MYP Biology 4.10 Tropisms Notes

What Are Tropisms?

1. These responses allow plants to position their leaves and roots in the most favorable conditions for survival.
2. Stems and roots respond in opposite ways to environmental cues:

Why Do Plants Need Tropisms?

1. Since plants can't move, they use controlled growth responses to adjust their position relative to their environment.
2. Key adaptive outcomes:
   1. Leaves grow where light is strongest for photosynthesis
   2. Roots grow where water and minerals are available
   3. Shoots grow away from gravity to access light
   4. Climbing plants use touch to locate supports

How Do Plants Detect Where Light Is?

1. Plants detect light direction using cells in the shoot tip that sense light intensity differences.
2. When light hits one side of the stem more strongly than the other, the shoot tip redistributes growth hormone to the shaded side, causing uneven growth.

How Does Phototropism Work in Shoots?

1. Shoots bend toward light because their shaded side grows faster than the lit side.
2. This unequal growth is controlled by auxin, a plant hormone that stimulates cell elongation in shoots.
3. Mechanism:
   1. Light hits one side of the shoot tip.
   2. Auxin moves to the shaded side.
   3. Auxin causes cells on the shaded side to elongate more.
   4. The shoot bends toward the light.

How Do Roots Detect the Direction of Gravity?

1. Roots detect gravity using specialised cells called statocytes that contain dense starch grains which settle under gravity.
2. These grains provide a positional “signal” that guides where auxin accumulates.

Why Do Roots Grow Downward Even Without Gravity Cues?

1. Roots grow downward because auxin inhibits cell elongation in roots, causing the lower side to grow more slowly.
2. Even in low-gravity or altered gravity environments, roots still grow downward due to internal signalling pathways evolved to respond to these auxin distribution patterns.
3. Mechanism:
   1. Gravity causes auxin to gather on the lower side of the root.
   2. In roots, high auxin levels inhibit elongation.
   3. The upper side grows faster, bending the root downward.
   4. Even if gravity cues are weak, auxin asymmetry + mechanosensitive pathways reinforce downward growth.

Why Is Auxin Central to All Tropisms?

1. Auxin regulates growth by altering how quickly cells elongate in different regions of a plant.
2. Its uneven distribution creates asymmetric growth, which produces bending.

Why Are Tropisms Critical for Plant Survival?

1. You should recognize tropisms are mechanisms for optimization.
2. It allowsplants to maximize photosynthesis, secure water and minerals, and anchor themselves effectively.
3. Plants with stronger or more responsive tropisms therefore have a competitive advantage in changing environments.