Subtopic 2.2: Subcellular compartments
Cytoskeleton is a structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement. The cytoskeleton is made up of microtubules, actin filaments, and intermediate filaments.
Endoplasmic reticulum (ER): It consists of a system of sac- and tube-like structures, which locally expand into cisterns. Its internal lumen is connected with the intermembrane space of the nuclear
membrane. Part of the ER is studded on the outside with ribosomes (rough ER), which take part in protein synthesis. The other part of the ER is free of ribosomes (smooth ER). Enzymes of the smooth ER are involved in the synthesis of fatty acids. The smooth ER also plays a role in detoxification by hydroxylation reactions.
Golgi apparatus: This organelle consists of stacks of flattened membrane sacs. Their main function is the further processing and sorting of proteins and their export to the final targets. In most cases, these are secreted or membrane proteins. In addition, the Golgi apparatus also produces polysaccharides.
Lysosomes: Lysosomes are vesicles enclosed by a lipid bilayer. These organelles are filled with many enzymes for polysaccharide, lipid, protein and nucleic acid degradation. They act also on intracellular material to be removed and even contribute to the apoptosis of their own cell. Lysosomes of special cells (e.g., macrophages) destroy bacteria or viruses as a defense mechanism.
Peroxisomes: Peroxisomes are surrounded by a single membrane. They are generated from components of the cytosol and do not bud from other membranes. The main task of these organelles is the performance of monooxygenase (hydroxylase) or oxidase reactions, which produce hydrogen peroxide (H2O2).
Mitochondria: In a typical eukaryotic cell, there are in the order of 2000 of these organelles, which are often of ellipsoidal shape. They have a smooth outer membrane and a highly folded inner membrane with numerous invaginations (cristae), which contain most of the membrane-bound enzymes of mitochondrial metabolism. Mitochondria are the site of respiration and ATP synthesis, but also of many other central reactions of metabolism, e.g., citrate cycle, fatty acid oxidation, glutamine formation, and part of the pathway leading to steroid hormones. Mitochondria are the only organelles which are equipped with their own (circular) DNA, RNA and ribosomes and thus can perform their own protein synthesis.
Nucleus: All eukaryotic cells show the presence of a separate nucleus, which contains the major portion of the genetic material of the cell (DNA). The nuclear DNA is organized in a number of chromosomes. The nucleus is surrounded by a double membrane of lipid bilayers with integrated proteins, called the nuclear membrane (also known as the nuclear envelope). Nuclear pores span the nuclear membrane and enable the transport of proteins, rRNA etc.