lipids and membranes

• Phospholipid bilayers have two layers with their hydrophilic head groups facing outside and inside, forming an impermeable barrier to water-soluble molecules.
• The hydrophobic tails are oriented toward the interior, while the polar heads face outward.
• The phosphate group is negatively charged at physiological pH, attracting positively charged cations such as Na+ or K+.
• Proteins are also present in cell membranes, providing specific functions such as transport across the membrane.
• amino acids with aliphatic side chains : the longer it is the more hydrophobic it is
• trans membrane proteins : the hydrophobic amino acids will be inside the membrane, hydrophilic ones will be outside
• alpha helices are hydrophobic and inside the membrane
• polar lipids are right after alpha helices to stabilize them to the membrane : they interact with the polar heads
• ways of association membrane/non trans membrane protein : lipid-binding domain, lipid modification
• lipid modification : palmitoylation, farmesyl, GPI enchor
• palmitoylation : covalent attachment of fatty acid to cysteine residue on cytoplasmic tail of non transmembrane protein
• farnesylation : addition of farnesyl group (C15) to C terminus of non transmembrane protein
• GPI anchor : glycosyl phosphatidylinositol - attaches to carboxyl terminal end of non transmembrane protein
• glycolipids have two functions : structural components of plasma membrane & recognition molecules
• Transient association protein : electrostatic, aliphatic helix
• amphipathic helix : soluble until associated with a sterol (?)
• properties of cellular membrane : asymmetric composition of the two layers, existence of electric potential across the membrane, presence of micro-domains and different phases
• around 60% of the membrane's surface are proteins
• lateral diffusion in membrane : fast and in liquid-disordered
• transverse diffusion in membrane : slow and in liquid-ordered or solid gel
• membrane fluidity is determined by temperature, fatty acid chain length, degree of saturation, cholesterol content
• fluidity increases as temperature increases
• melting temperature of a fatty acid decreases with the number of carbons
• melting temperature of a fatty acid decreases as the number of insaturations increases
• cholesterol makes the membrane more fluid and flexible, and helps to keep the membrane rigid
• cholesterol is synthesized in the endoplasmic reticulum but it's found mostly in the membrane
• lipid rafts are small domains within the plasma membrane that contain high concentrations of sphingomyelin and cholesterol
• lipid rafts have important roles in signal transduction pathways
• the atomic scale is in the angstrom range, where the size of an atom is 1 x 10^-10 m
• the molecular scale is in the nano-metre range, where the size of a molecule is 2 x 10^-9 m
• the macro molecular scale is 10 to 100 nm
• the organelle scale is 1 µm
• the cellular scale is 10 µm
• lysosome : a membrane-bound organelle that contains digestive enzymes
• all eukaryote cells contain the same organelles (except chloroplasts) in different quantities and efficiency depending on the cell's function
• le cytosquelette est composé de trois grandes familles de protéines : filaments intermédiaires, microtubules et actines
• microtubules : composant principal du fuseau mitotique (ségréation des chromosomes)
• actine : formation d'un réseau sous les plasmalemmes, les filopodes, qui permet le mouvement des vésicules vers l'intérieur ou l'extérieur de la cellule
• actine : les microvilli sont des prolongements de la membrane plasmatique qui augmentent sa surface pour une meilleure absorption
• actine : les lamellipodes propulsent le kératocyte (?)