Major Biomolecules: Structure, Function, and Cellular Roles

Major Biomolecules

Introduction

Biochemistry focuses on the study of biomolecules that are essential for life. The four major classes of biomolecules—proteins, polysaccharides (carbohydrates), lipids, and nucleic acids—each have unique structures, functions, and cellular placements. Understanding their properties is foundational for further study in biochemistry.

Proteins

Structure and Composition

• Proteins are polymers composed of amino acid monomers.

• Each amino acid contains:

  • Carboxylic acid group (-COOH), which is deprotonated to COO- at neutral pH.

  • Amino group (-NH2), which is protonated to NH3+ at neutral pH.

  • Side chain (R) unique to each amino acid, determining its properties.

• There are 20 common amino acids found in proteins.

• A polypeptide chain is a linear polymer of amino acids, with each amino acid in the chain referred to as a residue.

• Amino acids are joined by peptide bonds (amide linkages).

General structure of an amino acid:

• $\mathrm{H_2N{-}CHR{-}COOH}$

Functions of Proteins

• Structural: Provide support and shape to cells and tissues (e.g., collagen, keratin).

• Catalytic (Enzymes): Accelerate biochemical reactions (e.g., DNA polymerase, amylase).

• Storage and Transport: Store and transport molecules (e.g., hemoglobin transports oxygen).

• Communication and Recognition: Involved in cell signaling and immune recognition (e.g., hormones, antibodies).

• Defense: Protect against pathogens (e.g., immunoglobulins).

Example: The sequence of amino acids determines the protein's structure and function. For instance, enzymes have specific active sites formed by their unique amino acid sequences.

Polysaccharides (Carbohydrates)

Structure and Composition

• Polysaccharides are polymers of sugar monomers (saccharides).

• Sugars are carbohydrates composed primarily of carbon (C), hydrogen (H), and oxygen (O), and are polyhydroxyl compounds.

• Monosaccharide example: Glucose ($\mathrm{C_6H_{12}O_6}$)

Functions of Polysaccharides

• Energy storage: Starch in plants and glycogen in animals.

• Structural: Cellulose in plant cell walls, chitin in fungal cell walls and exoskeletons of arthropods.

• Recognition: Glycoproteins and glycolipids on cell surfaces are involved in cell-cell recognition.

Example: Glucose is a key energy source for cells and is the monomeric unit of many polysaccharides.

Lipids

Structure and Composition

• Lipids are rich in carbon and hydrogen, but poor in oxygen.

• They are insoluble in water but soluble in organic solvents due to their hydrophobic nature.

• Fatty acids (FA) are the simplest lipids, consisting of long hydrocarbon chains with a terminal carboxyl group (-COOH).

• Hydrophobic means 'water-fearing'; lipids tend to aggregate away from water.

Phospholipids and Membranes

• Phospholipids are amphipathic molecules, containing hydrophilic 'heads' (with phosphate groups) and hydrophobic fatty acid 'tails'.

• Phospholipids spontaneously form bilayer membranes in aqueous environments, with hydrophobic tails facing inward and hydrophilic heads facing outward.

Example: The cell membrane is composed of a phospholipid bilayer, which serves as a barrier and regulates transport into and out of the cell.

Nucleic Acids

Structure and Composition

• Nucleic acids are polymers of nucleotide monomers.

• Each nucleotide consists of:

  • A 5-carbon (pentose) sugar (ribose in RNA, deoxyribose in DNA)

  • A nitrogenous base (adenine, guanine, cytosine, thymine [DNA], uracil [RNA])

  • One or more phosphate groups

• Nucleotides are joined by phosphodiester bonds to form the nucleic acid backbone.

Functions of Nucleic Acids

• Genetic information storage: DNA stores genetic instructions in cells.

• Information transfer: RNA transmits genetic information and is involved in protein synthesis.

• Energy currency: ATP (adenosine triphosphate) is the main energy carrier in cells.

Example: DNA is double-stranded and stabilized by hydrogen bonds between complementary bases (A-T, G-C).

• $\text{ATP: Adenosine triphosphate}$

• $\text{Phosphodiester bond:} \; \mathrm{C{-}O{-}P{-}O{-}C}$

Cellular Organization

Cell Structure

• The cell is the basic unit of life, surrounded by a plasma membrane.

• Cytoplasm includes all materials enclosed by the plasma membrane, except the nucleus in eukaryotes.

• Cytosol is the aqueous portion of the cytoplasm, excluding subcellular structures.

Cell Types

• Prokaryotes: No membrane-bound nucleus (e.g., bacteria, archaea).

• Eukaryotes: Have a nucleus and other complex internal structures; typically 1000x greater in volume than prokaryotes.

Example: Mitochondria are the primary site of ATP generation in eukaryotic cells; chloroplasts in plant cells are the site of photosynthesis.

Cellular Compartments and Processes

• Endoplasmic reticulum (ER): Site of protein and lipid synthesis.

• Golgi complex: Modifies, sorts, and packages proteins for secretion.

• Transport vesicles: Move biomolecules between compartments.

• Secretory granules: Release biomolecules via exocytosis.

• Endocytosis: Uptake of biomolecules by invagination of the plasma membrane.

• Phagocytosis: Engulfment of large particles (e.g., bacteria) by cells such as macrophages.

• Lysosomes: Contain digestive enzymes; fuse with endosomes to digest material brought into the cell.

Metabolism and Energy

Overview

• Photosynthetic organisms capture sunlight energy to synthesize organic compounds.

• Organic compounds provide energy for all organisms.

Metabolic Pathways

• Metabolism: The collection of chemical reactions by which organisms synthesize and degrade compounds.

• Anabolism: Synthesis of complex molecules from simpler ones.

• Catabolism: Breakdown of complex molecules into simpler ones.

• Energetics: Study of energy changes during metabolic reactions.

Example: ATP is produced during catabolic reactions and consumed during anabolic reactions.

• $\text{General metabolic equation:} \; \text{Substrate} + \text{Enzyme} \rightarrow \text{Product}$

Summary Table: Major Biomolecules

Biomolecule	Monomer	Bond Type	Main Functions	Example
Protein	Amino acid	Peptide bond	Structural, catalytic, transport, defense	Hemoglobin, enzymes
Polysaccharide	Monosaccharide	Glycosidic bond	Energy storage, structure, recognition	Starch, cellulose
Lipid	Fatty acid, glycerol	Ester bond	Membranes, energy storage, signaling	Phospholipids, triglycerides
Nucleic acid	Nucleotide	Phosphodiester bond	Genetic information, energy transfer	DNA, RNA, ATP

Additional info: Some details, such as the specific structure of the cell membrane and the role of organelles, were expanded for clarity and completeness.