Chlamydomonas uses its flagella to swim toward light of moderate intensity but away from very bright light. What is the advantage of this behavior?

• A. Moves to optimum light intensity for photosynthesis.
• B. Senses water temperature changes.
• C. Uses mitochondria for photosynthesis.
• D. Light intensity has no effect on behavior.

Answer: (A)

Phototaxis helps the organism position itself where light conditions optimize energy capture for photosynthesis. Chlamydomonas swims toward moderate light because that level provides enough photons for efficient photosynthesis without risking damage from excess light. In moderate light, the photosynthetic apparatus can operate at a high rate without becoming overwhelmed, so the organism gains more energy for growth and survival. If light is too intense, photodamage and stress can occur, so moving away helps avoid that danger.

The other ideas don’t fit this behavior: sensing water temperature changes isn’t what triggers movement toward or away from light here, mitochondria aren’t involved in photosynthesis (chloroplasts carry that out), and saying light intensity has no effect contradicts the observed movement toward or away from light.