Endomembrane system
A unified system of membranes exchanging material within eukaryotic cells.
The endomembrane system is composed of the various membranes suspended in the cytoplasm of eukaryotic cells, which partition the cell into distinct functional and structural compartments known as organelles. These organelles include the nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, endosomes, and the cell membrane, among others. The system is defined as a single functional and developmental unit, with its components either connected directly or exchanging material through vesicle transport. Notably, it excludes the membranes of plastids and mitochondria, though it may have partially evolved from the actions of these organelles. The nuclear membrane, for instance, consists of a lipid bilayer that encloses the nucleus, while the endoplasmic reticulum serves as a synthesis and transport organelle branching throughout the cytoplasm. The Golgi apparatus comprises multiple compartments where molecules are packaged for delivery or secretion. Vacuoles, larger in plant cells, maintain cell shape and store waste. Vesicles are small, membrane-enclosed sacs for storage or transport, and the cell membrane acts as a protective barrier regulating entry and exit. A unique fungal organelle, the Spitzenkörper, is associated with hyphal tip growth. In prokaryotes, endomembranes are rare, though photosynthetic bacteria may have highly folded cell membranes or chlorosomes, and the *Thiomargarita* species possess a complex "pepin" system. The membranes share a lipid bilayer with associated proteins, but their thickness, composition, and metabolic behavior can be modified over time. The concept of a unified membrane system exchanging material was first proposed in 1974 by Morré and Mollenhauer, explaining organelle biogenesis through lipid flow from synthesis sites, though lipid transport mechanisms remain poorly understood.
- field
- Cell biology
- known_for
- Unifying concept of membrane compartments that exchange material via direct contact or vesicle transport
- components
- Nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, endosomes, cell membrane
- excludes
- Plastids and mitochondria
Lore & Background
The endomembrane system comprises the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, endosomes, and cell membrane. The nuclear envelope has two lipid bilayers, with the outer membrane continuous with the rough endoplasmic reticulum and featuring ribosomes. The endoplasmic reticulum, accounting for more than half the total membrane in eukaryotic cells, consists of flattened sacs and branching tubules forming a continuous sheet enclosing the ER lumen, which takes up about ten percent of cell volume. It produces transmembrane proteins and lipids for many organelles and delivers proteins destined for secretion or for the ER, Golgi, or lysosomes. The Golgi apparatus is a series of compartments where molecules are packaged for delivery or secretion. Vacuoles maintain cell shape and store waste, while vesicles are small membrane-enclosed sacs for storage or transport. The cell membrane is a protective barrier regulating entry and exit. A specialized organelle, the Spitzenkörper, is found only in fungi and is connected with hyphal tip growth.
Reader's Guide
The endomembrane system is significant as a foundational concept in cell biology, explaining how eukaryotic cells compartmentalize functions and transport materials. This concept accounts for the assembly of various lipid membranes from sites of synthesis, primarily the endoplasmic reticulum, with lipid transport mechanisms still poorly understood. The system's organelles are related through direct contact or vesicle transfer, yet their membranes differ in thickness, molecular composition, and metabolic behavior, all sharing a lipid bilayer. The nuclear envelope's pores facilitate high-traffic transport of RNA, ribosomal subunits, histones, and polymerases. The ER's role in cotranslational protein sorting and its resident proteins, like the chaperone BiP, underscore its central function in protein quality control. The system's exclusion of plastids and mitochondria highlights its distinct evolutionary and functional boundaries, while its presence in eukaryotes contrasts with rare endomembranes in prokaryotes, such as folded membranes in photosynthetic bacteria or chlorosomes in green sulfur bacteria.
Did You Know?
- The endomembrane system does not include the membranes of plastids or mitochondria, but might have evolved partially from the actions of the latter.
- The Spitzenkörper is an organelle of the endomembrane system found only in fungi, connected with hyphal tip growth.
- More than half the total membrane in eukaryotic cells is accounted for by the endoplasmic reticulum.
Frequently Asked Questions
Who is Endomembrane system?
Endomembrane system is the unifying concept in eukaryotic cell biology that groups together all internal membranes operating as one coordinated unit. Its members are linked either by direct physical contact or by shuttling cargo back and forth through vesicle transport.
What are Endomembrane system's powers/role?
Its core ability is carving the cytoplasm into specialized compartments so each can run distinct biochemical reactions without interference. It oversees the full pipeline from protein and lipid synthesis to sorting, packaging, and targeted degradation of cellular cargo.
How does Endomembrane system's story end?
As a structural framework it has no narrative finale—it simply persists for the entire lifespan of a living eukaryotic cell. Its 'final act' is the cell's own death, when membrane integrity collapses and the compartments dissolve.
Why is Endomembrane system important?
Without this coordinated network, a eukaryotic cell could not keep incompatible chemical reactions spatially separated and running simultaneously. It is the architectural reason complex cells outperform prokaryotes in metabolic efficiency.
Who are Endomembrane system's core team members?
The recognized roster includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, endosomes, and the plasma membrane. Mitochondria and plastids are explicitly excluded from the team despite also being membrane-bound organelles.
More in Cellular Biology 1-23
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record