Tropic responses Revision Notes

Gravitropism is the directional growth of plant parts in response to gravity.

Positive gravitropism means growth towards gravity; negative gravitropism means growth away from gravity.

• Roots exhibit positive gravitropism by growing downwards.
• Shoots exhibit negative gravitropism by growing upwards.

Gravity sensing cells detect the direction of gravity and influence hormone distribution, particularly auxin, to control growth direction.

This causes differential cell elongation, bending roots down and shoots up.

• Gravity sensing cells guide hormone movement.
• Auxin redistributes to control growth rates on different sides.

Roots growing downwards anchor the plant and absorb water and minerals from the soil.

Shoots growing upwards reach light for photosynthesis, essential for energy production.

• Gravitropism ensures roots and shoots orient correctly.
• This directional growth optimizes resource acquisition and survival.

Phototropism is a directional growth response where plant parts grow towards or away from a light source.

Positive phototropism is growth towards light; negative phototropism is growth away from light.

• Shoots and leaves usually show positive phototropism by growing towards light.
• Roots often show negative phototropism by growing away from light.

Plant hormones, especially auxin, redistribute unevenly in response to light direction.

Auxin accumulates on the shaded side, causing cells there to elongate more and the plant to bend towards the light.

• Unequal auxin distribution causes bending.
• Cells on shaded side elongate faster.

Growing towards light allows shoots and leaves to maximize light absorption for photosynthesis.

Roots growing away from light remain in the soil to absorb water and nutrients.

• Phototropism optimizes energy capture.
• It helps plants adapt to their environment.

Roots show positive gravitropism by growing downwards into the soil, anchoring the plant and absorbing water and minerals.

Shoots show negative gravitropism by growing upwards, away from gravity, to reach light.

• Roots grow downwards (positive gravitropism).
• Shoots grow upwards (negative gravitropism).

Shoots exhibit positive phototropism by bending towards light sources to maximize photosynthesis.

Roots generally show negative phototropism by growing away from light, remaining in the soil.

• Shoots bend towards light (positive phototropism).
• Roots grow away from light (negative phototropism).

Auxin hormone redistributes unevenly in response to gravity or light, causing cells on one side to elongate more and the plant to bend.

Investigations involve controlling variables like light direction or plant orientation, measuring growth angles or lengths, and repeating trials for reliability.

• Auxin controls differential cell elongation.
• Independent variables: light direction, gravity orientation.
• Dependent variables: angle or length of growth.
• Repeats improve reliability.
• Safety includes careful handling of plants and equipment.

In phototropism, auxin accumulates on the shaded side of the shoot.

This higher auxin concentration promotes cell elongation on that side, causing the shoot to bend towards the light source.

• Auxin redistributes to shaded side.
• Cells elongate more on shaded side.
• Shoot bends towards light.

In gravitropism, auxin accumulates on the lower side of the shoot due to gravity.

This causes cells on the lower side to elongate more, making the shoot grow upwards, against gravity.

• Auxin accumulates on lower side.
• Cells elongate more on lower side.
• Shoot bends upwards against gravity.

Auxin controls differential cell elongation, enabling directional growth responses to environmental stimuli.

This chemical regulation allows plants to optimize growth towards resources like light and away from unfavorable conditions.

• Hormones regulate growth direction.
• Unequal auxin distribution causes bending.
• Plants adapt growth for survival.

Auxin is synthesized in the shoot tip, the growing region at the top of the plant shoot.

From the shoot tip, auxin diffuses downwards through the plant tissues, moving throughout the shoot.

This movement allows auxin to influence growth in different parts of the shoot.

• Auxin is produced in the shoot tip.
• It moves by diffusion through the plant from the shoot tip.

When light shines on one side of the shoot, auxin accumulates more on the shaded side.

This uneven distribution causes cells on the shaded side to elongate more, bending the shoot towards the light source.

Similarly, gravity causes auxin to accumulate on the lower side of the shoot when it is horizontal.

This leads to greater cell elongation on the lower side, causing the shoot to bend upwards against gravity.

• Light causes auxin to concentrate on the shaded side of the shoot.
• Gravity causes auxin to concentrate on the lower side of the shoot.
• Uneven auxin distribution directs the shoot to grow towards light and upwards against gravity.

Auxin promotes elongation of cells in the shoot by loosening cell walls, allowing cells to expand.

Higher auxin concentration on one side results in more elongation of cells on that side.

This differential elongation causes the shoot to bend in the direction of the stimulus (light or gravity).

• Auxin stimulates cell elongation in shoots.
• Uneven auxin levels cause differential cell elongation.
• Differential elongation bends the shoot towards light or upwards against gravity.