Membranes and Membrane Transport

• Water composition of human body


  55–65% water, with water molecules being polar and capable of forming hydrogen bonds
• Hydrophilic molecules


  Easily form hydrogen bonds with water, dissolve in water
• Hydrophobic molecules


  Unable to form hydrogen bonds, interact differently
• Cell and organelle membranes


  • Composed of lipids, proteins, and small amounts of carbohydrates
• Lipids


  Amphipathic molecules with both hydrophobic and hydrophilic regions
• Phospholipids


  Most abundant lipids in membranes, consist of a glycerol backbone, a phosphate group, and two fatty acid chains
• Phospholipid organization in aqueous environment


  Spontaneously organize into a lipid bilayer, with hydrophobic tails pointing inward and hydrophilic heads outward
• Lipid bilayer


  • Cholesterol molecules present, contributing to its structure
• Lipid bilayer


  Acts as a barrier, regulating the movement of substances between the internal and external environment of cells
• Hydrophobic interior of bilayer


  Allows lipid-soluble molecules like steroids to pass through easily, while ions and large polar molecules are mostly impermeable
• Simple diffusion


  Passive process, allows molecules to move down their concentration gradient across the membrane
• Non-polar molecules


  Like oxygen and carbon dioxide can easily diffuse through the lipid bilayer
• Membrane proteins


  Crucial components of biological membranes, varying in location, structure, and function
• Types of membrane proteins


  • Integral proteins
  • Peripheral proteins
• Integral proteins


  Amphipathic, with hydrophobic regions interacting with the bilayer and hydrophilic regions facing the aqueous environment
• Peripheral proteins


  Hydrophilic, interact with integral proteins and phospholipid heads
• Functions of membrane proteins


  • Transport
  • Recognition
  • Receptor signaling
  • Enzymatic activity
  • Cell adhesion
  • Motility
• Osmosis


  Diffusion of water across a selectively permeable membrane, essential for maintaining cell volume, involves the movement of water from lower to higher solute concentrations
• Aquaporins


  Integral proteins facilitating the rapid movement of water across cell membranes through specific channels lined with hydrophilic side chains
• Diffusion


  Movement of solutes down their concentration gradient, assisted by proteins in facilitated diffusion
• Types of transport proteins


  • Channel proteins
  • Carrier proteins
• Channel proteins


  Form pores for the passage of polar molecules
• Carrier proteins


  Undergo conformational changes to transfer molecules across the membrane
• Ion channels


  Highly selective due to specific binding sites and pore size, can be gated, opening or closing in response to stimuli
• Carrier proteins


  Specific for certain solutes, such as the GLUT transporter for glucose
• Active transport


  Requires energy to move molecules against their concentration gradient, often mediated by pump proteins
• Types of active transport


  • Direct active transport
  • Indirect active transport
• Direct active transport


  Uses energy directly from ATP
• Indirect active transport


  Movement of one solute drives the movement of another against its gradient
• Membrane permeability


  Depends on the size and hydrophilic/hydrophobic nature of molecules, with facilitated diffusion and active transport providing selectivity through specific transport proteins
• Membrane carbohydrates


  • Form glycolipids or glycoproteins
• Glycolipids


  Amphipathic molecules with polar carbohydrate groups extending extracellularly and non-polar lipid components embedded in the bilayer
• Types of glycolipids


  • Glycoglycerolipids
  • Glycosphingolipids
• Glycolipids


  Contribute to membrane stability and cell recognition by forming hydrogen bonds with water molecules
• Glycoproteins


  Result from the bonding of oligosaccharides to proteins, with carbohydrate groups often protruding extracellularly
• Glycoproteins


  Play roles in cell recognition, adhesion, and signaling, acting as markers for immune recognition and facilitating cell-cell adhesion and signaling
• Glycocalyx


  Formed by carbohydrate groups of glycolipids and glycoproteins, aids in cell protection and communication
• Fluid mosaic model


  Describes the fluidity of the lipid bilayer and the arrangement of proteins as embedded within it, providing a framework for understanding membrane structure and function