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 (RER): Studded with ribosomes; synthesizes and packages proteins.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid synthesis and detoxification.

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

• Mitochondria: Site of ATP production; involved in energy metabolism.

• Lysosomes: Contain digestive enzymes; responsible for waste removal.

• Cytoskeleton: Provides structural support and facilitates cell movement.

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

Plasma Membrane: Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

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

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

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Glycoproteins and glycolipids act as cell recognition molecules.

Example: Glycoproteins on the cell surface help immune cells recognize foreign invaders.

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 directly powers transport proteins (e.g., Na+/K+ pump).

Example: The Na+/K+ pump maintains electrochemical gradients essential for nerve impulse transmission.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, facilitating communication and maintaining tissue integrity.

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

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

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

Example: Gap junctions allow ions to pass between cardiac muscle cells, enabling synchronized contraction.

Chemistry Comes Alive: Biomolecules

Basic Biochemistry

Biochemistry is the study of the chemical composition and reactions of living matter. Matter consists of atoms and elements, which combine to form molecules essential for life.

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

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

• Atoms: Composed of protons, neutrons, and electrons.

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

Example: Water (H2O) is formed by combining hydrogen and oxygen atoms.

Combining Matter: Chemical Bonds

Chemical bonds are formed by interactions between electrons in the outer shell of atoms. These bonds determine the structure and function of molecules.

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

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

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: Affect pH; acids donate protons, bases accept protons.

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

Organic Compounds

Organic compounds are molecules containing carbon and are fundamental to life. The main classes are carbohydrates, lipids, and proteins.

• Carbohydrates: Contain C, H, O; include 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 and membrane structure.

• 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 Junction Types

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

Biomolecules and Cellular Organization

Biomolecules form the basis of cellular structure and function, progressing from molecules to cells, tissues, organs, organ systems, and organisms.

• Hierarchy: Biomolecules → Cells → Tissues → Organs → Organ Systems → Organisms

Example: Proteins form muscle cells, which build muscle tissue, contributing to the muscular system.