Membrane Systems and Organelles
Membrane systems and organelles are the membrane-bounded compartments that subdivide the eukaryotic cell into specialised chemical environments. By enclosing distinct sets of enzymes and ionic conditions behind selectively permeable lipid bilayers, organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes and peroxisomes allow incompatible biochemical processes to proceed simultaneously within one cell.
Definition
Organelles are discrete subcellular structures, most of them bounded by one or two lipid-bilayer membranes, that compartmentalise specific metabolic and synthetic functions within the eukaryotic cell.
Scope
This area orients the learner to the principal membrane-bounded organelles of the eukaryotic cell and the endomembrane traffic that connects them. It groups topic entries on the endoplasmic reticulum, the Golgi apparatus and trafficking, mitochondria, the lysosomal and endosomal degradation system, and the peroxisome and centrosome. It treats these as structural and functional units of cell biology and histology, not as clinical conditions.
Sub-topics
Core questions
- How does membrane compartmentalisation allow a single cell to run incompatible chemistries at once?
- How is material moved selectively between organelles by vesicular and membrane-contact pathways?
- What distinguishes the single-membrane endomembrane organelles from the double-membrane mitochondrion?
- How are organelles built, maintained, and inherited at cell division?
Key concepts
- Lipid bilayer and selective permeability
- Compartmentalisation of metabolism
- Endomembrane system
- Vesicular transport and membrane traffic
- Single-membrane versus double-membrane organelles
- Organelle biogenesis and inheritance
- Membrane contact sites
Mechanisms
Each organelle is delimited by one or more lipid bilayers whose composition and embedded proteins define its identity and what can cross it. The endomembrane system — endoplasmic reticulum, Golgi apparatus, endosomes and lysosomes — is connected by a continuous flow of membrane carried in budding and fusing transport vesicles, so that lipids and proteins move directionally between compartments while each retains its characteristic environment. Mitochondria are bounded by two membranes and, with peroxisomes, are partly autonomous in their biogenesis. The differing lipid compositions of organelle membranes underpin their distinct behaviours and the sorting of cargo through the secretory and degradative routes.
Clinical relevance
Organelle structure and function underlie much of cell physiology, and many inherited and acquired diseases are understood as organelle dysfunction — for example lysosomal storage disorders, mitochondrial disease, and peroxisome biogenesis disorders. This area describes the normal cell biology that such conditions perturb; it is reference material for understanding mechanisms and is not a basis for diagnosis or treatment.
History
The systematic study of organelles followed the introduction of cell fractionation and electron microscopy in the mid-twentieth century. Albert Claude, Christian de Duve and George Palade, who shared the 1974 Nobel Prize, established subcellular fractionation and ultrastructural imaging as the tools that revealed the membrane-bounded compartments and their functions, including de Duve's discovery of the lysosome and the peroxisome.
Key figures
- Christian de Duve
- George Palade
- Albert Claude
- James Rothman
- Günther Blobel
Related topics
Seminal works
- rothman1994
- vanmeer2008
Frequently asked questions
- What is the difference between a single-membrane and a double-membrane organelle?
- Most endomembrane organelles (endoplasmic reticulum, Golgi, lysosomes, peroxisomes) are bounded by one bilayer, whereas mitochondria are bounded by two membranes, reflecting their distinct origin and biogenesis.
- Why do cells need organelles at all?
- Compartmentalisation lets a single cell run chemically incompatible reactions at once by confining specific enzymes and conditions behind selectively permeable membranes.