Cells & Tissues — revision sheet

The cell is the smallest unit of life, and every organ is built from just four basic tissues — epithelial, connective, muscle and nervous. This section covers eukaryotic cell structure, membrane transport, energy metabolism, the flow of genetic information, the cell cycle and cell death, and the histology of the basic tissues.

Cells in an adult body: ≈30–37 trillion (≈84 % erythrocytes by number)Typical cell diameter: ≈10–30 µm (erythrocyte 7.5 µm; oocyte ≈120 µm)Basic tissue types: 4 — epithelial, connective, muscle, nervousChromosomes (diploid): 46 = 22 pairs of autosomes + XX or XYHaploid genome: ≈3.1 × 10⁹ bp; ≈20 000 protein-coding genesDNA length per diploid nucleus: ≈2 mPlasma membrane thickness: ≈7.5–10 nmResting membrane potential: ≈ −70 mV (neuron); ≈ −90 mV (skeletal/cardiac muscle)ATP per glucose (aerobic): ≈30–32Cytoskeletal filament diameters: Actin 7 nm · intermediate 10 nm · microtubule 25 nmTypical human cell cycle: ≈24 h (G1 ≈11 h, S ≈8 h, G2 ≈4 h, M ≈1 h)Mitochondrial DNA: 16 569 bp, 37 genes, maternally inherited

Organelles

Eukaryotic cell (overview) Cell: The basic structural and functional unit of the body: a membrane-bounded, compartmentalised unit containing a nucleus, cytoplasmic organelles and a cytoskeleton.
Cytoplasm and cytosol Organelle: Everything between the plasma membrane and the nuclear envelope; its fluid part, the cytosol, is a crowded protein-rich gel where glycolysis, free-ribosome protein synthesis and many signalling reactions take place.
Ribosome Organelle: A non-membranous ribonucleoprotein machine (≈25–30 nm) that translates mRNA into protein; the eukaryotic 80S ribosome is built from a 60S and a 40S subunit.
Rough endoplasmic reticulum Organelle: A network of flattened membrane sacs (cisternae) studded with ribosomes, continuous with the outer nuclear membrane, where secretory, lysosomal and membrane proteins are made, folded and N-glycosylated.
Smooth endoplasmic reticulum Organelle: A network of ribosome-free, anastomosing membrane tubules, continuous with the RER, specialised for lipid and steroid synthesis, drug detoxification, glycogen metabolism and Ca²⁺ storage.
Golgi apparatus Organelle: A polarised stack of flattened cisternae (cis → medial → trans) that receives proteins from the ER, modifies their sugars, and sorts and packages them for secretion, the plasma membrane or lysosomes.
Secretory and transport vesicles Organelle: Small membrane-bounded carriers that shuttle cargo between compartments and to the cell surface, where they fuse with the plasma membrane to release their contents (exocytosis).
Endosome Organelle: A system of acidic membrane compartments that receives endocytosed material, sorts receptors for recycling and delivers cargo for degradation to lysosomes.
Lysosome Organelle: The cell's digestive organelle: a single-membrane vesicle with an acidic interior (pH ≈4.5–5) containing ≈40–60 hydrolytic enzymes that degrade macromolecules from endocytosis, phagocytosis and autophagy.
Peroxisome Organelle: A small single-membrane organelle that uses oxidases to break down very-long-chain and branched fatty acids, producing hydrogen peroxide that its catalase then destroys; it also makes plasmalogens for myelin.
Proteasome Organelle: A barrel-shaped, ATP-dependent protease complex (26S) in the cytosol and nucleus that destroys proteins tagged with chains of ubiquitin.
Mitochondrion Organelle: A double-membraned organelle with its own DNA that produces most of the cell's ATP by oxidative phosphorylation and also controls apoptosis, Ca²⁺ buffering and many metabolic pathways.
Cytoplasmic inclusions Organelle: Stored or accumulated materials in the cytoplasm — glycogen, lipid droplets, pigments and crystals — that are not metabolically active organelles.

Membrane & transport

Plasma membrane Organelle: The ≈8 nm lipid bilayer with embedded proteins that encloses the cell, controls what enters and leaves, and carries receptors, transporters and adhesion molecules.
Membrane transport Physiological process: The ways substances cross the plasma membrane: passively down their gradients (simple and facilitated diffusion, osmosis), actively against them using ATP or ion gradients, and in bulk by endocytosis and exocytosis.
Resting membrane potential Physiological process: The voltage across the plasma membrane of a resting cell (inside negative, typically −70 to −90 mV), generated mainly by K⁺ leaking out down the concentration gradient built by the Na⁺/K⁺-ATPase.
Cell signalling and receptors Physiological process: How cells detect chemical signals with receptors and convert them into intracellular responses through second messengers, protein kinases and changes in gene expression.

Nucleus & genetics

Nucleus Organelle: The largest organelle (≈5–10 µm), enclosed by a double-membraned nuclear envelope, that houses the genome as chromatin and is the site of DNA replication, transcription, RNA processing and ribosome assembly.
Nuclear envelope and nuclear pores Organelle: Two concentric lipid bilayers around the nucleus, separated by the perinuclear space and fused at thousands of nuclear pore complexes that control traffic between nucleus and cytoplasm.
Nucleolus Organelle: A dense, non-membranous nuclear body that forms around the ribosomal RNA genes and is the site of rRNA transcription, processing and ribosome subunit assembly.
Chromatin and chromosomes Molecule: The complex of DNA with histone and non-histone proteins that packages ≈2 m of DNA into the nucleus; it condenses into 46 visible chromosomes during cell division.
DNA replication Cell process: The semiconservative copying of the entire genome once per cell cycle during S phase, from tens of thousands of origins, with an error rate of only ≈1 in 10⁹–10¹⁰ nucleotides.
DNA repair Cell process: A set of pathways that continually detect and correct DNA damage and replication errors; inherited defects in them cause cancer-predisposition syndromes.
Transcription and RNA processing Cell process: Synthesis of RNA from a DNA template by RNA polymerase, followed in eukaryotes by capping, splicing and polyadenylation of pre-mRNA in the nucleus before export to the cytoplasm.
Translation and the genetic code Cell process: Decoding of mRNA into a polypeptide by ribosomes, using transfer RNAs that pair their anticodons with the triplet codons of the genetic code.
Regulation of gene expression Cell process: The control of when, where and how much each gene is expressed — at the level of chromatin, transcription, RNA processing, translation and protein stability — which makes cells with identical DNA different.

Cytoskeleton & cell surface

Cytoskeleton Organelle: A dynamic network of three protein filament systems — actin microfilaments (7 nm), intermediate filaments (10 nm) and microtubules (25 nm) — that shapes the cell, resists mechanical stress and, with motor proteins, moves organelles, chromosomes and the whole cell.
Centrosome and centrioles Organelle: The main microtubule-organising centre of animal cells: a pair of perpendicular centrioles (each nine microtubule triplets) embedded in pericentriolar material that nucleates microtubules and organises the mitotic spindle.
Cilia Organelle: Hair-like, microtubule-based projections of the cell surface: motile cilia (9+2 axoneme) beat in coordinated waves to move fluid, whereas single non-motile primary cilia (9+0) act as sensory antennae.
Flagellum Organelle: A single long whip-like motile projection with a 9+2 axoneme; in humans found only as the tail of the spermatozoon.
Microvilli Organelle: Finger-like, actin-supported projections of the apical plasma membrane (≈1 µm tall) that multiply the absorptive surface of epithelial cells, forming the brush border.
Cell junctions Organelle: Specialised protein complexes that seal epithelia (tight junctions), anchor cells to each other and to the basement membrane (adherens junctions, desmosomes, hemidesmosomes) and couple neighbouring cells chemically and electrically (gap junctions).

Cell processes

Cellular respiration Cell process: The stepwise oxidation of glucose (and fats and amino acids) to CO₂ and water that captures energy as ATP: glycolysis in the cytosol, then the link reaction, citric-acid cycle and oxidative phosphorylation in mitochondria — ≈30–32 ATP per glucose.
Apoptosis Cell process: Programmed, energy-dependent cell death executed by caspases, in which the cell shrinks and fragments into membrane-bound apoptotic bodies that are phagocytosed without inflammation.
Necrosis Cell process: Death of cells in living tissue after severe injury, characterised by swelling, loss of membrane integrity, leakage of cell contents and inflammation.
Autophagy Cell process: "Self-eating": the delivery of cytoplasmic components — damaged organelles, protein aggregates, microbes — to lysosomes for degradation and recycling, increased during starvation and stress.
Cell differentiation Cell process: The process by which unspecialised cells become specialised cell types by switching on cell-type-specific sets of genes, while keeping the same genome.

Cell cycle & division

Cell cycle Cell process: The ordered sequence of growth, DNA replication and division (G1 → S → G2 → M) by which a cell produces two genetically identical daughters, driven by cyclin–CDK complexes and guarded by checkpoints involving p53 and Rb.
Mitosis and cytokinesis Cell process: Division of the replicated nucleus into two genetically identical nuclei in five phases — prophase, prometaphase, metaphase, anaphase and telophase — followed by cytokinesis, the division of the cytoplasm.
Meiosis Cell process: Two successive divisions after a single round of DNA replication that halve the chromosome number (2n → n) and shuffle genetic material, producing haploid gametes.

Tissue overview

The four basic tissues Tissue: All organs are built from four basic tissues — epithelial, connective, muscle and nervous — each defined by its cells, its extracellular matrix and its function.
Body membranes Membrane: Sheets of tissue that cover or line body surfaces and cavities: mucous membranes (open to the exterior), serous membranes (closed body cavities), synovial membranes (joint cavities) and the cutaneous membrane (skin).

Epithelial tissue

Epithelial tissue Tissue: Sheets of closely packed, polarised cells with little extracellular matrix that cover body surfaces, line cavities and ducts, and form glands; avascular and resting on a basement membrane.
Simple squamous epithelium Tissue: A single layer of thin, flattened, polygonal cells with bulging central nuclei, specialised for rapid diffusion, filtration and smooth, slippery surfaces.
Simple cuboidal epithelium Tissue: A single layer of cube-shaped cells with central round nuclei, found in glandular ducts, kidney tubules and the thyroid follicles, where it secretes and absorbs.
Simple columnar epithelium Tissue: A single layer of tall cells with elongated, basally located nuclei, lining most of the digestive tract (non-ciliated, often with microvilli and goblet cells) and parts of the female reproductive tract (ciliated).
Pseudostratified columnar epithelium Tissue: A single layer of cells of different heights, all resting on the basement membrane but not all reaching the surface, so their nuclei lie at different levels and the tissue looks stratified; typically ciliated with goblet cells ("respiratory epithelium").
Stratified squamous epithelium Tissue: A multilayered epithelium in which basal cuboidal cells divide and move upward, flattening toward the surface; it protects against abrasion and, when keratinised (epidermis), against water loss.
Stratified cuboidal epithelium Tissue: An uncommon epithelium of usually two layers of cuboidal cells, lining the ducts of sweat glands and larger ducts of salivary, mammary and pancreatic glands.
Stratified columnar epithelium Tissue: A rare epithelium with columnar surface cells over one or more layers of smaller basal cells, found at transitions between other epithelia and in large ducts.
Transitional epithelium (urothelium) Tissue: A stratified epithelium unique to the urinary tract whose large dome-shaped surface "umbrella" cells flatten as the organ fills, forming a stretchable, urine-tight barrier.
Glandular epithelium Tissue: Epithelial cells specialised for secretion, forming exocrine glands (secreting via ducts onto surfaces) and endocrine glands (ductless, secreting hormones into the blood).
Basement membrane Membrane: A thin, sheet-like layer of specialised extracellular matrix — basal lamina plus reticular lamina — that underlies all epithelia and surrounds muscle cells, adipocytes and Schwann cells.

Connective tissue

Connective tissue Tissue: The most abundant and diverse tissue: relatively few cells scattered in an abundant extracellular matrix of fibres and ground substance, which binds, supports, protects, stores energy and transports.
Areolar (loose) connective tissue Tissue: The most widespread connective tissue: a loose, gel-rich meshwork of all three fibre types and many cell types that packages organs, underlies epithelia and surrounds vessels and nerves.
Adipose tissue Tissue: Loose connective tissue dominated by fat-storing adipocytes: white fat stores energy, insulates, cushions and secretes hormones; brown fat burns fat to generate heat.
Reticular connective tissue Tissue: A delicate three-dimensional network of thin reticular (type III collagen) fibres and reticular cells that forms the supporting stroma of lymphoid organs, bone marrow and the liver.
Dense regular connective tissue Tissue: Tightly packed, parallel bundles of type I collagen with rows of flattened fibroblasts, giving great tensile strength in one direction; forms tendons, most ligaments and aponeuroses.
Dense irregular connective tissue Tissue: Thick type I collagen bundles interwoven in many directions, resisting tension from all sides; forms the reticular dermis, organ capsules, periosteum and perichondrium.
Elastic connective tissue Tissue: Dense connective tissue rich in elastic fibres or sheets that stretch and recoil, found in the walls of large arteries, the ligamenta flava and the airways.
Extracellular matrix Molecule: The non-cellular material secreted by cells — collagen, elastic and reticular fibres embedded in a hydrated ground substance of glycosaminoglycans, proteoglycans and adhesive glycoproteins — that gives tissues their mechanical properties and signals to cells.

Cartilage

Hyaline cartilage Tissue: The most common cartilage: a glassy, avascular matrix of type II collagen and water-binding aggrecan, with chondrocytes in lacunae, that provides smooth, resilient support and the template for most of the fetal skeleton.
Elastic cartilage Tissue: Hyaline-like cartilage with a dense network of branching elastic fibres, giving flexibility and shape recovery; found in the auricle, external acoustic meatus, auditory tube and epiglottis.
Fibrocartilage Tissue: A tough blend of dense regular connective tissue and cartilage — rows of chondrocytes between thick bundles of type I collagen — that resists compression and shear; it has no perichondrium.

Muscle & nervous tissue (overview)

Muscle tissue (overview) Tissue: Tissue of elongated, excitable cells (fibres) packed with actin and myosin that contract to produce movement; three types — skeletal, cardiac and smooth — differ in structure, control and regenerative capacity.
Nervous tissue (overview) Tissue: Tissue of neurons, which generate and conduct electrical signals and communicate at synapses, and the more numerous supporting neuroglia; it forms the brain, spinal cord, ganglia and nerves.

Cell types

Cell types of the body (catalogue) Cell: A guide to the major specialised cell types of the human body, each a variation on the common eukaryotic plan adapted to a particular function.
Fibroblast Cell: The principal resident cell of connective tissue, which synthesises and remodels the extracellular matrix — collagen, elastic and reticular fibres and ground substance.
Adipocyte Cell: A cell specialised for storing triacylglycerol and releasing fatty acids on demand, and an endocrine cell secreting leptin and adiponectin; white adipocytes are unilocular, brown adipocytes multilocular.
Chondrocyte Cell: The cell of cartilage: chondroblasts secrete cartilage matrix and become chondrocytes trapped in lacunae, where they maintain the type II collagen and aggrecan of the matrix.
Stem cells Cell: Unspecialised cells that can both renew themselves and give rise to differentiated cell types; they are graded by potency from totipotent (zygote) to pluripotent, multipotent and unipotent.
Goblet cell Cell: A goblet-shaped, mucus-secreting unicellular exocrine gland scattered among the columnar cells of intestinal and respiratory epithelia.
Mesenchymal cell Cell: An unspecialised, stellate embryonic connective-tissue cell (and its adult descendant, the mesenchymal stem/stromal cell) that gives rise to fibroblasts, chondroblasts, osteoblasts, adipocytes and smooth muscle.

Mnemonics

Collagen types: Be So Totally Cool, Read Books — Type I: Bone, Skin, Tendon (≈90 % of body collagen); Type II: Cartilage (and vitreous, nucleus pulposus); Type III: Reticulin (blood vessels, granulation tissue, lymphoid stroma); Type IV: Basement membrane.
Phases of mitosis: PPMAT — "Please Pee on the MAT": Prophase, Prometaphase, Metaphase, Anaphase, Telophase (then cytokinesis).
Stop codons: U Are Away · U Are Gone · U Go Away — UAA, UAG, UGA. Start codon: AUG (methionine).
Microtubule motors: Kinesin kicks it out, dynein drags it in — kinesin moves cargo toward the plus end (anterograde, cell periphery/axon terminal); dynein moves toward the minus end (retrograde, centrosome/cell body).
Elastic cartilage: The 3 Es — Ear (auricle and external acoustic meatus), Epiglottis, Eustachian (auditory) tube.
G-protein-coupled receptors: Gq: "HAVe 1 M&M" — H1, α1, V1, M1, M3. Gi: "MAD 2s" — M2, α2, D2. Most others (β1–3, D1, H2, V2) are Gs.

From the Corpus anatomy atlas — for education only.