Q.Distinguish the terms Phototaxis and Photokinesis.

Taxis and Kinesis

Animals and even simple organisms constantly adjust their movement in response to what is around them — light, chemicals, heat, gravity. But there are two fundamentally different ways of "responding by moving," and the difference lies in whether direction is involved.

Taxis — a directed response

In taxis, the organism orients and moves along a direction set by the stimulus — either toward it or away from it. Because it is directional, taxis is named by the stimulus plus the direction:

• Phototaxis — movement in response to light (a moth flying toward a lamp is positive phototaxis; an earthworm moving away from light is negative).
• Chemotaxis — movement toward or away from a chemical (a bacterium swimming up a nutrient gradient).
• Geotaxis — response to gravity.

The key idea: the organism can sense which way the stimulus lies and steer accordingly.

Kinesis — a non-directional response

In kinesis, the stimulus changes the rate or amount of movement, but not its direction. The organism does not head toward or away from anything; it simply moves faster, slower, or turns more often depending on how strong the stimulus is. For example, woodlice move rapidly and randomly in dry air but slow down and settle in humid air — over time this accumulates them in the favourable damp patch, without any of them ever "aiming" for it. Photokinesis similarly means light altering the speed of movement, not its direction.

The contrast that matters

| Feature | Taxis | Kinesis | …