Plants and photosynthesis 1 of 47 Boardworks Ltd

Plants and photosynthesis 1 of 47 © Boardworks Ltd 2008

What is photosynthesis? 2 of 47 © Boardworks Ltd 2008

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? 3 of 47 © Boardworks Ltd 2008

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? carbon dioxide (from the air) water (from the soil) Plants use carbon dioxide and water to make their own food in a chemical reaction. What is the name of this reaction? 4 of 47 © Boardworks Ltd 2008

What is the name of the process? Plants make their food from carbon dioxide and water in a chemical reaction called photosynthesis. The food made by photosynthesis is the sugar, glucose. carbon dioxide (from the air) glucose water (from the soil) oxygen Oxygen gas is also made as a by-product of photosynthesis. 5 of 47 © Boardworks Ltd 2008

Where does the energy come from? Plants need energy for photosynthesis to take place. The energy for photosynthesis comes from the Sun. carbon dioxide (from the air) glucose water (from the soil) oxygen Where in a plant does photosynthesis take place? 6 of 47 © Boardworks Ltd 2008

Where does photosynthesis happen? Plant cells in the upper surface of leaves have chloroplasts which contain the green pigment called chlorophyll carbon dioxide (from the air) glucose water (from the soil) oxygen It is chlorophyll, which absorbs light energy from the Sun, that enables plants to carry out photosynthesis. 7 of 47 © Boardworks Ltd 2008

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? carbon dioxide + water light energy chlorophyll glucose + oxygen This equation can be read as: “Carbon dioxide and water, in the presence of light energy and chlorophyll, react to form glucose and oxygen”. 8 of 47 © Boardworks Ltd 2008

What is needed for photosynthesis? 9 of 47 © Boardworks Ltd 2008

What is the word equation? 10 of 47 © Boardworks Ltd 2008

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. 11 of 47 © Boardworks Ltd 2008

Leaves and glucose 12 of 47 © Boardworks Ltd 2008

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? 13 of 47 © Boardworks Ltd 2008

What are the useful features of leaves? Leaves often have many features that make them suitable for photosynthesis, including: l A leaf is broad and flat to capture lots of sunlight. l 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. l Certain plant cells contain chloroplasts filled with chlorophyll. l Small holes called stomata in the underside of a leaf allow gases in and out. 14 of 47 © Boardworks Ltd 2008

Inside a leaf 15 of 47 © Boardworks Ltd 2008

What do plants use glucose for? 16 of 47 © Boardworks Ltd 2008

Testing leaves for starch 17 of 47 © Boardworks Ltd 2008

Does light affect photosynthesis? 18 of 47 © Boardworks Ltd 2008

Photosynthesis 19 of 47 © Boardworks Ltd 2008

Photosynthesis 20 of 47 © Boardworks Ltd 2008

Roots and water 21 of 47 © Boardworks Ltd 2008

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? 22 of 47 © Boardworks Ltd 2008

How are roots adapted? Roots are branched and spread out for two reasons: l to absorb water (and mineral salts) from a large amount of soil l 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? 23 of 47 © Boardworks Ltd 2008

Why do plants need water? Water is needed by plants for photosynthesis but this important liquid is also used in many other ways: l to provide dissolved minerals that keep the plant healthy l to transport substances around the plant l to keep the plant rigid and upright l to keep the plant cool l to allow other chemical reactions to occur in plant cells. What happens to a plant if it does not get enough water? 24 of 47 © Boardworks Ltd 2008

25 of 47 © Boardworks Ltd 2008

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? tomato human onion human wheat (bread) human grass cow’s milk (cheese) 26 of 47 human © Boardworks Ltd 2008

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. How do plants make their food? Plants use light energy to carry out photosynthesis: carbon dioxide + water light energy glucose chlorophyll + oxygen 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? 27 of 47 © Boardworks Ltd 2008

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? 28 of 47 © Boardworks Ltd 2008

Food from plants 29 of 47 © Boardworks Ltd 2008

Other uses of plants Plant material can be used to make other products, including: l drugs – medicines, such as aspirin, are made from plants. l construction – wood is an important building material for houses and furniture. l clothing – materials, such as cotton, are made from plants. Some dyes are also made from plant extracts. l paper – wood pulp is used to make paper. 30 of 47 © Boardworks Ltd 2008

31 of 47 © Boardworks Ltd 2008

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: l nitrogen (N) – needed for healthy leaves l phosphorus (P) – needed for healthy roots l potassium (K) – needed for healthy flowers and fruit. 32 of 47 © Boardworks Ltd 2008

Healthy plant growth What happens when plants use up all the minerals in the soil, what do they look like? low low phosphorous potassium magnesium nitrogen Can you explain why potted plants often look sickly? 33 of 47 © Boardworks Ltd 2008

Healthy plant growth 34 of 47 © Boardworks Ltd 2008

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. 35 of 47 © Boardworks Ltd 2008

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? 36 of 47 © Boardworks Ltd 2008

Controlling weeds Weeds compete with crops for living resources and this can reduce crop yield. What can farmers do to control weeds? One way farmers can control weeds is to use chemicals called herbicides (or weedkillers). Weeds are part of the food web. How might using herbicides affect other organisms in the food web? 37 of 47 © Boardworks Ltd 2008

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. As the number of weeds decline, competition between the consumers increases. Is thistle a weed to a farmer? 38 of 47 © Boardworks Ltd 2008

Controlling insects Insects can cause a huge amount of damage to crops. They are thought of as pests as they compete with humans for food. One way farmers can control insects is to use chemicals called insecticides. Insecticides can kill the adult insect or prevent insect larvae hatching. However, nearly all types of insecticides can cause damage to local ecosystems and many are toxic to humans. 39 of 47 © Boardworks Ltd 2008

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. l Artificial fertilizers can be replaced with natural fertilizers and crop rotation to keep the soil rich in nutrients. l As an alternative to insecticides farmers may encourage the insects’ natural predators, such as birds and other insects, to feed on them. l 40 of 47 Instead of using herbicides weeds are removed by hand or mechanically. © Boardworks Ltd 2008

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. 41 of 47 © Boardworks Ltd 2008