2j Co-ordination and Response¶

Part of 2 Structure and Functions in Living Organisms.

Co-ordination links stimulus detection to effective response. The topic includes both plant responses and the faster nervous or hormonal control systems used in humans to maintain stable internal conditions.

Learning Objectives¶

Stimulus, Receptor and Effector¶

Organisms respond to environmental change so they can survive. Every coordinated response requires: - A stimulus — a change in the environment (e.g. heat, light, sound). - A receptor — a cell or organ that detects the stimulus (e.g. thermoreceptors in the skin). - An effector — a muscle or gland that carries out the response (e.g. sweat glands or muscles).

Homeostasis is the maintenance of a stable internal environment. In this course, key examples include the regulation of body temperature and blood water content. Homeostasis matters because enzymes and cells work properly only within narrow ranges of conditions.

Plant Responses¶

Plants cannot move from place to place, but they show directional growth responses to environmental stimuli called tropisms. Hormones called auxins co-ordinate these responses.

Phototropism is a response to light: - When a shoot is illuminated from one side, auxin moves to the shaded side. - Auxin stimulates cell elongation, so cells on the shaded side grow longer than those on the lit side. - The shoot bends towards the light source (positive phototropism). - Bending towards light enables more photosynthesis.

Gravitropism (geotropism) is a response to gravity: - In a shoot growing horizontally: auxin moves to the lower side, cells there grow more, and the shoot curves upwards (negative gravitropism). Growing away from gravity helps the shoot access more light. - In a root growing horizontally: auxin also moves to the lower side, but root cells are inhibited rather than stimulated by high auxin concentrations. Cells on the upper side grow more, bending the root downwards (positive gravitropism). This directs roots towards water and minerals and anchors the plant.

When auxin distribution becomes equal again, growth becomes straight.

Nervous and Hormonal Communication¶

The central nervous system (CNS) consists of the brain and spinal cord. The pathway for a nervous response is:

1. Receptor cells in sense organs detect a stimulus and convert it to an electrical impulse.
2. The impulse travels along sensory neurones to the CNS.
3. The CNS processes the information and sends a response impulse along motor neurones to effectors.
4. Effectors (muscles or glands) carry out the response.

Synapses are gaps between neurones. Transmission across a synapse is chemical: when an electrical impulse reaches the end of an axon, it triggers the release of neurotransmitters, which diffuse across the gap and bind to receptors on the next neurone, stimulating the next electrical impulse.

Reflexes and the Eye¶

The reflex arc is a rapid, automatic response to a dangerous stimulus that bypasses conscious thought:

1. Stimulus detected by receptors (e.g. thermoreceptors in fingertips detecting heat).
2. Impulse travels along a sensory neurone to the spinal cord.
3. The impulse is passed to a relay neurone in the spinal cord (CNS).
4. The impulse travels along a motor neurone to the effector.
5. The effector produces the response (e.g. biceps muscle contracts, arm moves away from heat).

The eye is a receptor organ adapted to detect light:

Accommodation (focusing): - Near object: ciliary muscles contract, suspensory ligaments slacken, the lens becomes fatter (more curved), light is refracted more strongly, and converges on the retina. - Distant object: ciliary muscles relax, suspensory ligaments are pulled taut, the lens becomes thinner (less curved), light is refracted less strongly, and still converges on the retina.

Pupil size: - Bright light: circular muscles in the iris contract and radial muscles relax, making the pupil smaller. This reduces the amount of light reaching the retina to prevent damage. - Dim light: radial muscles contract and circular muscles relax, making the pupil larger, allowing more light in to improve vision.

Temperature Regulation¶

Body temperature in humans is maintained at around 37 °C by the thermoregulatory centre in the brain, which receives information from thermoreceptors in the skin and monitors blood temperature directly.

When too hot: - Sweat is produced by sweat glands; water evaporating from the skin surface removes heat energy. - Vasodilation — blood vessels near the skin surface widen, so more blood flows close to the surface, increasing heat loss by radiation and conduction.

When too cold: - Sweating stops. - Skeletal muscles contract rapidly (shivering), generating heat from respiration. - Hairs stand on end, trapping a layer of warm air near the skin. - Vasoconstriction — blood vessels near the skin surface narrow, reducing blood flow to the surface and conserving heat.

Hormones¶

Hormones are chemical messengers secreted by endocrine glands into the bloodstream. They act on specific target organs.

Common Confusions¶

• Hormonal vs nervous control: Nervous control is fast and specific; hormonal control is slower, longer-lasting and carried in the blood to target organs.
• Phototropism direction in roots vs shoots: Both roots and shoots move auxin to the shaded/lower side. In shoots, auxin stimulates growth, so the shaded side grows longer and the shoot bends towards light. In roots, high auxin concentrations inhibit growth, so the upper side grows more and the root bends downwards.
• Accommodation: Ciliary muscles contract (not relax) to make the lens fatter for near objects.

Key Terms¶

• Homeostasis: maintenance of a stable internal environment.
• Receptor: a cell or organ that detects a stimulus.
• Effector: a muscle or gland that carries out a response to a stimulus.
• Phototropism: a directional growth response of a plant to light.
• Gravitropism (geotropism): a directional growth response of a plant to gravity.
• Auxin: a plant hormone that regulates cell elongation and directional growth.
• Synapse: a junction between two neurones where neurotransmitters are released.
• Neurotransmitter: a chemical that carries a signal across a synapse from one neurone to the next.
• Reflex arc: the nerve pathway involved in a rapid, automatic reflex response.
• Accommodation: the process by which the eye changes lens shape to focus on objects at different distances.
• Vasodilation: widening of blood vessels near the skin surface to increase heat loss.
• Vasoconstriction: narrowing of blood vessels near the skin surface to conserve heat.
• Insulin: a hormone that lowers blood glucose by promoting glycogen storage.
• Adrenaline: a hormone that prepares the body for increased activity.
• FSH: follicle-stimulating hormone; causes egg maturation in the ovary.
• LH: luteinising hormone; triggers ovulation.

Connected Pages¶

• 2i Excretion
• 3a Reproduction
• IGCSE Biology Key Terms Glossary