Control and Coordination

The One Thing

Control and coordination enable organisms to detect stimuli, process information and produce suitable responses. Animals use rapid nervous signals and slower, longer-lasting hormonal messages, whereas plants coordinate responses mainly through hormones and growth movements.

Reactions, Processes and Experiments

Key Terms

• Stimulus: A change in the internal or external environment that produces a response.
• Response: The reaction of an organism to a stimulus.
• Receptor: A specialised cell or sense organ that detects a stimulus.
• Neuron: The structural and functional unit of the nervous system that carries nerve impulses.
• Dendrite: A branched part of a neuron that receives signals.
• Cell body: The central part of a neuron containing the nucleus and most cell organelles.
• Axon: A long fibre that carries nerve impulses away from the cell body.
• Synapse: The small gap between two neurons across which a signal is transmitted using chemicals.
• Nerve impulse: An electrical signal that travels along a neuron.
• Reflex action: A rapid, automatic and involuntary response to a stimulus, such as withdrawing a hand from a hot object.
• Reflex arc: The pathway followed by a nerve impulse during a reflex action, usually involving a receptor, sensory neuron, spinal cord, motor neuron and effector.
• Central nervous system: The brain and spinal cord, which process information and coordinate responses.
• Peripheral nervous system: The nerves that connect the brain and spinal cord with the rest of the body.
• Brain: The main control centre that receives information, interprets it and coordinates responses.
• Forebrain: The region involved in thinking, memory, sensory interpretation, reasoning and voluntary actions.
• Midbrain: The region involved in controlling some reflex movements of the head, neck and eyes.
• Hindbrain: The region containing the cerebellum, pons and medulla; it helps control balance, posture and involuntary activities.
• Spinal cord: A nervous tissue column protected by the vertebral column that conducts impulses and controls many reflex actions.
• Effector: A muscle or gland that carries out a response.
• Hormone: A chemical messenger produced by an endocrine gland and transported through the blood to a target organ.
• Endocrine gland: A ductless gland that releases hormones directly into the bloodstream.
• Pituitary gland: A gland at the base of the brain that regulates growth and controls the activity of several other endocrine glands.
• Thyroxine: A hormone produced by the thyroid gland that regulates metabolism; iodine is needed for its production.
• Adrenaline: A hormone released during fear, anger or stress that prepares the body for emergency action.
• Insulin: A hormone produced by the pancreas that helps regulate blood glucose levels.
• Testosterone: A male sex hormone that promotes male secondary sexual characteristics and reproductive development.
• Oestrogen: A female sex hormone that promotes female secondary sexual characteristics and helps regulate the reproductive cycle.
• Auxin: A plant hormone that promotes cell growth and is involved in phototropism and other growth responses.
• Gibberellin: A plant hormone that promotes stem growth and seed germination.
• Cytokinin: A plant hormone that promotes cell division and delays ageing in plant tissues.
• Abscisic acid: A plant hormone that generally inhibits growth and helps plants respond to stress, including drought.
• Phototropism: Directional growth of a plant part in response to light; shoots usually grow towards light.
• Geotropism: Directional growth in response to gravity; roots generally grow towards gravity and shoots away from it.
• Hydrotropism: Growth of a plant part in response to water; roots usually grow towards moisture.
• Chemotropism: Growth in response to chemicals, such as the growth of a pollen tube towards the ovule.
• Thigmotropism: Growth in response to touch, such as tendrils coiling around a support.
• Nastic movement: A non-directional movement in response to a stimulus, such as folding of touch-me-not leaves.

Easily Confused

• Nervous control vs hormonal control: Nervous control is generally rapid, short-lived and transmitted through specific nerve pathways, whereas hormonal control is generally slower, longer-lasting and transported through the bloodstream.
• Reflex action vs voluntary action: A reflex action is rapid, automatic and involuntary, whereas voluntary actions involve conscious control and are associated particularly with the forebrain.
• Reflex arc vs central nervous system: A reflex arc is the pathway followed during a reflex action; the central nervous system consists of the brain and spinal cord, which process information and coordinate responses.
• Cerebellum vs medulla: The cerebellum helps maintain posture and balance and coordinates precise voluntary movements, whereas the medulla controls involuntary activities such as breathing, heartbeat, blood pressure and swallowing.
• Forebrain vs hindbrain: The forebrain is involved in thinking, memory, reasoning, sensory interpretation and voluntary actions, whereas the hindbrain helps control balance, posture and involuntary activities.
• Positive tropism vs negative tropism: Positive tropism is growth towards a stimulus, whereas negative tropism is growth away from it.
• Tropic movement vs nastic movement: Tropic movement is directional growth in response to a stimulus, whereas nastic movement is non-directional and may result from changes in cell water content.
• Auxin vs abscisic acid: Auxin generally promotes cell growth and elongation, whereas abscisic acid generally inhibits growth and helps plants respond to stress, including drought.
• Thyroxine vs insulin: Thyroxine regulates metabolism and requires iodine for production, whereas insulin helps regulate blood glucose levels.
• Endocrine gland vs exocrine gland: An endocrine gland is ductless and releases hormones directly into the bloodstream; an exocrine gland is not described by this material as releasing hormones directly into the blood.

What Gets Asked

• Questions may require the complete coordination pathway: stimulus → receptor → sensory neuron → central nervous system → motor neuron → effector → response. Omitting a component or confusing a sensory neuron with a motor neuron costs marks.
• Reflex-action questions may ask why the response is rapid. The required point is that the spinal cord can coordinate the response without waiting for detailed processing by the brain; the brain is informed soon after.
• Brain-function questions may distinguish the forebrain, midbrain, cerebellum, pons and medulla. A common error is assigning balance and precise voluntary movement to the medulla instead of the cerebellum, or assigning breathing and heartbeat to the cerebellum instead of the medulla.
• Endocrine questions may ask for the gland, hormone and effect. Specific pairings include thyroid–thyroxine, adrenal glands–adrenaline, pancreas–insulin and pituitary gland–growth hormone. Confusing the gland with the hormone loses marks.
• Questions on one-sided light may require the mechanism of phototropism: auxin accumulates on the shaded side of the shoot, causing greater elongation there and bending towards the light. Stating only that the shoot grows towards light omits the examined mechanism.
• Plant-movement questions may require distinctions among phototropism, geotropism, hydrotropism, chemotropism, thigmotropism and nastic movement. The key slip is treating all plant movements as non-directional or failing to identify the stimulus involved.