Biology - The Endomembrane System and Proteins
Together, the organelles of the endomembrane system form the core of a eukaryotic cell. From the outer membrane of the nuclear envelope and the ER, new proteins and lipids are synthesized. Through the Golgi complex, proteins and lipids are modified, sorted, and packaged for delivery to final destinations outside the cell or within one of the endomembrane organelles. Biomolecules also travel inward, trafficking through the endomembrane system from the plasma membrane toward internal destinations.
The endomembrane system includes the nuclear ..
The cytoskeleton forms a complex thread-like network throughout the cell consisting of three different kinds of protein-based filaments: microfilaments, intermediate filaments, and microtubules ([link]). The thickest of the three is the microtubule, a structural filament composed of subunits of a protein called tubulin. Microtubules maintain cell shape and structure, help resist compression of the cell, and play a role in positioning the organelles within the cell. Microtubules also make up two types of cellular appendages important for motion: cilia and flagella. Cilia are found on many cells of the body, including the epithelial cells that line the airways of the respiratory system. Cilia move rhythmically; they beat constantly, moving waste materials such as dust, mucus, and bacteria upward through the airways, away from the lungs and toward the mouth. Beating cilia on cells in the female fallopian tubes move egg cells from the ovary towards the uterus. A flagellum (plural = flagella) is an appendage larger than a cilium and specialized for cell locomotion. The only flagellated cell in humans is the sperm cell that must propel itself towards female egg cells.
Unlike most organelles, peroxisomes defy classification. Peroxisomes were originally described as separate from the endomembrane system because formation of these organelles appeared independent from endomembrane transport. Additionally, protein enzymes functioning inside peroxisomes were synthesized in the cytoplasm and not in the rough ER (the source of Golgi, lysosome, and secreted enzymes). However, scientific research over the last decade provides significant evidence that peroxisomes actually originate from the endoplasmic reticulum and that retrograde traffic from peroxisomes back to the ER also occurs. Combined, the current evidence supports peroxisomes as a part of the endomembrane system, but the subject remains an active area of scientific research.