biology

Photokinesis

Photokinesis is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photokinesis rather than just read about it. In short: Photokinesis is a change in the velocity of movement of an organism as a result of changes in light intensity. The alteration in speed is independent of the direction from which the light is shining.

Key takeaways

Photokinesis belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Photokinesis to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Photokinesis from memory before moving on to harder problems.

Reference excerpt

Photokinesis is a change in the velocity of movement of an organism as a result of changes in light intensity. The alteration in speed is independent of the direction from which the light is shining. Photokinesis is described as positive if the velocity of travel is greater with an increase in light intensity and negative if the velocity is slower. If a group of organisms with a positive photokinetic response is swimming in a partially shaded environment, there will be fewer organisms per unit of volume in the sunlit portion than in the shaded parts. This may be beneficial for the organisms if it is unfavourable to their predators, or it may be propitious to them in their quest for prey. In photosynthetic prokaryotes, the mechanism for photokinesis appears to be an energetic process. In cyanobacteria, for example, an increase in illumination results in an increase of photophosphorylation which enables an increase in metabolic activity. However the behaviour is also found among eukaryotic microorganisms, including those like Astasia longa which are not photosynthetic, and in these, the mechanism is not fully understood. In Euglena gracilis, the rate of swimming has been shown to speed up with increased light intensity until the light reaches a certain saturation level, beyond which the swimming rate declines. The sea slug Discodoris boholiensis also displays positive photokinesis; it is nocturnal and moves slowly at night, but much faster when caught in the open during daylight hours. Moving faster in the exposed environment should reduce predation and enable it to conceal itself as soon as possible, but its brain is quite incapable of working this out. Photokinesis is common in tunicate larvae, which accumulate in areas with low light intensity just before settlement, and the behaviour is also present in juvenile fish such as sockeye salmon smolts.

See also Kinesis (biology) Phototaxis Phototropism

References

Worked examples

Example 1 — a first encounter with Photokinesis

Start with the simplest possible case. Write down what Photokinesis claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photokinesis before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photokinesis ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photokinesis

In research: Photokinesis appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Photokinesis in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Photokinesis is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biology terminology, so understanding it makes those chapters shorter.
In everyday life: Look for Photokinesis outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Photokinesis” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Photokinesis in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Photokinesis means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Photokinesis out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photokinesis in simple terms?

Photokinesis is a change in the velocity of movement of an organism as a result of changes in light intensity. The alteration in speed is independent of the direction from which the light is shining.

Why does Photokinesis matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photokinesis?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photokinesis.