Subject: Integrated Science | Level: JHS (Basic 7 – 9) | Topic: Photosynthesis and Plant Nutrition | Difficulty: Intermediate
Introduction
Plants are unique among living things because they can make their own food. They do not need to eat other organisms. Instead, they capture the energy of sunlight and use it to convert simple raw materials into the sugar they need to grow and survive. This process is called photosynthesis. Understanding photosynthesis is essential for understanding ecology, food chains, agriculture, and even global climate because plants remove carbon dioxide from the air and release the oxygen that all living things breathe.
1. What is Photosynthesis?
Photosynthesis is the process by which green plants use light energy, water, and carbon dioxide to produce glucose (a type of sugar) and oxygen.
Word Equation for Photosynthesis:
Carbon dioxide + Water → Glucose + Oxygen
(in the presence of light energy and chlorophyll)
In symbols: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Where does Photosynthesis take place?
Photosynthesis takes place mainly in the leaves of a plant, specifically inside the chloroplasts found in the mesophyll cells of the leaf. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy (mainly red and blue wavelengths) and converts it into chemical energy.
2. Raw Materials and Products of Photosynthesis
Raw Materials (Inputs):
- Carbon dioxide (CO₂): Enters the leaf through tiny pores called stomata (singular: stoma). Stomata are found mainly on the underside of the leaf.
- Water (H₂O): Absorbed from the soil by the root hairs and transported up to the leaves through the xylem (water-conducting vessels).
- Light energy: Captured by chlorophyll from sunlight.
Products (Outputs):
- Glucose (C₆H₁₂O₆): Used by the plant for energy (through respiration), growth, and storage (converted to starch). Excess glucose is converted to starch and stored in the plant.
- Oxygen (O₂): Released into the atmosphere through the stomata as a by-product. This is the oxygen that animals, including humans, breathe.
3. The Structure of a Leaf and its Role in Photosynthesis
The leaf is the main organ of photosynthesis. Its structure is adapted for efficient photosynthesis in several ways:
- Broad and flat shape: Provides a large surface area to capture maximum sunlight
- Thin: Allows sunlight to penetrate all the way through the leaf
- Transparent upper epidermis: Allows light to reach the chloroplasts in the cells below
- Mesophyll cells (palisade layer): Tightly packed cells near the top of the leaf, each containing many chloroplasts; the main site of photosynthesis
- Spongy mesophyll: Air spaces allow carbon dioxide and oxygen to move freely within the leaf
- Veins (vascular bundles): Contain xylem (carries water and minerals) and phloem (carries glucose away from the leaf)
- Stomata: Allow CO₂ in and O₂ and water vapour out; can open and close to control gas exchange and water loss
- Guard cells: Pairs of cells surrounding each stoma that open and close the stoma depending on water content and light conditions
4. Factors Affecting the Rate of Photosynthesis
The speed at which photosynthesis occurs is called the rate of photosynthesis. Three main factors can limit this rate:
Light Intensity
As light intensity increases, the rate of photosynthesis increases – up to a point. Beyond a certain light level, other factors (like CO₂ concentration) become the limiting factor and the rate levels off.
Practical example: A plant in a dark room photosynthesises slowly; a plant placed in bright sunlight near a window photosynthesises much faster.
Carbon Dioxide Concentration
As CO₂ concentration increases, the rate of photosynthesis increases – up to a point. In greenhouses, farmers sometimes add extra CO₂ to increase plant growth.
Temperature
Photosynthesis involves enzymes. As temperature rises, enzyme activity increases and photosynthesis speeds up. However, if the temperature gets too high (above about 40°C for most plants), the enzymes are denatured (destroyed) and photosynthesis stops completely.
Limiting factor: The factor that is in shortest supply and is therefore controlling the rate of photosynthesis is called the limiting factor.
5. Testing a Leaf for Starch (Iodine Test)
To prove that photosynthesis has occurred, we test a leaf for starch (the stored product of glucose):
- Boil the leaf in water for 2 minutes to kill the cells and stop chemical reactions
- Boil the leaf in ethanol (alcohol) to remove the green chlorophyll, making the leaf pale/white
- Rinse the leaf in warm water to soften it
- Spread the leaf on a white tile and add iodine solution
- If starch is present, the leaf turns blue-black
- If no starch is present, the leaf stays brown-yellow (the colour of iodine)
Control experiment: A leaf kept in the dark (no photosynthesis) should NOT turn blue-black, proving that light is needed for starch production.
6. Mineral Nutrition in Plants
Besides photosynthesis, plants also need mineral salts from the soil for healthy growth. These minerals are absorbed through the root hairs along with water.
| Mineral | Function | Deficiency Effect |
|---|---|---|
| Nitrates | Making proteins and amino acids for growth | Stunted growth, yellow leaves |
| Phosphates | Making DNA and ATP (energy); root development | Poor root growth, purple leaves |
| Potassium | Enzyme function, water balance | Yellow leaf edges, poor flower/fruit production |
| Magnesium | Making chlorophyll | Yellowing between leaf veins (chlorosis) |
| Calcium | Cell wall formation | Poor growth of shoot tips and roots |
This is why farmers use fertilizers containing these minerals to improve crop yields. Common fertilizers used by Ghanaian farmers include NPK fertilizer (containing Nitrogen, Phosphorus, and Potassium).
7. Photosynthesis vs Respiration
Students often confuse photosynthesis and respiration. Here is a clear comparison:
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Occurs in | Plants (and some other organisms) | All living organisms |
| Inputs | CO₂ + H₂O + light energy | Glucose + O₂ |
| Outputs | Glucose + O₂ | CO₂ + H₂O + energy (ATP) |
| Site | Chloroplasts (leaves) | Mitochondria (all cells) |
| Time | Only during daylight | All the time (day and night) |
| Energy | Uses light energy (stores it) | Releases energy |
During the day, plants carry out BOTH photosynthesis and respiration. At night, only respiration occurs.
Summary
- Photosynthesis: CO₂ + H₂O → Glucose + O₂ (using light and chlorophyll)
- It occurs mainly in the palisade mesophyll cells of leaves, inside chloroplasts
- CO₂ enters through stomata; water is absorbed through roots via xylem
- Rate is affected by light intensity, CO₂ concentration, and temperature
- Starch test: blue-black = starch present = photosynthesis occurred
- Plants also need mineral salts (nitrates, phosphates, potassium, magnesium) for healthy growth
Practice Questions
- Write the word equation for photosynthesis.
- Name the structure in leaf cells where photosynthesis takes place.
- Explain TWO ways in which the structure of a leaf is adapted for efficient photosynthesis.
- A plant was kept in a dark cupboard for 48 hours. Its leaves were then tested with iodine. What result would you expect, and why?
- A farmer notices that his maize plants have yellow leaves and stunted growth. Which mineral deficiency is most likely responsible, and what should he do?
Answers: (1) Carbon dioxide + Water → Glucose + Oxygen (in presence of light and chlorophyll) (2) Chloroplasts (3) Any two: broad flat shape for maximum light absorption; thin for light penetration; palisade cells packed with chloroplasts near surface; stomata allow gas exchange; veins supply water and remove products (4) No blue-black colour; no starch because no photosynthesis occurred without light (5) Nitrate deficiency; farmer should apply nitrogen-containing fertilizer such as urea or NPK fertilizer
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