2.85 understand the role of auxin in the phototropic response of stems

This is commonly asked in exams! A few points need to be mentioned to get full marks:

What is the Role of Auxin in Phototropic response of Stems?

• Auxin is made in the tip of a root/shoot.
• When a shoot/stem is exposed to light, the shaded area accumulates more auxin and grows faster, causing it to bend towards the light.
• This is done via diffusion as the auxin diffuses from the tip down to the growing region. 
• Auxin stimulates mitosis and causes cellular elongation. 
• Auxin also changes the elasticity in cells. Increase in elasticity allows the cell to absorb more water, causing cell size to increase.

For more on Auxin and Tropisms click here.

2.84 describe the geotropic and phototropic responses of roots and stems

Plant growth is controlled by the plant hormone Auxin.

Auxin: plant hormones produced in the tip of a stem/root to control direction of growth.

This chemical is made in the tip of a growing shoot and when it diffuses down to the growing region, it can stimulate growth. Auxin causes cells in the shoot to divide via mitosis and can also increase the elasticity of cells, allowing it to absorb more water, leading to cellular elongation. 

Auxins and Positive Phototropism in the Shoot:

1. When a shoot tip is exposed to sunlight, it accumulates more auxin in the side that's in the shade.
2. This makes cells on the side of the shade grow faster than the side exposed to light, So the plant grows towards the light.

Auxins and Negative Geotropism in the Shoot:

1. Auxins will accumulate on the underside of the stem, against gravity.
2. This causes the lower side to grow faster, and the shoot grows upwards against gravity, hence the name 'negative' geotropism.

Auxin and Negative Phototropism in the Root:

1. More auxin will accumulate at the shaded side of the root. 
2. The auxin stimulates the root to grow downwards into the ground.

*Roots that grow underground arent EXPOSED to light. They grow downwards due to positive geotropism.

Auxin and Positive Geotropism:

1. More Auxin will accumulate on the lower side of the root
2. This means the root will grow downwards. 

2.83 understand that plants respond to stimuli

Plants increase their chances of survival by responding to environmental stimuli.

There are different tropisms in which plants respond to.

Tropism: growth response of a plant in which direction of stimulus stimulates direction of plant growth.

Examples of plant tropisms include:
1. Phototropism: plant response to the stimulus of light.
2. Geotropism: plant response to the stimulus of gravitational strength.
3. Thigmotropism: plant response to the stimulus of touch. (only certain plants respond to touch, such as Venus flytraps)