Carbohydrates, Proteins and Lipids
To have an understanding of the functioning of the human body, it is important to have a clear understanding of the roles taken up by carbohydrates, proteins, and lipids. This paper will look at the structural classification of carbohydrates and the levels of organization of protein structure. Further, the paper will discuss the role of lipids in maintaining the structure of the cell membrane.
Structural Classification of Carbohydrates
The importance of carbohydrates cannot be underestimated. They are the most abundant class of biological molecules and are made up of multiple hydroxyl group (Henrissat and Davies, 1997). The chemical composition of carbohydrates is dominated by carbon, hydrogen, and oxygen, chemically presented as (CH2O)n. Carbohydrates are generally classified into simple and complex carbohydrates. As indicated in the diagram below.
Simple carbohydrates are further divided into two categories; monosaccharides and disaccharides. Monosaccharides are the simplest forms of carbohydrates and they cannot be hydrolyzed further into simpler units. They are the sweet sugars with carbon number per molecule ranging from three to seven. Their names end with the suffix- ose. The chemical formula for glucose, the most common monosaccharide, for instance, is C6H12O6.
Disaccharides are formed when two monosaccharides undergo a dehydration reaction (condensation reaction, or dehydration synthesis) (Burke, 2000). In the process, the hydroxyl group of the monosaccharide combines with hydrogen of the other monosaccharide and releases water forming a covalent bond. For instance, sucrose on hydrolysis gives glucose and fructose. Common disaccharides include, lactose (found in milk), fructose (found in fruits), and galactose to mention a few.
Polysaccharides are rather complex forms of carbohydrates. They contain long monosaccharide units which are joined together by a glyosidic linkage (Englyst, & Cummings, 1988). Polysaccharides act as food storage, for instance, starch in plants. Starch, which is formed by combining glucose molecules, can be explained as a polymer of glucose. Other examples include cellulose in plants. Glycogen is stored in animal bodies in organs such as the liver, muscles and the brain. During times of glucose shortage, the body, using enzymes, breaks down glycogen to glucose.
Levels of Organization of Protein Structure
There are four levels of organization of protein structures; Primary structure, Secondary structure, Tertiary structure, and Quaternary structure. Primary structure is unique linear sequence of amino acids which are joined by a peptide bond to make up a protein or polypeptide chain. A mutation in the sequence leads to serious effects (Zarychanski et al., 2012), for instance, sickle cell anemia where HbA or normal hemoglobin is changed to HbS. Essentially, the primary structure is determined by covalent peptide bonds. The secondary structure is determined by hydrogen bonds that are formed between the backbone of the chain. The secondary structure contains the alpha helicase and beta pleated sheets that are folded by hydrogen bonds. The tertiary structure is formed after further folding and twisting a secondary structure to form a 3D arrangement of a functional protein. They are compact and if disrupted, it loses it activity. There are two major classes of tertiary structure proteins, fibrous which takes up structural roles, and globular which takes up complicated roles. Quaternary structure is formed by one or more polypeptide chain. This structure is highly sensitive to changes in temperature, pH, and other chemically significant conditions (Koga et al., 2012).
Role of Lipids in Maintaining the Structure of the Cell Membrane
According to Dowhan and Bogdanov (2002), lipids have a wide diversity in structural and biological functions. The most important and primary role taken up by lipids is the formation of subcellular organelles which take up the form of a lipid bilayer, and also in the formation of a permeability barrier of cells as explained by Dowhan and Bogdanov (2002). The most common lipid type defining bilayer is the glycerol-based phospholipid. Lipids have a structure that explains their function as a barrier. Just like fats, lipids are insoluble in water. Each molecule of lipids contains a hydrophilic region which is called a polar head region gets attracted to water condition, and a hydrophobic region also called a nonpolar or tail region which is repulsed by water conditions. Phospholipids are the most abundant class of lipids. Phospholipids organize themselves in a bilayer to hide their hydrophobic tail regions and expose their hydrophilic regions to water. The organization in this format is spontaneous and requires no energy. The layer is impermeable meaning that it allows only water and gases to pass through it. large and polar molecules cannot pass through it.
Dowhan and Bogdanov (2002) further their discussion of lipids and disclose that the diverse functions of lipids occur because a family of low molecular weight molecules which are physically fluid and easy to deform, enable the interaction of lipids with macromolecules in a specific manner. Fluidity is effected by the presence of proteins in the lipid bilayer. The bilayer’s fluidity allows structures mobility within the lipid layer and influenced membrane transport. Fluidity is affected by the structure of the fatty acid chains and temperature. At higher temperatures, fluidity reduces and the opposite is true.
Burke, L. M. (2000). Dietary carbohydrates. Nutrition in sport, 73.
Dowhan, W., & Bogdanov, M. (2002). Functional roles of lipids in membranes. In New comprehensive biochemistry (Vol. 36, pp. 1-35). Elsevier.
Englyst, H. N., & Cummings, J. H. (1988). Improved method for measurement of dietary fiber as non-starch polysaccharides in plant foods. Journal-Association of Official Analytical Chemists, 71(4), 808-814.
Henrissat, B., & Davies, G. (1997). Structural and sequence-based classification of glycoside hydrolases. Current opinion in structural biology, 7(5), 637-644.
Koga, N., Tatsumi-Koga, R., Liu, G., Xiao, R., Acton, T. B., Montelione, G. T., & Baker, D. (2012). Principles for designing ideal protein structures. Nature, 491(7423), 222.
Zarychanski, R., Schulz, V. P., Houston, B. L., Maksimova, Y., Houston, D. S., Smith, B., … & Gallagher, P. G. (2012). Mutations in the mechanotransduction protein PIEZO1 are associated with hereditary xerocytosis. Blood, blood-2012.
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