Photosynthesis in Plants
In plants, photosynthesis occurs mainly within the leaves. Since photosynthesis requires carbon dioxide, water, and sunlight, all of these substances must be obtained by or transported to the leaves. Carbon dioxide is obtained through tiny pores in plant leaves called stomata. Oxygen is also released through the stomata. Water is obtained by the plant through the roots and delivered to the leaves through vascular plant tissue systems. Sunlight is absorbed by chlorophyll, a green pigment located in plant cell structures called chloroplasts. Chloroplasts are the sites of photosynthesis. Chloroplasts contain several structures, each having specific functions:
Outer and inner membranes: protective coverings that keep chloroplast structures enclosed.
Stroma: dense fluid within the chloroplast. Site of conversion of carbon dioxide to sugar.
Thylakoid: flattened sac-like membrane structures. Site of conversion of light energy to chemical energy.
Grana: dense layered stacks of thylakoid sacs. Sites of conversion of light energy to chemical energy.
Chlorophyll: a green pigment within the chloroplast. Absorbs light energy.
Stages of Photosynthesis
Photosynthesis occurs in two stages. These stages are called the light reactions and the dark reactions. The light reactions take place in the presence of light. The dark reactions do not require direct light, however dark reactions in most plants occur during the day.
Light reactions occur mostly in the thylakoid stacks of the grana. Here, sunlight is converted to chemical energy in the form of ATP (free energy containing molecule) and NADPH (high energy electron carrying molecule). Chlorophyll absorbs light energy and starts a chain of steps that result in the production of ATP, NADPH, and oxygen (through the splitting of water). Oxygen is released through the stomata. Both ATP and NADPH are used in the dark reactions to produce sugar.
Dark reactions occur in the stroma. Carbon dioxide is converted to sugar using ATP and NADPH. This process is known as carbon fixation or the Calvin cycle. Carbon dioxide is combined with a 5-carbon sugar creating a 6-carbon sugar. The 6-carbon sugar is eventually broken-down into two molecules, glucose and fructose. These two molecules make sucrose or sugar.
In summary, photosynthesis is a process in which light energy is converted to chemical energy and used to produce organic compounds. In plants, photosynthesis occurs within the chloroplasts. Photosynthesis consists of two stages, the light reactions and the dark reactions. The light reactions convert light into energy (ATP and NADHP) and the dark reactions use the energy and carbon dioxide to produce sugar.