Biological Macromolecules: Structure, Function, and Diversity

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Carbon Skeletons and Molecular Diversity

Variation in Carbon Skeletons

Carbon atoms can form diverse molecular structures due to their ability to create four covalent bonds, resulting in a wide variety of carbon skeletons. This versatility is fundamental to the molecular diversity observed in biological systems.

Length and Shape: Carbon chains can be straight, branched, or arranged in rings, allowing for numerous structural possibilities.
Double Bonds: The presence and position of double bonds further increase diversity.
Isomerism: Structural isomers, cis-trans isomers, and enantiomers contribute to molecular variety.
Example: Glucose and fructose are both six-carbon sugars but differ in structure and properties.

Chemical Groups Important for Life

Functional Groups

Chemical groups attached to carbon skeletons influence molecular function and reactivity. The most biologically important groups include:

Hydroxyl (-OH): Found in alcohols; increases solubility in water.
Carbonyl (C=O): Found in ketones and aldehydes; affects reactivity.
Carboxyl (-COOH): Acts as an acid; found in amino acids and fatty acids.
Amino (-NH2): Acts as a base; found in amino acids.
Sulfhydryl (-SH): Important in protein structure (disulfide bonds).
Phosphate (-PO4): Involved in energy transfer (ATP, nucleic acids).
Methyl (-CH3): Affects gene expression and molecular recognition.
Why is it Important: These groups determine the chemical behavior and biological roles of molecules.

Macromolecules: Synthesis and Breakdown

Building and Breaking Macromolecules

Macromolecules are large, complex molecules essential for life. They are assembled and disassembled through specific chemical reactions:

Dehydration Synthesis: Monomers are joined by covalent bonds through the removal of water.
Hydrolysis: Polymers are broken down into monomers by the addition of water.
Equation:

Example: Formation and breakdown of starch from glucose monomers.

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as energy sources and structural components.

Monosaccharides: Simple sugars (e.g., glucose, fructose).
Disaccharides: Two monosaccharides joined (e.g., sucrose).
Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).
Functions: Energy storage (starch, glycogen), structural support (cellulose in plants, chitin in fungi).
Example: Cellulose provides rigidity to plant cell walls.

Lipids

Structure and Function

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. They play key roles in energy storage, membrane structure, and signaling.

Fats: Composed of glycerol and fatty acids; used for long-term energy storage.
Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.
Steroids: Four fused carbon rings; include hormones like cholesterol.
Functions: Energy storage, insulation, membrane structure, signaling.
Example: Phospholipids form the bilayer of cell membranes.

Proteins

Major Functions of Proteins

Proteins are polymers of amino acids and perform a wide variety of functions in cells.

1. Enzymatic: Catalyze biochemical reactions (e.g., amylase).
2. Structural: Provide support (e.g., collagen, keratin).
3. Storage: Store amino acids (e.g., ovalbumen in egg whites).
4. Transport: Move substances (e.g., hemoglobin transports oxygen).
5. Hormonal: Coordinate activities (e.g., insulin regulates blood sugar).
6. Receptor: Respond to chemical signals (e.g., nerve cell receptors).
7. Contractile and Motor: Movement (e.g., actin and myosin in muscles).
8. Defensive: Protect against disease (e.g., antibodies).

Levels of Protein Structure

Protein function depends on its structure, which is organized into four levels:

Primary Structure: Sequence of amino acids in a polypeptide chain.
Secondary Structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
Tertiary Structure: Overall 3D shape formed by interactions among side chains (R groups).
Quaternary Structure: Association of multiple polypeptide chains into a functional protein.
Example: Hemoglobin has quaternary structure with four polypeptide subunits.

Environmental Factors Affecting Protein Shape

Protein shape is sensitive to environmental conditions, which can lead to denaturation and loss of function.

Temperature: High temperatures can disrupt hydrogen bonds and unfold proteins.
pH: Changes in pH can alter ionic bonds and affect protein structure.
Salt Concentration: Alters ionic interactions and protein stability.
Example: Cooking an egg denatures ovalbumin, changing its texture.

Nucleic Acids

Purpose and Function

Nucleic acids, including DNA and RNA, store and transmit genetic information essential for life.

DNA (Deoxyribonucleic Acid): Stores genetic instructions for development and functioning.
RNA (Ribonucleic Acid): Involved in protein synthesis and gene regulation.
Monomers: Nucleotides, each consisting of a sugar, phosphate group, and nitrogenous base.
Example: mRNA carries genetic information from DNA to ribosomes for protein synthesis.