2 199.8 lb soil 200 lb soil In 1649, A Belgian physician, van Helmont, set up an experiment in which he planted a willow sapling, weighing 5 lb, in 200 lb of soil. How do plants get their food ? The soil was watered but nothing else was added. After 5 years, the tree had gained 169.2 lb in weight but the soil had lost only 2 pounds. van Helmont concluded that the tree had made 164lb of new growth from water alone.
3 van Helmont’s experiment was effective in showing that the plant’s food did not come from the soil. But he had overlooked the fact that airwas available to the plant as well as water. Could it be that the plant made 164 lb of material from just airand water? This might seem unlikely, but we now know that plants do indeed make their food using carbon dioxide from the air and water from the soil.
4 Feeding Animals get their food by eating plants, orother animals Carnivores eat animals Herbivores eat plants Plants make their own food They combine carbon dioxide from the air with water and dissolved salts from the soil Plants do NOTget their food from the soil The first stage by which plants make food is called PHOTOSYNTHESIS
Animals get their food … or (c) other animals ... plant products, by eating plants or ... 5 Plants make their food by photosynthesis
6 Photosynthesis Green plants take in carbon dioxide (CO2) from the air They take up water (H2O) from the soil The plants combine the CO2 with the H2O to make the sugar, glucose (C6H12O6) 6CO2 + 6H2O = C6H12O6 + 6O2 Oxygen (O2) is a by-product of this reaction
7 CO2 H2O CO2 H2O H2O CO2 CO2 H2O CO2 H2O CO2 H2O C6H12O6 + 6O2 6 molecules of carbon dioxide combine with 6 molecules of water to make one molecule of glucose and 6 molecules of oxygen
9 Energy It takes energy to make CO2 combine with H2O This energy comes from sunlight The energy is absorbed and used by a substance called chlorophyll
carbon dioxide sunlight (energy) water water 9
10 Chlorophyll Chlorophyll is a green coloured chemical It is present in the leaves of green plants The chlorophyll in the cells is packaged into tiny structures called chloroplasts The next slide shows a diagram of leaf cells with their chloroplasts
11 Leaf cells with chloroplasts cell wall chloroplast nucleus cytoplasm vacuole
12 All the reactions to combine CO2 and H2O take place in the chloroplast sunlight palisade cell of leaf water in the chloroplast, carbon dioxide and water combine to make sugar carbon dioxide
epidermis palisade cell ( photosynthesis) vessel (carries water) stoma (admits air) 13 Cell structure of a leaf The palisade cells are in the uppermost layers of the leaf
14 Carbohydrates • Glucose is one example of a carbohydrate • Other examples are starch, sucrose and cellulose (in cell walls) • Carbohydrate molecules contain the elements carbon, hydrogen and oxygen • Living organisms can easily change one carbohydrate into another
15 What happens to the glucose? The glucose made by the chloroplast is either (a) used to provide energy for the chemical processes in the cell ( by respiration) (b)turned into sucrose and transported to other parts of the plant or (c) turned into starch and stored in the cell as starch grains In darkness the starch is changed back into glucose and transported out of the cell
How Plants get their Food (2) How Plants Get Their Food (2) How plants get their food (2)
17 Other Food Glucose and starch are carbohydrates Carbohydrates can be oxidised during respiration to produce energy Plants need more than carbohydrates They need proteins for making new cytoplasm and cells for growth To make proteinsplants combine glucose with compounds of nitrogen, (nitrates)
18 fruits other sugars energy protein e.g. seed germination GLUCOSE cytoplasm starch cellulose cell walls storage e.g. starch in potato
19 Nitrates • Nitrates are present in the soil, dissolved in water • The plants take up nitrates in the soil water • The nitrates are conducted through the roots to the stem and then to the leaves • In the leaves, the nitrates and glucose are combined to make proteins • This process is called assimilation
20 Mineral salts • Nitrates are not the only salts that plants need to take in from the soil. • They need phosphates, sulphates, iron, potassium and magnesium salts. • This is the reason why farmers and gardeners add fertilizer to the soil. • These fertilizers usually contain nitrates, phosphates, and potassium (NPK).
21 Effects of fertilisers These are experimental strips of wheat. Varying amounts and types of fertiliser have been added to the soil to see which give the best plant growth
22 Averageyearly wheat yields from experimental plots
23 TO SUM UP Plants combine carbon dioxide from the air, and water from the soil to make glucose. The energy needed for this process comes from sunlight The sunlight is absorbed by chlorophyll contained in the chloroplasts of the leaf. The glucose can be used for energy or to make other substances. To make other substances, the glucose must be combined with other chemical elements such as nitrogen and potassium. These chemical elements are present in the soil and are taken up in solution by the roots.
24 QUESTIONS In the questions which follow, choose the best answer from the four alternatives
25 Question 1 For a plant to make glucose it needs (a) CO2 and H2O (b) CO2, H2O and sunlight (c) CO2, H2O, sunlight and chlorophyll (d) CO2, H2O, sunlight, chlorophyll and nitrates
26 Question 2 A by-product of photosynthesis is (a) Water vapour (b) Oxygen (c) Carbon dioxide (d) Nitrogen
27 Question 3 The plant needs to take in nitrates in order to make (a) Protein (b) Cellulose (c) Starch (d) Sugars
28 Question 4 Chlorophyll is present only in (a) The cytoplasm (b) The vacuole (c) The cell wall (d) The chloroplasts
29 Question 5 The food made by photosynthesis is transported round the plant in the form of (a) Glucose (b) Sucrose (c) Starch (e) Cytoplasm
30 Question 6 Which mineral ions are needed for making protein? (a) Magnesium ions (b) Sulphate ions (c) Phosphate ions (d) Nitrate ions