Photosynthesis is very important as a mode of nutrition. Nutrition is one of the characteristics of living organisms. It involves the process of obtaining energy through molecules obtained from the immediate environment for metabolic, growth and repair of body tissues. Different plant and animal species have developed different features which enables them to achieve nutrition.
This is the process by which 3 carbohydrates or sugars is manufactured in green plants containing chlorophyll by combination of carbon (iv) oxide and water with the help of solar energy (sunlight) while oxygen is given out as a by-product at the end of the process. Photosynthesis tends to be one of the most important biological activity as it ensures the production of food for both plants and animals. Plants that undergo photosynthesis is known as autotrophs (self-feeder).
Photosynthetic activity in green plants
Chlorophyll, an essential pigment in photosynthesis are found concentrated in structures known as chloroplast found in cells. The chloroplast are disc shaped in most plants, spiral in spirogyra and it’s star shaped in organisms like Zygnema. Chlorophyll molecules are capable of trapping light energy.
Carbon dioxide and water are inorganic raw materials of photosynthesis. Water molecules will be taken up from the root of plants while carbon dioxide will diffuse into the leaves through the stomata. During photosynthesis, these inorganic compounds will be converted into organic products which will be stored in plants as food. This form of nutrition is known as holophytic nutrition. Holophytic nutrition is only possible in green plants which undergo photosynthesis.
The first stable product or molecule of photosynthesis yields a simple sugar mainly glucose. The reaction represented in the chemical equation below;
Mechanism of Photosynthesis
The process or mechanism of photosynthesis is complex. It involves the catalytic activity of many enzymes. The mechanism is divided into two viz
It can be depicted that this mechanism involves light, thereby occurs naturally during the day. Light reaction of photosynthesis can be further split into different stages
A. Chlorophyll is energized: in this stage, the chlorophyll pigment traps sunlight energy and becomes energized. The energy from the activated chlorophyll is used for further stages of Photosynthesis.
B. Photolysis of water: the energy acquired by the chlorophyll is used to break water molecules into Hydrogen (H+)and hydroxy ion (OH-). The hydroxy ion gives off its negative ion to chlorophyll and forms water and oxygen, it takes 4OH- to undergo this. The oxygen is given of as a by-product of the reaction. The equation of reaction is denoted below
C. Transfer of Hydrogen to NADP (Nicotinamide adenosine dinucleotide phosphate): Hydrogen atom released above will be immediately picked by a coenzyme called NADP in order to prevent it’s escape from cells or recombining with oxygen to form water. NADP is now reduced to NADPH2 by accepting two Hydrogen ions and serve as an electron Carrier of Hydrogen ions.
D. Formation of ATP (Adenosine triphosphate): chloroplast contains low energy carrier molecule ADP (Adenosine diphosphate). The extra energy remaining from photolysis of water is used in phosphorylating ADP to form ATP. This reaction occurs in the grana.
Dark reaction (Calvin cycle)
It is called dark reaction because it does not require light energy. In this phase, carbon dioxide combines with two atoms of Hydrogen provided by NADPH2 to form sugars after of undergoing a series of reaction. Each stage of the reaction is catalyzed by an enzyme. Energy needed for the formation sugar is provided by ATP. These events takes place in the stroma of the chloroplast.
Organic compounds at the 3-carbon sugar stage is usually set aside, for the synthesis of oils, proteins and other carbohydrate derived substances.
Factors affecting photosynthetic activity
There are so many factors affecting the rate of photosynthesis. They include
- Light intensity
- Amountof carbon dioxide available
- Availabilityof water
- Environmental temperature
- Chlorophyll concentration
- Inhibitors such as herbicides
- High concentration of oxygen etc