The Chemistry of Life

Introduction to Chemistry

Chemistry is the scientific study of the composition, structure, and properties of matter. It explores the connections between the macroscopic world (what we can see and measure) and the microscopic world (atoms and molecules). Chemists use models and experiments to explain the properties of substances.

• Element: A pure substance consisting of only one type of atom.

• Compound: A substance formed when two or more different elements react chemically and combine in fixed proportions.

• Molecule: The smallest unit of a compound that retains the chemical properties of that compound.

• Inorganic: Substances not primarily composed of carbon and hydrogen.

• Organic: Substances primarily containing carbon and hydrogen atoms.

Properties of Water

Structure and Characteristics

Water, represented by the chemical formula H2O, is essential for life. It consists of two hydrogen atoms and one oxygen atom, held together by polar covalent bonds.

• Absorbs and releases heat slowly: This property helps regulate body temperature.

• Phases of water: Water can exist as a solid (ice), liquid (water), or gas (vapor).

• Density: Liquid water is more dense than ice because the molecules are packed more closely together. Ice is the least dense phase, which allows it to float.

• Universal solvent: Water is a polar substance capable of dissolving many other substances, making it vital for biological processes.

Dissociation of Water

Water can undergo dissociation, a process in which a molecule breaks down into smaller molecules or ions. When water dissociates, it forms two ions:

• Hydrogen ion (H+): A proton released when water loses an electron.

• Hydroxide ion (OH-): The remaining part of the water molecule after losing a hydrogen ion.

Acids, Bases, and the pH Scale

Definitions and Properties

• Acids: Molecules that dissociate in water to release hydrogen ions (H+).

• Bases: Molecules that either take up hydrogen ions or release hydroxide ions (OH-).

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution. It is defined as the negative logarithm of the hydrogen ion concentration:

• The scale ranges from 0 to 14.

• Low pH: High concentration of hydrogen ions (acidic).

• High pH: Low concentration of hydrogen ions (basic/alkaline).

• Adding an acid increases the concentration of hydrogen ions, lowering the pH.

• Adding a base decreases the concentration of hydrogen ions, raising the pH.

• pH < 7: Acidic (more hydrogen ions than hydroxyl ions).

• pH > 7: Basic (more hydroxyl ions than hydrogen ions).

• pH = 7: Neutral (equal number of hydrogen and hydroxyl ions).

• Each pH unit represents a tenfold difference in hydrogen ion concentration.

Buffers

Buffers are substances or mixtures of chemicals that help maintain a stable pH in a solution. They do this by taking up excess H+ or OH- ions, preventing drastic changes in pH. Buffers are crucial for maintaining homeostasis in living organisms. For example, the bicarbonate ion acts as a buffer in blood to prevent changes in pH when acids or bases are introduced.

Salts

When an acid reacts with a base, the products are a salt and water. A salt consists of the positive ion (cation) from the base and the negative ion (anion) from the acid.

Example reaction:

• HCl (acid) + NaOH (base) → NaCl (salt) + H2O (water)

Biologically Important Molecules

Proteins

Proteins are essential macromolecules used for structure, support, and as enzymes in biological systems. They are polymers made up of amino acids linked by peptide bonds.

• Proteins are composed of 20 different amino acids.

• The chemical properties of the amino acids determine the biological function of the protein.

• The sequence of amino acids (primary structure) contains the information needed for the protein to fold into its functional three-dimensional shape.

The 20 Amino Acids

• Alanine

• Arginine

• Asparagine

• Aspartic acid

• Cysteine

• Glutamic acid

• Glutamine

• Glycine

• Histidine

• Isoleucine

• Leucine

• Lysine

• Methionine

• Phenylalanine

• Proline

• Serine

• Threonine

• Tryptophan

• Tyrosine

• Valine

Essential Amino Acids

• Humans can synthesize 10 of the 20 amino acids; the remaining 10, called essential amino acids, must be obtained from the diet.

• Failure to obtain enough of even one essential amino acid can result in the breakdown of the body's proteins to supply the missing amino acid.

• The human body does not store excess amino acids for future use; they must be consumed regularly.

Protein Organization

• Primary structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding of the chain into structures such as alpha-helices and beta-sheets.

• Tertiary structure: The overall three-dimensional shape of a single polypeptide chain.

• Quaternary structure: The structure formed by the association of two or more protein molecules (subunits).

Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio (e.g., C6H12O6). They serve as a primary energy source for living organisms and are classified based on the number of sugar units.

• Monosaccharides: Simple sugars consisting of a single carbohydrate molecule. Examples include glucose (main energy source), fructose (found in fruit), and galactose (found in milk).

• Disaccharides: Composed of two monosaccharide molecules. Examples include maltose (two glucose units), sucrose (glucose + fructose), and lactose (glucose + galactose).

• Polysaccharides: Large molecules formed by combining many monosaccharides in a regular pattern. Examples include starch (plant energy storage), glycogen (animal energy storage), and cellulose (structural component in plant cell walls).

Lipids

Lipids are a diverse group of hydrophobic (water-insoluble) molecules, including fats, oils, fatty acids, soaps, and steroids. They are used for long-term energy storage and as structural components of cell membranes.

• Fats and oils: At room temperature, fats are solid and oils are liquid. Both are formed from one glycerol molecule and three fatty acid molecules (triglycerides). They provide energy, insulation, and protection for organs.

• Fatty acids: Long hydrocarbon chains ending with a carboxyl group. They can be saturated (no double bonds) or unsaturated (one or more double bonds). Unsaturated fatty acids are typically liquid at room temperature (oils).

• Soaps: Not true lipids, but derived from lipids. Soaps are salts formed from a fatty acid and an inorganic base. They act as emulsifiers, allowing water and oil to mix by forming fine oil droplets suspended in water.

• Steroids: Lipids with four fused carbon rings and various functional groups. Examples include cholesterol, aldosterone (regulates salt balance), and sex hormones.

Summary Table: Types of Biomolecules

Additional info: The above notes provide foundational knowledge for understanding organic chemistry, especially the structure and function of biomolecules, acid-base chemistry, and the role of water in biological systems.