Cells: The Living Units

Cell Structure and Function

The cell is the basic structural and functional unit of all living organisms. Each cell contains specialized organelles that perform distinct functions necessary for life.

• Nucleus: Contains condensed and uncondensed DNA; surrounded by a nuclear envelope; directs cellular activities.

• Rough Endoplasmic Reticulum (ER): Studded with ribosomes; synthesizes and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Packages and distributes proteins and lipids.

• Mitochondria: Site of ATP (energy) production; involved in cellular respiration.

• Lysosomes: Contain digestive enzymes; remove waste.

• Cytoplasm: Gel-like matrix that contains organelles and cytosol.

• Plasma Membrane: Surrounds the cell; regulates entry and exit of substances.

Example: The mitochondria generate ATP, which powers cellular processes such as muscle contraction and nerve transmission.

Plasma Membrane: Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It maintains cellular integrity and mediates communication with the environment.

• Phospholipids: Form the bilayer; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Cholesterol: Stabilizes membrane structure.

• Carbohydrates: Glycoproteins and glycolipids act as cell identifiers.

Example: Glycoproteins on the membrane surface help immune cells recognize self versus non-self.

Membrane Transport Mechanisms

The plasma membrane regulates the movement of substances into and out of the cell through passive and active transport mechanisms.

• Passive Transport: No energy required; includes simple diffusion, facilitated diffusion, and osmosis. Driven by concentration gradients.

• Active Transport: Requires ATP; includes phagocytosis, endocytosis, exocytosis, and membrane pumps. Moves substances against their concentration gradient.

• Primary Active Transport: ATP is used to move ions (e.g., Na+, K+) across the membrane.

Example: The sodium-potassium pump maintains the electrochemical gradient essential for nerve impulse transmission.

Cell Junction Types

Cell junctions are specialized structures that connect adjacent cells, providing structural integrity and communication.

• Tight Junctions: Integral proteins join cells, forming impermeable barriers.

• Desmosomes: Cadherin proteins join cells, providing mechanical strength.

• Gap Junctions: Connexon proteins form tunnels between cells, allowing direct communication.

Example: Gap junctions in cardiac muscle allow rapid transmission of electrical signals for synchronized contraction.

Chemistry Comes Alive: Biomolecules

Basic Biochemistry

Biochemistry is the study of the chemical composition and reactions of living matter. Understanding matter and energy is fundamental to cellular processes.

• Matter: Has mass and occupies space; exists as solids, liquids, or gases.

• Energy: Capacity to do work; forms include potential, kinetic, chemical, electrical, mechanical, and radiant.

• Atoms: Basic units of matter; composed of protons, neutrons, and electrons.

• Elements: Pure substances made of one type of atom; major elements in the body include carbon, hydrogen, oxygen, and nitrogen.

Example: Water (H2O) is composed of hydrogen and oxygen atoms.

Atomic Structure and Chemical Bonds

Atoms combine to form molecules and compounds through chemical bonds, determined by electron arrangements.

• Covalent Bonds: Electrons are shared between atoms; can be nonpolar (equal sharing) or polar (unequal sharing).

• Example: Water (H2O) is a polar covalent molecule.

Equation:

Inorganic Compounds

Inorganic compounds are essential for cellular function and include water, salts, and acids/bases.

• Water: Most abundant compound; high heat capacity, universal solvent, involved in hydrolysis and dehydration reactions.

• Salts: Ionic compounds; dissociate into electrolytes.

• Acids/Bases: Acids donate protons (H+), bases accept protons; used to adjust pH.

Example: Sodium chloride (NaCl) dissociates into Na+ and Cl- ions in solution.

Organic Compounds

Organic compounds are carbon-based molecules essential for life, including carbohydrates, lipids, and proteins.

• Carbohydrates: Contain C, H, O; classified as monosaccharides, disaccharides, and polysaccharides. Function as energy sources and structural components.

• Lipids: Contain C, H, O (and sometimes P); include triglycerides, phospholipids, and steroids. Function in energy storage, membrane structure, and signaling.

• Proteins: Composed of amino acids linked by peptide bonds; serve structural, contractile, and transport functions.

Example: Glucose is a monosaccharide used for cellular energy; hemoglobin is a protein that transports oxygen.

Summary Table: Cell and Biomolecule Relationships

The following table summarizes the relationships between biomolecules and levels of biological organization:

Additional info: These notes cover foundational concepts from Ch. 2 (Chemistry Comes Alive) and Ch. 3 (Cells: The Living Units) of Anatomy & Physiology, providing essential context for understanding cellular structure, membrane function, and biomolecular chemistry.