Chemical Reactions and the Carbon Basis of Life

Chemical Reactions in Biology

Atoms, Molecules, and Chemical Bonds

The structure of atoms and molecules determines their chemical behavior. Atoms combine through chemical bonding to form molecules, which are the basis of all biological processes.

• Atoms are the smallest units of matter that retain the properties of an element.

• Molecules are formed when two or more atoms bond together chemically.

• Chemical bonds include covalent, ionic, and hydrogen bonds, with covalent bonds being most common in biological molecules.

Chemical Reactions

Chemical reactions occur when substances are combined or broken down, resulting in the formation or breaking of chemical bonds. These reactions are fundamental to life and are often represented as chemical equations.

• Reactants are the starting substances in a reaction.

• Products are the substances formed as a result of the reaction.

• Example equation: $2H_2 + O_2 \rightarrow 2H_2O$

Chemical Equilibrium

Chemical equilibrium is reached when a reaction proceeds at equal rates in both directions, so the concentrations of reactants and products remain constant.

• Equilibrium can be disturbed by:

  • Changing the concentration of reactants or products

  • Changing the temperature

Energy in Biological Systems

Types of Energy

Energy is the capacity to do work or supply heat. In biological systems, energy exists in two main forms:

• Potential energy: Stored energy due to position or structure, such as energy stored in chemical bonds.

• Kinetic energy: The energy of motion, such as the movement of molecules.

Bond Energy and Electron Sharing

In molecules, potential energy is related to the position of shared electrons in covalent bonds.

• If shared electrons are far from the nuclei, bonds are long and weak (higher potential energy).

• If electrons are closer to the nuclei, bonds are shorter and stronger (lower potential energy).

Polar covalent bonds (unequal sharing) are shorter and stronger, while nonpolar covalent bonds (equal sharing) are longer and weaker.

The First Law of Thermodynamics

The first law of thermodynamics states that energy is conserved:

• Energy cannot be created or destroyed.

• Energy can be transferred or transformed from one form to another.

• Chemical reactions involve the transfer of energy, often from chemical bonds.

Spontaneous Reactions

Some chemical reactions are spontaneous, meaning they proceed without continuous external input of energy.

• Spontaneous reactions do not require added energy once started.

• Example: The reaction of hydrogen and oxygen to form water releases energy as heat and light.

Factors Driving Chemical Reactions

Two main factors determine whether a reaction will occur spontaneously:

1. Products are less ordered than reactants, so entropy (disorder) increases.

2. Products have lower potential energy than reactants (shared electrons are held more tightly in the products).

Second law of thermodynamics: In any energy transfer, entropy of the universe increases.

Metabolic Pathways

Definition and Types

A metabolic pathway is a series of chemical reactions in a cell, each catalyzed by a specific enzyme, that transforms a starting molecule into a product.

• Anabolic pathways build complex molecules from simpler ones (consume energy).

• Catabolic pathways break down complex molecules into simpler ones (release energy).

Each step in a pathway is catalyzed by a specific enzyme, ensuring regulation and efficiency.

Life is Carbon-Based

Importance of Carbon

Except for water, nearly all molecules in living organisms are based on carbon. Organic compounds are molecules that contain carbon bonded to other elements, especially hydrogen, oxygen, nitrogen, phosphorus, and sulfur.

• Carbon atoms can form four covalent bonds, allowing for a variety of molecular shapes (chains, rings, branches).

• This versatility enables the formation of a vast array of organic molecules essential for life.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms within molecules that determine the chemical properties and reactivity of those molecules. Important functional groups in biology include:

Chemical Group	Structure	Properties	Examples
Amino	–NH2	Acts as a base, attracts protons	Amino acids
Carboxyl	–COOH	Acts as an acid, donates protons	Amino acids, fatty acids
Carbonyl	–CO– (ketone), –CHO (aldehyde)	Can link molecules into larger compounds	Sugars
Hydroxyl	–OH	Acts as a weak acid, forms hydrogen bonds	Alcohols, carbohydrates
Phosphate	–OPO32–	Has two negative charges, stores energy	ATP, nucleic acids
Sulfhydryl	–SH	Forms disulfide bonds	Cysteine (amino acid)
Methyl	–CH3	Nonpolar, affects gene expression	Methylated DNA

Macromolecules and Polymerization

Monomers and Polymers

Cells use a limited set of small molecules (monomers) to build a huge variety of macromolecules (polymers):

• Proteins are made from 20 different amino acids.

• DNA and RNA are built from 4 types of nucleotides.

• Polysaccharides (complex carbohydrates) are built from monosaccharides.

Polymerization Reactions

Macromolecules are formed by linking monomers through condensation reactions (also called dehydration synthesis), which release water as a byproduct. The reverse process, hydrolysis, breaks polymers into monomers by adding water.

• Condensation: Monomer + Monomer → Polymer + Water

• Hydrolysis: Polymer + Water → Monomer + Monomer

These reactions are catalyzed by specific enzymes in cells.

Summary Table: Types of Biological Macromolecules

Macromolecule	Monomer	Example	Function
Proteins	Amino acids	Enzymes, antibodies	Catalysis, structure, transport
Nucleic acids	Nucleotides	DNA, RNA	Genetic information storage and transfer
Carbohydrates	Monosaccharides	Starch, cellulose	Energy storage, structure
Lipids	Fatty acids, glycerol	Fats, oils, phospholipids	Energy storage, membranes

Additional info: The notes above expand on the original slides by providing definitions, examples, and context for key terms and processes, ensuring a self-contained study guide for General Biology students.