In AP Biology, auxin is a plant hormone (commonly indole-3-acetic acid, or IAA) that coordinates growth and development, and its redistribution within a stem drives phototropism, a physiological response to an external environmental cue.
Auxin is the plant hormone behind a lot of how plants move and grow. The most common one is indole-3-acetic acid (IAA), which plants make from the amino acid tryptophan. When you hear "auxin," picture a chemical signal that tells plant cells where and how much to grow.
The classic example is phototropism, where a stem bends toward light. Auxin doesn't make the lit side grow. It actually moves to the shaded side of the stem, where it triggers cells to elongate. The shaded side stretches longer than the lit side, so the whole stem curves toward the light. Auxin also coordinates root growth and development, which is why it shows up in experiments on seedlings, coleoptiles (the protective sheath around a grass shoot), and root systems.
Auxin lives in Unit 8: Ecology, specifically topic 8.1 Responses to the Environment. It's the go-to plant example for learning objective AP Bio 8.1.A, which asks you to explain how an organism's behavioral and physiological response connects to changes in its internal or external environment. Auxin-driven phototropism is one of the CED's illustrative examples of exactly that kind of physiological response. The big idea here is that organisms detect environmental cues and respond in ways that help them survive. For a plant, bending toward light means more photosynthesis, which ties straight into fitness.
Keep studying AP® Biology Unit 8
Phototropism (Unit 8)
Phototropism is the response, and auxin is the mechanism behind it. The stem bends toward light because auxin piles up on the shaded side and makes those cells elongate. Learn them as a pair, not separately.
IAA (indole-3-acetic acid) (Unit 8)
IAA isn't a different thing from auxin, it's the specific auxin you'll see named on the exam. When an FRQ or question says IAA, it means auxin, and the 2019 Long FRQ even diagrammed how plants build IAA from tryptophan.
Photoperiodism (Unit 8)
Both are plant responses to light, but they answer different cues. Phototropism (auxin) responds to light direction, while photoperiodism responds to the length of day and night to time things like flowering.
Physiological Response (Unit 8)
Auxin-driven growth is the textbook physiological response in plants. It's the chemical, body-level reaction to an environmental cue, as opposed to a behavioral response like an animal fleeing a predator.
Auxin shows up in MCQ stems built around classic plant experiments: unidirectional light, oat or grass coleoptiles, and seedlings. A common stem asks which substance increases on the shaded side of a stem and causes the bend toward light. The answer is auxin. Another setup covers the coleoptile tip with an opaque cap and asks you to explain why bending stops. You reason that the tip detects light and a chemical signal (auxin) mediates the response. On the 2019 Long FRQ Q1, College Board used auxin directly, giving a model of the two-step enzymatic pathway that makes IAA from tryptophan and asking about root growth. The move you need to make: explain the mechanism. Don't just say "auxin makes it bend." Say auxin redistributes to the shaded side and causes cell elongation there, so the stem curves toward the light.
Phototropism is the observable response (the stem bending toward light). Auxin is the hormone that causes it. Saying "auxin" when you mean the bending, or "phototropism" when you mean the chemical, will cost you on a free-response explanation. The clean version: phototropism happens because auxin moves to the shaded side and elongates those cells.
Auxin is a plant hormone that coordinates growth, and indole-3-acetic acid (IAA) is the specific auxin you'll see named on the exam.
In phototropism, auxin moves to the shaded side of the stem and makes those cells elongate, which bends the stem toward the light.
Auxin is the classic physiological response to an environmental cue under learning objective AP Bio 8.1.A in Unit 8.
The 2019 Long FRQ used auxin and IAA directly, including the pathway that synthesizes IAA from the amino acid tryptophan.
On the exam, explain the mechanism (redistribution plus cell elongation), not just the outcome (the bend).
Auxin is a plant hormone that coordinates growth and development. The common form is indole-3-acetic acid (IAA), and its best-known job is driving phototropism, where a stem bends toward light.
No, it's the opposite. Auxin moves to the shaded side of the stem, where it makes cells elongate. That shaded side grows longer, so the stem curves toward the light.
Phototropism is the response you observe (the stem bending toward light), while auxin is the hormone that causes it. Auxin is the mechanism, phototropism is the result.
IAA (indole-3-acetic acid) is a specific, very common auxin, so when an exam question says IAA it's talking about an auxin. "Auxin" is the broader category of these plant hormones.
Yes. It's a core example in Unit 8, topic 8.1, used in MCQs about coleoptile and seedling experiments and in the 2019 Long FRQ, which covered the IAA synthesis pathway from tryptophan.
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