Membrane Structure and Function PDF

Lecture 8 CH 7 10/3/24 Membrane Structure and Function Life at the Edge - The plasma membrane is the boundary that separates the living cell from its surroundings - The plasma membrane exhibits selective permeability, allowing some substances to...

Lecture 8 CH 7 10/3/24 Membrane Structure and Function Life at the Edge - The plasma membrane is the boundary that separates the living cell from its surroundings - The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Cellular membranes are fluid mosaics of lipids and proteins - Phospholipids are the most abundant lipid in the plasma membrane - Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions - A phospholipid bilayer can exist as a stable boundary between two aqueous compartments - The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it - Proteins are not randomly distributed in the membrane Membrane Proteins and Their Functions - A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer - Proteins determine most of the membrane’s specific functions Peripheral proteins are bound to the surface of the membrane - Integral proteins penetrate the hydrophobic core - Integral proteins that span the membrane are called transmembrane proteins - Six major functions of membrane proteins - Transport - Enzymatic activity - Signal transduction - Cell-cell recognition - Intercellular joining - Attachment to the cytoskeleton and extracellular matrix (ECM) - HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell - HIV cannot enter the cells of resistant individuals that lack CCR5 The Role of Membrane Carbohydrates in Cell-Cell Recognition  - Cells recognize each other by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane - Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Membrane structure results in selective permeability - A cell must exchange materials with its surroundings, a process controlled by the plasma membrane - Plasma membranes are selectively permeable, regulation the cell’s molecular traffic The Permeability of the Lipid Bilayer - Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly - Hydrophilic molecules including ions and polar molecules do not cross the membrane easily Transport Proteins - Transport proteins allow passage of hydrophilic substances across the membrane - Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel - Channel proteins called aquaporins facilitate the passage of water - Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane - A transport protein is specific for the substance it moves Passive transport is diffusion of a substance across a membrane with no energy investment - Diffusion is the tendency for molecules to spread out evenly into the available space - Although each molecule moves randomly, diffusion of a population of molecules may be directional - At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other - Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases - No work must be done to move substances down the concentration gradient - The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen Effects of Osmosis on Water Balance - Osmosis is the diffusion of water across a selectively permeable membrane  - Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides Water Balance of Cells Without Cell Walls Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water Isotonic solution: solute concentration is the same as that inside the cell: no net water movement across the plasma membrane Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: solute concentration is less than that inside the cell; cell gains water - Hypertonic or hypotonic environments create problems for organisms - Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments Water Balance of Cells with Cell Walls - Cell walls help maintain water balance - A plant cell in a hypotonic solution swells until the wall opposes uptake; the is now turgid (frim) - If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp) - In a hypertonic environment, plant cells lose water - The membrane pulls away from the cell wall causing the plant to wilt, a usually lethal effect called plasmolysis Facilitated Diffusion: Passive Transport Aided by proteins - In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane - Transport proteins include channel proteins and carrier protein - Channel proteins provide corridors that a specific molecule or ion cross the membrane - Aquaporins facilitate the diffusion of water - Ion channels facilitate the diffusion of ions - Some ion channels, called gated channels, open or close in response to a stimulus - Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane The Need for Energy in Active Transport - Active transport moves substances against their concentration gradients - Active transport requires energy, usually in the form of ATP - Active transport is performed by specific proteins embedded in the membranes  - Active transport allows cells to maintain concentration gradients that differ from their surroundings - The sodium-potassium pump is one type of active transport system Active Transport Membrane Transport Cotransport: Coupled Transport by a Membrane Protein - Cotransport occurs when active transport of a solute indirectly drives transport of other substances - Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell - Bulk transport across the plasma membrane occurs by exocytosis and endocytosis - Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins - Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles - Bulk transport requires energy Exocytosis - In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell - Many secretory cells use exocytosis to export their products Endocytosis - In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane - Endocytosis is a reversal of exocytosis, involving different proteins - There are three types of endocytosis - Phagocytosis (“cellular eating”) - Pinocytosis (“cellular drinking”) - receptor -mediated endocytosis”) - receptor -mediated endocytosis - In phagocytosis a cell engulfs a particle in a vacuole - The vacuole fuses with a lysosome to digest the particle - In pinocytosis, molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles - In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation - A ligand is any molecule that binds specifically to a receptor site of another molecule