Exploring the Structure and Function of Cellular Organelles
Centrosome
Present only in animal cells, the centrosome serves as the microtubule organizing center. The diplosome consists of two centrioles arranged perpendicularly within a dense pericentriolar material. Microtubules extend from this material, forming the aster. Each centriole comprises nine microtubule triplets arranged cylindrically, with proteins maintaining their structure.
Function
Centrosomes play a crucial role in forming cilia, flagella, the spindle apparatus, and the cytoskeleton.
Cilia and Flagella
These cytoplasmic projections are located on the cell surface. Cilia create currents to move mucus, while flagella facilitate cell movement. Proteins like dynein provide the energy for microtubule movement.
Ribosomes
Ribosomes are membrane-less structures composed of proteins and rRNA. They can be found scattered in the cytosol or attached to the RER membrane. With a diameter of 200A and a sedimentation rate of 80s, ribosomes have a large subunit (60S) and a small subunit (40S). They are composed of approximately 80% water, 10% protein, and 10% rRNA.
Function
Ribosomes are responsible for protein synthesis by reading mRNA. Multiple ribosomes can read a single mRNA simultaneously, forming a structure called a polysome.
Endoplasmic Reticulum (ER)
The ER is a network of membranous sacs, tubules, and vesicles extending throughout the cytoplasm. It communicates with the outer nuclear membrane.
Rough Endoplasmic Reticulum (RER)
The RER consists of interconnected flattened sacs studded with ribosomes. It is involved in protein synthesis and transport.
Smooth Endoplasmic Reticulum (SER)
The SER is a network of tubules connected to the RER. It is responsible for lipid synthesis, storage, and transport, detoxification processes, and muscle contraction.
Golgi Apparatus
Discovered by Camillo Golgi in 1898, the Golgi apparatus consists of one or more dictyosomes. Each dictyosome is composed of 4-8 flattened sacs (cisternae) with accompanying vesicles, often located near the nucleus.
Function
The Golgi apparatus is involved in the transport, maturation, accumulation, and secretion of proteins from the RER. Proteins are modified and activated as they move through the Golgi cisternae from the cis face to the trans face. Modified proteins are then packaged into secretory vesicles, which may become lysosomes or fuse with the cell membrane for exocytosis. The Golgi apparatus also glycosylates lipids and proteins, synthesizes proteoglycans, and produces other carbohydrates.
Lysosomes
Lysosomes originate from the Golgi apparatus and contain digestive enzymes called acid hydrolases.
Function
Lysosomes are responsible for intracellular and extracellular digestion. Acid phosphatase is a key enzyme that cleaves phosphoesters. Intracellular digestion occurs when lysosomes fuse with vacuoles containing material to be digested. Extracellular digestion involves the release of lysosomal enzymes outside the cell.
Types of Lysosomes
- Primary Lysosomes: Contain only enzymes.
- Secondary Lysosomes: Actively involved in digestion.
Vacuoles
Vacuoles are membrane-bound vesicles containing an aqueous medium. In animal cells, vacuoles are small and often called vesicles. In plant cells, vacuoles are typically large and derived from the fusion of vesicles from the ER and Golgi apparatus. The vacuole membrane is called the tonoplast, and the entire set of plant cell vacuoles is called the vacuome.
Function
Vacuoles store water and other substances. They also facilitate transport between organelles of the endomembrane system and between these organelles and the extracellular environment. In animal cells, vacuoles have nutrient functions (phagocytic and pinocytotic vacuoles) or function as contractile vacuoles in ciliated protozoa.