Cells & Tissues
Cell structure and organelles, the four tissue types, and how they form the body's functional organization.
Interactive Diagram
Tap a teal label to learn about each structure.
The Cell and Its Organelles
The cell is the basic unit of life. The nucleus houses DNA and the nucleolus. Rough endoplasmic reticulum (ribosome-studded) synthesizes proteins for secretion or membranes; smooth ER handles lipid synthesis and detoxification. The Golgi apparatus modifies, sorts, and packages proteins. Mitochondria generate ATP by oxidative phosphorylation and have bacterial-like circular DNA — evidence of endosymbiotic origin and the basis for some antibiotic toxicity. Lysosomes and peroxisomes degrade material; the cytoskeleton (microtubules, microfilaments, intermediate filaments) provides structure and transport.
Membrane Transport
The plasma membrane is a fluid phospholipid bilayer with embedded proteins. Small or lipophilic molecules cross by simple diffusion; ions and polar molecules use channels and transporters. Active transport (Na+/K+-ATPase) uses ATP to move solates against gradients. Defects cause disease: cystic fibrosis (CFTR chloride channel), channelopathies in muscle and heart. The resting membrane potential and action potentials in excitable cells rely on ion gradients, the foundation of nerve and muscle physiology.
The Four Tissue Types
Epithelial tissue covers surfaces and lines cavities, with apical–basal polarity and little extracellular matrix; classified by shape (squamous, cuboidal, columnar) and layering (simple, stratified, pseudostratified). Connective tissue (loose, dense, cartilage, bone, blood) has abundant extracellular matrix with fibers (collagen, elastin) and cells (fibroblasts, chondrocytes, osteoblasts). Muscle tissue (skeletal, cardiac, smooth) generates contraction. Nervous tissue (neurons and glia) transmits electrical and chemical signals. Most organs combine all four.
Cell Junctions and Signaling
Tight junctions seal epithelial barriers (e.g., gut, blood–brain barrier). Adherens and desmosomes anchor cells mechanically (skin, heart — desmosome defects cause pemphigus). Gap junctions allow direct electrical and metabolic coupling (cardiac intercalated discs). Cells communicate by contact-dependent signaling, paracrine (local), endocrine (hormones via blood), and synaptic signaling. Receptors include ion-channel-linked, G-protein-coupled (GPCRs — the largest drug target family), enzyme-linked (receptor tyrosine kinases), and intracellular (steroid/thyroid hormone) receptors.
Cell Cycle and Apoptosis
The cell cycle (G1, S, G2, M) is regulated by cyclins and cyclin-dependent kinases, with checkpoints controlled by p53 and Rb. Most mature neurons are arrested in G0; many epithelial and marrow cells cycle rapidly. Apoptosis is programmed cell death via intrinsic (mitochondrial, Bcl-2 family, cytochrome c, caspases) and extrinsic (death receptor, Fas) pathways. Dysregulation drives disease: cancer (loss of cell-cycle control, p53 mutation), neurodegeneration (excess apoptosis), and failure of apoptosis permits autoimmunity and autoimmune lymphoproliferation.
