Composed of grey matter (neuronal cell bodies and unmyelinated axons) and white matter (myelinated axons)

Peripheral nervous system (PNS): - All nerves outside the CNS - Somatic nervous system: voluntary control of skeletal muscles; conscious sensory information from the skin, sense organs and muscles - Autonomic nervous system: involuntary control of smooth muscle, cardiac muscle and glands

The autonomic nervous system has two divisions with opposing (antagonistic) effects:

Both divisions have a preganglionic neurone releasing ACh onto a ganglion cell, and a postganglionic neurone reaching the effector. The transmitter released at the effector differs between the two divisions.

The Human Brain¶

The brain is the integrating centre of the CNS. Major regions and their functions:

Cerebral Lobes and Localisation¶

Different parts of the cerebrum are specialised for different functions — a concept called localisation of function: - Parietal lobe: somatosensory processing, spatial awareness, orientation - Occipital lobe (visual cortex): processes visual information - Temporal lobe (auditory cortex): processes sound; also involved in memory and language - Frontal lobe: voluntary motor control (motor cortex); planning, decision-making, personality

Control of Heart Rate¶

The heart is myogenic — it can initiate its own contractions without nervous input because the cardiac muscle contains cells that spontaneously depolarise. However, heart rate is regulated by the nervous system via the sinoatrial node (SAN).

Cardiac Conduction System¶

1. Sinoatrial node (SAN): a region of specialised muscle cells in the right atrium wall; the pacemaker; generates the electrical wave that initiates each heartbeat
2. The wave spreads across both atria, causing simultaneous atrial contraction
3. The wave reaches the atrioventricular node (AVN) between the atria; a slight delay here allows the atria to fully contract before ventricular contraction begins
4. The wave passes down the bundle of His (bundle of conducting fibres) in the interventricular septum
5. It spreads via Purkyne fibres up through the ventricle walls from the apex upwards, causing simultaneous ventricular contraction that forces blood up into the aorta and pulmonary artery

Nervous Control of Heart Rate¶

The SAN receives two nerves from the cardiovascular centre in the medulla oblongata:

Factors That Alter Heart Rate¶

The cardiovascular centre receives inputs from several receptor types:

• Chemoreceptors in the carotid arteries, aortic arch and medulla: detect falling pH (rising CO₂ from exercise) → send impulses to cardiovascular centre → sympathetic stimulation → increased heart rate
• Stretch receptors in muscles and joints: detect movement → signal exercise is occurring → increased heart rate
• Baroreceptors in the carotid sinus and aortic arch: detect blood pressure; falling pressure → increased heart rate; rising pressure → decreased heart rate
• Adrenaline (from adrenal medulla during stress): binds directly to SAN cells → increased rate and force of contraction

Skeletal Muscle and the Sliding Filament Theory¶

Skeletal Muscle Structure¶

Skeletal muscle is composed of muscle fibres — elongated multinucleate cells enclosed in a plasma membrane called the sarcolemma.

Sarcomere Structure¶

During contraction, the I bands and H zone shorten, but the A band length stays the same — because the myosin filaments do not change length, only the degree of overlap changes. This is the sliding filament theory.

Mechanism of Contraction (Cross-Bridge Cycle)¶

1. Action potential arrives at the neuromuscular junction; ACh released; sarcolemma depolarised
2. Depolarisation travels down T-tubules into the fibre
3. Sarcoplasmic reticulum releases Ca²⁺ ions into the sarcoplasm
4. Ca²⁺ binds to troponin (a regulatory protein on the actin filament)
5. Troponin changes shape, moving tropomyosin away from the myosin-binding sites on actin
6. Myosin heads bind to actin, forming cross-bridges
7. Myosin heads perform a power stroke — they pivot, pulling the actin filament towards the centre of the sarcomere
8. ATP binds to the myosin head, causing it to detach from actin (cross-bridge detachment requires ATP)
9. Hydrolysis of ATP (by myosin ATPase) provides energy for the myosin head to return to its original position
10. If Ca²⁺ is still present, the cycle repeats; the actin slides further inward

When the action potential ceases, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum (using ATP). Tropomyosin covers the myosin-binding sites again and the muscle relaxes.

ATP is needed for: - Providing energy for the power stroke (myosin head return) - Detaching myosin from actin - Pumping Ca²⁺ back into the sarcoplasmic reticulum after contraction - Pumping Na⁺/K⁺ to restore resting potential in the sarcolemma

Sources of ATP for Muscle Contraction¶

Slow-Twitch and Fast-Twitch Muscle Fibres¶

Key Terms¶

• Tropism: a directional growth response of a plant to a directional stimulus.
• Phototropism: growth response to light.
• Gravitropism: growth response to gravity.
• Auxin: plant growth regulator that promotes cell elongation in shoots and redistributes during tropic responses.
• Apical dominance: inhibition of lateral bud growth while the apical bud remains active.
• Abscission layer: zone of cells at the base of a leaf stalk where separation occurs during leaf fall.
• Gibberellin: plant growth regulator involved in stem elongation and seed germination.
• Abscisic acid (ABA): plant growth regulator associated with stomatal closure and seed dormancy.
• Ethene: gaseous plant hormone involved in fruit ripening and abscission.
• Cytokinin: plant growth regulator that promotes cell division and delays senescence.
• Photoperiodism: sensitivity of plants to the relative lengths of light and dark periods, used to align responses to the seasons.
• Phytochrome: light-sensitive plant pigment that exists in two interconvertible forms (Pr and Pfr) and mediates photoperiodic responses.
• Etiolation: rapid, spindly growth in the absence of light, driven by high gibberellin levels, allowing a seedling to reach light.
• Alkaloid: nitrogen-containing bitter compound used as a chemical defence against herbivores (e.g. caffeine, nicotine, morphine).
• Volatile organic compound (VOC): airborne plant chemical released in response to herbivory that can attract predators of the herbivore or warn neighbouring plants.
• Autonomic nervous system: involuntary division of the nervous system controlling smooth muscle, cardiac muscle, and glands.
• Sarcomere: repeating functional unit of a myofibril responsible for muscle contraction.

Connected Pages¶

• 5.1.1 Communication and homeostasis (negative feedback and thermoregulation)
• 5.1.3 Neuronal communication (action potentials, neuromuscular junction)
• 5.1.4 Hormonal communication (adrenaline, second messenger)
• 5.2.2 Respiration (ATP generation for muscle contraction)
• 6.2.1 Cloning and biotechnology (commercial uses of plant hormones)
• Module 5: Communication, homeostasis and energy