Photosynthesis is the reverse of Cellular Respiration.

The main end products of respiration are CO2 (carbon dioxide) and water, which are used as the starting material for Photosynthesis, and photosynthesis converts them into glucose and O2 (oxygen).

Photosynthesis is the ultimate source of all energy rich carbon compounds used by all organisms; it is responsible for the continual supply of atmospheric O2 (oxygen), without which all the aerobic organisms, that use oxygen would not exist.

Green plants, algae, some unicellular green flagellates and 2 bacteria groups are the only organisms that photosynthesize. Each year they release half of all the O2 (oxygen) in the atmosphere.

Plants use CO2 (carbon dioxide) when they produce O2 (oxygen). CO2 is converted to O2 during photosynthesis. At the same time, animals through their respiration process use this O2 from their metabolism and replace it with CO2, which is then used by plants to begin the cycle again.

Photosynthesis is a solar powered process. SUNLIGHT is a key component of the process.

Light is a form of ELECTROMAGNETIC ENERGY. When light meets matter, it can be reflected, transmitted or ABSORBED.

PIGMENT absorbs light. Plant pigment, CHLOROPHYLL (the main light-absorbing molecule of green plants), is a pigment that absorbs LIGHT ENERGY.

Chlorophyll is found in specialized structures called CHLOROPLAST; they give plants their green color. Each chloroplast contains all the chlorophyll and enzymes needed to complete the complex chemical reactions of photosynthesis.

Chlorophyll participates directly in LIGHT REACTIONS.

The site of photosynthesis is typically the leaf of green plants. Each cell has about 30 to 40 chloroplast.

The large amount of chlorophyll in the leaves of plants allows it to produce most of their own FREE ENERGY, by using photosynthesis.

Cells of green leaves-> Contain chloroplast organelles-> Filled with chlorophyll molecules-> Absorb light energy during photosynthesis

Photosynthesis involves 2 linked sets of chemical reactions:
1. The 1st is called the LIGHT-DEPENDENT REACTIONS
2. The 2nd set, which does the actual synthesizing of chemicals, is called LIGHT-INDEPENDENT or the CALVIN CYCLE.

In these 2 processes green organisms use energy from sunlight to make sugar.

Photosynthesis = Light-dependent reactions (produce energy) + Calvin Cycle (produce sugar)

1. LIGHT-DEPENDENT REACTIONS
– Light-Dependent Reactions are the conversion of light energy into chemical energy.
– In the first set of reactions, the chlorophyll absorbs energy that is striking the plants surface from sunlight. The Light-Reactions absorb the energy of sunlight and converts it to energy that is stored in chemical bonds.
– Once the light is absorbed the electrons of the chlorophyll become excited (a process where an electron gains energy). This energy absorption agitates the electrons within the chlorophyll. Some of these energized electrons are transferred where they can be used by the organism.
– The energy released from the electrons in the chlorophyll is used to do 2 things:
1. Split water molecules. When the water molecule is split, O2 (oxygen) is released into the atmosphere.
2. Make ATP. Some of the energy from the electrons in chlorophyll is used to change ADP to ATP.
– This energy is passed along until it reaches a particular pair of molecules that can process the energy. The energy released by the transferred electrons helps add a phosphate group to ADP, creating more ATP.
– Some of the produced ATP provides energy to run the 2nd stage of photosynthesis, the Calvin Cycle.
– As a result of the Light-Dependent Reactions, water is split and O2 (oxygen ) is given off. The plant uses some of the O2 for CELLULAR RESPIRATION and some is released into the atmosphere.

2. CALVIN CYCLE
– The 2nd stage, called the Calvin Cycle, uses the ATP given off by the 1st set of reactions.
– This step involves the storage of chemical energy into sugar molecules.
– It is called a “CYCLE” because it begins and ends at exactly the same point: Carbon Dioxide (CO2) molecules.
– The Calvin Cycle takes place in the chloroplast. The simple inorganic molecules of CO2 (carbon dioxide) is used to make a complex organic molecule.
– In this step, CO2 (carbon dioxide) is chemically reduced from hydrogen ions and turned into a carbohydrate (sugar molecule/glucose).

CO2 (Carbon Dioxide) + H (Hydrogen Ions) = CH2O (Carbohydrate)

– One sugar molecule is to be used by the plant cell.
– Many plants make more sugar than they need; this sugar is converted to starch and stored in the roots, tubers and fruits of plants, food that other organisms eat.
– Most organisms need the food manufactured by green plants in the process of photosynthesis.

The following reaction summarizes the chemical process of Photosynthesis:

6 CO2(Carbon Dioxide) + 6 H2O(Water){REACTANTS} -> C6 H12 O6(Glucose) + 6 O2(Oxygen) + 6 H2O(Water){PRODUCTS}