AQA GCSE Investigating the effect of light or gravity on the growth of newly germinated seedlings(Biology)

Investigating the effect of light on the growth of newly germinated seedlings

Aim

To investigate how light affects the growth direction of newly germinated seedlings.

Equipment

Petri dishes or pots
Cotton wool
Water
Seeds (e.g., cress or bean seeds)
Ruler
Labels
Light source (lamp or window)

Method

Place seeds on damp cotton wool in a Petri dish or pot.

Expose the seeds to different lighting conditions

Effect of light: Place seeds in light from one direction and compare with seeds in darkness.

Water the seeds as needed.

After a seven days, measure the length of the seedlings (shoots and roots).

Record observations using labelled drawings showing the direction of growth.

Results

Condition	Shoot length after 7 days (cm)		Root length after 7 days (cm)		Direction of shoot growth
	Seeding 1	Seedling 2	Seedling 1	Seedling 2
Light	3.2	3.0	2.5	2.4	All shoots grow towards the light
Darkness	2.5	2.6	2.6	2.7	Directly upwards

Observations / Biological Drawings

Include careful, labelled drawings showing:

Shoots bending towards light (phototropism)

Roots growing downwards (geotropism)

Differences between seedlings in different conditions light vs dark

Draw each seedling roughly to scale (not necessarily exact, but proportional).

Label clearly: shoot, root, direction of growth, light source

Diagram below shows seedling 1 and 2 for both dark and light conditions

Analysis

Describe patterns observed: e.g., shoots grow towards light, roots grow in direction of gravity.

Explain why this happens using plant hormones (auxins):

Auxins accumulate on the shaded side of shoots. so cells on the shaded side will elongate. As a result the shoot bends towards light.

Auxins in roots inhibit growth on the lower side of the root (when it is horizontal, so the root bends downwards.

Investigating the effect of gravity on the growth of newly germinated seedlings

Aim

To investigate how gravity affects the growth direction of newly germinated seedlings.

Equipment

Petri dishes or pots
Cotton wool
Water
Seeds (e.g., cress or bean seeds)
Labels
Clinostat machine to remove gravitational effects

Method

Place some seeds horizontally onto petri dish and others vertically.

Ensure that equal numbers of seeds point in horizontal and vertical directions.

Effect of gravity: Grow seeds both horizontally and vertically with and without a clinostat machine to observe how roots and shoots respond.

Water the seeds as needed.

After a few days, observe how the roots and shoots have grown

Record observations using labelled drawings showing the direction of growth.

Observations / Biological Drawings

Include careful, labelled drawings showing:

Shoots bending towards light (phototropism)

Roots growing downwards (geotropism)

Differences between seedlings in different conditions shoot and root growing horizontally with and without a clinostat machine

Draw each seedling roughly to scale (not necessarily exact, but proportional).

Label clearly: shoot, root, direction of growth, with or without clinostat

Diagram below shows seedling 1 with the shoot and root growing horizontally, notice that they will both bend with the effect of gravity.

Diagram below shows seeding 2 with the shoot and root growing vertically, they maintain their vertical position.

In the image below now the petri dish is turned slowly using machine called a clinostat. The clinostat removes the effect of gravity that both the shoot and root experience. As a result both the shoot and root will continue to grow in their current direction without bending.

Analysis

In this investigation, seedlings were grown in different orientations and conditions to observe how gravity affects growth.

The vertical seedling grew normally. The shoot grew upwards and the root grew downwards, showing that shoots are negatively gravitropic and roots are positively gravitropic.

The horizontal seedling initially grew sideways, but after some time the shoot bent upwards and the root bent downwards. This bending shows that the plant can detect the direction of gravity and adjust its growth.

In the experiment using a clinostat, the seedling was slowly rotated. Because the direction of gravity constantly changed, the plant could not detect a consistent gravitational stimulus. As a result, the seedling grew more straight and showed little or no bending.

This happens because gravity normally causes the plant hormone auxin to move to the lower side of the root or shoot. This creates unequal growth on the two sides, causing the plant to bend. When the plant is rotated on a clinostat, the auxin is distributed more evenly, so the seedling grows straight rather than bending.

Practice Questions

1.Explain why the root of a horizontally placed seedling bends downwards.

2.Explain why the shoot of a horizontally placed seedling bends upwards.

3.Describe what happens to auxin distribution when a plant is placed on its side.

4.Why does a seedling grown on a clinostat show less bending?