Carbohydrate foods—fats, sugars, and starches—are used to supply the energy needed to power the processes that allow living things to function. Even when we are lying down quite still, our bodies require energy. The minimum amount they need is known as the basal metabolic rate (BMR), and it is measured in a person who has slept for at least eight hours and is lying motionless and who has not eaten for at least 12 hours—because digestion uses energy. The BMR for an adult person is 1,200-1,800 kcal day-1 (5-7.5 MJ day-1), although it varies widely between individuals.

There are two processes that release energy from carbohydrates. The first, and least efficient, is anaerobic respiration. This takes place under airless conditions and only some single-celled organisms use it. Fermentation, used by yeasts, is a type of anaerobic respiration. The other process is aerobic respiration. It requires the presence of oxygen. All plants and animals use aerobic respiration, which should not be confused with breathing—passing air or water across lung or gill membranes in order to absorb oxygen from it into the bloodstream. Respiration, both anaerobic and aerobic, takes place as a sequence of chemical reactions and they have a similar purpose. This is to release energy that is used to attach a phosphate group of atoms to a molecule of adenosine diphosphate (ADP), thereby converting it to adenosine triphosphate (ATP). ATP is transported through the organism, and when it arrives at a location where there is a deficit of energy, the phosphate group is detached (ATP ^ ADP), releasing the energy that was used to attach it. All living organisms transport and use energy by this ADP o ATP reaction.


The process by which certain bacteria and all cyanobacteria and green plants use energy derived from sunlight (photo-) to construct (synthesize) sugars from carbon dioxide (CO2) and water (H2O) is known as photosynthesis. The overall sequence of reactions can be summarized as:

The arrow pointing upward indicates that oxygen (O2) is released as a gas; C6H]2O6 is glucose, a simple sugar.

Photosynthesis proceeds in two stages. One, dependent on light energy, is called the light-dependent or light stage. The other is the light-independent or dark stage. These are shown in a simplified form in the diagram.

During the light stage, chlorophyll, a chemical compound present in the cells of photosynthesizing bacteria, including cyanobacteria, and in bodies called chloroplasts in plant cells, absorbs light energy. An electron escapes from the chlorophyll molecule, is captured by an adjacent molecule, and then passes from one molecule to another along an electron-transport chain, eventually being used to split water molecules:

The hydroxyl (OH-) donates its extra electron to the chlorophyll, neutralizing it, and hydroxyls then combine to form water, with the release of oxygen:

Free hydrogen atoms attach themselves to molecules of nicotinamide adenine dinucleotide phosphate (NADP), converting it to NADPH. This completes the light-dependent stage.

NADPH loses its H during the light-independent stage, and the NADP returns to the light-dependent stage.

The light-independent stage begins when a CO2 molecule attaches to a molecule of ribulose biphosphate (RuBP) in the presence of the enzyme RuBP carboxylase (rubisco). The carbon from the CO2 then enters a series of reactions that end with the synthesis of glyceraldehyde phosphate, a three-carbon sugar (a sugar with three carbon atoms in each of its molecules) that is used to make other compounds such as glucose, and the reconstruction of RuBP RuBP is then ready to enter a further sequence. This cycle of reactions was discovered by the American biochemist Melvin Calvin (1911-97) and is known as the Calvin cycle.

Respiration is a process in which carbon is oxidized. In aerobic respiration the oxygen is obtained from the air or from oxygen that is dissolved in water. Anaerobic respiration, including fermentation, obtains oxygen from oxygen-containing compounds. The oxidation of carbon produces carbon dioxide, and this is the waste product from all forms of respiration. Aerobic respiration, for example, on which we depend, can be summarized as:

The CO2 returns to the air or dissolves in water. All of the carbon dioxide that is removed from the atmosphere by photosynthesis eventually returns to it through respiration.

Photosynthesis. The series of chemical reactions by which plants use the energy of sunlight to manufacture sugars has two stages. Water is broken down into hydrogen and oxygen in the light stage, and in the dark stage carbon from carbon dioxide is used to make sugars.

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