Biology. Topic: Nutrition in plants and animals. Introduction to the topic.

Transcription

1 Biology Topic: Nutrition in plants and animals. Introduction to the topic. Organisms obtain food in various ways to survive and carry out physiological processes like respiration, growth, and osmo-regulation. Organisms that have chlorophyll (green pigment) such as plants and protests (i.e. Euglena) make food by combining inorganic substances namely water and carbon dioxide to make carbohydrates in a process called photosynthesis. This process involves the use of energy (solar energy) from the sun. Some use energy from the oxidation of certain substances to make food in a process called chemosynthesis. Organisms that make their own food are referred to as autotrophs. Some organisms obtain complex food substances made by other organisms and break them down to simple substances while others make their own food from simple inorganic substances. Some organisms obtain food by breaking down dead organic substances. These are called saprophytes and the type of feeding is called saprophytism. For example, fungi feed on a dead decaying wood. Therefore, fungi are saprophytes. Some organisms obtain food by associating with other organisms in particular kinds of relationships. Organisms that obtain food by associating with others of not their own kind, is a relationship called parasitism. In this relationship, the organism that benefits is called a parasite while the other that does not benefit (may be harmed) is called a host e.g a tick on a cow. In other relationships, both organisms benefit from the relationship. This is called mutualism e.g the gut bacteria. In some relationships, one organism may benefit or not. This is called commensalism and the organisms involved are called commensals. All relationships where two organisms live together are called symbiosis. Organisms which obtain food by breaking down complex food substances to simple absorbable molecules are called heterotrophs. 1

2 Sub-topic: Nutrition in green plants Time required: 8 periods (320 minutes) Brief description of unit. Green plants differ from other organisms because they take in simple materials such as carbon dioxide, water and mineral salts (i.e. nitrates, phosphates and sulphates) to build more complex substances including carbohydrates, oils and proteins. The building up of carbohydrates takes place in the presence of the sunlight in a process called photosynthesis. Therefore, the raw materials for photosynthesis are water and carbon dioxide. During photosynthesis the plants give off oxygen according to the equation below. Light energy 6H C0 2 C 6 H Chlorophyll Some carbohydrates are later changed into lipids and proteins. This unit therefore deals with: - The process of photosynthesis. Rate of photosynthesis. Factors affecting photosynthesis. The structure of a leaf and its adaptations to photosynthesis. Experiments on photosynthesis. The importance of major plant nutrient elements. Main content and concepts to emphasize; Definition of photosynthesis. Raw materials for photosynthesis namely carbon dioxide and water. The two main stages of photosynthesis: dark and light stage. 2

3 Factors affecting photosynthesis namely carbon dioxide, light, chlorophyll. Products of photosynthesis namely carbohydrates and oxygen gas. Testing of a leaf for starch. Adaptations of a leaf to photosynthesis. Mechanism or process of photosynthesis including equation of photosynthesis (the word equation and chemical equation) Experiment to show that: i) Carbon dioxide is necessary for photosynthesis. ii) Light is necessary for photosynthesis. iii) Chlorophyll is necessary for photosynthesis. iv) Oxygen is given off during the process of photosynthesis The importance of major plant nutrients and their effects on photosynthesis. Teaching / Learning Materials, activities and guidance: Polythene bags, cotton wool, bell jars/ 1 litre mineral water bottles, cut into a half, petroleum jelly, concentrated sodium hydroxide solution, potted destarched plants, dark paper, knife, pins, Iodine solution, variegated leaf, water; submerged leaf, plant, spirogyra, test tubes,glowing splints, filter funnel,beaker. Guidance to the teacher: - The teacher should set up in advance a number of potted plants and should destarch these plants by keeping them in darkness for a bout 48 hours. To avoid experimental errors, take the necessary precautions, for example before proceeding to test if the plant has been destarched by testing for presence of starch. If no starch is present proceed with the experiment. On the other hand, if starch is present destarch the plants further. Control experiments should be set up in order to be able to make conclusions. 3

4 The process of photosynthesis is very extensive, however, you should not teach details of the process be guided by the syllabus for depth. This unit needs a lot of preparation and common seedlings like those of beans or some other suitable plants with at least 3-5 leaves should be readily available. Due to experimental work, the topic may take more than 2 weeks. Glossary: Photosynthesis: - Is a chemical process by which green plants make organic compounds from carbon dioxide and water in the presence of sunlight. Chlorophyll: - refers to a green pigment found in all organisms that carry out photosynthesis. It absorbs light energy. Destarch: - Is a process of removing or eliminating starch from the leaves of a plant. It is done by keeping the plant in the dark for 48 hours. Variegated leaf: - A leaf that has green and non-green parts in its lamina. Limiting factor: - Any environmental factor that by its presence or absence, either decrease or increase limits the growth, metabolic process or distribution of organisms or populations. In this context, the limiting factor would slow down the rate of photosynthesis. Macronutrient: -Is a chemical element needed by plants in relatively large amounts. Macro elements are sometimes called essential elements or major elements. Micro- nutrient: - A chemical element needed by plants in relatively small quantities. They are sometimes called trace elements or minor elements. 4

5 Note that: The division between macro and micro is based on quantities and does not mean that one is more important than the other. Autotrophic Nutrition: - Is a type of nutrition in which organisms synthesize organic materials they need from inorganic sources. Teacher s notes: The process of photosynthesis is summed up in the word and chemical equation below: Water + carbon dioxide Glucose + Oxygen (Raw materials) sun light and (Products) Chlorophyll. 6CO 2 + 6H 2 0 Sun light C 6 H 12 O 6 + 6O 2 (g) (l) Chlorophyll Mechanism of photosynthesis. Photosynthesis occurs in two phases; namely light- dependent stage (light reaction/ stage) and light independent stage (dark reaction/stage.) Light dependent stage Radiant energy is absorbed by chlorophyll. The chlorophyll is activated and it converts light energy into chemical energy in the form of ATP. [Adinosine Triphosphate] Water is split into hydrogen ions and hydroxyl ions. Light H 2 O [ H ] + + [OH] Chlorophyll 5

6 Oxygen gas is formed from reactions, involving hydroxyl ions. The products of the light dependent stage are ATP, H + ions and oxygen. Light independent stage The hydrogen ions and chemical energy that were produced during the light dependent phase are used to reduce carbon dioxide to form glucose. Adaptations of a leaf for photosynthesis: Transverse section of a leaf showing some of the adaptations. INTERNAL STRUCTURE OF A LEAF Cuticle: - Is non-cellular, water proof and transparent. It allows light to pass through. Upper epidermis: - Is a single layer of cells on the upper surface of a leaf. It allows light to pass to the cells below. Palisade mesophyll - has cells vertically arranged such that many can fit into a small space. The cells have large number of chloroplasts. Chloroplasts: - contain chlorophyll for absorbing sunlight. Spongy mesophyll: - Have large air spaces for fast diffusion of gases to and from the photosynthesizing cells. The cells have few chloroplasts. 6

7 Veins (Vascular bundles): - They act as drain pipes distributing raw materials to the leaves and conducting away manufactured food to other parts of a plant. Stoma: - A pore that allows gaseous exchange to take place. Carbon dioxide enters and oxygen leaves the leaf through stomata. Thin lamina: - Provides a short distance over which diffusion of gases take place. Broad lamina: - Provides a large surface area for absorbing sunlight energy. Factors affecting the rate of photosynthesis. When plenty of carbon dioxide, sunlight and water are provided to a plant, photosynthesis will be at its maximum rate; sometimes the rate of photosynthesis is not as high as expected due to inadequacy of factors that include light intensity, water, temperature and carbon dioxide concentration. By providing plenty of each of these factors to a plant increases the rate of photosynthesis. However, if one of these factors is not adequate, the rate of photosynthesis may become low. A factor that is inadequate is called a limiting factor. Light intensity: in case of dim light, the rate of photosynthesis is low. As light intensity increases, the rate of photosynthesis increases. Carbon dioxide concentration: the more carbon dioxide a plant is given, the faster is the rate of photosynthesis, until a maximum is reached. Temperature: photosynthesis is an enzyme-controlled reaction. Increase in temperature increases the rate of photosynthesis while a decrease lowers it. 7

8 Stomatal opening and closing regulates the amount of carbon dioxide entering a plant which it uses for photosynthesis. If the stomata are closed then photosynthesis cannot take place and when they are open, carbon dioxide enters and the rate of photosynthesis increases. Table : Macro and micronutrients and their uses. Macro nutrients Uses Deficiency Nitrogen Amino acid and protein synthesis - Chlorosis with purple colourings Phosphorus Protein formation - Reddish purple leaves and stunted growth. Sulphur Formation of certain amino acids - Stunted growth - yellow patches on leaves Calcium Formation of cell wall (middle lamella) - Poor development of leaves at shoot apex Potassium Concerned with synthesis of carbohydrates and protein metabolism in - Chlorosis, and yellowing of leaves at the margins, tips. - Retarded growth. young leaves. Magnesium Chlorophyll formation. Facilitates enzyme - Chlorosis of older leaves. - Stunted growth. activity. Iron Required for the formation of chlorophyll but not part of the molecule. - Chlorosis with pale leaves 8

9 Micro nutrients Cobalt, Copper, Zinc, Manganese, Boron They are necessary for the manufacture of enzymes. - Stunted growth of leaves. Mineral salts are necessary for normal plant growth. Several elements are known for normal plant growth and development. Some of these elements are Carbon, Hydrogen and Oxygen. Other elements are Nitrogen, Sulphur, Phosphorus, Potassium, Calcium and Magnesium. The last category of important elements includes Iron III, Manganese, Zinc, Boron and Molybdenum. These elements in last category are called trace elements because they are needed in small amounts for normal growth. Apart from the first category, the plant obtains the nutrients it needs from the soil. The effects of these chemical elements can be discovered by growing plants in water solutions containing balanced amounts of salts necessary for healthy plant growth. This solution is called a culture solution. If any of the elements is left out; its effect can be noted. Table II. shows that certain elements are necessary for normal plant growth. Table II Content Culture experiment result Function of element Complete solution Distilled water. Normal healthy growth Virtually no growth Component of all 9

10 No Nitrogen. Very little growth proteins No Phosphorus Thin lanky growth, poor root development and reddish leaves Component of certain enzymes and in the nucleus is substances like DNA No Sulphur No Potassium No Calcium No Magnesium No Iron Some growth but less than it would be in complete solution. Little growth. Leaves turn orange brown. Stunted growth. Stunted growth and leaves turn yellow. Poor growth, leaves turn yellow. Present in certain proteins Needed in cell formation Needed in cell wall formation Chlorophyll contains magnesium Needed in chlorophyll synthesis For the teacher and students: Carbohydrates are the products of photosynthesis. Simple sugars are the first to be formed but are immediately converted into starch, the first visible products of photosynthesis. It is therefore possible to find out if a plant has carried out photosynthesis by testing for starch. Presence of starch implies that photosynthesis has occurred. 10

11 STUDENTS ACTIVITIES. Worksheet I Aim: - To test a leaf for starch. Apparatus and Materials Beaker, boiling tube, heat source, test tube holder Iodine solution, ethanol, water and a leaf that has been exposed to light. Procedure -Half fill a beaker with water and boil the water. Place the leaf in boiling water for one or two minutes. This denatures the enzymes and stops chemical reactions. Put the leaf into a boiling tube and pour enough ethanol to cover the leaf. Place the boiling tube into the beaker of boiling water. A water bath is used to prevent accidents that may arise from alcohol being inflammable. Remove the flame when the ethanol boils. Re-introduce the flame when the ethanol stops boiling. Repeat until the leaf is decolourised. Diagram to illustrate boiling a leaf in alcohol using a water bath. (Testing a leaf for starch) - When the chlorophyll has been removed (the leaf looks white) remove the leaf from ethanol (the leaf will be brittle). - Dip the leaf in water to soften it and make it permeable. 11

12 - Using a pipette, add dilute iodine solution to the leaf surface. Student s discussion: 1. When boiling the leaf in ethanol, what happened to the colour of ethanol? Explain your observation. 2.What colour did the leaf turn when you added iodine? 3. What do these results show? Worksheet II To show that carbon dioxide is necessary for photosynthesis. Aim: To show that carbon dioxide is necessary for photosynthesis. Apparatus and Materials: - Two well watered destarched potted plants, polythene bags, cotton wool, 2 bell jars, petroleum jelly, 100cm beaker, concentrated sodium hydroxide solution, boiling tube, heat source, test tube holder, cork, Iodine solution, ethanol, water, soda lime. Procedure / method Procedure: - Set up the experiment as shown above and expose to light for 2-6 hours. 12

13 Obtain a leaf from each plant and test it for starch. Result - Leaf from jar A stains brown. - Leaf from jar B turns black or dark blue. Conclusion. Carbon dioxide is necessary for photosynthesis. Worksheet III Aim: To show that light is necessary for photosynthesis. Apparatus and Materials: well watered destarched potted plant, dark paper, scissors / knife / scalpel, pins/clips Method / procedure: The set up is as shown below: - Fix the paper on one of the leaves as shown in the diagram below. Diagram to show the set up. - Make a sketch of the leaf showing the covered and uncovered parts. 13

14 - The experimental set up is exposed to sunlight for 2-6 hours. - The paper is removed and the leaf is tested for starch. - Make a labelled diagram to show the results. Results. Diagram showing results. (the parts that were covered are stained brown, and the rest of the leaf turns blue or black) Conclusion. Starch is produced only in areas that received light, showing that light is necessary for photosynthesis. Worksheet IV Aim: To show that chlorophyll is necessary for photosynthesis. 14

15 Apparatus and Materials Plant that has variegated leaves, ethanol, beaker, boiling tube, water, heat source Iodine solution. Method / procedure: - Leave the plant with variegated leaves in light for 2-6 hours. - Remove one leaf and make a labelled diagram to show the green and non-green parts. - Test the leaf for starch. Drawing of a leaf to show the distribution of chlorophyll (A variegated leaf before and after testing for starch with iodine solution) Conclusion: Starch is made only in areas of the leaf with chlorophyll, showing that chlorophyll is necessary for photosynthesis. Worksheet V 15

16 Aim: To show that oxygen is given off during photosynthesis. Apparatus and materials: - Beaker, short stemmed funnel, 2 thick wooden cubes, sodium hydrogen carbonate powder, test tube, water plant e.g. Elodea. (Spirogyra can be used too), glowing splint, Method / procedure: Set up the experiment as shown below. Expose it to light for about 2 hours. (The supports ensure circulation of air. Sodium hydrogen carbonate adds carbon dioxide.) Test any gas collected in a test tube using a glowing splint. Observations. A gas collects in the test tube. The gas re-lights a glowing splint or makes it glow brighter. Conclusion: Oxygen is given off during photosynthesis. Evaluation questions for students. Section A 16

17 Objectives type questions: - Which of the following are raw materials for photosynthesis? a) Water and Glucose b) Starch and Mineral salts c) Water and carbon dioxide d) Glucose and starch. 2. Which carbohydrate does a plant store? a) Glucose b) Sucrose c) Starch d) Galactose. 3. The light dependant stage of photosynthesis yields the following. a) Water and oxygen. b) Hydrogen ions, energy and oxygen c) Water and energy d) Oxygen and energy. 4. Why is glucose not very good as storage material in plants? a) It is insoluble b) It is soluble c) It is a disaccharide. d) It is in short supply. 5. What substance does a plant need to be able to convert glucose into proteins? a) Nitrogen b) Oxygen c) Hydrogen d) Starch. 17

18 Section B Structured questions 1. An experiment was performed to find out how fast a plant photosynthesised as the concentration for carbon dioxide in the air around it was varied. The results were as follows. Carbon dioxide concentration. Rate of photosynthesis in arbitrary units, % of by volume in air Low intensity of light. High intensity of light a) Plot these results on a graph using same axes. b) What is the carbon dioxide concentration of normal air? c) What is the rate of photosynthesis at this carbon dioxide concentration in high light intensity? d) Up to what values does carbon dioxide act as a limiting factor at high light intensity? 2. (a) Distinguish between autotrophic nutrition and holozoic nutrition. (b) What factors affect the rate of photosynthesis? (c) How is a leaf adapted to carry out photosynthesis? 18

19 3.Photosynthesis is a plant s way of making food. (a) List four things that a leaf must have for photosynthesis to occur. (b) Which of these are raw materials 4. In an experiment to find out that light is necessary for photosynthesis, the set up below was used. (a) What should be done to plant before and why? (b) If you tested the leaf for starch after 24 hours, what results would you expect? (c) (i). Where is the control in this experiment? (ii) Is the control satisfactory? If not, can you suggest a better one? Section C Essay question. a) What is a limiting factor? b) Name three factors that may affect the rate of photosynthesis in a plant. c) Explain how each of the factors named in (b) above limits the rate of photosynthesis. 19

20 Answers to evaluation questions: Section A Objective type questions 1.C 2.C 3.B 4.B 5.A SECTION B Structured questions. 1 (a) In autotrophic nutrition, plants take in simple inorganic molecules that they build up into complex organic molecules using light energy whereas in holozoic nutrition an organism obtains nutrients by taking in complex organic food, which it breaks down into simple absorbable molecules. Holozoic nutrition does not involve use of light energy. (b) Light intensity, carbon dioxide, water, and temperature. (c) - a waxy cuticle to minimise water loss. - vascular tissue to transport water to and manufactured food away from the leaves. - stomata that allow carbon dioxide to enter and oxygen to leave - has chloroplasts, These contain chlorophyll which absorbs sunlight. - spongy mesophyll cells that have large air spaces where gases can diffuse easily to and from photosynthesizing cells. - thin lamina which provides a short distance over which diffusion takes place. - broad lamina which provides a large area surface for absorbing sunlight. 20

21 Section C Essay question. a) A limiting factor is that whose supply decreases the rate of a metabolic reaction e.g. light intensity limiting the rate of photosynthesis. b) Carbon dioxide concentration temperature, stomata. Stomata allow in carbon dioxide, which is used, for photosynthesis. When the stomata open more carbon dioxide enters and photosynthesis is high. When the stomata close less carbon dioxide enters and photosynthesis is slow Students Discussion questions 1. When finding out if a plant needs light, a destarched plant is used. Why did you do this? 2. Why would it be necessary to set up two plants, one in the dark and the other in light? 3. When testing a leaf for starch, certain precautions may be taken. Write out the instructions and suggest the precautions to be taken. 4. Give 2 reasons why photosynthesis is important to humans. 5. Photosynthesis involves a pair of raw materials, a pair of conditions and a pair of products, Name them. References: - 1.Mckean D.G (1973), introduction to Biology, Hodder Murry (UK) London. 2. Ian J. Burton (2001), the Cambridge Revision Guide GCE, level Biology, Cambridge University Press. 3.Maxwell Ojo.B (1998), Modern Tropical Biology, Evans and Brothers London(UK) 4.Stone R.H and Cozens A. B. (2002), New Tropical Biology (3 rd Edition) Longmans, London (UK). 21