LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- explain reflexes and synapses
- describe eye responses
- compare nervous and hormonal control
- explain homeostasis and tropisms
THE BIG IDEA
How nervous, hormonal and plant systems detect change and coordinate responses.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
SECTION 01
Nervous pathways
The CNS is brain and spinal cord; the PNS is all other nerves. Impulses travel along sensory, relay and motor neurones.
DETAILED EXPLANATION
- Reflex: receptor → sensory → relay → motor → effector.
- At a synapse, an impulse releases neurotransmitter; it diffuses, binds receptors and triggers the next impulse.
- Asymmetric release and receptors make transmission one-way.
SECTION 02
Sense organs and the eye
Sense organs contain receptor cells that respond to particular stimuli, including light, sound, touch, temperature and chemicals. In the eye, the cornea refracts; the iris controls the pupil; the lens focuses; the retina detects; and the optic nerve transmits. The blind spot lacks receptors.
DETAILED EXPLANATION
- Bright light: circular muscles contract and radial muscles relax, so the pupil narrows. Dim light reverses these actions.
- Near focus: ciliary muscles contract, suspensory ligaments slacken and the lens becomes thicker. Distance focus reverses this.
- Rods support dim vision; three cone types give colour; the fovea has many cones and provides sharp vision.
SECTION 03
Hormones and homeostasis
Hormones travel in blood to target organs; they act more slowly and for longer than nerve impulses. Negative feedback reverses movement from a set point.
DETAILED EXPLANATION
- Adrenal glands release adrenaline; the pancreas releases insulin and glucagon; testes release testosterone; ovaries release oestrogen.
- Adrenaline raises breathing rate, heart rate, pupil diameter and blood glucose.
- Insulin lowers glucose; glucagon raises it through liver responses. Type 1 diabetes is managed with insulin.
- Skin hairs and fatty tissue reduce heat loss. Sweat glands cool by evaporation, and shivering releases heat through muscle contraction.
- Vasodilation sends more blood through surface capillaries to increase heat loss; vasoconstriction reduces this flow. The brain coordinates these responses.
SECTION 04
Tropisms
Phototropism responds to light and gravitropism to gravity. Auxin from the shoot tip becomes unevenly distributed and stimulates cell elongation.
DETAILED EXPLANATION
- More elongation on the shaded side bends a shoot towards light.
- Shoots are normally positive to light and negative to gravity; roots show opposite directions.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Reflexes are rapid and automatic.
- Synapses use neurotransmitter.
- The eye controls light and focus.
- Negative feedback maintains conditions.
- Auxin differences cause shoot bending.