Cell Structure and Function

A comprehensive vocabulary review of cell biology, covering historical discoveries, microscopy, prokaryotic versus eukaryotic structures, cellular organelles, and the endomembrane system.

Cell theory

The unifying biological principle stating that every organism is made up of one or more cells, cells are the functional units of multicellular organisms, and all cells arise from preexisting cells.

Matthias Schleiden

The German botanist who concluded in 1838 that cells and substances produced by cells form the basic structure of plants and that plant growth occurs by adding new cells.

Theodor Schwann

The German biologist who concluded in 1839 that animal tissues are composed of cells, collaborating with Schleiden to establish cells as fundamental units of life.

Rudolf Virchow

The German physician who completed the cell theory in 1855 by concluding that all cells come from previously existing cells.

Robert Hooke

The English scientist and inventor who in 1665 viewed thin slices of cork oak bark with a primitive light microscope and coined the term "cells" after the tiny rooms occupied by monks.

Anton van Leeuwenhoek

The Dutch microscopist who in the 1670s constructed simple microscopes with superior lenses and observed a previously unknown living world of microscopic organisms.

Diffusion

The relatively slow physical process by which molecules dissolved in fluids move, limiting cell diameter to generally $1\text{ to }100\,\mu\text{m}$ so all parts maintain ready access to materials and waste removal.

Plasma membrane

An extremely thin membrane consisting of a phospholipid bilayer with embedded proteins that encloses the cell, isolates its contents, and regulates the transport of materials.

Cytoplasm

All the fluid and cellular structures that lie inside the plasma membrane but outside of the nucleus in a eukaryotic cell.

Cytosol

The fluid portion of the cytoplasm containing water, salts, and an assortment of organic molecules where most metabolic activities occur.

Cytoskeleton

A dynamic network of protein filaments within the cytoplasm that provides structural support, transports materials, alters cell shape, and facilitates cell division.

Deoxyribonucleic acid (DNA)

The genetic material in all cells that encodes inherited instructions in nucleotide segments called genes to direct cell construction and cellular activities.

Ribosomes

Cellular "workbenches" composed of ribosomal RNA ($\text{rRNA}$) and proteins that serve as the sites of protein synthesis.

Nucleoid

A distinct, non-membrane-enclosed region in a prokaryotic cell that contains a single circular chromosome made of coiled DNA.

Plasmids

Small, circular rings of DNA found in most prokaryotic cells that carry specialized genes independent of the nucleoid, such as genes that deactivate antibiotics.

Peptidoglycan

A polymer composing bacterial cell walls in which short peptides link chains of sugar molecules that contain amino functional groups.

Capsules and slime layers

Polysaccharide coatings secreted outside bacterial cell walls that facilitate adherence to surfaces and host tissues to form films.

Attachment pili

Short, abundant surface proteins projecting from bacterial cell walls that help bacteria adhere to host tissues and environmental structures.

Sex pili

Elongated, specialized surface proteins on bacteria that bind to nearby cells of the same type and form a cytoplasmic bridge to transfer plasmids.

Organelles

Membrane-enclosed internal cellular structures specialized to carry out particular biochemical functions in eukaryotic cells.

Extracellular matrix (ECM)

A reinforcing framework of adhesive and supporting proteins embedded in a polysaccharide gel secreted by animal cells that provides biochemical and structural support.

Nuclear envelope

A double membrane perforated by protein-lined pores that isolates the eukaryotic nucleus and selectively regulates molecular passage.

Chromatin

The nuclear material consisting of DNA molecules associated with proteins, visible as extended strands in non-dividing cells and condensed chromosomes during division.

Nucleolus

A specialized nuclear structure composed of ribosomal RNA, proteins, ribosome-coding chromosomal regions, and assembling ribosomes, serving as the site of ribosome synthesis.

Polyribosomes

Clusters of multiple ribosomes strung along single strands of messenger RNA ($\text{mRNA}$) in the cytoplasm that simultaneously synthesize identical polypeptide chains.

Mitochondria

Double-membrane eukaryotic organelles that extract energy from food molecules through aerobic metabolism to produce ATP.

Cristae

Deep, convoluted folds formed by the inner membrane of a mitochondrion that hold energy-extracting enzymes.

Matrix

The fluid-filled space enclosed within the inner membrane of a mitochondrion where initial food-breakdown reactions take place.

Chloroplasts

Double-membrane organelles present in plants and photosynthetic protists that capture solar energy to synthesize sugars.

Thylakoid

An individual hollow, membranous sac stacked within the stroma of a chloroplast, possessing membrane-bound chlorophyll pigments.

Stroma

The fluid portion enclosed by the inner membrane of a chloroplast surrounding the thylakoid stacks, where sugar synthesis occurs.

Endosymbiont hypothesis

The theory proposing that mitochondria and chloroplasts evolved from ancient free-living bacteria that established endosymbiosis inside host cells approximately $1.7\text{ billion}$ years ago.

Plastids

Double-membrane organelles found exclusively in plants and photosynthetic protists that synthesize and store pigments or food molecules such as starch.

Endomembrane system

A collective network of internal eukaryotic membranes—including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles—that compartmentalizes cellular processes.

Endocytosis

A cellular transport mechanism where the plasma membrane extends around extracellular materials, fuses, and pinches inward to form a membrane-bound vesicle.

Exocytosis

The cellular process in which an intracellular vesicle fuses with the plasma membrane to discharge its contents into the extracellular environment.

Rough endoplasmic reticulum

A network of membranous sacs continuous with the nuclear envelope and studded with ribosomes, responsible for synthesizing, modifying, and sorting secretory and membrane proteins.

Smooth endoplasmic reticulum

A membranous network devoid of ribosomes that synthesizes membrane phospholipids and steroid sex hormones, stores calcium ions in muscle, and detoxifies drugs and wastes.

Golgi apparatus

An organelle consisting of flattened, interconnected membranous sacs that chemically modifies, sorts, and packages proteins and lipids received from the rough ER.

Lysosomes

Membrane-bound digestive vesicles containing hydrolytic enzymes operating optimally at about $\text{pH } 5$ to break down food, damaged organelles, and cellular debris.