Energy can be converted from one form to another but can be neither created nor destroyed. Ultimately, the energy used by all but a few living things comes from the sun.

For human beings, burning a meal all at once to produce energy would be harmful to us thus we have a few specific requirements for the energy conversion process:

• Energy must be released from food gradually, since excess energy could harm us.

• Energy must be stored in readily accessible forms as glycogen and fat (triacylglycerides).

• Release of energy from storage must be finely controlled so that it is available exactly when and where it is needed.

• Just enough energy must be released as heat to maintain constant body temperature.

• Energy in a form other than heat must be available to drive chemical reactions that are not favourable at body temperatures.

The greater the amount of free energy released, the farther a reaction proceeds toward product formation before reaching equilibrium.

Reactions in which the products are higher in energy than the reactants can also take place, but such unfavourable reactions cannot occur without the input of energy from an external source; such reactions are endergonic.

Pathway is a series of enzyme-catalysed chemical reactions that are connected by their intermediates, that is, the product of the first reaction is the reactant for the second reaction, and so on.

There are two main categories of cells where the energy generating reactions can take place: prokaryotic cells, found in single-celled organisms (e.g., bacteria and blue-green algae), and eukaryotic cells, found in some single-celled organisms, such as yeast, and all plants and animals.

Cytoplasm is the region between the cell membrane and the nuclear membrane in a eukaryotic cell.

Cytosol is the fluid part of the cytoplasm surrounding the organelles within a cell, contains dissolved proteins and nutrients

The mitochondria (singular, mitochondrion), often called the cell’s “power plants,” are the most important of the organelles for energy production and produce about 90% of the body’s energy-carrying molecule, ATP.

• A mitochondrion is a roughly egg-shaped structure composed of a smooth outer membrane and a folded inner membrane. The space enclosed by the inner membrane is the mitochondrial matrix. Within the matrix, the citric acid cycle and production of most of the body’s adenosine triphosphate (ATP) take place.

• The coenzymes and proteins that manage the transfer of energy to the chemical bonds of ATP are embedded in the inner membrane of the mitochondrion.

• Mitochondria contain their own DNA, synthesize some of their own proteins, and multiply using chemicals moved from the cell cytosol into the mitochondrial matrix. The number of mitochondria is greatest in eye, brain, heart, and muscle cells, where the need for energy is greatest.

All of the chemical reactions that take place in an organism constitute its metabolism. Most of these reactions occur in the reaction sequences of metabolic pathways, a sequence of reactions where the product of one reaction serves as the starting material for the next.

• Such pathways may be linear (a series of reactions that convert a reactant into a specific product through a series of intermediate molecules and reactions), cyclic (a series of reactions that regenerates one of the first reactants), or spiral (the same set of enzymes progressively builds up or breaks down a molecule).

Catabolism is a metabolic reaction pathways that break down food molecules and release biochemical energy.

Anabolism is a metabolic reactions that build larger biological molecules from smaller pieces.

The process can be roughly divided into the four stages:

• STAGE 1: Digestion Enzymes in saliva, the stomach, and the small intestine convert the large molecules of carbohydrates, proteins, and lipids to smaller molecules.

  • Carbohydrates are broken down to glucose and other sugars; proteins are broken down to amino acids; and triacylglycerols, the lipids commonly known as fats and oils, are broken down to glycerol plus long-chain carboxylic acids, termed fatty acids.

  • These smaller molecules are transferred into the blood for transport to cells throughout the body.

• STAGE 2: Acetyl-coenzyme A production The small molecules from digestion follow separate pathways that separate their carbon atoms into two-carbon acetyl groups.

  • The acetyl groups are attached to coenzyme A by a high-energy bond between the sulphur atom of the thiol 1 ¬SH2 group at the end of the coenzyme A molecule and the carbonyl carbon atom of the acetyl group.

  • The resultant compound, acetyl-coenzyme A, which is abbreviated acetyl-CoA, is an intermediate in the breakdown of all classes of food molecules.

  • It carries the acetyl groups into the common pathways of catabolism—Stage 3, the citric acid cycle and Stage 4, electron transport and ATP production.

• STAGE 3: Citric acid cycle Within mitochondria, the acetyl-group carbon atoms are oxidized to the carbon dioxide that we exhale.

  • Most of the energy released in the oxidation leaves the citric acid cycle in the chemical bonds of reduced coenzymes (NADH, FADH2).

  • Some energy also leaves the cycle stored in the chemical bonds of ATP or a related triphosphate.

• STAGE 4: ATP production Electrons from the reduced coenzymes are passed from molecule to molecule down an electron-transport chain.

  • Along the way, their energy is harnessed to produce more ATP.

  • At the end of the process, these electrons—along with hydrogen ions from the reduced coenzymes—combine with oxygen we breathe in to produce water.

  • Thus, the reduced coenzymes are in effect oxidized by atmospheric oxygen, and the energy that they carried is stored in the chemical bonds of ATP molecules.

The metabolic strategy for dealing with what would be an energetically unfavourable reaction is to couple it with an energetically favourable reaction so that the overall energy change for the two reactions is favourable.

The net result of catabolism is the oxidation of food molecules to release energy. Many metabolic reactions are therefore oxidation–reduction reactions, which means that a steady supply of oxidizing and reducing agents must be available.

Citric acid cycle is the series of biochemical reactions that breaks down acetyl groups to produce energy carried by reduced coenzymes and carbon dioxide.

Electron-transport chain is the series of biochemical reactions that passes electrons from reduced coenzymes to oxygen and is coupled to ATP formation. It is also called the respiratory chain.

Electron transport proceeds via four enzyme complexes held in fixed positions within the inner membrane of mitochondria, along with two electron carriers that move through the membrane from one complex to another.

• The complexes and mobile electron carriers are organized in the sequence of their ability to pick up electrons. The four fixed complexes are very large assemblages of polypeptides and electron acceptors.

The most important electron acceptors are of three types:

(1) various cytochromes that are proteins that contain heme groups in which the iron cycles between Fe2+ and Fe3+;

(2) proteins containing iron–sulphur groups in which the iron also cycles between Fe2+ and Fe3+;

(3) coenzyme Q (CoQ), often known as ubiquinone because of its ubiquitous (widespread) occurrence and because its ring structure with the two ketone groups is a quinone.

ATP synthesis:

• ATP generation is accomplished by a series of enzyme complexes in the inner memranes of mitochondria.

• Electrons and hydrogen ions enter the first two complexes of the electron-transport chain from succinate (in the citric acid cycle), NADH, and FADH2, where they are transferred to coenzyme Q.

• Then, the electrons and hydrogen ions proceed independently; the electrons gradually give up their energy to the transport of hydrogen ions across the inner mitochondrial membrane to maintain different concentrations on opposite sides of the membrane.

• The hydrogen ions return to the matrix by passing through ATP synthase, where the energy they release is used to convert ADP to ATP.