Introduction to Cytology and Biological Macromolecules

Introduction to Biology

What is Biology?

Biology is the scientific study of life and living organisms, encompassing their structure, function, growth, origin, evolution, and distribution. It covers a wide range of topics from the molecular mechanisms within cells to the interactions of organisms with their environment.

• Prokaryotes: Single-celled organisms without a nucleus, including Bacteria and Archaea.

• Eukaryotes: Organisms with cells that contain a nucleus, including animals, plants, fungi, and protists.

• Animals: Divided into vertebrates (mammals, fish, amphibians, birds) and invertebrates (insects, crustaceans).

• Plants: Multicellular, photosynthetic organisms.

Example: Escherichia coli is a prokaryote, while humans are eukaryotes.

Elements and Chemical Bonds in Biology

Periodic Table of Elements for Biology

Biological systems primarily use a subset of elements from the periodic table, with carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) being the most abundant in living organisms.

• Electronegativity: The tendency of an atom to attract electrons. Oxygen is more electronegative than carbon or hydrogen.

• Nonpolar Covalent Bonds: Electrons are shared equally (e.g., C-H bonds).

• Polar Covalent Bonds: Electrons are shared unequally, leading to partial charges (e.g., O-H bonds).

Example: Water (H2O) has polar covalent bonds, making it an excellent solvent for biological molecules.

Macromolecules in Biology

Classes of Macromolecules

Living organisms are primarily composed of four classes of macromolecules: carbohydrates, lipids, nucleic acids, and proteins. These macromolecules are polymers, large molecules made by linking together smaller subunits called monomers.

• Carbohydrates: Energy storage and structural support.

• Lipids: Energy storage, membrane structure, signaling.

• Nucleic Acids: Information storage and transfer (DNA, RNA).

• Proteins: Catalysis, structure, transport, signaling.

Additional info: The principle of molecular economy suggests that organisms use a limited set of building blocks to construct a wide variety of macromolecules.

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen (CHO). They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with 5 or 6 carbons, often forming rings in solution.

• Disaccharides: Two monosaccharides linked together (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose, chitin).

Roles:

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

• Structural support: Cellulose in plant cell walls, chitin in fungal cell walls and arthropod exoskeletons.

Example: Potatoes store energy as starch; paper and wood are made of cellulose.

Carbohydrate Bonding and Isomerism

• Glycosidic Bonds: Covalent bonds that link monosaccharides in polysaccharides. Can be α(1→4) or β(1→4) linkages, affecting structure and digestibility.

• Isomers: Molecules with the same chemical formula but different structures.

Type of Isomer	Example	Description
Structural Isomer	Glucose vs. Fructose	Different covalent bond arrangement
Geometric Isomer	Glucose vs. Galactose	Same skeleton, different spatial arrangement

All have the formula C6H12O6.

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) store and transmit genetic information. They are polymers of nucleotides, each consisting of a sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base.

• DNA: Double-stranded, stores genetic information.

• RNA: Single-stranded, involved in protein synthesis.

• Nitrogenous Bases: Adenine (A), Thymine (T, DNA only), Cytosine (C), Guanine (G), Uracil (U, RNA only).

Genetic Code: The sequence of bases encodes information for protein synthesis.

Example: In a DNA sample with 30% adenine, there will also be 30% thymine, and the remaining 40% will be split equally between cytosine and guanine.

Other Nucleotides

• ATP (Adenosine Triphosphate): The energy currency of the cell.

• NAD+ (Nicotinamide Adenine Dinucleotide): Electron carrier in cellular respiration.

• FAD (Flavin Adenine Dinucleotide): Another electron carrier.

Proteins

Amino Acids and Protein Structure

Proteins are polymers of amino acids, which have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain).

• 20 standard amino acids: 13 are essential for humans (must be obtained from diet).

• Complete proteins: Contain all 20 amino acids (e.g., animal proteins).

• Incomplete proteins: Lacking one or more essential amino acids (e.g., cereal grains, legumes).

Peptide Bond: Covalent bond formed between amino acids during protein synthesis.

Levels of Protein Structure

• Primary: Amino acid sequence.

• Secondary: Local folding (α-helix, β-sheet) via hydrogen bonding.

• Tertiary: 3D shape due to interactions among R groups.

• Quaternary: Arrangement of multiple polypeptide subunits.

Chaperones: Proteins that assist in folding other proteins.

Denaturation: Loss of structure due to heat or chemicals, often irreversible.

Protein Functions

• Enzymes: Catalyze biochemical reactions.

• Transport: Move substances across membranes.

• Structural: Provide support (e.g., collagen).

• Signaling: Hormones and receptors.

Lipids

Structure and Function

Lipids are hydrophobic molecules composed mainly of carbon, hydrogen, and oxygen. They include fats, oils, phospholipids, and steroids.

• Fatty Acids: Long hydrocarbon chains with a carboxyl group (COOH).

• Triglycerides: Three fatty acids linked to a glycerol molecule; used for energy storage.

• Phospholipids: Glycerol, two fatty acids, and a phosphate group; major component of cell membranes.

• Steroids: Four fused carbon rings (e.g., cholesterol).

Properties: Nonpolar and hydrophobic; energy-rich due to many C-H bonds.

Example: Phospholipids form the bilayer of cell membranes, creating a barrier between the cell and its environment.

Summary Table: Major Biological Macromolecules

Macromolecule	Monomer	Main Elements	Functions
Carbohydrates	Monosaccharide	C, H, O	Energy storage, structure
Lipids	Fatty acid, glycerol	C, H, O (sometimes P)	Energy storage, membranes
Proteins	Amino acid	C, H, O, N, S	Catalysis, structure, transport
Nucleic Acids	Nucleotide	C, H, O, N, P	Information storage, transfer