What is the fundamental difference between a taxis and a kinesis?

A taxis is a directional response where the organism moves toward or away from a stimulus. A kinesis is a non-directional response where the organism changes its speed or rate of turning randomly to stay in or find a favorable environment.

How does the effect of IAA differ between plant shoots and plant roots?

In shoots, a high concentration of IAA stimulates cell elongation, causing growth toward light. In roots, a high concentration of IAA inhibits cell elongation, causing the root to grow downward with gravity.

Compare the connectivity of rod cells and cone cells to bipolar neurones.

Multiple rod cells share a single bipolar neurone (spatial summation), which increases sensitivity but lowers acuity. Each cone cell usually connects to its own bipolar neurone, which results in high visual acuity but lower sensitivity to light.

What is a common mistake when describing the role of the brain in a reflex arc?

Students often assume the brain is not involved at all; however, while the reflex action itself is processed in the spinal cord for speed, sensory information is still sent to the brain so the stimulus can be consciously perceived after the event.

Why is it incorrect to say that the nervous system 'starts' the heartbeat?

The heart is myogenic, meaning the Sinoatrial Node (SAN) generates its own electrical impulses to initiate contraction. The nervous system (via the autonomic branch) only speeds up or slows down this pre-existing rhythm.

What error occurs if a student describes IAA as causing 'cell division'?

IAA influences cell elongation (the stretching of existing cells), not cell division (mitosis). Confusing these two processes leads to a misunderstanding of how plant tropisms actually occur.

Define 'Generator Potential'.

A generator potential is a non-propagated, graded change in the membrane potential of a receptor cell in response to a stimulus. If it exceeds a threshold, it triggers an action potential in the sensory neurone.

What are 'stretch-mediated sodium channels'?

These are specialized ion channels in the membrane of a Pacinian corpuscle that open physically when the membrane is deformed by pressure, allowing sodium ions to enter the neurone.

Myogenic refers to muscle contraction that is initiated from within the muscle itself, rather than by external nerve impulses. The heart is the primary example of myogenic tissue in the body.

How does the sympathetic nervous system affect heart rate?

The sympathetic nervous system sends impulses to the SAN via the accelerator nerve, releasing noradrenaline. This increases the frequency of electrical impulses generated by the SAN, thereby increasing heart rate.

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Organisms Respond to Changes in their Environments (A Level only)

Survival & Response

Summary

Survival and response encompasses the physiological and behavioral mechanisms by which organisms detect environmental changes and coordinate appropriate actions to maintain homeostasis and increase the probability of survival. This involves a complex interplay between sensory receptors, coordination systems (nervous and hormonal), and effectors that execute responses ranging from simple plant growth movements to rapid animal reflexes and autonomic heart rate regulation.

1. The Stimulus-Response Pathway

Stimuli and Detection: A stimulus is a detectable change in the internal or external environment of an organism. These changes are detected by specialized cells called receptors, which are specific to certain types of energy, such as light, heat, or pressure.
Coordination Systems: Once a stimulus is detected, the information is sent to a coordinator. In animals, this is typically the nervous system (using rapid electrical impulses) or the endocrine system (using slower, long-lasting chemical hormones) to process the information and determine the appropriate response.
Effectors and Response: The coordinator sends signals to effectors, which are muscles or glands that carry out the physical response. This sequence ensures that organisms can find food, avoid predation, and maintain optimal conditions for metabolic processes.

Flowchart showing the sequence from stimulus to response.

2. Plant Growth Factors: Indoleacetic Acid (IAA)

Mechanism of Auxins: Plants respond to directional stimuli through tropisms, which are growth responses controlled by chemical growth factors like IAA. Unlike animal hormones, these factors are produced in growing regions (tips) and move to other tissues via diffusion or phloem transport.
Phototropism in Shoots: In shoots, IAA moves toward the shaded side of the tip. High concentrations of IAA stimulate cell elongation in shoot cells, causing the shaded side to grow faster than the light side, resulting in the shoot bending toward the light source.
Gravitropism in Roots: In roots, gravity causes IAA to accumulate on the lower side. Crucially, high concentrations of IAA inhibit cell elongation in roots; therefore, the upper side grows faster than the lower side, causing the root to bend downward into the soil.

3. Simple Animal Responses: Taxes and Kineses

4. The Reflex Arc and Sensory Reception

5. Key Distinctions in Visual Perception

6. Exam Strategy & Tips