LipoproteinA lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, surrounded by a phospholipid outer shell, with the hydrophilic portions oriented outward toward the surrounding water and lipophilic portions oriented inward toward the lipid center. A special kind of protein, called apolipoprotein, is embedded in the outer shell, both stabilising the complex and giving it a functional identity that determines its role.
Cellular compartmentCellular compartments in cell biology comprise all of the closed parts within the cytosol of a eukaryotic cell, usually surrounded by a single or double lipid layer membrane. These compartments are often, but not always, defined as membrane-bound organelles. The formation of cellular compartments is called compartmentalization. Both organelles, the mitochondria and chloroplasts (in photosynthetic organisms), are compartments that are believed to be of endosymbiotic origin.
Endoplasmic reticulumThe endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae (in the RER), and tubular structures in the SER.
GlycosylationGlycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not always in chemistry), glycosylation usually refers to an enzyme-catalysed reaction, whereas glycation (also 'non-enzymatic glycation' and 'non-enzymatic glycosylation') may refer to a non-enzymatic reaction. Glycosylation is a form of co-translational and post-translational modification.
Endomembrane systemThe endomembrane system is composed of the different membranes (endomembranes) that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane among others.
Low-density lipoproteinLow-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall density naming convention), very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL delivers fat molecules to cells. LDL is involved in atherosclerosis, a process in which it is oxidized within the walls of arteries.
O-linked glycosylationO-linked glycosylation is the attachment of a sugar molecule to the oxygen atom of serine (Ser) or threonine (Thr) residues in a protein. O-glycosylation is a post-translational modification that occurs after the protein has been synthesised. In eukaryotes, it occurs in the endoplasmic reticulum, Golgi apparatus and occasionally in the cytoplasm; in prokaryotes, it occurs in the cytoplasm. Several different sugars can be added to the serine or threonine, and they affect the protein in different ways by changing protein stability and regulating protein activity.
N-linked glycosylationN-linked glycosylation, is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom (the amide nitrogen of an asparagine (Asn) residue of a protein), in a process called N-glycosylation, studied in biochemistry. The resulting protein is called an N-linked glycan, or simply an N-glycan. This type of linkage is important for both the structure and function of many eukaryotic proteins.
High-density lipoproteinHigh-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80–100 proteins per particle (organized by one, two or three ApoA). HDL particles enlarge while circulating in the blood, aggregating more fat molecules and transporting up to hundreds of fat molecules per particle.
Cell membraneThe cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extracellular space). The cell membrane consists of a lipid bilayer, made up of two layers of phospholipids with cholesterols (a lipid component) interspersed between them, maintaining appropriate membrane fluidity at various temperatures.
