Cell membrane- transport

• Cell membranes are approximately 7nm thick and are composed of 75% phospholipids, 25% transmembrane and peripheral proteins, cholesterol, and polysaccharides.
• The fluid mosaic model describes the cell membrane as a fluid/elastic phospholipid bilayer with proteins floating in it and small amounts of carbohydrates giving a mosaic appearance.
• Phospholipids are the fundamental building blocks of all cell membranes, amphipathic molecules consisting of two hydrophobic fatty acid chains linked to a phosphate-containing hydrophilic head group.
• Phospholipids spontaneously form bilayers in aqueous solutions due to the poor solubility of their fatty acid tails in water.
• Phospholipid bilayers form a stable barrier between two aqueous compartments and represent the basic structure of all biological membranes.
• Phospholipids are amphiphilic and possess both hydrophilic (a polar head) and hydrophobic moieties (a nonpolar tail).
• A polar head of a phospholipid is constantly in touch with the aqueous environment.
• Lipids form aggregates in the aqueous environment, including monolayers, micelles, and bilayers having both hydrophilic and hydrophobic parts.
• Micelles form when phospholipids have just one fatty acid chain (conical) and bilayers form when phospholipids have two fatty acid chains (cylindrical).
• Spherical bilayers form when phospholipids at the extreme ends curve round to protect themselves.
• The "eat me signal" is represented as ".".
• Cholesterol has several functions in cell membranes: it decreases permeability, prevents membrane crystallisation, and mediates membrane fluidity.
• These spherical bilayers form because of the amphiphilic properties of phospholipids.
• Cylindrical phospholipids with two fatty acids form bilayers.
• Conical phospholipids with one fatty acid form micelles.
• Mammalian membrane lipids include four major types: phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin.
• Glycolipids are another type of mammalian membrane lipid.
• Cholesterol makes up 50 to 60% of the total membrane lipid and 40% of the total membrane lipid.
• Cell membrane fluidity (CMF) is a parameter describing the freedom of movement of protein and lipid constituents within the cell membrane.
• Lipid bilayers behave as two-dimensional fluids in which individual molecules, both lipids and proteins, are free to rotate and move in lateral directions.
• Factors affecting membrane fluidity include lipid composition, temperature, and cholesterol content.
• Membrane fluidity provides a perfect balance: the cell can move, substances can move freely across the cell membrane, but it's not too fluid otherwise, there will be no control of what substances entering or leaving the cell.
• The phospholipid bilayer gives fluidity and elasticity to the membrane but there are some restrictions to this provided by binding of membrane components to the extracellular matrix and protein-cytoskeleton associations.
• Cell membranes have many functions: they keep toxins out of the cell, control movement of water and macromolecules, provide structural support and protection, provide a site for catalysis, establish chemical and electrical gradients, and they are involved in cell-cell recognition and cell signalling.
• The plasma membrane of organelles separates incompatible biochemical processes.
• The phospholipid bilayer (PLB) is fluid with a consistency similar to salad dressing.
• Transverse diffusion between monolayers, also known as 'flip-flopping', can take several hours.
• Lipids laterally diffuse within their monolayer at a rate of 10^7 times per second.
• Cholesterol flip-flops rapidly.
• Several factors affect membrane fluidity: lipid composition, temperature, and cholesterol concentration.
• Lipid composition affects membrane fluidity: long fatty acid chains reduce fluidity, while double (cis) bonds in unsaturated fatty acids increase fluidity.
• Organisms adjust lipid structure in relation to temperature, for example, shorter chains in cold climates to facilitate release of energy (heat).
• Membrane asymmetry refers to the fact that the two sides of the membrane are structurally and functionally different.
• Lipid asymmetry provides the two sides of the plasma membrane with different biophysical properties and influences numerous cellular functions.
• Proteins are responsible for carrying out specific membrane functions, with one protein molecule per every 50 to 100 molecules of lipid.
• Membrane behaviour: asymmetry involves transverse asymmetry, where the inner and outer monolayers differ, and lateral asymmetry, where transient micro/nanodomains exist within monolayers.
• The asymmetry in cell membranes is maintained because "flip-flop" movement is rare.
• Sphingolipids are not exchanged between monolayers and remain in the monolayer where they were synthesized.
• Membrane (lateral) asymmetry involves lipid rafts, which despite free lateral movement of lipids, cell membranes are not homogeneous.
• Aggregations of specific molecules form transient lipid rafts characterised by highly ordered and tightly packed lipid molecules, with cholesterol acting as a glue in it.