PHOTOSYNTHESIS
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy
into chemical energy in the form of glucose or other sugars. This process occurs in the
chloroplasts of plant cells and is essential for the survival of almost all living things on Earth.
The process of photosynthesis can be divided into two stages: the light-dependent reactions
and the light-independent reactions.
The light-dependent reactions occur in the thylakoid membranes of the chloroplasts. These
reactions involve the transfer of electrons from water molecules to produce ATP (adenosine
triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). This process also
releases oxygen as a byproduct.
The light-dependent reactions are powered by light energy, which is absorbed by pigments such
as chlorophyll. The absorbed light energy is used to excite electrons, which are then transferred
from molecule to molecule through a series of electron carriers. The energy from these excited
electrons is used to produce ATP and NADPH through the process of chemiosmosis.
In chemiosmosis, the transfer of excited electrons through the electron carriers creates an
electrochemical gradient across the thylakoid membrane. This gradient is used to drive the
synthesis of ATP from ADP (adenosine diphosphate). The synthesis of ATP occurs through the
movement of protons across the thylakoid membrane, from the stroma (the fluid-filled region of
the chloroplasts) to the thylakoid space.
The light-independent reactions, also known as the Calvin cycle, occur in the stroma of the
chloroplasts. These reactions use the energy from ATP and NADPH to convert carbon dioxide
into glucose. This process occurs in a series of enzyme-catalyzed reactions and involves the
incorporation of carbon atoms from CO2 into the carbohydrate molecule.
The light-independent reactions begin with the fixation of CO2, which is converted into a stable
intermediate compound called 3-phosphoglycerate. The energy from ATP and NADPH is used
to convert 3-phosphoglycerate into glucose through a series of intermediate compounds.
Overall, photosynthesis is a vital process that allows plants to convert light energy into chemical
energy and produce the oxygen that is necessary for the survival of most living organisms on
Earth. In addition to providing a source of energy for plants, photosynthesis also plays a critical
role in the global carbon cycle, as it removes CO2 from the atmosphere and converts it into
organic matter.