Chemistry of Life

Atoms, Molecules, and Bonds

The foundation of biology is chemistry, as all living things are composed of atoms and molecules. Understanding atomic structure and chemical bonding is essential for studying biological molecules and cellular processes.

• Atoms: The smallest stable unit of matter, composed of subatomic particles:

  • Protons: Positively charged, found in the nucleus.

  • Neutrons: No charge, found in the nucleus.

  • Electrons: Negatively charged, orbit the nucleus in energy levels (shells).

• Electron Shells: Electrons occupy shells around the nucleus. The first shell holds 2 electrons, the second 8, and the third up to 18. The valence shell (outermost shell) determines chemical reactivity.

• Octet Rule: Atoms are most stable when their valence shell has 8 electrons.

• Atomic Number: Number of protons in an atom; defines the element.

• Mass Number: Number of protons plus neutrons.

• Isotopes: Atoms with the same number of protons but different numbers of neutrons. Some are radioisotopes and decay, emitting radiation (used in dating fossils).

Major Elements in Biology

• Four elements make up 96% of living matter: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N).

• Other essential elements: Sodium (Na), Calcium (Ca), Potassium (K), Sulfur (S), Chlorine (Cl), Magnesium (Mg).

• Trace elements (needed in small amounts): Iodine (I), Iron (Fe), Copper (Cu), Zinc (Zn).

Chemical Bonds

• Ionic Bonds: Formed when one atom gives up an electron to another, creating ions (charged atoms). Example:

  • Ionic bonds dissociate in water, forming electrolytes.

• Covalent Bonds: Formed when atoms share electrons. Can be single, double, or triple bonds. Covalent bonds are strong and do not dissociate in water.

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom (like O or N). Important for the structure of water, proteins, and DNA.

Properties of Water

Unique Properties and Biological Importance

Water is vital for life due to its unique chemical and physical properties.

• Solvent: Water dissolves many substances, making it a universal solvent. This allows for chemical reactions and transport of materials in organisms.

• High Heat Capacity: Water absorbs and retains heat, helping to stabilize temperature in organisms and environments.

• High Heat of Vaporization: Water requires a lot of energy to evaporate, aiding in cooling mechanisms (e.g., sweating).

• Density: Water is less dense as a solid (ice) than as a liquid, so ice floats, insulating aquatic life in winter.

• Cohesion and Adhesion: Water molecules stick to each other (cohesion) and to other surfaces (adhesion), contributing to surface tension and capillary action.

• Polarity: Water is a polar molecule, with partial positive and negative charges, allowing it to interact with other polar molecules and ions.

• Hydrophilic vs. Hydrophobic:

  • Hydrophilic: Molecules that mix with water (e.g., salts, sugars).

  • Hydrophobic: Molecules that do not mix with water (e.g., fats, oils).

Acids, Bases, and pH

Definitions and Biological Relevance

• Acid: Substance that releases hydrogen ions () in solution. Example:

• Base: Substance that releases hydroxide ions () in solution. Example:

• Strong acids/bases dissociate almost completely; weak acids/bases dissociate partially.

• Neutralization: Acid and base react to form water and a salt. Example:

• pH Scale: Measures hydrogen ion concentration. Ranges from 0 (acidic) to 14 (basic), with 7 as neutral.

  • Each pH unit represents a tenfold change in concentration.

• Buffers: Substances that minimize changes in pH by binding or releasing . Example:

Major Macromolecules

Carbohydrates

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

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

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

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose, chitin). Used for energy storage and structural support.

Lipids

Lipids are hydrophobic molecules, including fats, oils, and waxes. They are important for energy storage, insulation, and cell membranes.

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

  • Saturated fatty acids: No double bonds; solid at room temperature.

  • Unsaturated fatty acids: One or more double bonds; liquid at room temperature.

  • Trans fats: Artificially hydrogenated unsaturated fats; associated with health risks.

• Triglycerides: Three fatty acids linked to glycerol; main form of stored energy in animals.

• Phospholipids: Two fatty acids and a phosphate group attached to glycerol; major component of cell membranes.

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

Membranes and the Cell

Cell Membrane Structure and Function

The cell membrane is a selectively permeable barrier that controls the movement of substances in and out of the cell.

• Phospholipid Bilayer: Double layer of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Embedded in the membrane; function as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids; involved in cell recognition.

• Cholesterol: Maintains membrane fluidity.

Transport Across Membranes

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement of larger or polar molecules via transport proteins.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement of substances against their concentration gradient using energy (ATP).

• Bulk Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Osmosis and Tonicity

• Tonicity: Describes the effect of a solution on cell volume.

  • Isotonic: No net water movement; cell volume remains constant.

  • Hypotonic: Water enters the cell; cell may swell or burst.

  • Hypertonic: Water leaves the cell; cell shrinks.

Table: Comparison of Major Macromolecules

Additional info:

• Proteins and nucleic acids are also major macromolecules, though not detailed in the provided images. Proteins are polymers of amino acids and perform a wide range of functions, including catalysis (enzymes), structure, and transport. Nucleic acids (DNA and RNA) store and transmit genetic information.