Biochemistry Study Guide
Biochemistry is the study of the chemistry of living things. The biomolecules in this lesson give an insight into all living things, including humans.
Carbohydrates (or sugars) serve as the main source of energy for living organisms. They are composed of carbon, hydrogen, and oxygen with the general molecular formula Cx(H2O)y, a hydrate of carbon. Carbohydrate names have the suffix –ose (e.g., such as glucose or fructose).
Monosaccharides are the simplest carbohydrate structure composed of one ring that can contain five C atoms (pentose, such as ribose, which is a constituent of RNA) or six C atoms (hexose, such as galactose derived from milk sugar lactose).
Disaccharides are dimeric sugar made of two monosaccharides joined together in a reaction that releases a molecule of water (dehydration). The bond between the two sugar molecules is called glycosidic linkage and can have either an axial (α-glycoside) or an equatorial (β-glycoside) orientation with respect to the ring conformation. Examples include the following:
- Maltose (two glucose molecules joined together), found in starch
- Lactose (one galactose joined to one glucose), found in milk
- Sucrose (one fructose joined to one glucose), table sugar
Polysaccharides are polymers or a long chain of repeating monosaccharide units. Examples include the following:
- Starch is a mixture of two kinds of polymers of α-glucose (linear amylose and amylopectin).
- Amylose contains glucose molecules joined together by α-glycosidic linkages, and amylopectin additionally has a branching at C-6. They are storage polysaccharides in plants.
- Glycogen consists of glucose molecules linked by α-glycosidic linkage (C-1 and C-4) and branched (C-6) by α-glycosidc linkage. Glycogen is the storage form of glucose in animals (liver and skeletal muscle).
- Cellulose consists of glucose molecules joined together by β-glycosidic linkage. Cellulose is found in plants and is not digested by humans (lacking the necessary enzyme).
Condensation and Hydrolysis
Condensation is the process of bonding together separate monosaccharide subunits into a disaccharide and/or a polysaccharide. It is also called dehydration synthesis as one molecule of water is lost in the process. It is carried out by specific enzymes.
Hydrolysis is the reverse process of condensation as a water molecule and specific enzymes break all the glycosidic linkages in disaccharides and polysaccharides into their constituting monosaccharides.
Lipids are a diverse group of compounds that are insoluble in water and polar solvents but are soluble in nonpolar solvents. Lipids are stored in the body as a source of energy (twice the energy provided by an equal amount of carbohydrates).
Triglycerides are lipids formed by the condensation of glycerol (one molecule) with fatty acids (three molecules). They can be saturated (from fatty acid containing only C-C single bonds) or unsaturated (the presence of one or more C = C double bonds). Triglycerides are found in the adipose cells of the body (neutral fat) and are metabolized by the enzyme lipase (an esterase) during hydrolysis, producing fatty acids and glycerol.