Cells: The Living Units

Overview of Cell Structure

Cells are the basic structural and functional units of all living organisms. Each cell contains specialized structures called organelles, each with distinct roles essential for cellular function and survival.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Chromatin/Chromosomes: DNA in uncondensed (chromatin) or condensed (chromosome) form; chromosomes are visible during cell division.

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Ribosomes: Sites of protein synthesis; can be free in cytoplasm or attached to rough endoplasmic reticulum (ER).

• Rough ER: Studded with ribosomes; synthesizes and transports proteins.

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

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Powerhouse of the cell; site of ATP (energy) production through cellular respiration.

• Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

• Centrioles: Involved in organizing microtubules during cell division.

• Cytoskeleton: Provides structural support and facilitates cell movement.

• Plasma Membrane: Encloses the cell, regulates entry and exit of substances.

Plasma Membrane: Structure and Function

The plasma membrane is a selectively permeable barrier that separates the cell from its external environment. It is described by the Fluid Mosaic Model, which highlights its dynamic and flexible nature.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, forming a double layer.

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

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); function in cell recognition and signaling.

• Cholesterol: Stabilizes membrane fluidity.

Membrane Transport Mechanisms

The plasma membrane is selectively permeable, allowing some substances to pass while restricting others. Transport mechanisms are classified as passive or active.

• Passive Transport: Does not require energy (ATP). Includes:

  • Simple Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Movement via protein channels or carriers.

  • Osmosis: Diffusion of water across a semipermeable membrane.

• Active Transport: Requires ATP to move substances against their concentration gradient.

  • Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses energy from the movement of another substance.

  • Endocytosis/Exocytosis: Bulk transport into (endo-) or out of (exo-) the cell.

Cell Junctions

Cells are connected by specialized junctions that facilitate communication and maintain tissue integrity.

• Tight Junctions: Integral proteins join cells to form impermeable barriers.

• Desmosomes: Cadherin proteins anchor cells together, providing mechanical strength.

• Gap Junctions: Connexon proteins create channels for communication between adjacent cells.

Chemistry Comes Alive: Biomolecules

Basic Chemistry for Anatomy & Physiology

Chemistry underpins all biological processes. Understanding atoms, elements, and chemical bonds is essential for studying biomolecules and cellular function.

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

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

• Atoms: Smallest units of elements; composed of protons (+), neutrons (0), and electrons (-).

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

Chemical Bonds and Molecules

Atoms combine to form molecules and compounds through chemical bonds, which determine the properties of substances in the body.

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

• Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions.

• Hydrogen Bonds: Weak attractions between polar molecules, important in water and biological macromolecules.

Biochemistry: Inorganic and Organic Compounds

Biochemistry is the study of the chemical composition and reactions of living matter. Compounds are classified as inorganic or organic.

• Inorganic Compounds:

  • Water: Most abundant compound in cells; high heat capacity, solvent properties, and reactivity.

  • Salts: Ionic compounds that dissociate into electrolytes in solution.

  • Acids and Bases: Affect pH; acids release H+, bases accept H+.

• Organic Compounds:

  • Carbohydrates: Contain C, H, O; main energy source. Includes monosaccharides (glucose), disaccharides (sucrose), and polysaccharides (glycogen).

  • Lipids: Contain C, H, O (sometimes P); includes triglycerides, phospholipids, and steroids. Functions in energy storage, insulation, and cell membranes.

  • Proteins: Composed of amino acids linked by peptide bonds; functions include structural support, enzymes, transport, and signaling.

Hierarchy of Biological Organization

Biomolecules combine to form cells, which organize into tissues, organs, organ systems, and ultimately, the organism.

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

Additional info: This hierarchy illustrates the increasing complexity from chemical to organismal level, foundational for understanding anatomy and physiology.