COMPREHENSIVE BIOLOGY

 Composition, structure, and functions of biomolecules:

Carbohydrates:

Carbohydrates are essential biomolecules that play diverse and vital roles in living organisms. They are broadly defined as polyhydroxy aldehydes or ketones, or compounds that produce these units upon hydrolysis.

 They are primarily composed of three elements: carbon (C), hydrogen (H), and oxygen (O).

Structure:

Monosaccharides (Simple Sugars):

• These are the simplest form of carbohydrates and cannot be hydrolyzed into smaller sugar units.

• They typically contain three to seven carbon atoms.

E.g.: Glucose (C6​H12​O6​): A six-carbon sugar (hexose) and an aldose (contains an aldehyde group).

Disaccharides:

• Formed when two monosaccharide units are joined together by a glycosidic bond (a covalent bond formed by a dehydration reaction, where a water molecule is removed).

E.g.: Sucrose (table sugar): Glucose + Fructose (α-1,2-glycosidic bond).

Oligosaccharides:

• Comprise a short chain of three to ten monosaccharide units linked by glycosidic bonds.

Examples include raffinose (found in beans, cabbage, etc.), a trisaccharide of glucose, galactose, and fructose.

Polysaccharides (Complex Carbohydrates):

• Long chains of many (hundreds to thousands) monosaccharide units linked by glycosidic bonds.

• They can be linear or branched.

E.g.: Glycogen: The primary energy storage polysaccharide in animals (stored mainly in the liver and muscles).

Functions of Carbs:

• Primary Energy Source,

• Energy Storage.

• Building Blocks for Macromolecules.

• Cell Recognition and Signaling.

• Dietary Fiber.

Lipids:

Lipids are a diverse group of organic compounds characterized by their insolubility in water (hydrophobic nature) and solubility in nonpolar organic solvents.

Some lipids may contain phosphorus (P), nitrogen (N), and sulfur (S).

Structure:

Triglycerides (Fats and Oils):

• The most common type of lipid serves as the primary form of energy storage in the body and foods.

• 
  

• Composed of one glycerol molecule esterified to three fatty acid molecules. The fatty acids can be the same or different and can be saturated or unsaturated.

Phospholipids:

• Similar to triglycerides, but one of the fatty acids is replaced by a phosphate group (often with an additional polar head group attached, like choline, serine, or ethanolamine).

• They have a hydrophilic (water-loving) head (the phosphate group and its attached polar group) and two hydrophobic (water-fearing) fatty acid tails.

Steroids:

• Characterized by a unique carbon skeleton consisting of four fused rings (three six-membered rings and one five-membered ring).

E.g., cholesterol.

Waxes:

• Esters of long-chain fatty acids with long-chain alcohols.

• They are highly hydrophobic and serve as protective coatings.

E.g., beeswax.

Functions:

• Long-term Energy Storage.

• Insulation and Protection.

• Hormone Production and Signaling.

• Absorption of Fat-Soluble Vitamins.

• Nutrient Source and Transport.

Proteins:

Proteins are polymers made up of smaller monomeric units called amino acids. There are 20 common types of amino acids found in proteins.

Amino acids are linked together by peptide bonds, which are covalent bonds formed between the carboxyl group of one amino acid and the amino group of another, with the removal of a water molecule (a dehydration or condensation reaction)

Structure:

Primary Structure:

• This is the linear sequence of amino acids in a polypeptide chain, from the N-terminus (amino end) to the C-terminus (carboxyl end).

• It is determined by the genetic code (DNA sequence) and is unique to each protein.

Secondary Structure:

Alpha-helix (α-helix): A spiral staircase-like structure where the polypeptide backbone coils around a central axis. Hydrogen bonds form between every fourth amino acid.

Beta-pleated sheet (β-sheet): A zigzag, sheet-like structure formed by hydrogen bonds between adjacent strands of the polypeptide chain, which can be parallel or anti-parallel.

Tertiary Structure:

The three-dimensional shape of a single polypeptide chain results from interactions between the side chains (R-groups) of the amino acids.

Composed of different bonds, like covalent, ionic, etc.

Quaternary Structure:

• This level of structure exists only in proteins composed of two or more polypeptide chains (subunits) that associate together to form a functional complex.

• The interactions holding these subunits together are similar to those in tertiary structure (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bonds).

Functions:

• Enzymatic Catalysis.

• Structural Support.

• Transport and Storage.

• Immune Defense.

• Signaling and Communication.

• Regulation of Gene Expression.

• Fluid Balance.

Nucleic acids:

Nucleic acids are essential biomolecules that play a central role in the storage, transmission, and expression of genetic information in all known forms of life.

A Pentose (5-Carbon) Sugar:

• In DNA, the sugar is deoxyribose (hence "deoxyribonucleic acid"). Deoxyribose lacks a hydroxyl group (-OH) at the 2' carbon position compared to ribose.

• In RNA, the sugar is ribose (hence "ribonucleic acid").

A Nitrogenous Base: These are nitrogen-containing, ring-shaped molecules that provide the "code" for genetic information. There are five main types, classified into two categories:

• Purines (double-ring structure):

  • Adenine (A)

  • Guanine (G)

• Pyrimidines (single-ring structure):

• Cytosine (C)

• Thymine (T) (found only in DNA)

• Uracil (U) (found only in RNA, replaces thymine).

Structure:

Primary Structure:

• This is the linear sequence of nucleotides linked together by phosphodiester bonds.

• A phosphodiester bond forms between the phosphate group of one nucleotide (attached to its 5' carbon) and the hydroxyl group at the 3' carbon of the sugar of the next nucleotide.

• The sequence is always read from the 5' end (which has a free phosphate group) to the 3' end (which has a free hydroxyl group).

Secondary Structure:

• DNA: The most famous secondary structure is the double helix, elucidated by Watson and Crick.

• It consists of two polynucleotide strands coiled around a central axis, running in antiparallel directions (one 5' to 3' and the other 3' to 5').

• RNA molecules are typically single-stranded, but they can fold back on themselves to form localized secondary structures like hairpin loops or stem-loops. These structures often involve intramolecular base pairing (e.g., A-U, G-C, and sometimes non-canonical pairs) and are crucial for RNA's diverse functions.

Tertiary Structure:

• Refers to the overall three-dimensional shape of a nucleic acid molecule.

• For DNA, this involves the supercoiling of the double helix.

• For RNA, tertiary structure is critical for the function of many RNA molecules, especially those involved in catalysis (ribozymes) or protein synthesis (tRNA, rRNA), allowing them to adopt specific binding pockets or catalytic sites.

 Quaternary Structure:

• Some complex nucleic acid structures, particularly in viruses or in cellular machinery, involve the association of multiple nucleic acid molecules or the association of nucleic acids with proteins to form larger functional complexes.

Function: 

• Storage of Genetic Information (DNA).
• Transmission of Genetic Information (DNA Replication).
• Expression of Genetic Information (RNA and Protein Synthesis).
• Regulation of Gene Expression.

Vitamins:

Vitamins are a diverse group of organic compounds that are essential micronutrients. Vitamins are organic compounds, meaning they contain carbon and hydrogen, often along with oxygen, nitrogen, and sometimes sulfur or other elements.

Structure:

Water-Soluble Vitamins:

• These vitamins dissolve in water and are generally not stored in large amounts in the body (except B12).

• Excess amounts are typically excreted in urine, meaning they need to be consumed regularly.

Fat-Soluble Vitamins:

• These vitamins dissolve in fats and oils and are absorbed along with dietary fats.

• They are stored in the body's fatty tissues and liver, which means they can accumulate to toxic levels if consumed in excessive amounts over time.

Function:

• Energy Metabolism (B-complex Vitamins).
• Antioxidant Protection (Vitamins C and E).
• Bone Health (Vitamins D and K).
• Blood Clotting (Vitamin K).
• Immune System Support (Vitamins A, C, D, E, B6, B9, B12).
• Nervous System Function (B-complex Vitamins).