11.1.1

Plant Tropisms

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Directional Responses in Plants

Plants are not mobile organisms so they cannot move to avoid a harmful environment. Instead they regulate growth in response to directional stimuli. Directional growth responses are called tropisms.

Phototropism

Phototropism

  • Phototropism is growth in response to direction of light:
    • Shoots are positively phototropic - they grow towards light.
    • Roots are negatively phototropic - they grow away from light.
Gravitropism

Gravitropism

  • Gravitropism is growth in response to direction of gravity:
    • Shoots are negatively gravitropic - they grow upwards (away from the gravitational pull).
    • Roots are positively gravitropic - they grow downwards (towards the gravitational pull).

Indoleacetic acid (IAA)

IAA is a type of auxin that controls the tropic response in plants.

Distribution of IAA

Distribution of IAA

  • IAA can be transported over short distances (by diffusion or active transport) and longer distances (in the phloem) around the plant.
  • When a plant detects directional stimuli, IAA is transported to different parts of the plant.
    • This creates an uneven distribution of IAA.
  • Where the distribution of IAA is uneven, a directional growth response occurs.
Phototropism - shoots

Phototropism - shoots

  • Shoots are positively phototropic.
  • If a shoot is exposed to an uneven light source, IAA is transported to the more shaded part.
  • A higher concentration of IAA in the shaded parts cause cells in the shaded area to elongate.
  • Cell elongation causes the shoot to bend towards the light.
Phototropism - roots

Phototropism - roots

  • Roots are negatively phototropic.
  • If a root is exposed to an uneven light source, IAA is transported to the more shaded part.
  • A higher concentration of IAA in the shaded parts inhibits cell elongation on the shaded part.
  • The root bends away from the light.
Gravitropism - shoots

Gravitropism - shoots

  • Shoots are negatively gravitropic.
  • If a shoot is exposed to an uneven gravitational pull, IAA is transported to the underside (closer to the pull of gravity).
  • A higher concentration of IAA in the underside cause cells in the underside to elongate.
  • Cell elongation causes the shoot to bend away from the gravitational pull.
Gravitropism - roots

Gravitropism - roots

  • Roots are positively gravitropic.
  • If a root is exposed to an uneven gravitational pull, IAA is transported to the underside (closer to the pull of gravity).
  • A higher concentration of IAA in the underside inhibits cell elongation on the lower side..
  • The root bends towards the gravitational pull.
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Cell Respiration

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Viruses

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Unity & Diversity - Organisms

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Evidence for Evolution

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Natural Selection

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Form & Function - Molecules

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Proteins

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6

Form & Function - Cells

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7

Form & Function - Organisms

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Gas Exchange

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Transport

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8

Form & Function - Ecosystems

8.1

Species, Communities & Ecosytems

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8.2

Energy Flow

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Carbon Cycle

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Climate Change

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9

Interaction & Interdependence - Molecules

9.1

Enzymes

9.1.1Enzymes9.1.2Factors Affecting Enzyme Activity9.1.3Enzyme-Controlled Reactions9.1.4End of Topic Test - Enzymes
9.2

Metabolism

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9.3

Cell Respiration

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9.4

Photosynthesis

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10

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11

Interaction & Interdependence - Organisms

11.1

Integration of Body Systems

11.1.1Plant Tropisms
11.2

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Interaction & Interdependence - Ecosystems

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Continuity & Change - Molecules

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Continuity & Change - Cells

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15

Continuity & Change - Organisms

15.1

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16

Continuity & Change - Ecosystems

16.1

Natural Selection

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16.2

Stability & Change

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Practice questions on Plant Tropisms

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which of the following statements about plant tropisms are true?True / false
  2. 2
    Which of the following statements about IAA are true?True / false
  3. 3
    Which of these is NOT an effect of IAA in the shoots?Multiple choice
  4. 4
    Steps involved in gravitropism in the shoots:Put in order
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