Carbon: The Backbone of Life

Importance of Carbon in Biological Molecules

Carbon is a fundamental element in biology due to its unique ability to form large, complex, and diverse molecules. Although not the most abundant element in Earth's crust, it is central to the composition of living organisms.

• Organic compounds are carbon-based and include proteins, DNA, carbohydrates, and lipids.

• Inorganic compounds lack carbon-hydrogen (C-H) bonds and include substances like carbon dioxide.

• Carbon's versatility arises from its four valence electrons, allowing it to form single, double, and triple bonds with other atoms.

• Carbon can create both polar and nonpolar bonds, serving as the backbone for a wide variety of molecules.

Example: Diamond and graphite are both pure carbon but differ in structure and properties due to different arrangements of carbon atoms.

Organic vs. Inorganic Compounds

The distinction between organic and inorganic compounds is based on the presence of carbon and hydrogen bonds.

Functional Groups in Organic Molecules

Role of Functional Groups

Functional groups are specific groups of atoms attached to the carbon skeleton that determine the chemical reactivity and properties of organic molecules.

• Functional groups impart distinctive chemical characteristics to molecules.

• Tiny differences in functional groups can lead to major differences in biological properties (e.g., estrogen vs. testosterone).

Example: The presence or absence of a methyl group can change a molecule's function dramatically.

Macromolecules: Polymers and Monomers

Definition and Formation

Macromolecules are large molecules composed of thousands of covalently connected atoms. Most biological macromolecules are polymers, which are chains of repeating units called monomers.

• Monomers are the basic molecular units (e.g., glucose, amino acids, nucleotides).

• Polymers are long chains of monomers (e.g., glycogen, polypeptide, DNA).

• Polymer formation occurs via dehydration (condensation) reactions, where two monomers bond through the loss of a water molecule.

• Polymer breakdown occurs via hydrolysis, which adds water to break bonds between monomers.

Equation:

Major Classes of Biological Macromolecules

Overview

There are four major groups of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Three of these (carbohydrates, proteins, nucleic acids) are polymers.

• Carbohydrates: Energy storage and structural support

• Lipids: Long-term energy storage, insulation, and membrane structure

• Proteins: Catalysis, structure, transport, and regulation

• Nucleic acids: Information storage and transfer

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio (CH2O). They serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose)

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose)

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

Equation:

Polysaccharides: Types and Roles

Polysaccharides can be divided into storage and structural types.

Example: Cellulose provides rigidity to plant cell walls and is a major component of dietary fiber.

Lipids

Structure and Function

Lipids are hydrophobic molecules that do not form polymers. They are important for energy storage, insulation, and membrane structure.

• Fats (Triglycerides): Composed of glycerol and three fatty acids; used for long-term energy storage.

• Phospholipids: Major component of cell membranes; consist of two fatty acids, a phosphate group, and glycerol; amphipathic (hydrophilic head, hydrophobic tails).

• Steroids: Include cholesterol and hormones; characterized by four fused rings.

Equation:

Saturated vs. Unsaturated Fats

Example: Animal fats are typically saturated, while plant and fish fats are unsaturated.

Phospholipids and Membranes

Phospholipids form the bilayer structure of cell membranes, providing selective permeability and physical protection.

• Phospholipid bilayer is fluid and contains embedded proteins.

• Cholesterol modulates membrane fluidity, especially in animal cells.

• Plants adjust fatty acid composition to maintain membrane fluidity in response to temperature changes.

Proteins

Structure and Function

Proteins are polymers of amino acids and account for more than 50% of the dry mass of most cells. They perform a wide range of functions, including catalysis, structure, transport, and regulation.

• Amino acids are organic molecules with amino and carboxyl groups, differing by their side chains (R groups).

• There are 20 standard amino acids in proteins.

• Proteins are formed by peptide bonds between amino acids.

Equation:

Levels of Protein Structure

Example: Hemoglobin has quaternary structure with four polypeptide chains; collagen is a fibrous protein with three coiled chains.

Protein Function and Disease

A change in the primary structure can affect protein function, as seen in sickle-cell disease, which results from a single amino acid substitution in hemoglobin.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit genetic information. DNA and RNA are polymers of nucleotides, each consisting of a phosphate group, a pentose sugar, and a nitrogenous base.

• DNA (Deoxyribonucleic acid): Contains deoxyribose sugar; bases are adenine (A), cytosine (C), guanine (G), and thymine (T).

• RNA (Ribonucleic acid): Contains ribose sugar; bases are adenine (A), cytosine (C), guanine (G), and uracil (U).

• Nucleotides are joined by phosphodiester bonds between the 3' and 5' carbons of the sugar.

Equation:

DNA Structure and Base Pairing

DNA is a double helix with antiparallel strands held together by hydrogen bonds between complementary bases.

• Adenine pairs with thymine (A-T); cytosine pairs with guanine (C-G).

• Base pairing ensures accurate replication and information transfer.

RNA Structure and Function

RNA is typically single-stranded and plays roles in protein synthesis and gene regulation.

• Messenger RNA (mRNA) carries genetic information from DNA to ribosomes.

• Ribosomal RNA (rRNA) forms part of the ribosome.

• Transfer RNA (tRNA) brings amino acids to the ribosome during translation.

Adenosine Triphosphate (ATP)

ATP is a special nucleotide that stores energy in its phosphate bonds, used to power cellular processes.

Genetic Information and Protein Synthesis

Genes are segments of DNA that encode the primary sequence of proteins. DNA directs its own replication and controls protein synthesis via mRNA.

• Each gene carries information for a specific protein.

• Gene expression involves transcription (DNA to RNA) and translation (RNA to protein).

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard General Biology curriculum.