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A tropism is a directional growth response. In a shoot responding to unilateral light, auxin distribution can promote greater elongation on the shaded side so the shoot bends toward light. Gravitropic responses differ between roots and shoots because tissues respond differently to auxin concentration. Do not apply one concentration-response curve identically to every organ. A curvature result requires a difference in growth between sides, not simply more growth everywhere.
Video by Cognito. Foundation recap: the written lesson develops the A-level detail.
Viewing focus: Explain auxin effects in shoots and roots
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is a tropism?
A directional growth response. Direction relates to the stimulus.
After watching 2: Which signal is associated with fruit ripening?
Ethene. Its effect depends on tissue and stage.
What you'll be able to do
A tropism is a directional growth response. In a shoot responding to unilateral light, auxin distribution can promote greater elongation on the shaded side so the shoot bends toward light. Gravitropic responses differ between roots and shoots because tissues respond differently to auxin concentration. Do not apply one concentration-response curve identically to every organ. A curvature result requires a difference in growth between sides, not simply more growth everywhere.
Gibberellins can promote processes including germination and stem elongation, while ethene affects ripening, senescence and abscission in appropriate contexts. Abscisic acid contributes to stress responses such as stomatal closure. Hormones interact and tissue sensitivity matters. In a germinating cereal seed, signalling can promote enzyme production that mobilises stored reserves for embryo growth. State the tissue, signal and effect to make the causal chain clear.
Compare treated and untreated material with matched age, genotype and growing conditions. Control light direction, temperature, water and hormone application method. Use repeated independent plants rather than repeatedly measuring one plant as if it were many biological replicates. Commercial uses include rooting, ripening control and weed control, but optimal concentration and timing depend on the intended effect. A result can support a role for a signal without proving every step in its transport mechanism.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Why is one plant measured ten times not equivalent to ten independently treated plants?
It is repeated measurement, not ten biological replicates.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.