To execute their various activities, such as building polymers, pumping chemicals across membranes, moving, and reproducing, living cells require energy transfusions from outside sources. The puffin gets its energy from sand eels and other aquatic creatures, whereas many other animals get theirs from photosynthetic species like plants and algae.

The energy contained in food's organic components is ultimately derived from the sun. Energy enters an ecosystem as sunlight and departs as heat; nevertheless, the chemical components required for life are recycled.

Photosynthesis produces oxygen as well as organic compounds that are utilized as fuel by eukaryotic mitochondria.

• The term cellular respiration refers to Including both aerobic and anaerobic processes. However, it originated as a synonym for aerobic respiration because of the relationship of that process to organismal respiration, in which an animal breathes in oxygen. Thus, cellular respiration is often used to refer to the aerobic process.

Energy flows into an ecosystem as sunlight and ultimately leaves as heat, while the chemical elements essential to life are recycled, as shown in the image attached. Catabolic pathways are metabolic processes that release stored energy by breaking down complex compounds.

The transfer of electrons from fuel molecules (such as glucose) to other molecules is critical in these pathways.

Because of the arrangement of electrons in the bonds between their atoms, organic molecules have potential energy.

Fuels are compounds that can engage in exergonic processes.

A cell destroys complex organic compounds with high potential energy to simpler waste products with lower energy through the action of enzymes.

Fermentation is a catabolic process that involves the partial breakdown of sugars or other organic fuels without the use of oxygen.

However, aerobic respiration is the most effective catabolic route, in which oxygen is used as a reactant alongside the organic fuel (aerobic is derived from the Greek aer, air, and bios, life).

Aerobic respiration is possible in the cells of most eukaryotic and many prokaryotic species. In a similar mechanism that harvests chemical energy without the need for oxygen, certain prokaryotes employ molecules other than oxygen as reactants.

The reaction releases energy to the surroundings because the electrons lose potential energy when they end up being shared unequally, spending more time near electronegative atoms such as oxygen as shown in the image attached.

The major combustion reaction that happens at the burner of a gas stove is the oxidation of methane by oxygen.

In a car engine, the burning of gasoline is likewise a redox reaction; the energy released drives the pistons.

The oxidation of glucose and other compounds in food, however, is the energy-yielding redox activity of most interest to biologists.

In general, organic molecules with a lot of hydrogen are good fuels because their bonds are a source of “hilltop” electrons, which can release energy when they “fall” down an energy gradient during their conversion to oxygen.

According to the respiration summary equation, hydrogen is transported from glucose to oxygen. The essential feature, which is not evident in the simplified equation, is that the energy state of the electron changes when hydrogen (with its electron) is transferred to oxygen.

The oxidation of glucose in respiration transports electrons to a lower energy state, releasing energy for ATP production.

Carbohydrates and lipids, the primary energy-producing nutrients, are repositories of electrons coupled with hydrogen, frequently in the form of C—H bonds. Only the activation energy barrier prevents the stream of electrons from reaching a lower energy state (see Figure 8.13). Without this barrier, a dietary item like glucose would react with O2 almost instantly.

When we provide the activation energy by lighting glucose, it burns in the air, producing 686 kcal (2,870 kJ) of heat per mole of glucose (about 180 g). Of course, the body temperature is not high enough to cause burning. Instead, if you consume glucose, enzymes in your cells will reduce the activation energy barrier, allowing the sugar to be oxidized.

If energy is released from a fuel all at once, it cannot be harnessed efficiently for constructive work.