Carbon dioxide – Photosynthesis

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Photosynthesis is a process where various organisms (plants, algae and bacteria) convert energy from the sun (light) into chemical energy. The process stores carbon through absorbation of CO2 from the atmosphere and releases oxygen to the atmosphere.

During the photosynthesis, energy from light is used by green plants, some algae and bacteria to combine CO2 from the atmosphere with water, to create carbohydrates (sugar) and oxygen.

Photosynthesis occurs in plants or parts of plants that contain green chlorophyll pigments, which absorbs energy from light.

In addition to green plants, chlorophyll may be found in red and brown algae and some bacteria.

The overall equation for photosynthesis may be expressed as:

6CO2 + 12H2O(Water) + photons(Light energy) → C6H12O6 + 6O2 + 6H2O

CO2 – Carbon dioxide
H2O – Water
Photons – Light energy
C6H12O6 – Carbohydrate (Sugar)
O2 – Oxygen

Chemosynthesis is the only alternative process to photosynthesis occurring within some bacteria where chemical energy is used to transform carbon dioxide and water into organic elements.

The photosynthesis can be divided into two phases, the light dependent phase (“the light phase”) and the light independent phase (“the dark phase”)

The light phase
The leaf of the plants absorbs 80 % – 90 % of the light. 1% -2 % of the absorbed energy is used for the photosynthesis; the rest is lost as heat.

The photosyntyhesis itself is taking place in organelles called chloroplasts located within the leaves or other green parts of the plant.

During this phase one molecule of the pigment chlorophyll absorbs one photon and loses one electron.

Simplified, this phase produces oxygen (O2) and electron carriers (through NADPH which is a reducing agent and ATP, an agent transporting energy within cells) used during the dark phase.

The overall equation for the light-dependent reactions may be expressed as:

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

The dark phase
The reaction taking place during light independent phase, also called the Calvin cycle, may simplified be described as CO2 caption (the enzyme RuBisCO), fixation and transformation. Using energy from the NADPH and the ATP formed during the light phase CO2 is transformed into Carbohydrates.

The overall equation for the light-independent reactions may be expressed as:

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3– (phosphate) + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

The carbohydrates fomed by photosynthesis are used to form other organic compounds, such as sucrose, starch and cellulose.

This way, CO2 is removed from the atmosphere and stored in the structure of the plant.

Quantities
Cellular respiration (the plants exchange or utilization of carbohydrates) releases back to the atmosphere 40% – 70 % of the carbondioxide captured during the photosynthesis.

Some estimates indicate that about 120 Gt of carbon is annually absorbed and stored through photosynthesis as carbohydrates and around 60 Gt carbons is converted back and released into the atmosphere as CO2.

It is estimated that the out of the total amount of biomass about 39,5% is carbon.

Further it is assumed that about 66% of the net production of biomass is on land production and that about 34% is marine production.

There are various estimates for annual total net production of biomass and the corresponding carbon quantities. The net production of biomass is estimated to 210 – 270 Gt/year.

Assuming the total annual production of biomass is 265 Gt, then 39% carbon yields about 105GtC/year the corresponding amount of CO2 assimilated through photosyntesis is: (105/12(the atom weight of carbon)) * 44 (the mol weight of CO2) = 385 GtCO2/year.

Estimates indicate that the atmosphere contain about 2750 Gt CO2, corresponding to 750 Gt Carbon.