Glucose is mainly produced by plants and other photoautotrophs. The plants use carbon dioxide, water, inorganic salts, and light energy to perform photosynthesis. The sunlight is captured by chlorophyll, which is used to produce carbohydrates, water, and oxygen.

The polysaccharides are formed by joining monomeric units via glycosidic bonds that release water as a byproduct, thus the reaction is called hydration reaction.

Saccharification is the process of degradation of carbohydrates into their simpler forms. It entails hydrolysis. The higher animals including humans convert carbohydrates into simpler forms through the action of salivary amylase. In the small intestine, the enzymes maltase, lactase, and sucrase break down disaccharides into monosaccharides.

The epithelial cells of the small intestine absorb the monosaccharides from the intestinal lumen through the sodium ion- glucose symport system. The GluT proteins facilitate the transport of monosaccharides, which are then released into capillaries by facilitated diffusion.

The glucose is converted into glucose-6-phosphate inside the cell and then take participate in one of the metabolic pathways: (1) glycolysis (2) glycogenesis and (3) pentose phosphate pathway. The glycolysis occurs in the cytosol of the cell. It is a series of reactions that convert a molecule of glucose into two molecules of pyruvate and produce a small amount of energy in the form of ATP and NADH.

The process is followed by the Krebs cycle and oxidative phosphorylation. Anaerobic respiration takes place in the absence of oxygen. In gluconeogenesis, the pyruvate is converted into glucose, thus it is referred to as reverse glycolysis. Here, non-carbohydrate precursors are used to forming glucose such as pyruvate, lactate, glycerol, etc. the process is shuttled from the endoplasmic reticulum into the cytoplasm.

The metabolic process of producing glycogen from glucose is called glycogenesis. The process is mainly used for the storage of sugar.

Glycogenesis primarily occurs in liver and muscle cells because they are the responsible organs for carbohydrate storage. The glycogen is broken down into glucose when the body needs energy. The process is called glycogenolysis. Thus, glycogenolysis is referred to as the reverse reaction of glycogenesis.

The pentoses and NADPH are synthesized in the cytosol by the glucose metabolic pathway. The process completes in the liver, adrenal cortex, adipose tissues, testis of animals. The pentose-phosphate pathway is an alternative metabolic route in the breakdown of glucose.

A part of the pathway functions in the formation of hexoses from CO2 in plants. Here fructose takes part in glycolysis. Fructose is metabolized in the adipose tissues, and in the kidney in animals. The milk sugar is comprised of galactose that is derived from lactose.

The phosphorylation of galactose completes via the enzyme galactokinase and then it enters glycolysis. To ensure proper metabolism of sugar, there should be appropriate assimilation of it. Their levels of glucose in the body should be regulated.

In humans, pancreatic cells secrete hormones such as insulin and glucagon that regulate blood sugar levels. The glucagon is released in case of low glucose levels in the blood whereas insulin is produced in a high level of glucose in the body. Glucogen stimulates the production of sugar and also stimulates the conversion of stored glycogen into glucose.

On the other hand, insulin enhances the rate of uptake of glucose from the bloodstream by muscle cells and adipose tissues. Thus the excess glucose is converted into glycogen through the process of glycogenolysis.