Cell organelles are specialized sub-cellular structures embedded within the cytoplasm that perform specific metabolic functions. In eukaryotes, these organelles are membrane-bound, allowing for compartmentalization—a feature that enables diverse biochemical processes to occur simultaneously without interference.
The Endomembrane System
The endomembrane system consists of a group of membranes and organelles that work together to modify, package, and transport lipids and proteins. It includes the Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, and Vacuoles.
Endoplasmic Reticulum (ER)
The ER is a network of tiny tubular structures scattering the cytoplasm. It divides the intracellular space into luminal (inside ER) and extra-luminal (cytoplasm) compartments.
- Rough ER (RER): Studded with ribosomes; actively involved in protein synthesis and secretion.
- Smooth ER (SER): Lacks ribosomes; the major site for lipid synthesis and steroid hormone production in animal cells.
Golgi Apparatus
Discovered by Camillo Golgi (1898), it consists of flat, disc-shaped sacs called cisternae.
- Function: It is the “post office” of the cell, responsible for packaging materials into vesicles for delivery to intracellular targets or secretion outside the cell.
- Fact: It is the important site for the formation of glycoproteins and glycolipids.
Lysosomes
These are membrane-bound vesicular structures formed by the Golgi apparatus.
- Suicidal Bags: They contain almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrates) active at acidic pH.
- Function: Digestion of intracellular waste, worn-out organelles, and foreign particles.
Vacuoles
- Plant Cells: Occupy up to 90% of the volume. The vacuole is bound by a single membrane called the tonoplast, which facilitates the transport of ions against concentration gradients.
- Animal Cells: Much smaller; often used for storage or osmoregulation (e.g., contractile vacuoles in Amoeba).
Energy-Transforming Organelles
These organelles are semi-autonomous as they possess their own DNA and ribosomes.
Mitochondria
Known as the “Powerhouse of the Cell.”
- Structure: Double-membrane bound; the inner membrane forms folds called cristae to increase surface area.
- Function: Site of aerobic respiration and production of cellular energy in the form of ATP (Adenosine Triphosphate).
- Trivia: Mitochondria are inherited maternally in humans (Mitochondrial DNA).
Plastids
Found in all plant cells and Euglenoids. They are classified based on pigments:
- Chloroplasts: Contain chlorophyll and carotenoids; site of photosynthesis.
- Chromoplasts: Contain fat-soluble carotenoid pigments (carotene, xanthophylls), giving plants yellow, orange, or red colors.
- Leucoplasts: Colorless plastids used for storage (e.g., Amyloplasts store starch, Elaioplasts store oils, Aleuroplasts store proteins).
Chloroplast vs mitochondria vector illustration. Labeled structure scheme.
Non-Membrane Bound Organelles
Ribosomes
Discovered by George Palade (1953), these are composed of RNA and proteins.
- Types: 70S (found in prokaryotes, mitochondria, and chloroplasts) and 80S (found in eukaryotic cytoplasm).
- Function: The “Protein Factories” of the cell.
Centrosome and Centrioles
- Found predominantly in animal cells.
- Function: They form the basal body of cilia/flagella and give rise to spindle fibers during cell division, crucial for organizing microtubule assembly.
Summary Table: Organelle Functions and Features
| Organelle | Key Feature | Primary Function |
| Mitochondria | Double membrane; Own DNA (70S Ribosomes) | ATP Production (Aerobic Respiration) |
| Chloroplast | Contains Chlorophyll; Double membrane | Photosynthesis |
| Ribosome | Non-membranous; Palade particles | Protein Synthesis |
| Lysosome | Acid Hydrolases; Single membrane | Intracellular Digestion |
| Golgi Body | Cis and Trans faces | Packaging and Secretion |
| Centriole | 9+0 microtubule arrangement | Spindle formation in animal cells |
UPSC Prelims Fact File
- Endosymbiotic Theory: Suggests that mitochondria and chloroplasts were once free-living prokaryotes that entered into a symbiotic relationship with eukaryotic cells. This explains why they have their own DNA and 70S ribosomes.
- Cytoskeleton: A network of filamentous proteinaceous structures (microtubules, microfilaments) that provide mechanical support, motility, and maintenance of cell shape.
- Cilia and Flagella: Hair-like outgrowths of the cell membrane. Cilia are small and work like oars; flagella are longer and responsible for cell movement. Both have a 9+2 microtubule arrangement in eukaryotes.
- Peroxisomes: Small vesicles containing oxidative enzymes; they play a role in lipid metabolism and chemical detoxification (e.g., neutralizing H2O2).