PLANT NUTRITION A British clergyman and chemist demonstrated in the year 11772, that when a plant or animal was kept alone in an airtight jar, it died, but when a plant and animal were put together in an airtight jar, both lived. Jan Ingen-Housz later showed that sunlight was necessary for plants to produce oxygen although he knew nothing about oxygen. Jean Senebier was was a Swiss Pastor showed that plants use carbon dioxide when they produce oxygen; he suggested that carbon dioxide was converted to oxygen during photosynthesis. Senebier called this gas that was converted "fixed air." EARLY EXPERIMENT TO SHOW THAT CHLOROPHYLL IS IMPORTANT DURING PHOTOSYNTHESIS AND THAT CHLOROPLASTS DO NOT ABSORB GREEN WAVELENGTHS. T. W. Engelmann, in the year 1883 studied the algae Spirogyra sp., which has distinctively long, spiral chloroplasts. He used a prism to direct specific wavelengths of light to different parts of the Spirogyra. After this, he put oxygen-dependent (aerobic) bacteria to the solution containing the Spirogyra, expecting that when the Spirogyra was getting the best light for photosynthesis, the Spirogyra would release the most oxygen. He was of the believe that, the bacteria would cluster where more oxygen is found. It was found out that the greatest numbers of bacteria clustered where the chloroplasts were absorbing the bands of red and blue light not by the green wavelengths. This experiment concluded that oxygen was being produced where the red and blue wavelengths were being absorbed.therefore, chloroplasts do not absorb green wavelengths from spectrum. Green plants obtain their food through: 1. Photosynthesis and 2. Mineral salts uptake. Explanation why most leaves look green: This is because while the leaves absorb the red and blue wavelengths, the green passes through. That is when white light falls on a leaf, it absorbs all the components of light except the green component. This green component is reflected, hence giving a green appearance of leaf. Examples of photosynthetic pigments include: chlorophyll, carotenoids, xanthophylls and anthocyanins. PHOTOSYNTHESIS There are two major types of nutrition, they are autotrophic (self-feeders) and heterotrophic (dependent-feeders). Plants, algae and green bacteria are all examples of autotrophs, while animals, fungi and most bacteria are all examples of heterotrophs. Photosynthesis is an example of autotrophic nutrition. Photosynthesis This is a biochemical process in green plants, in which, their chlorophyll (found in the chloroplasts) manufacture food/sugar (organic food) using inorganic substances such as carbon dioxide and water in the presence of sunlight, giving off oxygen as a by-product. 6CO2 + 6H2O →C6H12O6 + 6O2 Note: Photosynthesis is an example of holophytic nutrition because inorganic compounds are converted into organic compounds. Mechanism/process/stages of photosynthesis The process of photosynthesis is broadly grouped into two: 1. Light reaction; 2. Dark reaction. Light Reaction This stage involves: a. Capturing of light energy from the sunlight. Certain wavelengths of the light energy are been trapped by the chlorophyll. Red and blue light energy are trapped the most efficiently, while green light energy is trapped the least. The chlorophyll is therefore energized. b. The conversion of light energy into chemical energy. The energy released from the electrons in the chlorophyll when it traps light energy is used to do two things: (i) split water molecules and (ii) make ATP. The process of splitting water is called photolysis. During photolysis, water molecule is splited to form hydrogen ions (H+) and hydroxyl ions, while oxygen is given out as a by-product from the hydroxyl ion. i.e 4H2O → 4H+ + 4OH- 4(OH-) → 2H2O + O2 The hydrogen atom (H2) released is collected by a coenzyme called NADP (Nicotinamide adenosine dinucleotide phosphate), this is done to prevent its escape from the cell or recombining with oxygen to form water. This process reduces the NADP to NADPH2, because it (NADP) have accepted hydrogen atoms and serve as electron carrier of hydrogen ions. This coenzyme brings the hydrogen atom to the next step of photosynthesis process. In the next step, the light energy which has been stored as a low chemical energy carrier called ADP (Adenosine diphosphate) gets more energy from the extra or unused energy from photolysis to form a high chemical energy carrier called ATP. This energy is stored in the grana part of chloroplasts for dark reaction. The major products of light stage/light-dependent stage of photosynthesis and their importances include: 1. ATP (Adenosine triphosphate)/energy, to generate energy for the dark stage reaction. 2. H+ (Hydrogen ion), reduces NADP to NADPH2. 3. OH- (Hydroxyl ion), splits into water and oxygen escapes. 4. NADPH2 (reduced nicotinamide adenine dinucleotide phosphate), generates hydrogen ions/to reduce carbon dioxide to form carbohydrate in the dark stage. 5. O2 (Oxygen gas), is given off into the atmosphere form animals/other organisms to use for respiration. DARK REACTION. This is also called Calvin cycle. This stage does not require absorption of light energy. During this process, carbon dioxide is reduced by the hydrogen ions produced in the light stage. This is done together with the energy provided by ATP. This process leads to formation of sugars through a series of reactions, controlled by enzyme. These processes take place in the stroma of the chloroplasts. i.e 4H + CO2 → CH2O + H2O. This is a reduction process and enzyme is involved. 6CO2 + 12H2 →C6H12O6 + 6H2O (This equation only shows the overall reaction of the dark stage). The overall mathematical reactions of photosynthesis look like this: Light stage: 12H2 O → 12H2 + 6O2............(i) Dark stage: 6CO2 + 12H2 →C6H12O6 + 6H2O.........(ii) Add equation (i) and (ii). 12H2 O + 6CO2 + 12H2 → 12H2 + 6O2 + C6H12O6 + 6H2O Collect like terms: 12H2 O - 6H2 O + 6CO2 →C6H12O6 + O2 6H2 O + 6CO2 →C6H12O6 + 6O2 Note: 1. Dark reaction is not called the dark reaction because it occurs at night, but because the steps of this process do not involve the absorption of light energy. 2. In the reaction for dark stage, the name of the first real carbohydrate produced is a 3-carbon molecule called PGAL (phosphoglyceraldehyde), which is used to make other macromolecules such as fats, oils, proteins and carbohydrate derivatives. 3. Chloroplast is a special organelle found in plants, which contains a special pigment molecule called chlorophyll. Chlorophyll is found in little sacs called thylakoids (a coin like structures). The thylakoids stack up on each other like coins to form grana. The grana are surrounded by a fluid called stroma. Factors/conditions (materials) that affect the rate of food production in a green plant or in photosynthesis The rate of photosynthesis is affected by the following factors: 1. The wavelength of light available/ light intensity as moderately high light intensity favours photosynthesis. 2. An optimum temperature is required as very high or too low temperature may alter enzymes reaction. 3. The carbon dioxide must be adequately supplied to the chlorophyll for faster photosynthesis. 4. The concentration of chlorophyll available within the organisms determine the rate of photosynthesis; if its low, photosynthesis rate will be slower. 5. Water. Water must be adequate. 6. Oxygen concentration. The higher the concentration level in the atmosphere, the slower the rate of photosynthesis. 7. Leaf structure. A broad leaf will undergo more photosynthesis than a leaf that is reduced (such as to a spine). 8. Inhibitors, such as herbicides and urea will alter rate of photosynthesis. 9. Pollutants, such as sulphur dioxidea dn ozone damage leaves and hence, alter photosynthesis. Experiment to show that oxygen is given out as a by-product of photosynthesis Method/procedure: Water plant such as Elodea or Spirogyra is placed in a beaker of water covered with a funnel and a test tube filled with water is turned upside down/inverted over the funnel stem. The experimental set-up is placed in sunlight for about 3 hours/few hours while an identical control experiment is setup and placed in a dark cupboard for about 3 hours/few hours to prevent photosynthesis taking place. Observation: Bubbles of gas were observed in the test-tube of the experimental setup, while no gas bubbles were observed in the control. The gas was tested with glowing splint which was rekindled/burst into flames. Conclusion: This showed that oxygen gas is produced during photosynthesis. How leaf of a flowering plant is adapted for photosynthesis 1. The guard cells have chloroplast for the absorption of sunlight. 2. Large vacuoles of the palisade cells store photosynthetic products. 3. Phloem transports manufactured food to other parts of the plants. 4. Xylem conducts water into the leaves for photosynthesis. 5. Broad/flat leaf lamella exposes large surface area for maximum absorption of light. 6. Large intracellular air spaces in the spongy mesophyll allow oxygen/carbon dioxide to diffuse in/out of the chlorophyllus cells/gaseous exchange. 7. Thin lamella allows light penetration into leaf tissue of mesophyll. 8. Palisade mesophyll cells contain a lot of chloroplast/chlorophyll for maximum absorption of light. 9. The stomata open easily when it becomes turgid for diffusion of gases/carbon dioxide/oxygen. 10. The bean-shaped structure of the guard cell is to allow oxygen/carbon dioxide exchange. Importance of photosynthesis to humans 1. Food produced by photosynthesis is consumed/eaten by all humans. 2. Provides humans with fossil fuel. 3. Oxygen produced during photosynthesis is taken in by man during breathing/for respiration. 4. Provides raw materials for industries, such as medicinal herbs. 5. It maintains carbon dioxide balance in the atmosphere by reducing them, because they could become a pollutant. It means it brings purification of air. Importance of photosynthesis to living things/nature 1. Production of food for both plants and animals. 2. Purification of the atmosphere by maintaining oxygen-carbon balance. 3. It releases oxygen for plants and animals to use for respiration. 4. It is the building blocks for other substances in the body of organisms, such as building of fats, proteins etc. EXPERIMENTS ON PHOTOSYNTHESIS Activities on Photosynthesis: 1. To show the materials and conditions necessary for photosynthesis. The following external conditions are necessary for photosynthesis: a. Sunlight b. Carbon dioxide c. Chlorophyll d. Water a. Sunlight: ✓ Ensure that the leaf used is a fresh green leaf, which is still attached to the parent plant. This will allow the leaf receives a continuous supply of water and mineral salts; ✓ Cover both surfaces of the leaf with strips of black paper with a pattern cut on it; ✓ Make sure the plant-leaf is left in sunlight for few hours; and ✓ Pluck the leaf after some few hours, and test for starch. ✓ Only the parts of the leaf exposed to sunlight turn blue-black. ✓ This change on the the part of the leaf exposed to sunlight shows that sunlight is needed for photosynthesis. b. Carbon dioxide: • Enclose the leaf of a potted plant attached to the plant in a flask/bell jar containing caustic soda/caustic potash (sodium hydroxide/potassium hydroxide) solution. The caustic soda/caustic potash will absorb carbon dioxide; • Ensure that the experiment set-up is done early in the morning, and allow to stand in exposed bright sunlight for 4-6 hours; and • Detach the leaf, and test for starch. The result will be no sign of starch. • Set up a control experiment labelled ‘B’, ensure that there is no soda lime, also, let water replaces caustic soda/potash. Hence, ‘B’ receives sufficient carbon dioxide from the air. It will be observed that only leaf in ‘B’ is tested positive to starch presence, because it receives carbon dioxide. • This change in B shows that carbon dioxide is necessary for photosynthesis. Precaution: 1. If you are using conical flask, smear the flask with Vaseline at the neck so that it is air-tight, but if you are using bell jar, use soda lime to prevent entry of more carbon dioxide. 2. Make sure that the experiment start early before day light so as to ensure that plant has not started manufacturing food. 3. Ensure that in both set up, the soil is covered with polythene to prevent the release of carbon dioxide by the micro-organisms in the soil. Another description of Experiments to show that carbon dioxide is necessary for photosynthesis Experiment 1: Method: Two potted plants are de-starched by placing them in a dark cupboard for 24 hours; a leaf each from both plants are then tested for starch to ensure they have no starch/have been de-starched; the potted plants are placed on a board each; a beaker containing caustic soda is placed beside plant A to absorb carbon dioxide, while a beaker containing water is placed beside plant B as control. They are both covered with bell jars labelled A and B respectively. The mid/edge is smeared with vaseline/petroleum jelly to make it air tight. The setup is exposed to sunlight/light for 4-6 hours, then a leaf each from both plants are plucked and tested for starch. Observation: It is observed that leaf from plant A is negative/does not contain starch/remains brown in colour while leaf from plant B is positive/contains starch/turns blue-black. Conclusion: Carbon dioxide is necessary for photosynthesis. Experiment 2: Method: De-starch a well-watered potted plant by putting it in darkness for 48 hours. Insert a leaf into a flask containing KOH/potassium hydroxide/caustic potash, ensure both flasks are air tight by smearing the split corks with vaseline. Leave the potted plant in a well-lit place for 6 to 9 hours. Detach the leaves and test them for starch. Observation: It is observed that the leaf in the flask containing caustic potash contains no starch while the leaf in the controlled experiment contained starch Conclusion: Carbon dioxide is necessary for photosynthesis. c. Water: ▪ Take a potted plant and get it well watered; ▪ Keep the potted plant in a dark cupboard for at least 24hours or overnight. This will destarch the leaves; ▪ Enclose the destarched potted plant in a conical flask containing alkaline pyrogallol solution. The alkaline pyrogallol solution will absorb water, and remove any trace of water inside the flask; ▪ Set-up a control experiment, without destarching the leaves; exposed to all conditions necessary for photosynthesis; ▪ The destarched leaves show negative result, by showing a yellowish-brown colour, while the control leaves turn blue-black, when iodine test was conducted. ▪ The negative result in test experiment shows that water is released during photosynthesis. d. Chlorophyll: • Take a variegated leaf (e.g. leaf of Croton, Acalypha,or Coleus plant) that has been exposed to sunlight for a few hours; • Draw a diagram to show the distribution of the green colour of the leaf; and • Test the leaf for starch. Only the green parts containing starch will turn blue-black, others will not. • The change in green colour to blue-black when tested with iodine shows that chlorophyll is needed for photosynthesis. Variegated leaf: A leaf with different colour parts/patches/green and non-green parts. The only part of it that would be positive when treated with iodine solution is the green part, because there is presence of chlorophyll for tapping solar energy to manufacture starch which turns blue-black with iodine. 2. To show that starch is a product of photosynthesis. I. Pluck the leaf of a plant which has been exposed to sunlight for a few hours to ensure that food is manufactured; II. Place the plucked leaf in boiling water for 30 seconds to kill the leaf; III. Remove the leaf from boiling water, then place it in hot alcohol (use a water bath) to decolorize the leaf; IV. Dip the decolorized leaf in hot water to soften it; V. Place the leaf on tile and add iodine solution to it to test for starch presence. Experiment to show the effect of light intensity on photosynthesis/or how the intensity of light affects the rate of photosynthesis Get a table lamp/torch, a beaker, a water plant such as Elodea or Spirogyra, pond water, prepare 0.5% solution of sodium bicarbonate/baking soda/bicarbonate of soda and a beaker. Prepare the set-up as follows in the dark room: Fill the beaker with the pond water, add a small amount of 0.5% sodium bicarbonate solution to the pond water to saturate the water with carbon (IV) oxide and thus make it readily available to the plant for photosynthesis. Insert the water plant through the test-tube. Then, place the lamp/torch 15 cm from the set-up, switch on the lamp. Take three 1-minute counts of the number of bubbles produced. The bubbles are oxygen given off by the plant; which can be confirmed using a glowing splint placed over the mouth of the test-tube after the experiment. The splint is rekindled/glows brighter, which confirms that the gas is rich in oxygen. Calculate the average number of counts. Repeat the procedure several times, each time moving the lamp further away from the set-up. The farther it is, the slower the bubbles counts. This shows that light intensity affects rate of photosynthesis. Fate of photosynthetic products The only product of photosynthesis is the sugar. The sugar is used as either glucose or stored as starch, which may by used to make other substances. The glucose is used to generate energy during respiration. The energy produced is stored as ATP, which is used for life activities such as growth and reproduction. Excess starch are broken down to form glucose and as well converted to substances such as sucrose, and oils, which are stored in the plant for other uses. Such uses may be in the plant tissues or useful to human when we eat plant, where they provide energy to human and to build-up the many organic compounds that we need. Fate of photosynthetic by-product The by-product of photosynthesis is oxygen. Oxygen gas is released and is used by animals for respiration and as well by plants during respiration. It breaks down sugar (glucose) to release energy for the organisms cells and tissues in order to carry out life activities. Mineral requirements of plants Plants obtain mineral salts majorly from the soil, while others are gotten from gases (such as carbon, hydrogen and oxygen) and use them to react with the carbohydrates made during photosynthesis to make proteins and as well for healthy growth. For example, nitrogen is needed to make proteins. Mineral salts are not required in the same quantity, hence they are grouped into: a. Macro-nutrients or major elements and b. Micro-nutrients or trace elements. a. Macro-nutrients or major/essential elements: These are elements required by plants in large quantities for growth and development. Examples include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulphur, calcium, iron and magnesium. b. Micro-nutrients or trace/minor/non-essential elements: These are elements that are needed in plants, which must be present for growth and development but in small quantities. They are required for the activity of enzymes. Their absence or over dose is disastrous to plants. Examples include copper, boron, cobalt, chlorine, molybdenum, zinc and manganese When a plant lacks any of these elements, it shows certain signs called deficiency symptoms. The functions and deficiency symptoms of these elements are shown below: Element Function/importance Deficiency symptoms Nitrogen (N) 1. Protein synthesis/synthesis of amino acids 2. Synthesis of nucleic acids/nitrogenous bases 3. Synthesis/formation of chlorophyll 1. Stunted growth. 2. Chlorosis/yellow leaves 3. Small/reduced leaves Phosphorus (P) 1. Formation of nucloeproteins 2. Formation of coenzymes 3. Formation of ATP, DNA and RNA 4. Controls nuclear division 5. Controls stem and root formation 1. Poor/stunted growth 2. Stems and leaves appear purplish/reddish brown 3. Poor root development 4. Mottling of lower leaves Potassium (K) 1. Cell formation 2. Protein synthesis 3. Regulates respiration and photosynthesis 1. Leaves/leaves margins turn orange or brown 2. Delayed growth/ poor growth 3. Premature death Sulphur (S) Forms components of some amino acids and proteins 1. Yellow leaves/ chlorosis 2. Stunted growth 3. Weak stem/ slender stem Calcium (Ca) 1. Formation of cell wall 2. Neutralizes organic acids 3. Activates some enzymes 4. Gives rigidity to plant 1. Stunted growth 2. Weak stem 3. Poor root development Magnesium (Mg) 1. Formation of chlorophyll 2. Promotes growth 1. Yellow leaves 2. Stunted growth Iron (Fe) 1. Formation of chlorophyll 2. Formation of protein 1. Yellow leaves 2. Poor growth Manganese (Mn) 1. Activates some enzymes 2. Regulates certain cell activities such as respiration 1. Death of shoot Zinc (Zn) 1. Activates some enzymes 2. Necessary for the synthesis of the starting material of auxin 1. Poor growth 2. Poor leaf formation Molybdenum (Mo) 1. Activates some enzymes such as the enzyme that reduces nitrates to nitrite 2. Aids nitrogen fixation 1. Retarded growth 2. Necrosis of leaf tissue Water Culture This is the method of growing plants in distilled water containing all the necessary elements in the correct quantities in a medium. Examples of water culture solution prepared in the laboratory include Knop's culture solution and Sach's culture solution. Both culture solutions are called complete culture solutions. Water Culture Experiments We can determine the deficiency symptoms of some elements using water culture experiments, by preparing media where each medium lacks a specific element. A control experiment where seedlings are grown in a complete culture medium is done. Precautions to be taken while carrying out the water culture experiment 1. Use dry cotton wool or stoppers to hold the stem to prevent rottening of the stem. 2. Cover the gas jar with black paper or paint it black to prevent alagbo growth in the culture solution. 3. All the jars used must be kept under the same light and temperature to prevent the effect of other variables. 4. All culture vessels must be sterilized before the experiment to avoid contamination with pollutants and microorganims. 5. Air/oxygen must be blown in daily into the the culture solution through the bent glass tubing to aerate/replenish the oxygen used up by the plant. 6. Use healthy growing seedlings of the same age and size to give accurate growth rate. 7. Replace culture solutions every two weeks to avoid depletion of nutrients. Experiment on mineral deficiency/ to show the effects of mineral deficiency in plants