Tropic Responses (Tropisms)

A tropism is a directional growth response of a plant organ to an external stimulus. The plant grows either towards the stimulus (positive tropism) or away from it (negative tropism). Tropisms are controlled by plant hormones called auxins.

How Phototropism Works

1. Light strikes one side of the plant shoot.
2. Auxins (plant hormones) move to the darker side of the shoot.
3. Higher auxin concentration on the dark side causes faster cell elongation.
4. The shoot bends toward the light.

Plant Hormones

Auxins are the first discovered plant hormones (by Fritz Went, 1928). They are found in shoot tips, buds, young leaves, and germinating seeds. Auxins:

• Promote cell elongation
• Stimulate cell division and differentiation
• Increase cell wall elasticity

Key rule: High auxin concentration stimulates shoot growth, while low concentration stimulates root growth. This explains why shoots grow toward light (positive phototropism) and roots grow toward gravity (positive geotropism).

Structure of a Neuron

A neuron (nerve cell) is the basic unit of the nervous system. Neurons are specialized for excitability (responding to stimuli) and conductivity (carrying nerve impulses).

Key parts of a neuron:

• Cell body (soma): Contains the nucleus and organelles
• Dendrites: Short branches that receive impulses from other neurons
• Axon: Long fiber that carries impulses away from the cell body
• Myelin sheath: Fatty covering that insulates and speeds up impulse transmission
• Nodes of Ranvier: Gaps in the myelin sheath that allow saltatory conduction
• Axon terminals: End of axon where impulses are passed to other neurons or effectors

Types of Neurons

How a Nerve Impulse Travels

1. Stimulus activates a sensory receptor (e.g., touch, heat).
2. Sensory receptor generates an electrical impulse.
3. Impulse travels along sensory neuron to the central nervous system.
4. Relay neuron processes and passes the impulse.
5. Motor neuron carries impulse to effector (muscle or gland).
6. Effector produces a response (movement or secretion).

Sense organs contain sensory receptors that detect specific stimuli and convert them into nerve impulses.

The Eye (Sense of Vision)

The eye detects light and converts it into electrical signals for the brain.

Key structures:

• Cornea: Transparent front layer that refracts light
• Iris: Colored part; controls pupil size to regulate light entry
• Pupil: Opening in the iris; appears black because light is absorbed
• Lens: Focuses light onto the retina
• Retina: Light-sensitive layer containing photoreceptors (rods and cones)
• Optic nerve: Carries impulses from retina to the brain

How the eye works: Light enters through the cornea and pupil, the lens focuses it on the retina, photoreceptors convert light into electrical impulses, and the optic nerve carries these to the brain for interpretation.

The Ear (Sense of Hearing and Balance)

The ear detects sound waves and helps maintain balance.

Parts of the ear:

• Outer ear: Pinna collects sound waves; ear canal directs them to the eardrum
• Middle ear: Contains ossicles (malleus, incus, stapes) that amplify vibrations; Eustachian tube equalizes pressure
• Inner ear: Cochlea (hearing); semicircular canals (balance)

Hearing process: Sound waves → pinna → ear canal → eardrum (vibration) → ossicles → oval window → cochlea fluid → hair cells → electrical impulse → brain via auditory nerve.

The endocrine system uses hormones—chemical messengers released into the bloodstream—to coordinate body functions.

Key Endocrine Glands and Their Functions

Nervous System vs. Endocrine System

Both systems work together: For example, when you see a snake, your nervous system causes an immediate "fight or flight" response, while your endocrine system releases adrenaline to maintain that state.

Scenario: A student touches a hot stove.

1. Stimulus: Heat from the stove
2. Receptor: Temperature receptors in the skin
3. Sensory neuron: Carries impulse from skin to spinal cord
4. Relay neuron: In spinal cord, connects to motor neuron
5. Motor neuron: Carries impulse to arm muscles
6. Effector: Arm muscles contract, pulling hand away

This reflex arc happens in milliseconds, protecting the body from damage.

In Tanzania, understanding plant tropisms helps farmers maximize crop yields. For example, rice farmers in Morogoro can use knowledge of phototropism to understand why rice seedlings lean toward sunlight, while understanding hydrotropism helps them design irrigation systems that encourage roots to grow deeper toward water sources, making plants more drought-resistant during dry seasons in regions like Dodoma.