BIOL 1P91 - Chapter 5 STUDENT 2024 Membrane Structure, Synthesis, and Transport PDF

Membrane Structure, Synthesis, and Transport Chapter 5 1  Chapter 5 Outline Membrane Structure Fluidity of Membranes Synthesis of Membrane Components in Eukaryotic Cells Overview of Membrane Transport Proteins that Carry Out M...

Membrane Structure, Synthesis, and Transport Chapter 5 1  Chapter 5 Outline Membrane Structure Fluidity of Membranes Synthesis of Membrane Components in Eukaryotic Cells Overview of Membrane Transport Proteins that Carry Out Membrane Transport Exocytosis and Endocytosis 2  Introduction “The cell membrane holds the contents of a cell in one place so that the chemistry of life can occur.” Plasma membrane: Biomembrane that separates the internal contents of a cell from its external environment Regulates the traffic of substances into and out of the cell Provides an interface to carry out many vital cellular activities 3  Membrane structure  Framework is a phospholipid bilayer  = Two layers of phospholipids  Phospholipids are amphipathic  Membranes also contain:  Proteins  Carbohydrates attached to lipids and proteins, forming glycolipids & glycoproteins.  Other lipids, e.g. cholesterol (animal cells) or phytosterols (plant cells) Click 5.1 to edit Master Membrane text structure 4  Fluid-mosaic model  Overall, the plasma membrane is a mosaic of lipid, protein, and carbohydrate  It resembles a fluid because lipids and proteins can move relative to each other within the membrane  Leaflet: Half of a phospholipid bilayer  Each faces a different region  e.g. cytoplasm or cell exterior  Two leaflets are asymmetrical (not the same)  e.g. glycolipids found primarily in the extracellular leaflet 5 Fluid-mosaic model of membrane structure 6  Membrane Proteins  Participate in many important cellular processes  Transport, energy transduction, cell signalling, secretion, cell recognition, and cell-to-cell contact  Are important medically  Approximately 70% of medications exert effects by binding to membrane proteins  Estimated percentage of transmembrane proteins is substantial  20%–30% of all genes 7  Types of Membrane Proteins Transmembrane proteins span from one side of the membrane, through the hydrophobic interior, to the other side of the bilayer. Lipid-Anchored proteins: Covalent attachment of a lipid to the protein anchors it to the membrane by inserting into the bilayer. Peripheral membrane proteins are non-covalently bound to other proteins or lipids on the membrane surface (without interacting with the membrane interior) 8  Membranes are Semifluid  Biomembranes exhibit properties of fluidity, but lipid molecules move freely in only two dimensions, not three  Around their long axes (rotational) and laterally within the membrane leaflet  = semifluid  Movements are energetically favourable because fatty acyl tails stay within hydrophobic interior Click 5.2 to editof Fluidity Master text Membranes 9  Bilayer Fluidity  Optimal level of bilayer fluidity is essential for normal cell function, growth, and division  Membranes are less fluid at low temperatures and more fluid at high temperatures  Organisms can alter lipid composition to ensure optimal fluidity  Fluidity is influenced by:  Length of phospholipid tails  Double bonds in phospholipid tails  Presence of cholesterol 10  Length of phospholipid tails  Lipid tails range from 14 – 24 carbon atoms  16 – 18 carbons most common  Shorter tails interact less with each other, making the membrane more fluid Less fluid More fluid 11  Double bonds in phospholipid tails  Double bonds create kinks in the lipid tails  = unsaturated fatty acids  Reduces interactions between adjacent tails, making bilayer more fluid Less fluid More fluid 12  Membrane Cholesterol  Polar head group aligns with hydrophilic heads of phospholipids  Nonpolar hydrocarbon tail associates with hydrophobic phospholipid tails  Ring structure provides rigid structure that also impairs dense packing of phospholipids 13  Cholesterol & Fluidity  Tends to stabilize membranes  Effect depends on temperature  At higher temperatures, cholesterol makes membrane less fluid  At lower temperatures, cholesterol makes membrane more fluid Low or High Low temperature High temperature temperature + Viscous Liquid disordered membrane sterols Liquid ordered 14  Membranes are Selectively Permeable  If plasma membranes were composed of only phospholipids, transport would be limited  Substantial amounts of proteins create a membrane that is selectively permeable  Membrane structure ensures …  Essential molecules enter  Metabolic intermediates remain  Waste products exit Click 5.4 to edit Master Overview text of Membrane Transport 15  Ways to Move Across Membranes  Passive transport: Does not require an input of energy  Molecules diffuse down a concentration gradient  Simple diffusion: Without the aid of a transport protein  Directly through the phospholipid bilayer  Facilitated diffusion: Diffusion across a membrane with the aid of transport proteins  Active transport moves molecules against a concentration gradient using energy from ATP 16 Movement Across a Biological Membrane 17  Simple Diffusion  Hydrophobic interior is a barrier to movement of ions and hydrophilic molecules  Four factors affect the ability of solutes to pass through a lipid bilayer:  Size  Polarity  Charge  Concentration  Highest permeability observed for gasses and small, uncharged molecules 18 19 Membrane gradients Living cells maintain a relatively constant internal environment that is distinctly different from their external environment 20  Osmosis  Movement of water across membranes in response to solute concentration gradients  Bilayer is relatively impermeable to many solutes, but is somewhat permeable to water  If solute cannot cross membrane to reach equilibrium, water may move to the environment with the higher solute concentration 21  Osmosis  In a hypertonic environment, cells will lose water to the environment and shrink  In animal cells this process is known as crenation  In plants and algae it is called plasmolysis  Hypertonic environments are generally rare and only regularly occur for organisms in salt water  In a hypotonic environment, cells will take up water  Animal cells will swell and may lyse  Cell wall of plant cells prevents major expansion  Generates osmotic pressure that stops the net flow of water 22 Effects of Osmosis 23  Transport Proteins  Transmembrane proteins that provide a pathway for the movement of specific ions and hydrophilic molecules across membranes  Bypass the phospholipid bilayer  Allow biological membranes to be selectively permeable to small molecules and ions  Two classes: Channels and transporters Click 5.5 to edit Master Transport text Protein 24  Channels  Transmembrane proteins that form a passageway for the facilitated diffusion of ions or molecules across the membrane  Most channels are gated  Often in response to a ligand  Can open to allow the diffusion of solutes and close to prohibit it  When channel is open, movement of solutes can be very rapid 25  Transporters Membrane proteins that bind a solute and undergo a conformational change that moves it to the other side of the membrane Slower than channels 26  Categories of Transporters  Uniporters: Bind a single molecule or ion and transport it across the membrane  Symporters or co-transporters: Bind two or more different types of ions or molecules and transport them in the same direction  Antiporters: Bind two or more different types of ions or molecules and transport them in opposite directions 27  Active Transport  Movement of a solute across a membrane against its gradient, from a region of low concentration to higher concentration  Primary active transport: Uses energy directly to transport a solute against a concentration gradient  Transporters that uses energy sources to change conformation are also called pumps  E.g. An ATP-driven pump hydrolyzes ATP to actively transport solutes against a gradient  Secondary active transport: Uses a pre-existing gradient to drive the active transport of another solute  E.g. In the H+/sucrose symporter, hydrogen ions move down their gradient, while sucrose is actively transported against its gradient 28 29  Na+/K+ pump  A single pump exports sodium ions Nerve cell (Na+), and imports potassium ions (K+) against their gradients using ATP Extracellular  = Antiporter environment Na+/K+ -ATPase High [Na+] 3 Na+ Low [K+]  An example of an electrogenic pump ADP + Pi  Generates an electrical gradient ATP 2 K+  Net export of one positive charge Low [Na+] Cytosol High [K+] 30 Mechanism of Pumping 31