BIOLOGICAL MACROMOLECULES

Bio-molecules are a subset of organic molecules. Organic molecules are defined as molecules or compounds that contain both carbon and hydrogen. Most organic molecules are a component of living organisms or have been produced from them. All other types of molecules are inorganic molecules including water, salts (e.g., sodium chloride), acids (e.g., carbonic acid), and bases (e.g., sodium hydroxide).

The four major classes of organic biological macro-molecules (bio-molecules) are lipids, carbohydrates, nucleic acids, and proteins.

General Characteristics

Bio-molecules have diverse carbon skeletons with varying amounts of hydrogen and functional groups attached to them. 

Three of the four major classes of organic bio-molecules, including complex carbohydrates, nucleic acids, and proteins-exist as polymers, chemical structures that are composed of repeating identical or similar monomers. Lipids are not polymers. All of the classes are formed by dehydration synthesis reactions and digested through hydrolysis.

1-Lipids 

Plasma Membrane

Lipids are a very diverse group of fatty like water insoluble molecules that include triglycerides, phospholipids, steroids, and eicosanoids. Triglycerides are composed of glycerol and three fatty acids and are generally used for long-term energy storage. Phospholipids are made up of glycerol, two fatty acids, and a phosphate functional group with various organic groups attached to it. Phospholipids are amphipathic molecules containing a polar head and two nonpolar tails that form membranes. Steroids are distinct multiring structures formed predominantly of hydrocarbons and include cholesterol, steroid hormones, and bile salts. 

Eicosanoids are modified 20-carbon fatty acids that are synthesized as needed from arachidonic acid, a common component of both plasma membranes and nuclear membranes. Eicosanoids act locally. Other lipids include glycolipids and fat-soluble vitamins.

2-Carbohydrates 

Carbohydrate is an organic molecule composed of carbon, hydrogen and oxygen. Carbohydrates are molecules with the following chemical formula: (CH2O)n, where n indicates the numbers of carbon atoms that are in a molecule. Carbohydrates exist in increasing levels of complexity that include monosaccharides, disaccharides, and polysaccharides. 

The simplest type of carbohydrates are simple sugar monomers called monosaccharides. All monosaccharides have between three and seven carbon atoms. But in common, they have a back bone of five or six-carbon atoms. Carbohydrates formed from two monosaccharides are disaccharides, and those with many monosaccharides are polysaccharides. 

Glucose is the most common monosaccharide in the human body and is used for energy. When in excess, glucose is stored as the polysaccharide called glycogen in liver and skeletal muscle tissue. Other types of carbohydrates include the monosaccharides galactose, fructose, ribose, and deoxyribose; the disaccharides maltose, sucrose, and lactose; and the polysaccharides starch and cellulose.

3-Nucleic Acids

Nucleic acids are macromolecules that store and transfer genetic or hereditary information in cells. Two classes of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), polymers formed from nucleotide monomers. These molecules ultimately determine the type of proteins synthesized by cells. Adenosine triphosphate (ATP) is the energy currency molecule of a cell. 

3.1- Proteins

Proteins are macromolecules made of one or more linear strands of amino acid monomers. Proteins, once synthesized, function within the cell, plasma membrane, blood plasma, and in other body fluids. Proteins serve many different functions in the body. Proteins are polymers that differ in the number and sequence of 20 different amino acid monomers arranged in a unique linear sequence.

3.2-Function of Proteins

Proteins serve a vast array of functions. For example, proteins 

-Serve as catalysts (enzymes) in most metabolic reactions of the body

-Act in defense, for example, immunoglobulins (antibodies) attach to foreign substances and help in their elimination.

-Aid in transport, such as haemoglobin molecules transporting respiratory gases within the blood

-The protein contributes to structural support, such as collagen. Collagen is a major component of ligaments and tendons

-Perform function in movement, occur when myosin and actin proteins interact during contraction of muscle tissue.

-Perform regulation, such as occurs When the peptide hormone insulin helps control blood glucose levels.

-It can provide storage, such as ferritin. The ferritin stores iron in liver cells

DNA

3.3-Protein Structure

The three-dimensional structure of proteins is dependent upon the linear sequence of its amino acids. 

3.3.a- Categories of Amino Acids 

The 20 amino acids can be categorized as nonpolar, polar, charged, and those with special functions. 

3.3.b-Levels of Protein Structure 

Amino acids bonded together form protein polymers. There are four increasingly complex levels of organization. (a) The primary structure is the linear sequence of amino acids in the protein. (b) The secondary structures of a protein arise as a polypeptide chain which may include alpha helixes and beta-pleated sheets. (c) The tertiary structure is the completed 3-dimensional shape or conformation of the protein, which occur when loops, helices and sheets fold up into a domain. So, this structure may be a globular or fibrous protein. (d) A quaternary structure is another shape of protein which is formed in some complex proteins when two or more protein molecules associate to form the final protein. The shape arises from the interaction between multiple polypeptide subunits of the same protein.

Protein Structure

3.2.c- Amino Acid Sequence and Protein Conformation

Protein organization includes the primary, secondary, tertiary structures and a quaternary structure if there are two or more protein chains. These levels of organization ultimately determine the structure and function of a protein. Denaturation usually results in the loss of biological activity of a protein in response to the change in its three-dimensional shape that may have occurred through an increase in temperature or a change in pH and fat-soluble vitamins.