29D Plants for Food Plants as food Fertilizers and competition Contents9D Plants for FoodPlants as foodFertilizers and competitionPests and pesticidesSummary activities
3Humans in food chainsHumans 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?wheat(bread)humangrasscow’s milk(cheese)humantomatohumanpepperhumanWhat do these and all food chains have in common?
4carbon dioxide water glucose oxygen Plants as producersAll 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 energycarbondioxidewaterglucoseoxygenchlorophyllThe 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?
5Food from plantsPlants provide food in many different forms – some foods are from the leaf, the stem or the root of a plant, others are the seed or the fruit of a plant.Which parts of plants are these foods from?
79D Plants for Food Plants as food Fertilizers and competition Contents9D Plants for FoodPlants as foodFertilizers and competitionPests and pesticidesSummary activities
8Healthy plant growthPlants 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.
9Fertilizers 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?
10Competition between plants Organisms that share a habitat have to competewith each other for limited living resources.A weed is any plant thatis growing in the wrong place.Weeds are a problem for farmers as they competewith the crops for resources such as light, water, living space and mineral salts.How does competitionaffect the growth of crops?
11Controlling weedsWeeds 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?
129D Plants for Food Plants as food Fertilizers and competition Contents9D Plants for FoodPlants as foodFertilizers and competitionPests and pesticidesSummary activities
13Pests and plant growthPests are animals that eat and damage crops and so canbe 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 toa type of plant such as cabbages.Snails and 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?
14Controlling pests What can farmers do to control pests? One method of pest control is theuse 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?
15Pesticide in a food chain 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:plantplanktonzooplanktonsmallfishlargegrebe(bird)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 chainabsorbed some of the DDT from the water.How did this affect the rest of the food chain?
16Pesticide in a food chain plantplanktonzooplanktonsmallfishlargefishgrebe(bird)DDTabsorbed5 ppm10 ppmEach zooplankton ate lots of plant plankton and got several doses of DDT. Each zooplankton contained 5 ppm of 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 contained 10 ppm of DDT.
17Pesticide in a food chain plantplanktonzooplanktonsmallfishlargefishgrebe(bird)DDTabsorbed5 ppm10 ppm250 ppmEach 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?
18Pesticide in a food chain plantplanktonzooplanktonsmallfishlargefishgrebe(bird)DDTabsorbed5 ppm10 ppm250 ppm1600 ppmEach grebe had the amazing amount of 1600 ppm of DDTin its tissue which could kill the bird.In most cases, this amount of toxin made the birds’ eggshave very thin shells. These eggs broke very easily and sonot 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.
19Pesticide in a food chain 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?grebe (bird)large fishsmall fishzooplanktonplanktonbefore sprayingafter spraying
20Pesticide 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?
219D Plants for Food Plants as food Fertilizers and competition Contents9D Plants for FoodPlants as foodFertilizers and competitionPests and pesticidesSummary activities
22Glossarycompetition – 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.