🌿 Photosynthesis
1. Energy is the ability to do work.
2. Living things need energy from food to carry out life processes.
3. The Sun is the main source of energy for all living things on Earth.
1. Plants make their own food using:
- (a) light energy obtained from the Sun,
- (b) carbon dioxide from the air and,
- (c) water from the soil.
2. The process through which plants make their own food is called photosynthesis.
3. A green substance found in plant cells helps to trap light energy from the Sun. This substance is called chlorophyll.
4. Chlorophyll is mostly found in the leaves of plants.
- Stomata — tiny openings on leaves where carbon dioxide enters
- Chlorophyll — traps light energy from the Sun in the leaves
- Water — absorbed by roots and transported to the leaves
1. During photosynthesis, plants produce:
- (a) food in the form of sugar, which will provide the plants with the energy needed for their life processes, and
- (b) oxygen gas, which is used by living organisms.
2. Excess food in plants is converted to starch, which can be stored in plant parts such as roots, stems and fruits.
Carbon dioxide + Water → Sugar + Oxygen
(light energy is trapped by chlorophyll in the leaves)
- Water absorbed by the roots is transported through the water-carrying tubes to the leaves.
- Carbon dioxide enters the leaf through the stomata.
- Chlorophyll traps light to make food through photosynthesis.
- Food made by the leaves is transported through the food-carrying tubes to all parts of the plant.
- Oxygen is released through the stomata.
1. Animals cannot make their own food unlike plants because they do not have chlorophyll that traps light energy needed for photosynthesis.
2. Instead, animals obtain the energy needed for life processes by eating food.
3. Animals can be grouped according to the types of food they eat. The three groups are:
- (a) herbivores — animals that eat only plants,
- (b) carnivores — animals that eat only animals, and
- (c) omnivores — animals that eat both plants and animals.
1. The rate of photosynthesis refers to the speed at which plants make food in the presence of light.
2. Factors that affect the rate of photosynthesis include:
- (a) intensity and colour of light,
- (b) amount of chlorophyll in the leaves,
- (c) amount of carbon dioxide in the air, and
- (d) temperature.
1. From 0 °C to 30 °C, as temperature increases, the rate of photosynthesis increases.
2. However, beyond 30 °C, increasing the temperature causes the rate of photosynthesis to decrease.
1. Chlorophyll is a green pigment found in the chloroplasts of plant cells.
2. It enables plants to trap light for photosynthesis.
3. Chlorophyll is found mainly in the leaves of most plants and in the stem of the cactus plant.
Green, red and purple parts of a leaf contain chlorophyll. The reddish or purplish appearance is due to other pigments masking the green colour of chlorophyll.
Yellow and white parts of a leaf do not contain chlorophyll and cannot undergo photosynthesis.
1. As the intensity of light increases, the rate of photosynthesis increases.
2. However, past a certain point, even if light intensity continues to increase, the rate of photosynthesis remains the same.
3. This happens because the plant has reached its maximum rate of photosynthesis, which is limited by other factors such as the amount of carbon dioxide available.
1. Carbon dioxide is required for photosynthesis to take place.
2. Carbon dioxide enters the leaves through tiny openings in the leaves called stomata.
3. The greater the amount of carbon dioxide in the surroundings, the greater the rate of photosynthesis.
4. Past a certain point, even as the amount of carbon dioxide increases, the rate of photosynthesis remains the same — the plant has reached its maximum rate, limited by an insufficient amount of other factors such as light intensity.
1. A data logger can be used to measure changes in the amount of carbon dioxide and oxygen in a closed system to determine whether photosynthesis has occurred and the rate at which it took place.
2. Indicator solutions that change colour in response to changes in carbon dioxide or oxygen levels can also be used to determine if photosynthesis has occurred.
In the presence of light, hydrilla undergoes photosynthesis. The leaves produce oxygen, which appears as bubbles.
- The rate can be found by counting the number of oxygen bubbles produced in a fixed period of time, or
- by measuring the volume of oxygen collected in the measuring cylinder after a fixed period of time.
1. A starch test can be used to determine if a plant has undergone photosynthesis and converted excess sugar to starch.
2. When iodine solution (yellowish-brown) is added to a substance containing starch, a dark blue colour is observed.
- A leaf is plucked from a plant and placed in boiling water for 10 seconds to kill the cells and soften the cell walls.
- The leaf is then soaked in alcohol and placed in a water bath at 80 °C to decolourise it, so starch test results can be observed clearly.
- Iodine solution is dropped onto the decolourised leaf. If starch is not present, the iodine solution remains yellowish-brown. If starch is present, the iodine solution turns dark blue.
1. A plant should be destarched before it is used in a photosynthesis experiment by leaving it in a dark place for at least 48 hours.
2. This prevents photosynthesis and causes the plant to use up any stored starch for respiration, ensuring no starch is present in the leaves at the start of the experiment.