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KS3 Biology. 9D Plants for Food. Contents. 9D Plants for Food. Plants as food. Fertilizers and competition. Pests and pesticides. Summary activities. Humans in food chains. wheat (bread). human. grass. cow’s milk (cheese). human. tomato. human. pepper. human.
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KS3 Biology 9D Plants for Food
Contents 9D Plants for Food Plants as food Fertilizers and competition Pests and pesticides Summary activities
Humans in food chains wheat (bread) human grass cow’s milk (cheese) human tomato human pepper human Humans eat many different foods and so are at the top of many food chains. What are the food chains for the ingredients in this slice of pizza? What do these and all food chains have in common?
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: light energy carbon dioxide water glucose oxygen chlorophyll 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 from the leaf, the stemor the root of a plant,others are the seedor the fruitof a plant. Which parts of plants are these foods from?
Contents 9D Plants for Food Plants as food Fertilizers and competition Pests and pesticides Summary activities
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? Mineral salts 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.
Fertilizers and plant growth Plants take in mineral salts from the soil and in time there are less nutrients available in the soil. Farmers add chemicals called fertilizers to soil to make sure that their crops get enough mineral salts. Manufactured fertilizers can be expensive. So why do farmers choose to use them? Fertilizers help crops to grow well and so increase the farmer’s crop yield. What other types of fertilizer are there?
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? One way farmers can control weeds is to use chemicals called herbicides(orweedkillers). Weeds are part of the food web. How might using herbicides affect other organisms in the food web?
Contents 9D Plants for Food Plants as food Fertilizers and competition Pests and pesticides Summary activities
Pests and plant growth Pestsare animals that eat and damage crops and so can be a problem for farmers. Pests reduce crop yield and compete with humans for food. What animals might be considered as pests by farmers? • Caterpillars are pests specific to a type of plant such as cabbages. • Snailsand slugs are pests that eat the leaves of many plants. • Other common pests include insects, birds and mice. If the number of pests is reduced, what happens to crop yield?
Controlling pests What can farmers do to control pests? One method of pest control is the use of pesticides that kill pests. Pesticides contain poisonous chemicals called toxins. Pesticides can kill useful animals as well as the pests that they were meant to kill. Pests are part of the food web and the toxins in pesticides can affect other organisms in a food chain or food web. What are the advantages and disadvantages of pesticides?
Pesticide in a food chain plant plankton zooplankton small fish large fish grebe (bird) Introducing a pesticide at the bottom of a food chain can have huge effects on the organisms above. Consider the following food chain from a lake: A pesticide called DDT was sprayed on the lake to control mosquito larvae. DDT is a toxin that does not break down in the environment and so stays in animals’ bodies if it is eaten. The plant plankton at the bottom of the food chain absorbed some of the DDT from the water. How did this affect the rest of the food chain?
Pesticide in a food chain plant plankton zooplankton small fish large fish grebe (bird) 5ppm 10ppm DDT absorbed Each zooplankton ate lots of plant plankton and got several doses of DDT. Each zooplankton contained5 ppmof DDT. Each small fish ate many zooplankton and so consumed even more DDT. How much DDT do you think each small fish contained? Each small fish contained10 ppmof DDT.
Pesticide in a food chain plant plankton zooplankton small fish large fish grebe (bird) 5ppm 10ppm 250ppm DDT absorbed Each large fish ate several small fish and so consumed even more DDT. How much DDT do you think each large fish contained? Each large fish had 250 ppm of DDT in it. Each grebe ate several large fish therefore getting more than one dose of DDT. How much DDT do you think each grebe contained?
Pesticide in a food chain plant plankton zooplankton small fish large fish grebe (bird) 5ppm 10ppm 250ppm 1600ppm DDT absorbed Each grebe had the amazing amount of1600 ppmof DDT in its tissue which could kill the bird. In most cases, this amount of toxin made the birds’ eggs have very thin shells. These eggs broke very easily and so not many chicks were born alive. This example is actually based on real events that took place in the USA in the 1950s. It shows how a toxin can be passed on in a food chain and gets more concentrated at each step. This is called bioaccumulation.
Pesticide in a food chain before spraying after spraying grebe (bird) large fish small fish zooplankton plankton DDT lead to the decline many species of birds. What do these pyramids of numbers show about the effect of spraying the lake with the toxin DDT on the numbers of organism in a food chain?
Pesticide in a food chain DDT was considered as a safe pesticide when it was first used to kill insect pests. The problem was that DDT does not break down in the environment and the levels of this toxin that built up in top carnivores proved to be a major hazard. Today, many countries, including the UK, have banned the use of DDT to protect the environment. Alternative chemicals are now used as pesticides instead of DDT. These new pesticides break down quickly in the environment . Why aren’t these new pesticides hazardous to wildlife?
Contents 9D Plants for Food Plants as food Fertilizers and competition Pests and pesticides Summary activities
Glossary • competition – The demand by two or more organisms for limited shared resources, such as nutrients, space or light. • crop–A plant that is grown to be of use to humans. • fertilizer –A chemical that is added to soil to provide plants with the mineral salts needed for healthy growth. • herbicide –A chemical used to kill weeds. • pest–An animal that damages crops and competes with humans for food. • pesticide –A chemical used to kill pests. • toxin–A poisonous chemical. • weed –A plant growing in the wrong place that competes with a crop.
