Topic: Plant Nutrition
Lesson Objectives: by the end of the lesson, learners should be able to:
i. Define photosynthesis,
ii. Explain the mechanism of photosynthesis,
iii. State and explain the materials and conditions necessary for photosynthesis,
iv. Demonstrate the necessary experiments on photosynthesis,
v. Discuss all elements required by plants for their growth and development.
One of the main characteristics of living things is their ability to feed. This is called nutrition. Both plants and animals feed. While some organisms called autotrophs can manufacture their own food, other organisms called heterotrophs cannot. Therefore, most autotrophs can manufacture their own food through a process called photosynthesis.
Photosynthesis is defined as the process by which green plants manufacture their food (organic compounds) making use of carbon dioxide and water in the presence of sunlight.
6CO2 + 6H2O C6H12O6 + 6O2
From the chemical equation above, water combines with carbon dioxide in the presence of sunlight within the chlorophyll of leaves to manufacture food (glucose) and oxygen is liberated as a by product. Photosynthesis occurs in all green parts of plants.
Mechanism of Photosynthesis: Photosynthesis is an endothermic reaction and it consists of a series of complex reactions. These reactions to during photosynthesis is grouped into two stages called Light reaction and Dark reaction
Light Reaction Stage
Light reaction stage occurs during the day or in the presence of sunlight. The light energy or solar energy is captured by the chlorophyll and electrons are excited. The energy so trapped is used to split water into hydrogen ion (H+) and hydroxyl ion (OH-). This splitting of water is called photolysis of water.
Photolysis of water is represented by the equation:
During this process, oxygen is given out as by-product as shown in the chemical equation above. At the same time, a compound, co-enzyme or NADP is reduced by hydrogen ion to NADPH and ATP (Adenosine Triphosphate) is formed.
Dark Reaction Stage
Dark reaction occurs at night or in the absence of light. Together with the energy provided by ATP, the reduced compound Nicotinamide Adenine Dinucleotide (NADP) then lead to the assimilation of carbon dioxide. Through a series of steps, each controlled by a specific enzyme, a three carbon compound (CH2O) or sugar is formed. The formation of this three carbon compound can be represented by a chemical equation as follows;
Materials and Conditions Necessary for Photosynthesis
For photosynthesis to take place, certain materials or conditions must be available. These are carbon(iv)oxide, water, cholorophyII, sunlight, mineral salts, suitable temperature and enzymes.
While carbon(iv)oxide , water, sunlight, mineral salts and suitable temperature are external conditions or factors; cholorophyII and enzymes are internal conditions or factors necessary for photosynthesis to take place.
i. Carbon (iv)oxide: Carbon(iv) oxide is derived from the atmosphere and it diffuses into the intercellular spaces through the stomata of the leaves. From the intercellular spaces, carbon(iv)oxide diffuses further into the mesophyll containing chloroplasts.
ii. Water and mineral salts: Water and mineral salts are derived from the soil. They pass into the roots of plants through the root hairs by a process called osmosis. Water and dissolved mineral salts are conducted by xylem from the roots through the stem and finally to the mesophyll cells containing cholorophyII of the leaves.
iii. Sunlight: Sunlight is obtained from solar energy. The light from the sun is trapped by the cholorophyII of the leaves. The sunlight is used to split water into hydrogen ions and hydroxyl ions in a process called photolysis.
iv. Optimum temperature: Temperature is derived partly from the solar energy and partly from chemical reactions within the leaves during which heat is generated. Suitable temperature is important for enzymes to enable them function properly during photosynthesis.
v. Chlorophyll: Cholorophyll is the green colouring pigment found in the palisade and spongy mesophyll of the leaves. The cholorophyll represents sites where food can be synthesized and it helps to trap solar energy and convert it to chemical energy.
Adaptation of Leaf for Photosynthesis
i. Broad/flat leaf lamella, exposes larger surface area for maximum absorption of light;
ii. Palisade mesophyll cells contact lot of chloroplast cholorophyll, for maximum absorption of light;
iii. Position of leaf to stem; for optimum reception of light Ray’s/sunlight;
iv. Thin lamella; allows light penetration into leaf tissue of mesophyll;
v. Large vacuoles of the palisade cells; photosynthesis products.
vi. Large intracellular air spaces in the spongy mesophyll; allow oxygen carbondioxide to diffuse in/out of the chlorophyllus cells/gaseous exchange.
vii. The epidermal cells have thin cell walls; for easy penetration of sunlight to the mesophyll;
viii. Guard cells have chloroplast; for the absorption of sunlight.
ix. The stomata; open easily when it becomes turgid for diffusion of gases/carbondioxide oxygen.
x. Xylem; conducts water into the leaves for photosynthesis.
Evidence of Photosynthesis in Plants
Formation of sugar and starch: The main product formed during photosynthesis is simple sugar. The simple sugar formed partly used by the plant and excess of it is converted to starch immediately for shortage. The starch is then transported to other parts of the plant through the phloem vessels for shortage. This process is called translocation.
The best way to show the evidence of photosynthesis in plant is to test the leaf for starch. The test for starch can be carried out through the following experiments:
Aim: To test for the presence of starch in the leaf.
Materials required: Fresh green leaves from outdoor plants, beakers, burner, boiling tubes, dropping tube, white tile and iodine solution.
Method: First boil the leaf in water for 4 – 6 minutes so as to kill the cells, inactive the enzymes and burst starch grains present.
Then dip the leaf into a test tube containing 70% alcohol to decolourise the leaf. After that, the decolourise leaf is dipped into a beaker containing hot water to soften it. Finally, place the leaf in a white tile and pour few drops of iodine solution on the leaf. In the control experiment, a leaf from a plant kept in the dark cupboard is plucked and tested for starch.
Fig 6.2: To test for the presence of starch in a leaf.
Observation: It is seen that the leaf that was plucked from the potted plant outside turned blue-black with the iodine solution while the other leaf (control experiment) remained colourless.
Conclusion: Since the leaves in the real experiment has turned blue-black with iodine solution, it shows that photosynthesis has taken place or starch is formed by the leaf.
Aim: To show that sunlight is necessary for photosynthesis.
Material required: A potted plant, strip of black paper, clips, cupboard.
Method: The potted plant is first de-starched by putting it in a dark cupboard for 1 – 2 days. This is to remove all traces of starch formed in the leaves. After this, the middle of one of the leaves is covered by a strip of black paper, both at the front and back with the aid of clips. The whole plant is then placed in sunlight. After about 3 – 5 hours, the paper is removed. The leaf is then tested for starch.
Observation: Only the exposed parts, i.e, the top and bottom of the leaf turned blue-black with the iodine solution which shows the presence of starch while the area that was covered with black paper will remain colourless, indicating that starch is absent.
Conclusion: The experiment shows that sunlight is necessary for photosynthesis.
Aim: To show that carbon(iv)oxide is necessary for photosynthesis.
Materials required: A potted plant, Vaseline, conical flask, split cork, retort stand and caustic soda (sodium hydroxide) solution.
Method: Use a leaf attached to a potted plant (fig 6.4). The leaf is enclosed in the flask containing caustic soda (sodium hydroxide) solution. This solution absorbs the carbon(iv)oxide inside the flask. The flask mouth is corked and smeared with Vaseline at the neck to make it air tight. The whole experiment is now exposed to sunlight for several hours. Two leaves (one inside the flask) and the other outside the flask (control experiment) are plucked and tested for starch.
Observation: At the end of the test for starch, the leaf inside the flask did not show blue-black colour, indicating absence of starch formation because of the lack of carbon(iv)oxide inside the flask while the leaf outside the flask shows blue-black colour, indicating the presence of starch.
Conclusion’ The experiment shows that carbon(iv)oxide is necessary for photosynthesis to take place.
Fig 6.4: To show that carbon(iv)oxide is necessary for photosynthesis.
Aim: To show that cholorophyII is neessary for photosynthesis.
Materials required: A variegated plant like Croton, ice-plant or acalypha plant. A variegated plant has green and white patches on the leaves
Method: The potted variegated plant is exposed to sunlight for about 3 – 5 hours after which a variegated leaf is plucked fresh from the plant during the day time when there is sunlight. Make a drawing of the leaf and map out the green and white patches. The variegated leaf is then tested for starch. Spread out the leaf and then, compare the iodine map with the chlorophyll map.
Observation: It will be noticed that the green parts of the variegated leaf are stained blue-black with iodine solution while the white parts remain colourless.
Conclusion: This experiment shows that cholorophyII is necessary for photosynthesis to take place.
Aim: To show that oxygen is given out as a by-product during photosynthesis.
Materials required: A water plant, e.g Elodea, glass funnel, beaker, water, test tube and splinter.
Method: Fill up the beaker with water. Then place the water plant (e.g Elodea) at the bottom of the beaker. This is followed by the filling of the test tube with water and then invert it over the stem of the funnel. The whole set-up is then placed in the sunlight as shown in fig 6.6 for several hours. Tiny bubbles of gas will start to appear on the surface of the leaves and these break off and accumulate at the top of the test tube. The gas is tested with a glowing splinter.
Observation: It is seen that the gas formed at the top of the test tube rekindles a glowing splinter showing the presence of oxygen.
Fig 6.6: To show that oxygen is given off as a by-product during photosynthesis
Conclusion: This experiment shows that oxygen is given off as a by-product during photosynthesis.
Importance of Photosynthesis
Photosynthesis is very important both to plants and animals for the following reasons:
i. Production of food: Photosynthesis provides food for both plants and animals. All green plants are able to manufacture their food through the process of photosynthesis while animals depend directly or indirectly on these green plants for their food.
ii. Purification of the atmosphere: Waste product like carbon(iv)oxide released during respiration by both plants and animals is removed from the atmosphere by plants for use during photosynthesis.
iii. Release of oxygen to the environment: Oxygen needed for respiration by plants and animals is released into the environment during photosynthesis.
iv. It serves as building blocks for other substances: Photosynthesis provides the building block or carbon skeleton on which other food substances such as proteins, fats, oil etc are built.
Done studying? See all previous lessons in Biology
Take a quick test for this lesson
i. Define Photosynthesis. Describe the light and dark reaction stages of photosynthesis.
ii. Name five conditions necessary for photosynthesis to take place.
iii. Desrcibe an experiment to show that light is necessary for photosynthesis.
iv. In the test for starch, why is green leaf boiled in (i) water (ii) alcohol?
v. Explain how the leaf of a flowering plants is adapted for photosynthesis.
Questions answered correctly?
Do stay connected for more lesson contents.