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.
Comparison of Photosynthesis and Respiration
Basic Reaction
Photosynthesis CO2 + H2O + light energy ---> (CH2O)n (eg glucose) + O2
Respiration (CH2O)n (eg glucose) ---> CO2 + H2O + chemical energy
Feature Photosynthesis Respiration
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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
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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.