Photosynthesis is the process by which plants, algae, and cyanobacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of food (sugars). It occurs in two stages: (1) the light reactions where sunlight is absorbed and converted to chemical energy in the form of ATP and NADPH, and (2) the dark reactions where carbon dioxide is fixed using the ATP and NADPH to produce sugars like glucose. The overall equation is: 6CO2 + 6H2O + sunlight → C6H12O6 (glucose) + 6O2. Photosynthesis provides a critical source of food for organisms and oxygen for respiration.

2. what is photosynthesis?
Photosynthesis converts massive amount of Sunlight
into electrical and then chemical energy. The input is
carbon dioxide (CO2), water (H2O), minerals and light,
and the output is carbohydrates (food) that we need
for our nourishment, and oxygen that we need to
breathe. This oxygenic photosynthesis occurs in
higher plants (e.g., rice, maize, wheat, mosses, ferns,
forest trees, shrubs, etc); in green, red, brown and
yellow algae, and even blue-green cyanobacteria.

3. Equation of photosynthesis
6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O
6 molecules of carbon dioxide (6CO2) and 12
molecules of water (12H2O) are consumed in the
process, while glucose (C6H12O6), six molecules of
oxygen (6O2), and six molecules of water (6H2O) are
produced.
This equation may be simplified as: 6CO2 + 6H2O +
light → C6H12O6 + 6O2.

4. The set of photosynthetic reactions are arbitrarily
divided into :
(1) The light phase (that produces the reducing power
and ATP, the energy currency of life).
(2) The dark phase (where the products of light phase
are used to convert CO2 to carbohydrates).
Division of photosynthesis

5. The light phase
(occur in the presence of light)
It is a photochemical phase. 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.

6. (cont.)
This process further has two more phases :
1.Photolysis of water or hill reaction:
Chemical process by which molecules are broken down into
smaller units through the absorption of light.
ADP+inorganic phosphate+energy ATP
(photophosphoryation)
Assimilatory molecules are: NADPH2 and
ADP
2.Formation of assimilating power
molecules:The light reactions result in the
formation of the high-energy compounds
ATP and NADPH. While these compounds
can be used to drive metabolic processes,
one additional critical reaction must occur: the fixation of
carbon dioxide.

7. The dark reaction
(do not need direct light)
It is a biosynthetic phase . 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.

8. summary
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.