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; site of genetic material storage and transcription.

• Cytoplasm: Contains organelles and cytosol; site of metabolic activities.

• Mitochondria: Involved in energy (ATP) production; known as the "powerhouse" of the cell.

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

• Smooth ER: Synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Lysosomes: Contain digestive enzymes; involved in waste removal.

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

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 inward.

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

• Cholesterol: Stabilizes membrane structure.

• Carbohydrates: Glycoproteins and glycolipids act as cell identifiers.

Membrane Transport Mechanisms

The plasma membrane controls 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. Movement is driven by concentration gradients.

• Active Transport: Requires ATP; includes phagocytosis, endocytosis, exocytosis, and membrane pumps. Allows movement against concentration gradients.

• Primary Active Transport: Example: Sodium-potassium pump ( ATPase$) moves ions across the membrane using ATP.

Cell Junction Types

Cell junctions are specialized structures that connect cells and facilitate communication and adhesion.

• 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.

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, and gases.

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

• Atoms: Smallest units of elements; composed of protons, neutrons, and electrons.

• Elements: Pure substances; major elements in the body include carbon, hydrogen, oxygen, and nitrogen.

Atomic Structure and Chemical Bonds

Atoms combine to form molecules and compounds. Chemical bonds are determined by electron interactions.

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

• Examples: , , (polar)

Inorganic Compounds

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

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

• Salts: Ionic compounds; dissociate into electrolytes.

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

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 (sometimes P); include triglycerides (fats), phospholipids, and steroids. Function in energy storage and membrane structure.

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

Summary Table: Cell Junction Types

The following table summarizes the main types of cell junctions and their functions:

Biomolecules to Organism Hierarchy

The organization of life follows a hierarchical structure:

• Biomolecules → Cells → Tissues → Organs → Organ Systems → Organism

Example: Proteins (biomolecules) form muscle cells, which make up muscle tissue, contributing to the muscular system in the human body.

Additional info: This hierarchy is fundamental to understanding how molecular interactions translate to physiological functions at the organism level.