Science notes, unit 4

Plants

Textbook Science Year 4 (DLP), pages 69–86

What this chapter covers

You can say that plants respond to stimuli such as water, gravity, light and touch (4.1.1)
You can name the parts of a plant that respond to each stimulus (4.1.2)
You can carry out investigations on plant responses and draw conclusions (4.1.3, 4.1.4)
You can explain what photosynthesis means (4.2.1)
You can state the needs of photosynthesis (4.2.2)
You can state the products of photosynthesis and write the word equation (4.2.3)
You can explain why photosynthesis is important to living things (4.2.4, 4.2.5)

4.1How plants respond to stimuli

A plant cannot walk away, but it still reacts to the world around it. A stimulus is anything that makes a living thing react. The reaction is called a response. Plants respond to four stimuli: water, gravity, light and touch.

Stimulus: something that causes a response, such as water, light, gravity or touch.
Response: the way a living thing reacts to a stimulus.
Gravity: a force that pulls objects towards the Earth.

Part of plant	Stimulus	Response
Roots	Water	Grow towards the water
Roots	Gravity	Grow downwards, in the direction of the pull of gravity
Shoots	Light	Grow towards the light
Leaves of some plants	Touch	Droop and fold inwards

The Mimosa pudica and the Venus flytrap are two plants whose leaves respond to touch.

Remember: roots respond to two stimuli (water and gravity). Shoots respond to light. Leaves of some plants respond to touch.

Not every plant folds its leaves when touched. Only some plants, such as Mimosa pudica and Venus flytrap, do so. Do not write that "all leaves" respond to touch.

Facts: Science Year 4 (DLP) textbook, pp. 70-71.

The investigations

The textbook uses four fair tests. In each one the aim says what we want to find out, and the hypothesis is a testable statement about what we expect to find.

Investigation	What is done	Result and conclusion
Roots and water	Green bean seedlings on a petri dish. Cotton on the left and right sides (A and B). Water is added to cotton A only.	Roots grow towards cotton A. Roots respond to water.
Roots and gravity	Seedlings placed in different positions in a moist petri dish, held upright for a week.	All roots grow downwards. Roots respond to gravity.
Shoots and light	Chilli plants in boxes A and B, with a hole on the left of A and the right of B, in the sunlight.	Shoots bend towards the hole in each box. Shoots respond to light.
Leaves and touch	Touch the leaves, the flower and the stem of a Mimosa pudica.	Only the leaves fold inwards. Leaves of Mimosa pudica respond to touch.

Example: write a conclusion

In the shoe box test, the shoot in box A bent to the left and the shoot in box B bent to the right.
Each shoot bent towards the hole where light came in. Conclusion: shoots respond to light. The hypothesis is accepted.

A seedling is placed on its side. After a few days its root bends downwards. Which stimulus caused this?

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Gravity. Roots respond to gravity by growing downwards.

4.2Photosynthesis

People and animals can move to find food. Plants cannot, so they make their own food. This process is called photosynthesis.

Photosynthesis: the process in which plants make their own food using sunlight, chlorophyll, carbon dioxide and water.

What plants need

Need	What it is and where it comes from
Sunlight	The main source of energy
Chlorophyll	The green substance in plants that absorbs sunlight
Carbon dioxide	A gas in the air that enters through the leaves
Water	Absorbed through the roots

A plant that is not green, such as one with red leaves, can still carry out photosynthesis if it has chlorophyll.

What plants make

Chlorophyll absorbs sunlight. The plant changes water and carbon dioxide into glucose and oxygen. The glucose is joined together to form starch, which is stored in the leaves, stems, roots, seeds, flowers and fruits. Oxygen is released into the air through the leaves.

A tomato plant. Labels show glucose stored as starch as the food made, and oxygen released into the air through the leaves — Products of photosynthesis

Picture: Science Year 4 (DLP) textbook, p. 78.

Word equation
carbon dioxide + water → glucose + oxygen
(in the presence of sunlight and chlorophyll)

Sunlight and chlorophyll are needs, not products. They are written on the arrow, not after it.

(a) Name the green substance that absorbs sunlight. (b) Name the two products of photosynthesis.

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(a) Chlorophyll (b) Glucose (stored as starch) and oxygen

4.3Why photosynthesis is important

It supplies food. Plants are food for humans and animals.
It supplies oxygen. Humans and animals breathe in the oxygen that plants release.
It keeps the balance of gases in the air. Plants take in carbon dioxide, which humans and animals breathe out.

Plant responses help photosynthesis. Roots grow towards water and shoots grow towards light, so the plant gets what it needs.

Example: predict and explain

Plant X and plant Y are given the same water. Plant X is in the sunlight. Plant Y is in a black box.
After some days plant X stays healthy, but plant Y turns pale and weak. Reason: plant Y gets no sunlight, so it cannot carry out photosynthesis to make food.

Give two reasons why photosynthesis is important to living things.

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Any two: it supplies food; it supplies oxygen; it maintains the balance of gases (carbon dioxide and oxygen) in the air.

Facts: Science Year 4 (DLP) textbook, pp. 76-84.