Control and Coordination
In animals, control and coordination are provided by the nervous system and the hormonal (endocrine) system. In plants, coordination is achieved only through chemical substances (plant hormones), as they have no nervous system. A change in the environment that an organism responds to is called a stimulus.
Nervous System
Neuron (Nerve Cell)
The neuron is the structural and functional unit of the nervous system — the longest cell in the body. Parts:
- Dendrites — receive information (stimulus) and generate a nerve impulse.
- Cell body (cyton) — contains nucleus and cytoplasm; the impulse travels through it.
- Axon — long fibre that carries the impulse away to the nerve ending.
- Nerve ending / Synapse — the gap between the nerve ending of one neuron and the dendrite of the next. Chemicals (neurotransmitters) cross this gap to pass the impulse.
Impulse flow: Dendrite → Cell body → Axon → Nerve ending → Synapse → next neuron. At the synapse, electrical impulse triggers release of chemicals which cross the gap and start a new impulse in the next neuron. The synapse ensures impulse travels in one direction.
Reflex Action and Reflex Arc
A reflex action is a rapid, automatic, involuntary response to a stimulus (e.g. pulling hand away from a hot object). It does not involve conscious thinking by the brain; it is controlled by the spinal cord, making the response quicker.
The pathway taken by the nerve impulse in a reflex action is the reflex arc:
Receptor (skin) → Sensory neuron → Spinal cord (relay neuron) → Motor neuron → Effector (muscle)
Human Brain
The brain is the main coordinating centre, protected by the skull and cerebrospinal fluid. Three major parts:
- Fore-brain (Cerebrum) — main thinking part; controls voluntary actions, seat of sensations, memory, reasoning, intelligence; has separate regions for hearing, smell, sight, etc.
- Mid-brain — connects fore- and hind-brain; controls involuntary actions like changes in pupil size and some reflexes.
- Hind-brain — has three parts: Cerebellum (precision of voluntary actions and maintaining posture/balance), Pons, and Medulla oblongata (controls involuntary actions like blood pressure, salivation, vomiting).
Spinal Cord
The spinal cord is a cylindrical structure extending from the medulla; it is involved in reflex actions and connects the brain with the rest of the body. Together the brain and spinal cord form the Central Nervous System (CNS); the nerves form the Peripheral Nervous System (PNS).
Nervous tissue limitations: electrical impulses are fast but reach only cells connected by nervous tissue, not every cell; and cells take time to reset. Hence chemical (hormonal) communication is also used.
Coordination in Plants
Plants respond to stimuli without a nervous system, using two types of movement:
- Immediate response to touch (nastic) — e.g. the sensitive plant Mimosa pudica (touch-me-not) folds its leaves when touched. This is independent of growth; caused by changes in water content of cells.
- Growth-dependent movement (tropism) — directional growth response to a stimulus.
Tropisms (Directional Growth Movements)
| Tropism | Stimulus | Example |
|---|---|---|
| Phototropism | Light | Shoots grow towards light (positive); roots away (negative) |
| Geotropism | Gravity | Roots grow towards gravity (positive); shoots away (negative) |
| Hydrotropism | Water | Roots grow towards water |
| Chemotropism | Chemicals | Pollen tube grows towards the ovule |
| Thigmotropism | Touch | Tendrils coil around a support |
In phototropism, the plant hormone auxin diffuses to the shaded side of the shoot tip, causing cells there to grow longer, so the shoot bends towards light.
Plant Hormones (Phytohormones)
| Hormone | Function |
|---|---|
| Auxin | Promotes cell elongation; controls phototropism |
| Gibberellin | Helps in the growth of the stem |
| Cytokinin | Promotes cell division; more in fruits and seeds |
| Abscisic acid (ABA) | Inhibits growth; causes wilting/closing of stomata (stress hormone) |
Hormones in Animals (Endocrine System)
Hormones are chemical messengers secreted by endocrine glands (ductless glands) directly into the blood, which carries them to target organs. They act in very small amounts.
| Gland | Hormone | Function |
|---|---|---|
| Pituitary | Growth hormone | Controls growth of the body (deficiency → dwarfism; excess → gigantism) |
| Thyroid | Thyroxine | Regulates carbohydrate, protein and fat metabolism; needs iodine (deficiency → goitre) |
| Pancreas | Insulin | Regulates blood sugar level (deficiency → diabetes) |
| Adrenal | Adrenaline | Prepares body for emergency ("fight or flight"); raises heartbeat, breathing, blood pressure |
| Testis (male) | Testosterone | Controls male secondary sexual characters |
| Ovary (female) | Oestrogen | Controls female secondary sexual characters |
Feedback Mechanism
The timing and amount of hormone released are regulated by feedback mechanisms. For example, when blood sugar rises, the pancreas releases more insulin; as sugar falls to normal, insulin secretion decreases. This keeps hormone levels balanced.
Exam tips: Neuron parts in impulse order (dendrite→cell body→axon→synapse). Reflex arc is controlled by the SPINAL CORD, not the brain. Cerebellum = balance/posture; Medulla = involuntary actions (heartbeat, BP). Iodine is needed for thyroxine (why iodised salt is advised). Adrenaline = emergency hormone. Draw a neat labelled diagram of neuron and reflex arc when asked.
Note on rationalisation: The sub-topic "Movements in Plants" is retained; some detailed content on the chemical nature of hormones and additional examples were trimmed in the latest NCERT edition. Study the sections above as per the current textbook.