Water lies at the core of energy capture and processing by higher living things The splitting of the water molecule during photosynthesis and its re-formation through aerobic respiration is the heart of energy processing by living things.
Focusing on water is the key to understanding the relationship between these processes.
Photosynthesis CO2 + H2O + light energy ---> (CH2O)n (e.g. glucose) + O2
Respiration (CH2O)n (e.g. glucose) ---> CO2 + H2O + chemical energy
Feature |
Photosynthesis |
Respiration |
Energy |
Stored |
Released |
Reactants |
CO2 and H2O |
Glucose |
Products |
Glucose |
CO2 and H2O |
Electron Transport chain |
Yes |
Yes |
Products of Electron Transport |
ATP + NADPH |
ATP |
Electron Transport chain Pumps H+ ions |
Yes |
Yes |
H+ gradient drives ATP formation |
Yes |
Yes |
ATP synthase |
Yes |
Yes |
Start molecule |
Water |
NADH |
End molecules |
NADH |
Water |
Photosynthesis and respiration comprise electron transfer between proteins, which often contain metal ions, e.g. iron, in specific electron-transport chains.
The electron-transport proteins in photosynthesis as well as in respiration are organized as complicated molecular aggregates bound to membrane systems of two specific cell organelles, chloroplasts and mitochondria.
The energy liberated during the electron transport is used to pump protons across the membranes, so that a difference in pH and electrical potential between the two sides is created.
This electrochemical potential is then used to drive the synthesis of adenosine triphosphate (ATP), the universal energy storage molecule in living cells, according to the chemiosmotic mechanism formulated by the British biochemist Peter Mitchell (Nobel Prize for chemistry 1978).
The photosynthetic apparatus in bacteria is simpler than in algae and higher plants.
The structural work has, however, shown that there is a close relationship between the bacterial reaction center and the oxygen-evolving protein complex in higher plants.
Many central biological functions in addition to photosynthesis and cell respiration are associated with membrane-bound proteins.
Membranes as well as carbon and water are key features of life. Examples are transport of chemical substances between cells, hormone action and nerve impulses.
>>Next: Electron transport chain