Photosynthesis is a chemical pathway that’s integral to producing energy in plants and other primary producers. Energy in the form of molecules of glucose is produced from light, water and carbon dioxide while oxygen is released. This occurs in several complex steps, photosynthesis is a rate limited reaction, depends on several factors including carbon dioxide concentration, ambient temperature and light intensity; the energy is retrieved from photons, I.e. particles of light, and water is used as a reducing agent.

In the light reactions, occuring within the thykaloid, and stroma of the chloroplasts, water supplies the pigment chlorophyll with replacement electrons for the ones removed from photosystem II. Additionally, water (H2O) split by light during photolysis into H+ and OH- acts as a source of oxygen along with functioning as a reducing agent; it reduces the molecule NADP to NADPH by providing H+ ions. NADP and NADPH are integral to the dark reactions, or Calvin cycle where monosaccharides or sugars like glucose are produced after the modification of several molecules.

Respiration in the mitochondria utilizes oxygen for the production of ATP in the Krebs’s cycle via the oxidization of pyruvate (through the process of glycoysis). The electron transport chain, in which oxygen functions as the terminal electron acceptor, occurs in both plants and animals. Respiration includes:

Glycolysis: occurs in the cytoplasm 2 molecules of ATP are used to cleave glucose into 2 pyruvates, 4 ATP and 2 electron carrying NADH molecules.
The Kreb's cycle: in the mitochondrial matrix- 6 molecules of CO2 are produced by combining oxygen and the carbon within pyruvate, 2 ATP oxygen molecules, 8 NADH and 2 FADH2.
The electron transport chain, ETC: in the inner mitochondrial membrane, 34 ATP, electrons combine with H+ split from 10 NADH, 4 FADH2, renewing the number of electron acceptors and 3 oxygen; this forms 6 H2O, 10 NAD+, 4 FAD.