Tropisms, Taxes, Kineses and Reflexes

A taxis is directional movement towards or away from a stimulus, such as movement relative to light. A kinesis is a non-directional change in movement rate or turning rate associated with stimulus intensity. Faster movement in an unfavourable region can reduce time spent there even without the organism orienting towards a destination.

A distribution in a choice chamber does not by itself show which mechanism produced it. Track paths, speeds or turning as well as final counts. In a tropism, a plant’s growth direction responds to a directional stimulus; it is not locomotion of the whole rooted plant.

Indoleacetic acid, IAA, is a growth regulator produced in growing regions and transported to other tissues. Its effect on elongation depends on concentration and tissue: roots and shoots differ in sensitivity, and a concentration promoting shoot elongation can inhibit root elongation.

In a shoot exposed to unilateral light, greater IAA action on the shaded side can promote elongation there, bending the shoot towards light. In a horizontal root, unequal IAA distribution and greater root sensitivity can inhibit elongation on the lower side, allowing the upper side to elongate more and bend the root downwards. Use the tissue and concentration data given rather than claiming IAA always promotes growth.

A receptor detects a potentially harmful change and a sensory neurone carries impulses to a coordinating region. Synaptic transmission through a relay neurone and then a motor neurone activates an effector such as skeletal muscle. The three-neurone pathway produces a rapid response without requiring conscious decision before movement.

The reflex still involves nervous processing and can be influenced by the wider nervous system. Specify the receptor, pathway and effector in an unfamiliar example. A protective consequence, such as withdrawing from a damaging surface, helps explain the response’s survival value but is not a substitute for the mechanism.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.