Practical - effect of light or gravity on newly germinated seedlings

Homeostasis

What is the Independent variable?

Light intensity

What is the dependent variable?

Height of seedlings.

First step

Place cotton wool in 3 petri dishes, soaked in equal volumes of water (control)

Second step

Place 10 mustard seeds in each dish (control)

Third step

Leave dishes in a warm place and allow seeds to germinate.

How must you look after seedlings before they germinate?

Watering them with the same volume of water everyday.

Fourth step

Seeds germinate. We must check that all 3 dishes have the same number of seedlings (CV)

Fifth step

Use a ruler to measure height of seedlings, holding to ensure the stem is straight. Be careful not to damage seedlings.

Sixth step

Place all in different conditions.

What are the 3 conditions?

Full sunlight, partial light and darkness

Seventh step

Measure height of each seedling everyday for at least 5 consecutive days. Record in a table

What do we do when experiment is finished?

Calculate a mean seedling height for each day.

What is the typical result for full sunlight and partial light?

Grow to similar heights.

Why do they grow to same height?

Because chlorophyll is very efficient at absorbing light energy.

What direction do these seedlings grow?

Towards light source due to phototropism

How is auxin involved in this phototropism?

It concentrates on the side of the seedling furthest from the light and causes this side to grow faster.

What happens to seedlings in the dark?

They grow the longest.

Why do they grow the longest?

Seeds usually germinate underground and grow rapidly to reach the light. In dark, they continue growing rapidly to reach the light.

What happens to the leaves in the dark?

They are small and yellow

Why do they go small and yellow?

Once seedlings have used all the energy in their energy stores, they cannot carry out photosynthesis in the dark.

How to place seedlings when investigating effect of gravity?

Dish of seedlings is placed on its side in the dark.

How do the shoots grow in effect to gravity?

Seedlings grow upwards, in opposite direction to gravity, and roots grow downwards in direction of gravity.

How does auxin affect this?

It inhibits cell growth in roots. Gravity causes auxin to build up on lower side of the root. This now grows more slowly than the upper side and makes roots grow in direction of gravity.