PLANT NUTRITION
Photosynthesis
Photosynthesis is the process whereby light energy is absorbed by chlorophyll and
transformed into chemical energy used in the synthesis of carbohydrates from water
and carbon dioxide. Oxygen is released during the process. Water and carbon dioxide
are the raw materials for photosynthesis.
It occurs in two stages namely the light-dependent stage and the light independent or
dark stage.
Light dependent stage
Light energy is absorbed by chlorophyll and then converted into chemical
energy.
Light is also required to split water molecules into oxygen and hydrogen i.e
photolysis of water.
Light-independent stage
Hydrogen produced is then used to reduce carbon dioxide to carbohydrate
(glucose)
The chemical energy for this process comes from the light stage.
Since the formation of glucose from carbon dioxide does not directly require
light it is called the dark stage
Enzymes are involved in the dark stage.
Overall Equation for the reaction
Importance of Photosynthesis
1. Carbohydrates are produced during photosynthesis. Proteins, fats and other
organic compounds are then formed from these carbohydrates. All these food
substances eventually become the food of animals since animals depend
directly (herbivores) or indirectly on plants.
2. The ultimate source of energy for living things is the Sun. Light energy is
converted to chemical energy which is stored within the carbohydrate
molecules. When animals feed on plants, they obtain this energy directly from
them.
, 3. Coal is a fossil formed from trees and it contains a store of energy derived
from sunlight through photosynthesis. When coal is burnt, this energy is used
in cooking, driving machines to work etc
4. Photosynthesis also helps to maintain the balance of gases in the atmosphere.
Carbon dioxide is absorbed and at the same time oxygen is produced. Living
organisms then use this oxygen in respiration.
Fate of glucose in leaves
1. When glucose is first formed, it is used by the cells during tissue respiration to
provide energy for cellular activities
2. It is used to make cellulose cell walls
3. Excess glucose is converted to sucrose which is transported to storage organs
e.g seeds, stem tubers, root tubers and corms to be stored as starch or in other
forms depending on the plant. In sugarcane plant, it is stored in the stem as
sucrose
4. During the day, the rate of photosynthesis is so great that sugars are formed
faster than they can be removed. A large amount accumulates in the leaf and is
converted to starch for temporary storage in the leaf. At night, photosynthesis
stops and the starch is reconverted by enzymes in the leaf to simple sugar
5. The glucose in the leaf can react with nitrates and other mineral salts brought
to the leaf to form amino acids. These are then combined to form proteins
which are converted into new protoplasm within the cells.
6. Fats are also formed from glucose. Some of the sugars that reach the storage
organs may also be converted to fats and stored within these organs.
, The LEAF as an ORGAN for PHOTOSYNTHESIS
Adaptations of the leaf for photosynthesis
Adaptation Purpose
Broad and flat shape Offers a large surface area for maximum absorption of
sunlight
Thin lamina Faster diffusion for carbon dioxide to reach inner cells
as there is a shorter distance
Network of veins Provides water supply to photosynthesizing cells. No
cell is very far away from a water-conducting vessel in
one of those veins
Perpendicular Prevent overlapping. They are arranged perpendicularly
arrangement of leaves at right angles to the light rays
Petiole Holds the lamina away from the stem so that the lamina
can obtain sufficient sunlight
Photosynthesis
Photosynthesis is the process whereby light energy is absorbed by chlorophyll and
transformed into chemical energy used in the synthesis of carbohydrates from water
and carbon dioxide. Oxygen is released during the process. Water and carbon dioxide
are the raw materials for photosynthesis.
It occurs in two stages namely the light-dependent stage and the light independent or
dark stage.
Light dependent stage
Light energy is absorbed by chlorophyll and then converted into chemical
energy.
Light is also required to split water molecules into oxygen and hydrogen i.e
photolysis of water.
Light-independent stage
Hydrogen produced is then used to reduce carbon dioxide to carbohydrate
(glucose)
The chemical energy for this process comes from the light stage.
Since the formation of glucose from carbon dioxide does not directly require
light it is called the dark stage
Enzymes are involved in the dark stage.
Overall Equation for the reaction
Importance of Photosynthesis
1. Carbohydrates are produced during photosynthesis. Proteins, fats and other
organic compounds are then formed from these carbohydrates. All these food
substances eventually become the food of animals since animals depend
directly (herbivores) or indirectly on plants.
2. The ultimate source of energy for living things is the Sun. Light energy is
converted to chemical energy which is stored within the carbohydrate
molecules. When animals feed on plants, they obtain this energy directly from
them.
, 3. Coal is a fossil formed from trees and it contains a store of energy derived
from sunlight through photosynthesis. When coal is burnt, this energy is used
in cooking, driving machines to work etc
4. Photosynthesis also helps to maintain the balance of gases in the atmosphere.
Carbon dioxide is absorbed and at the same time oxygen is produced. Living
organisms then use this oxygen in respiration.
Fate of glucose in leaves
1. When glucose is first formed, it is used by the cells during tissue respiration to
provide energy for cellular activities
2. It is used to make cellulose cell walls
3. Excess glucose is converted to sucrose which is transported to storage organs
e.g seeds, stem tubers, root tubers and corms to be stored as starch or in other
forms depending on the plant. In sugarcane plant, it is stored in the stem as
sucrose
4. During the day, the rate of photosynthesis is so great that sugars are formed
faster than they can be removed. A large amount accumulates in the leaf and is
converted to starch for temporary storage in the leaf. At night, photosynthesis
stops and the starch is reconverted by enzymes in the leaf to simple sugar
5. The glucose in the leaf can react with nitrates and other mineral salts brought
to the leaf to form amino acids. These are then combined to form proteins
which are converted into new protoplasm within the cells.
6. Fats are also formed from glucose. Some of the sugars that reach the storage
organs may also be converted to fats and stored within these organs.
, The LEAF as an ORGAN for PHOTOSYNTHESIS
Adaptations of the leaf for photosynthesis
Adaptation Purpose
Broad and flat shape Offers a large surface area for maximum absorption of
sunlight
Thin lamina Faster diffusion for carbon dioxide to reach inner cells
as there is a shorter distance
Network of veins Provides water supply to photosynthesizing cells. No
cell is very far away from a water-conducting vessel in
one of those veins
Perpendicular Prevent overlapping. They are arranged perpendicularly
arrangement of leaves at right angles to the light rays
Petiole Holds the lamina away from the stem so that the lamina
can obtain sufficient sunlight
