Cells: The Living Units

Overview of the Cell

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

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

• Cytoplasm: Gel-like substance where organelles are suspended.

• Organelles: Specialized structures such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes.

• Plasma Membrane: Outer boundary that regulates what enters and exits the cell.

Plasma Membrane Structure: Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. This structure is described by the fluid mosaic model, which highlights the dynamic and flexible nature of the membrane.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

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

• Carbohydrates: Attach to proteins (glycoproteins) or lipids (glycolipids) for cell recognition and signaling.

• Cholesterol: Stabilizes membrane fluidity.

Membrane Transport Mechanisms

The plasma membrane controls the movement of substances into and out of the cell through selective permeability. Transport mechanisms are classified as passive or active.

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

  • Simple diffusion

  • Facilitated diffusion (via channels or carriers)

  • Osmosis (water movement)

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient. Includes:

  • Primary active transport (e.g., Na+/K+ pump)

  • Secondary active transport

  • Vesicular transport (endocytosis, exocytosis, phagocytosis)

Cell Junctions

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

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

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

• Gap Junctions: Connexon proteins form channels that allow communication and passage of ions between cells.

Chemistry Comes Alive: Biomolecules

Basic Chemistry for Anatomy & Physiology

Chemistry underlies all physiological processes. Understanding matter, energy, atoms, and elements is essential for studying biomolecules and cellular function.

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

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

Atomic Structure and Chemical Bonds

Atoms combine to form molecules and compounds through chemical bonds, which are determined by the arrangement of electrons in the outer shell.

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

• 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 molecules.

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, universal solvent, involved in hydrolysis and dehydration reactions.

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

  • Acids and Bases: Acids donate H+; bases accept H+. pH measures hydrogen ion concentration.

• Organic Compounds:

  • Carbohydrates: Contain C, H, O; include monosaccharides, disaccharides, and polysaccharides. Main energy source for cells.

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

  • Proteins: Made of amino acids linked by peptide bonds; serve structural, enzymatic, contractile, and transport roles.

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 foundational knowledge is essential for understanding all subsequent topics in Anatomy & Physiology, including tissue structure, organ function, and physiological processes.