General Biology: Chemical Context, Water, Macromolecules, Cell Structure, and Membrane Transport

The Chemical Context of Life

Elements and Compounds in Living Organisms

Living organisms are composed primarily of a small number of chemical elements. Understanding the chemical context of life is fundamental to biology.

• Element: A substance that cannot be broken down into simpler substances by chemical means. Examples: carbon (C), hydrogen (H), oxygen (O), nitrogen (N).

• Compound: A substance formed when two or more elements are chemically bonded together. Example: water (H2O).

• Periodic Table: Organizes elements by atomic number and properties. Biological systems primarily use about 25 elements.

Water and the Fitness of the Environment

Properties of Water and Their Biological Significance

Water's unique properties make it essential for life. Its molecular structure and hydrogen bonding result in several important characteristics.

• Polarity: Water is a polar molecule, meaning it has a partial positive charge on one side and a partial negative charge on the other.

• Hydrogen Bonding: The polarity of water allows it to form hydrogen bonds, leading to high cohesion, adhesion, and surface tension.

• High Specific Heat: Water can absorb or release large amounts of heat with little temperature change, helping to stabilize temperatures in organisms and environments.

• Solvent Properties: Water is known as the "universal solvent" because it dissolves many substances, facilitating chemical reactions in cells.

• Density of Ice: Ice is less dense than liquid water, so it floats, insulating aquatic environments in cold climates.

Example: The high heat capacity of water helps maintain stable temperatures in lakes and oceans, supporting aquatic life.

Carbon and Molecular Diversity of Life

Carbon's Bonding Versatility

Carbon atoms can form four covalent bonds, allowing for a diversity of stable organic molecules with various shapes and functions.

• Hydrocarbons: Molecules consisting only of carbon and hydrogen. They are nonpolar and hydrophobic.

• Functional Groups: Specific groups of atoms attached to carbon skeletons that confer specific chemical properties (e.g., hydroxyl, carboxyl, amino, phosphate).

Example: The presence of a carboxyl group (-COOH) makes a molecule acidic, as in amino acids.

The Structure and Function of Large Biological Molecules

Macromolecules: Carbohydrates, Proteins, Lipids, and Nucleic Acids

Large biological molecules, or macromolecules, are essential for structure and function in living organisms. They are typically polymers made from monomer subunits.

• Carbohydrates: Serve as fuel and building material. Monosaccharides (e.g., glucose) are simple sugars; polysaccharides (e.g., starch, glycogen, cellulose) are complex carbohydrates.

• Proteins: Polymers of amino acids. Functions include catalysis (enzymes), structure, transport, and signaling.

• Lipids: Hydrophobic molecules including fats, phospholipids, and steroids. Fats store energy; phospholipids form cell membranes.

• Nucleic Acids: DNA and RNA store and transmit genetic information.

Comparison Table: Triglycerides vs. Phospholipids

Property	Triglyceride	Phospholipid
Structure	Glycerol + 3 fatty acids	Glycerol + 2 fatty acids + phosphate group
Function	Energy storage	Main component of cell membranes
Polarity	Nonpolar	Amphipathic (hydrophilic head, hydrophobic tails)

Example: Cellulose is a polysaccharide that provides structural support in plant cell walls, while starch is used for energy storage.

A Tour of the Cell

Prokaryotic vs. Eukaryotic Cells

Cells are the basic units of life. They are classified as prokaryotic or eukaryotic based on their internal structure.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. Example: Bacteria.

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles. Examples: Plants, animals, fungi, protists.

• Organelles: Specialized structures within eukaryotic cells (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

Example: The mitochondrion is the site of cellular respiration, converting glucose into ATP.

Membrane Structure and Function

Plasma Membrane Composition and Transport Mechanisms

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates the movement of substances into and out of the cell.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins floating in or on the fluid lipid bilayer.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Membrane Proteins: Include transport proteins, receptors, enzymes, and structural proteins.

• Transport Mechanisms:

  • Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis, facilitated diffusion).

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

Example: Aquaporins are membrane proteins that facilitate the rapid transport of water across the cell membrane.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, causing the cell to shrink.

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, causing the cell to swell or burst.

Example: If a freshwater organism is placed in salty water (hypertonic), it will lose water and may shrink or die.

Practice Questions: Membrane Transport and Tonicity

Key Definitions and Applications

• Isotonic, Hypertonic, Hypotonic: See definitions above.

• Ideal Tonicity for Cells: Animal cells function best in isotonic environments; plant cells prefer hypotonic environments for turgor pressure.

• Solute Concentration of a Living Cell: Typically around 0.9% NaCl for animal cells.

• Effects of Changing Environments: Placing a freshwater organism in saltwater causes water loss; placing plant cells in distilled water causes them to swell.

• Laboratory Applications: Adjusting the tonicity of solutions can be used to lyse (rupture) red blood cells for protein extraction.

Example: If an Elodea plant is placed in distilled water (hypotonic), water enters the cells, causing them to become turgid but not burst due to the cell wall.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard General Biology curriculum.