How plants-get-food
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The basics of photosynthesis
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How plants-get-food
- 1. How Plants Get Their Food (1)
- 2. How do plants get their food ? The soil was watered but nothing else was added. After 5 years, the tree had gained 74kg in weight but the soil had lost only 52g. van Helmont concluded that the tree had made 74kg of new growth from water alone 2 90.72kg soil In the 17th Century, A Belgian physician, van Helmont, set up an experiment in which he planted a willow sapling in a weighed amount of soil. 90.20kg soil
- 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 air was available to the plant as well as water. Could it be that the plant made 74kg of material from just air and water? This might seem unlikely but we now know that plants do indeed make their food from carbon dioxide from the air and water from the soil. 3
- 4. Feeding Animals get their food by eating plants, or other 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 NOT get their food from the soil The first stage by which plants make food is called PHOTOSYNTHESIS 4
- 5. Animals get their food … by eating plants or ... ... plant products, or (c) other animals Plants make their food by photosynthesis 5
- 6. Photosynthesis 6 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. C6H12O6 CO2 CO2 CO2 CO2 CO2 CO2 H2O H2O H2O H2O H2O H2O 6O2 + 6 molecules of carbon dioxide combine with 6 molecules of water to make one molecule of glucose and 6 molecules of oxygen 7
- 8. 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 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 10
- 11. Leaf cells with chloroplasts cell wall nucleus chloroplast cytoplasm vacuole 11
- 12. All the reactions to combine CO2 and H2O take place in the chloroplast sunlight water carbon dioxide in the chloroplast, carbon dioxide and water combine to make sugar 12 palisade cell of leaf
- 13. Cell structure of a leaf The palisade cells are in the uppermost layers of the leaf epidermis palisade cell ( photosynthesis) vessel (carries water) stoma (admits air) 13
- 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 14
- 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 15
- 16. 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 proteins plants combine glucose with compounds of nitrogen, (nitrates) 17
- 18. GLUCOSE storage e.g. starch in potato starch fruitsother sugars e.g. seed germination energy cytoplasm protein cell walls cellulose 18
- 19. When a salt such as potassium nitrate dissolves in water it separates into two ions, a potassium ion and a nitrate ion Ions KNO3 K+ + NO3 - The potassium ion (K+ ) carries a positive charge. The nitrate ion (NO3 - ) carries a negative charge These ions move freely and independently in the soil water and it is in this form that they are taken up by plants
- 20. Nitrates Nitrate ions are present in the soil, dissolved in water The plants take up nitrate ions in the soil water The nitrate ions are conducted through the roots to the stem and then to the leaves In the leaves, the nitrate ions and glucose are combined to make proteins This process is called assimilation 19
- 21. Mineral ions Nitrates are not the only ions that plants need to take in from the soil They need phosphate, sulphate, iron, potassium and magnesium ions This is the reason why farmers and gardeners add fertiliser to the soil These fertilisers usually contain nitrates, phosphates and potassium (NPK) 20
- 22. 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 21
- 23. Average yearly wheat yields from experimental plots 0 500 1000 1500 2000 2500 3000 Kgperhectare No manure Farmyard manure Chemical fertilizer No phosphate No nitrate No magnesium 22
- 24. 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 as ions in the soil and are taken up in solution by the roots. 23 TO SUM UP
- 25. QUESTIONS In the questions which follow, choose the best answer from the four alternatives 24
- 26. 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 25
- 27. Question 2 A by-product of photosynthesis is (a) Water vapour (b) Oxygen (c) Carbon dioxide (d) Nitrogen 26
- 28. Question 3 The plant needs to take in nitrate ions in order to make (a) Protein (b) Cellulose (c) Starch (d) Sugars 27
- 29. Question 4 Chlorophyll is present only in (a) The cytoplasm (b) The vacuole (c) The cell wall (d) The chloroplasts 28
- 30. Question 5 The food made by photosynthesis is transported round the plant in the form of (a) Glucose (b) Sucrose (c) Starch (e) Cytoplasm 29
- 31. Question 6 Which mineral ions are needed for making protein? (a) Magnesium ions (b) Sulphate ions (c) Phosphate ions (d) Nitrate ions 30
- 32. Incorrect 31
- 33. Correct 32
Editor's Notes
- Notice how little the leaves overlap each other. This enables them to trap the maximum amount of sunlight. the importance of this will be explained later in the presentation
- Most fungi and bacteria get their food by breaking down organic matter, such as plant and animal remains (detritus). They then absorb the soluble breakdown products. These organisms are called detritivores. The name ‘Photosynthesis’ is derived from ‘photo’ (light) and ‘synthesis’ (building up). Plants synthesize their food with the aid of sunlight. (Slides 9 and 10).
- When a plant is photosynthesising, it is taking in carbon dioxide and giving out oxygen. Plants which live in ponds, streams etc. are immersed in the water they need for photosynthesis
- This reaction is summarised by the equation 6CO2 + 6H2O = C6H12O6 + 6O2
- Without sunlight, photosynthesis could not take place. Without photosynthesis, plants could not survive. Without plants, most animals would die out because, ultimately, animals depend on plants for their food. e.g. sunlight >>>> plants >>>> herbivores >>>> carnivores
- The carbon dioxide comes from the air. The water comes from the soil. The energy comes from sunlight.
- It is not only the leaves that contain chlorophyll; any green part of a plant, leaves, leaf stalk, stem, sepals will contain chloroplasts.
- These are called palisade cells and they are present in the upper layers of a leaf where most sunlight is absorbed. The chloroplasts are present in the cytoplasm lining the cell
- The water travels from the roots, through the stem and into the leaf in a system of vessels. The carbon dioxide diffuses into the leaf through tiny pores called stomata.
- The palisade layer traps most of the sunlight. The vessel carries water from the stem to the leaf. Carbon dioxide enters through the stoma and diffuses through the air spaces between the cells.
- The chemistry of carbohydrates is dealt with in the section ‘Chemicals of Living Cells’.
- Before it is transported, glucose is converted to sucrose. Two glucose molecules combine to make a molecule of sucrose. 2C6H12O6 = C12H22O11 + H20 glucose sucrose It is the sucrose which is transported throughout the plant
- Pyracantha. Some of the food made in the leaves is sent to the berries
- Carbohydrates may be (a) oxidised to provide energy for chemical reactions. (b) turned into starch and stored in storage organs such as potatoes and parsnips. (c) turned into cellulose which builds the cell walls. (d) Combined with nitrogen (from nitrates) to make amino acids , which are combined to make proteins
- Proteins are needed for making the cell structures, e.g. cytoplasm, nucleus, chloroplasts. The plant can grow only by making new cells and cell structures Strictly speaking, it is not nitrates salts that are taken up but nitrate ions. When a salt such as potassium nitrate, is dissolved in water it splits into positively charged potassium ions and negatively charged nitrate ions. KNO3 becomes K+ and NO3- The plant may take up either or both of these ions.
- Nitrates are needed for making proteins. Phosphates are needed for DNA and for chemical reactions involving energy release. Sulphates are needed for some proteins. Iron is needed for certain enzyme reactions. Magnesium is needed for making chlorophyll.
- Some of these plots have received different types of fertiliser. Some have had only manure added to the soil. Some had neither manure nor fertiliser for many years
- The chemical fertilizer contains all the mineral ions needed by the plants. The other fertilizers are lacking in one of the essential ions. Which of these mineral ions appears to be the most important?