CELL BIOLOGY 2 (membrane structure)

• The plasma membrane is a protein-studded, fatty film that separates and protects a cell's chemical components from the outside environment
• Without membranes, there would be no cells and no life
• The plasma membrane consists of a two-ply sheet of lipid molecules about 5 nm thick, into which proteins have been inserted
• The plasma membrane serves as a barrier to prevent the contents of the cell from escaping and mixing with molecules in the surrounding environment
• For a cell to survive and grow, nutrients must pass inward across the plasma membrane, and waste products must make their way out
• The plasma membrane is penetrated by highly selective channels and transporters that allow specific small molecules and ions to be imported and exported
• Proteins in the membrane act as sensors or receptors that enable the cell to receive information about changes in its environment and respond appropriately
• The plasma membrane can enlarge in area by adding new membrane without losing its continuity, and it can deform without tearing, allowing the cell to move or change shape
• All cell membranes, regardless of location, are composed of lipids and proteins and share a common general structure
• Cell membranes are composed of lipids and proteins and share a common general structure
• The lipids in cell membranes combine hydrophilic and hydrophobic properties in a single molecule
• Phospholipids, cholesterol, and glycolipids are amphipathic membrane lipids that drive lipid molecules to assemble into bilayers in an aqueous environment
• Membrane lipids are amphipathic, with a hydrophilic head and one or two hydrophobic tails
• The hydrophilic head in different types of membrane lipids includes serine phosphate, an -OH group, and sugar galactose plus an -OH group
• Phosphatidylcholine is the most common phospholipid in cell membranes
• Phosphatidylcholine is built from five parts: choline linked to a phosphate group, two hydrocarbon chains forming the hydrophobic tails, and a molecule of glycerol linking the head to the tails
• A kink in one of the hydrocarbon chains occurs where there is a double bond between two carbon atoms
• Hydrophilic molecules dissolve readily in water due to charged or uncharged polar groups that can form electrostatic attractions or hydrogen bonds with water molecules
• Hydrophobic molecules are insoluble in water because their atoms are uncharged and nonpolar, forcing adjacent water molecules to reorganize into a cagelike structure around them
• Amphipathic molecules, like membrane lipids, have a hydrophilic head attracted to water and hydrophobic tails that seek to aggregate with other hydrophobic molecules
• The conflict in amphipathic molecules is resolved by the formation of a lipid bilayer, where hydrophilic heads face water on both surfaces and hydrophobic tails are shielded within the bilayer interior
• The lipid bilayer is self-sealing; any tear in the sheet creates a free edge that is energetically unfavorable, leading to spontaneous rearrangement to eliminate the free edge
• The prohibition on free edges leads to the formation of a boundary around a closed space, making amphipathic molecules assemble into self-sealing containers defining closed compartments
• The lipid bilayer is a flexible two-dimensional fluid, able to bend and move about within the plane of the membrane, crucial for membrane function and integrity
• The fluidity of a lipid bilayer is crucial for membrane function and depends on its phospholipid composition
• The fluidity of a cell membrane is determined by the ease with which its lipid molecules move within the plane of the bilayer
• Factors affecting the fluidity of a lipid bilayer include the phospholipid composition, hydrocarbon tail length, and the number of double bonds in the tails
• Shorter chain length in hydrocarbon tails reduces their tendency to interact, increasing the fluidity of the bilayer
• Hydrocarbon tails with double bonds create kinks, making it harder for tails to pack tightly, resulting in increased fluidity
• Unsaturated hydrocarbon tails with double bonds increase the fluidity of lipid bilayers compared to saturated tails
• In bacterial and yeast cells, hydrocarbon tail lengths and saturation levels are adjusted to maintain consistent membrane fluidity in varying temperatures
• Cholesterol in animal cells stiffens the bilayer by filling spaces between phospholipid molecules, reducing flexibility and permeability
• Membrane fluidity is essential for various cellular functions, including protein diffusion, cell signaling, lipid and protein distribution, cell division, and membrane fusion
• New phospholipids in eukaryotic cells are manufactured in the endoplasmic reticulum and distributed evenly between the bilayer monolayers by scramblases
• Flippases in the Golgi apparatus maintain membrane asymmetry by moving specific phospholipids from one side of the bilayer to the other
• In single-celled organisms, substances can easily enter the cell due to a short distance that needs to be crossed
• In multicellular organisms, the distance for substances to enter the cell is much larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen due to their higher surface area to volume ratio
• Membrane proteins can associate with the lipid bilayer in different ways:

  • Some proteins are located almost entirely in the cytosol and are associated with the cytosolic half of the lipid bilayer by an amphipathic α helix
  • Other proteins are linked to either side of the bilayer solely by a covalently attached lipid molecule
  • Some proteins are bound indirectly to one face of the membrane, held in place only by their interactions with other membrane proteins
• Proteins directly attached to the lipid bilayer, whether transmembrane, associated with the lipid monolayer, or lipid-linked, can only be removed by disrupting the bilayer with detergents