Cell respiration and photosynthesis are essentially opposite processes. Photosynthesis is the process by which organisms make high-energy compounds – the sugar glucose in particular – through the chemical "reduction" of carbon dioxide (CO₂). Cellular respiration, on the other hand, involves the breakdown of glucose and other compounds through chemical "oxidation." Photosynthesis consumes CO₂ and produces oxygen. Cellular respiration consumes oxygen and produces CO₂.

In photosynthesis, energy from light is converted into chemical energy of bonds between atoms that power processes within cells. Photosynthesis emerged in organisms 3.5 billion years ago, has evolved complex biochemical and biophysical mechanisms, and today occurs in plants and single-celled organisms. It is because of photosynthesis that Earth's atmosphere and seas contain oxygen.

In photosynthesis, CO₂ and sunlight are used to produce glucose (sugar) and molecular oxygen (O₂). This reaction takes place through several steps in two stages: the light phase and the dark phase.

In the light phase, energy from light powers reactions that split water to release oxygen. In the process, high-energy molecules, ATP and NADPH, are formed. The chemical bonds in these compounds store the energy. Oxygen is a byproduct, and this phase of photosynthesis is the opposite of oxidative phosporylation of the cellular respiration process, discussed below, in which oxygen is consumed.

The dark phase of photosynthesis is also known as the Calvin Cycle. In this phase, which uses the products of the light phase, CO₂ is used to make the sugar, glucose.

Cellular respiration is the biochemical breakdown of a substrate through oxidation, wherein electrons are transferred from the substrate to an "electron acceptor," which can be any of a variety of compounds, or oxygen atoms. If the substrate is a carbon- and oxygen-containing compound, such as glucose, carbon dioxide (CO₂) is produced through glycolysis, the breakdown of glucose.

Glycolysis, which takes place in the cytoplasm of a cell, breaks glucose down to pyruvate, a more "oxidized" compound. If enough oxygen is present, pyruvate moves into specialized organelles called mitochondria. There, it is broken down into acetate and CO₂. The CO₂ is released. The acetate enters a reaction system known as the Krebs Cycle.

In the Krebs Cycle, acetate is broken down further so that its remaining carbon atoms are released as CO₂. This is opposite of one aspect of photosynthesis, the binding of carbons from CO₂ together to make sugar. In addition to CO₂, the Krebs Cycle and glycolysis use energy from the chemical bonds of substrates (such as glucose) to form high-energy compounds such as ATP and GTP, which are used by cell systems. Also produced are high-energy, reduced compounds: NADH and FADH2. These compounds are the means by which electrons, which hold the energy derived initially from glucose or another food compound, are transferred to the next process, called the electron transport chain.

In the electron transport chain, which in animal cells is located mostly on the inner membranes of mitochondria, reduced products such as NADH and FADH2 are used to create a proton gradient -- an imbalance in the concentration of unpaired hydrogen atoms on one side of the membrane vs. the other. The proton gradient, in turn, drives the production of more ATP, in a process called oxidative phosphorylation.

Overall, photosynthesis involves the energizing of electrons by light energy to reduce (add electrons to) CO2 to build a larger compound (glucose), producing oxygen as a byproduct. Cellular respiration, on the other hand, involves taking electrons away from a substrate (glucose, for instance), which is to say oxidation, and in the process the substrate is degraded so that its carbon atoms are released as CO2, while oxygen is consumed. Thus, photosynthesis and cellular respiration are nearly opposite biochemical processes.