1 of 47© boardworks ltd 2008 plants and photosynthesis

1 of 47 © Boardworks Ltd 2008

Plants and photosynthesis


What is photosynthesis?


How do plants grow?

Many years ago, people thought that plants ate soil and that made them grow.

But think about it, if that really happened then there wouldn’t be any soil left!

Plants actually grow by making their own food, not by eating soil.

So where does the stuff that plants are made of come from?


How do plants make their own food?

One of the raw materials that plants need to make food does come from the soil, the other comes from the air.

What are these two raw materials?

Plants use carbon dioxide and water to make their own food in a chemical reaction. What is the name of this reaction?

carbon dioxide(from the air)

water(from the soil)


What is the name of the process?

Plants make their food from carbon dioxide and water in a chemical reaction called photosynthesis.

Oxygen gas is also made as a by-product of photosynthesis.

glucose

oxygen



The food made by photosynthesis is the sugar, glucose.


Where does the energy come from?

Plants need energy for photosynthesis to take place. The energy for photosynthesis comes from the Sun.

Where in a plant does photosynthesis take place?

glucose

oxygen




Where does photosynthesis happen?

Plant cells in the upper surface of leaves have chloroplasts which contain the green pigment called chlorophyll.

It is chlorophyll, which absorbs light energy from the Sun, that enables plants to carry out photosynthesis.

glucose

oxygen



chlorophyll


What is the equation?

Plants make their own food through photosynthesis.

In this chemical reaction, chlorophyll in plant cells absorbs light energy to change carbon dioxide and water into glucose and the by-product oxygen.

What is the word equation for photosynthesis?

This equation can be read as:“Carbon dioxide and water, in the presence of light energy and chlorophyll, react to form glucose and oxygen”.

light energy

chlorophyll

carbon dioxide + water oxygen+glucose


What is needed for photosynthesis?


What is the word equation?


Photosynthesis and the atmosphere

Before plants the atmosphere was mainly made up of carbon dioxide and water vapour.

Gradually carbon dioxide was converted into oxygen as photosynthesis in plants and algae evolved.

The increased level of oxygen slowly made the planet more suitable for other forms of life.


Leaves and glucose


How are leaves adapted?

Leaves can be thought of as small ‘factories’ that produce food for plants by photosynthesis.

Leaves are adapted so that photosynthesis can take place.

Plants need carbon dioxide, water, sunlight and chlorophyll to carry out this important process.

What features of leaves make them suitable for photosynthesis?


What are the useful features of leaves?

Leaves often have many features that make them suitable for photosynthesis, including:

A leaf is broad and flat to capture lots of sunlight.

Veins carry water to the leaf and take food from the leaf to the rest of the plant. Veins also help to support the leaf.

Certain plant cells contain chloroplasts filled with chlorophyll.

Small holes called stomata in the underside of a leaf allow gases in and out.


Inside a leaf


What do plants use glucose for?


Testing leaves for starch


Does light affect photosynthesis?


Photosynthesis


Roots and water


How does water enter a plant?

Water is one of the raw materials needed for plants to carry out photosynthesis.

How does water enter a plant?

Water from the soil enters a plant through the roots.

You can’t normally see them but roots are a very important part of a plant.

Why are roots branched and spread out through the soil?


How are roots adapted?

Roots are branched and spread out for two reasons:

to absorb water (and mineral salts) from a large amount of soil

to anchor the plant in the soil.

Taking a closer look, roots are covered in root hair cells.

Root hair cells have thin walls and a large surface area to help them absorb lots of water.

How are roots adapted to their job?


Why do plants need water?

Water is needed by plants for photosynthesis but this important liquid is also used in many other ways:

to provide dissolved minerals that keep the plant healthy

to transport substances around the plant

to keep the plant rigid and upright

to keep the plant cool

to allow other chemical reactions to occur in plant cells.

What happens to a plant if it does not get enough water?


Humans in food chains

Humans eat many different foods and so are at the top of many food chains.

What are the food chains for the ingredients in a slice of pizza?

wheat(bread)

human

grasscow’s milk(cheese)

human

tomato human

onion human


Plants as producers

All food chains start with a producer.

Plants are called producers because they produce the food that humans and all other consumers depend on.

Plants use light energy to carry out photosynthesis:

How do plants make their food?

light energy

chlorophyll

carbon dioxide + water oxygen+glucose

The glucose produced by plants is converted into starch for storage or used to make proteins, fats and other substances.

Which parts of plants can be eaten?


Food from plants

Plants provide food in many different forms – some foods are the leaf, the stem or the root of a plant, others are the seed or the fruit.

Which plant parts are these foods from?


Food from plants


Other uses of plants

Plant material can be used to make other products, including:

drugs – medicines, such as aspirin, are made from plants.

construction – wood is an important building material for houses and furniture.

clothing – materials, such as cotton, are made from plants. Some dyes are also made from plant extracts.

paper – wood pulp is used to make paper.


Healthy plant growth

Plants need carbon dioxide and water for photosynthesis, but they also need small amounts of mineral salts for healthy growth.

Where do plants get mineral salts from?

Minerals are dissolved in water in the soil and so plants absorb these nutrients in the water they take in from the soil.

The three main elements in mineral salts are:

nitrogen (N) – needed for healthy leaves

phosphorus (P) – needed for healthy roots

potassium (K) – needed for healthy flowers and fruit.



What happens when plants use up all the minerals in the soil, what do they look like?

Can you explain why potted plants often look sickly?

lownitrogen

lowmagnesium

lowpotassium

lowphosphorous


Fertilizers and plant growth

Plants take in mineral salts from the soil and over time there are fewer nutrients available in the soil.

Farmers and gardeners often add manure or chemicals, called fertilizers, to soil to make sure that their crops get enough mineral salts.

Manufactured fertilizers help crops to grow well and so increase the farmer’s crop yield, but they can be very expensive.

If fertilizers aren’t used responsibly they can run into rivers, causing water pollution.


Competition between plants

Organisms that share a habitat have to compete with each other for limited living resources.

A weed is any plant that is growing in the wrong place.

Weeds are a problem for farmers as they compete with the crops for resources such as light, water, living space and mineral salts.

How does competition affect the growth of crops?


Controlling weeds

Weeds compete with crops for living resources and this can reduce crop yield. What can farmers do to control weeds?

Weeds are part of the food web. How might using herbicides affect other organisms in the food web?

One way farmers can control weeds is to use chemicals called herbicides (or weedkillers).


Types of herbicides

Non-selective herbicides kill most of the plants that they are sprayed on. This type of herbicide can be used to clear large patches of ground.

Selective herbicides only kill specific types of plants.

Many of the weeds that are killed using herbicides are important sources of food or shelter for consumers like insects and butterflies.

Is this thistle a weed to a farmer?

As the number of weeds decline, competition between the consumers increases.


Controlling insects

One way farmers can control insects is to use chemicals called insecticides. Insecticides can kill the adult insect or prevent insect larvae hatching.

Insects can cause a huge amount of damage to crops. They are thought of as pests as they compete with humans for food.

However, nearly all types of insecticides can cause damage to local ecosystems and many are toxic to humans.


Alternative farming methods

Organic farming is an alternative farming method. It is thought that this method may have less impact on the environment as fewer artificial chemicals are used.

As an alternative to insecticides farmers may encourage the insects’ natural predators, such as birds and other insects, to feed on them.

Artificial fertilizers can be replaced with natural fertilizers and crop rotation to keep the soil rich in nutrients.

Instead of using herbicides weeds are removed by hand or mechanically.


The pros and cons of organic farming

There are both advantages and disadvantages of organic farming.

Using fewer artificial chemicals may decrease the risk of toxic substances remaining on food and could reduce the impact farming has on the environment.

However, organic farming is more labour-intensive and may result in a lower yield. This means that organic farming requires more labour and space, making organic food more expensive.


Organic food


Which chemical?


Summary activities


Glossary


Anagrams


Multiple-choice quiz

1 of 47© boardworks ltd 2008 plants and photosynthesis

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