2. Cell membrane, intracellular compartmentalization Flashcards
lipids
Organic compounds formed in living organisms with varying composition and structure that have a common feature that they are well soluble in apolar solvents. Hydrolyzable lipids include neutral fats and phospholipids. Neutral fats are built from glycerol and three fatty acid molecules, while phospholipids also contain a polar part formed by phosphoric acid and an alcohol molecule. Steroids and poly-isoprenoids (e.g., carotenoids) belong to the group of nonhydrolyzable lipids.
amphipathic molecule
Molecules (ions) containing apolar and polar (or ionic) moieties are called amphipathic substances. Such molecules are e.g. the phospholipids. Amphipathic molecules in water (or in other polar solvents) - based on the similar solves similar principle - are turned towards each other by their apolar (hydrophobic) hydrocarbon chains forming a micelle.
Polar, apolar
Molecules or parts of molecules with unequal charge distribution (dipoles) are polar. Polar particles interact readily whit water. A molecule or part of a molecule that does not undergo charge separation is apolar. Therefore, it does not enter into significant interaction with water.
cell cortex
A three-dimensional protein network on the intracellular side of the cell membrane. It is composed primarily of actin filaments and actin-binding proteins, but in many cells (e.g. RBC), spectrin dimers are also important components.The net-like structure is anchored to transmembrane proteins by anchoring proteins. They play a role in maintaining plasma membrane integrity, determining cell shape, cell motility and regulating the function of various membrane proteins. Disruption of their structure can lead to pathological conditions (e.g. spherocytosis, elliptocytosis in RBC).
glycocalyx
A viscous coating layer on the extracellular side of the cell membrane composed of carbohydrates (glycoproteins, proteoglycans and glycolipids), mainly associated with proteins and less frequently with lipids. It plays a role in cell surface protection, cell adhesion and signal transduction. The composition of the glycocalyx helps immune cells to differentiate between normal, self and abnormal or non-self cells. It may help in the cellular entry of various pathogens. Increased levels can play a role in the development of resistance to anticancer therapies.
asymmetry of the lipid bilayer
The lipid composition of the outer and inner layers of the cell membrane is different, which is a consequence of active translocation by flippase and floppase enzymes. The different geometrical structure of the major lipid componenets in the two layers contributes to the spontaneous curvature of membranes, which is necessary for the shaping of cells and vesicles. Phosphatidylserine is found only in the inner layer of the plasma membrane in intact cells, but in apoptotic cells it also appears in the outer layer and acts as an ‘eat-me’ signal resulting in the clearance of the dying cell by phagocytes.
scramblase, flippase, floppase
Enzyme proteins involved in the vertical asymmetry of lipid bilayers that facilitate the otherwise extremely slow spontaneous translocation of phospholipids between the two layers of the membrane. Scramblases facilitate the random shuffling of lipids between the two layers without energy investment, and are non-specific. Flippases translocate lipids inwards, floppases translocate lipids outwards, they are active transporters (hydrolyse ATP to function) and are specific for their substrate.
lipid rafts
Lipid rafts are 10-200 nm diameter, cholesterol-, glyco- and sphingolipid-rich domains in the cell membrane, held together mostly by lipid-lipid and protein-lipid interactions. They are characterized by lower fluidity, increased thickness, rigidity and packing density compared to other regions of the membrane. They are able to assemble or segregate various proteins, dynamically regulating the quality and quantity of molecules interacting in them, and thus the efficiency of signal transduction processes. They play a role in membrane trafficking, cell adhesion and migration, synaptic transmission, cytoskeletal organisation and pathogen entry into the cell.
signalosome
Supramolecular protein complex of various signaling elements, whose association and activities are primarily regulated by protein-protein interactions. The composition and assembly of the signalosome is dynamically changes in space and time and thereby ensures the specificity and speed of signal transduction. High concentration of membrane proteins provided by lipid platforms as well as scaffold proteins can serve to enhance the interaction with downstream signaling molecules. Synapses and immune synapses are prominent examples.
endoplasmic reticulum (ER)
An intracellular membrane system characteristic of eukaryotic cells located in the cytoplasm near the nucleus. The surface of the rough ER (RER) binds ribosomes, which synthesize proteins that need to be in the membrane or in the interior (=lumen) of the RER, where their final structure is obtained, and are further modified. Then they are packaged into vesicles and transported to the Golgi apparatus. The smooth ER (SER) lacks ribosomes, it is important for metabolic reactions, including the synthesis of phospholipids and fatty acids, and various detoxification processes.
Golgi complex
An organelle usually located near the nucleus, which is composed of flattened membrane disks, the cisternae, stacked upon each other. It has a polarized organization with various compartments characterized by different enzymes: the cis-Golgi network, the cis-, medial- and trans-cisternae, and the trans-Golgi network. Its function is the posttranslational modification of proteins coming from the endoplasmic reticulum (e.g. N-glycosylation, O- glycosylation, hydroxylation, sulfation), sorting and shipping of proteins to their intended destinations (e.g. to the lysosomes, to the cell membrane, to the outside of the cell by constitutive secretion, or regulated secretion).
lysosome
A membrane-bound organelle with a variable appearance. It is the most important place where degradation of materials inside the cells takes place. The material to be degraded can be of intracellular and extracellular origin. Lysosomes contain acid hydrolases which can only work at an acidic pH generated by H+-ATPases located in their membrane. A primary lysosome contains only the enzymes, but not the materials to be digested. Secondary lysosomes contain both the acid hydrolases and the materials to be degraded.
peroxisome
A membrane-bound organelle, which may have an endosymbiotic origin and it is present in large amount in liver and kidney cells. The enzymes of peroxisomes (e.g. peroxidases) are used in oxidative decomposition processes (mainly fatty acids, alcohol and amino acids). Catalase, another characteristic peroxisomal enzyme, neutralizes hydrogen-peroxide generated as a by- product in peroxisomal oxidation by converting it to H2O.
storage disease
Diseases characterized by lysosomal dysfunction arising as a result of an inherited mutation of one of the lysosomal enzymes. The lack of enzyme activity results in accumulation of undigested substrates in the lysosomes. E.g: Tay – Sachs disease, Fabry disease, I-cell disease.
