molecular_biology_of_the_cell_2_complete_summary.2022.pdf

Lipid bilayers contain: 1) Phospholipids a. Phosphoglycerides b. Sphingolipids c. Inositol phospholipids 2) Glycolipids 3) Cholesterol Phospholipids are the most abundant membrane lipids The main phospholipids in eukaryotes are phosphoglycerides (3C-backbone and two long- chain fatty acid chains) Many different phosphoglycerides can be made by combining several different fatty acids and head groups. Most abundant ones are phosphatidylethanolamine, phosphatidylserine and phosphatidylcholine in mammalian cell membranes. Another class of phospholipids are sphingolipids (sphingosine i.o. glycerol. NH2 and two hydroxyl groups at one end). Sphingomyelin is the most abundant in mammals. A minor class of phospholipids are inositol phospholipids. In the cytosolic leaflet of the plasma membrane they play an important part in cell signalling: in response to extracellular signals, specific lipid kinases phosphorylate the head groups of these lipids to form docking sites for cytosolic signalling proteins. Cholesterol is a sterol. It contains a rigid ring structure, to which a single polar hydroxyl group and a short nonpolar hydrocarbon chain are attached. Glycolipids resemble sphingolipids, but i.o. a phosphate-linked head group they have sugars attached. Bilayers are spontaneously formed by phospholipids since the hydrophobic tails point inwards. Another possibility is that spherical micelles are formed Liposomes: Spherical lipid bilayer vesicles Since individual phospholipids are confined to their own monolayer, newly made phospholipids from the cytosolic monolayer of the ER membrane would not be able to migrate. This is solved by phospholipid translocators, or flippases, which catalyse the flip- flop of phospholipids from one monolayer to the other Lipid rafts: Small region of a membrane enriched in sphingolipids and cholesterol The lipid bilayer is asymmetrically charged. This is exploited to distinguish between live and dead cells, as ell as converting extracellular signals into intracellular ones. Glycolipids have the most extreme asymmetry, as they are only found in the monolayer facing away from the cytosol. Gangliosides are the most complex of the glycolipids. Their presence alters the electrical field across the membrane and the concentrations of ions at the membrane surface. Besides, they function in cell-recognition processes. Lumen: The space inside a hollow structure Carbohydrate layer: The carbohydrate-rich zone on the eukaryotic cell surface attributable to glycoproteins, glycolipids, and proteoglycans of the plasma membrane. Lectin: Protein that binds tightly to a specific sugar. Abundant lectins from plant seeds are used as affinity reagents to purify glycoproteins or to detect them on the surface of cells. By its affinity for lectins, the carbohydrate layer can be visualized. Chapter 11: Transporters bind specific solutes and transfer them across the lipid bilayer by undergoing conformational changes that alternately expose the solute-binding site on one side of the membrane and then on the other. Some transporters move a single solute “downhill,” whereas others can act as pumps to move a solute “uphill” against its electrochemical gradient, using energy provided by ATP hydrolysis, by a downhill flow of another solute such as Na+ or H+, or by light to drive the requisite series of conformational changes in an orderly manner. Transporters belong to a small number of protein families. Each family evolved from a common ancestral protein, and its members all operate by a similar mechanism. The family of P-type transport ATPases is an important example; each of these ATPases sequentially phosphorylates and dephosphorylates itself during the pumping cycle. The superfamily of ABC transporters is the largest family of membrane transport proteins and is especially important clinically. It includes proteins that are responsible for cystic fibrosis, for drug resistance in both cancer cells and malaria-causing parasites, and for pumping pathogen- derived peptides into the ER for cytotoxic lymphocytes to reorganize on the surface of infected cells. Passive transport  down the concentration/electrical gradient - channels Active transport  against the concentration/electrical gradient - transporters The concentration and electrical gradient together are called the electrochemical gradient Active transport is carried out in three main ways: 1) Coupled transporters, harness the energy stored in concentration gradients to couple the uphill transport of one solute across the membrane to the downhill transport of another 2 2) ATP-driven pumps, couple uphill transport to the hydrolysis of ATP 3) Light- or redox-driven pumps, couple uphill transport to an input of energy from light or from a redox reaction Uniporters: Passively mediate the movement of a single solute from one side of the membrane to the other at a rate determined by their Vmax and Km (aka one solute is being transported) Symporters: Simultaneously transfer a second solute in the same direction (aka one solute is lifting) Antiporters: Transfer a second solute in the opposite direction (aka