Eukaryotic cell (overview)
Cellula
Also known as: cell, animal cell, human cell, cell theory
The basic structural and functional unit of the body: a membrane-bounded, compartmentalised unit containing a nucleus, cytoplasmic organelles and a cytoskeleton.
Overview
Cell theory (Schleiden, Schwann, Virchow) states that all organisms are made of cells, that the cell is the basic unit of life, and that cells arise only from pre-existing cells (omnis cellula e cellula). Human cells are eukaryotic: unlike prokaryotes (bacteria), they have a nucleus enclosed by a double membrane, membrane-bound organelles, a cytoskeleton with motor proteins, linear chromosomes packaged with histones, and 80S cytoplasmic ribosomes.
Each cell has three main regions: the plasma membrane, the cytoplasm (cytosol, organelles, inclusions and cytoskeleton) and the nucleus. Cell size is limited by the surface-area-to-volume ratio — as a cell grows, its volume (metabolic demand) rises faster than its surface (exchange capacity) — which is why most cells are 10–30 µm across and why absorptive cells amplify their surface with microvilli and folds.
Compartmentalisation lets incompatible reactions run side by side: acid hydrolysis in lysosomes, oxidation in peroxisomes, protein folding in the ER, ATP synthesis in mitochondria. In a hepatocyte the cytosol occupies about half of the cell volume, mitochondria about a fifth, and the ER, Golgi, nucleus, lysosomes, peroxisomes and endosomes most of the rest.
- Location
Every tissue; human cells range from ≈4–5 µm (cerebellar granule cells, platelets as cell fragments ≈2–3 µm) to ≈120 µm (secondary oocyte).
Parts & subdivisions
- Selectively permeable lipid bilayer with proteins; boundary of the cell.
- Cytosol plus organelles, inclusions and cytoskeleton.
- Contains the genome; site of DNA replication and transcription.
- ER, Golgi apparatus, endosomes, lysosomes, peroxisomes, mitochondria, vesicles.
- Ribosomes, proteasomes, centrosome/centrioles, cytoskeleton, cilia, microvilli.
Function
- Maintains homeostasis of its internal environment.
- Obtains energy, synthesises macromolecules and grows.
- Responds to signals and communicates with other cells.
- Reproduces (mitosis/meiosis) and differentiates into specialised cell types.
- Physiology
Most cell activities fit three themes: energy (glycolysis, oxidative phosphorylation), information (DNA replication, transcription, translation, signalling) and traffic (membrane transport, the secretory and endocytic pathways, cytoskeletal motors).
Test yourself
The epithelium lining the urinary bladder, which can change from about six cell layers to two or three as it fills, is:
- Stratified squamous non-keratinised
- Transitional epithelium (urothelium)
- Pseudostratified columnar
- Stratified cuboidal
Answer: Transitional epithelium (urothelium)
Urothelium has large dome-shaped umbrella cells with uroplakin plaques and tight junctions that make an impermeable blood–urine barrier and flatten when the bladder is distended.
The trachea is lined by:
- Simple columnar epithelium with microvilli
- Stratified squamous non-keratinised epithelium
- Pseudostratified ciliated columnar epithelium with goblet cells
- Simple cuboidal epithelium
Answer: Pseudostratified ciliated columnar epithelium with goblet cells
"Respiratory epithelium": all cells touch the basement membrane but not all reach the lumen, so nuclei lie at different levels. Cilia and goblet-cell mucus form the mucociliary escalator.
After ionising radiation, p53 halts the cell cycle in G1 mainly by inducing:
- Cyclin D
- The CDK inhibitor p21
- MDM2 degradation of Rb
- Telomerase
Answer: The CDK inhibitor p21
DNA damage activates ATM/ATR kinases, which stabilise p53. p53 transcribes p21 (CIP1/WAF1), which inhibits G1/S cyclin–CDKs, keeping Rb hypophosphorylated and E2F sequestered. If damage is irreparable p53 triggers apoptosis (BAX, PUMA, NOXA).
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.