Membrane Transport Lecture 3 PDF

Objectives Understand the structure of the cell membrane Understand concept of fluid mosaic model Understand the factors which influence the transport of a substance across the cell membrane Describe the different mechanisms by which substances cross the cell membrane Cell membrane...

Objectives Understand the structure of the cell membrane Understand concept of fluid mosaic model Understand the factors which influence the transport of a substance across the cell membrane Describe the different mechanisms by which substances cross the cell membrane Cell membrane Cells are surrounded by a membrane – Separates cell contents from extracellular environment – Regulate passage of substances in/out of cells and between cell organelles and cytosol – Detect chemical messengers arriving at cell surface – Link adjacent cells together by membrane junctions – Anchor cells to extracellular matrix Cell membrane AKA plasmalemma, plasma membrane, or cytoplasmic membrane Plasma membranes 6 - 10 nm thick – Requires electron microscope Electron micrograph of a human red blood cell plasma membrane. Cell membrane videos - Watch both https://www.youtube.com/watch?v=CNbZDcibegY https://www.youtube.com/watch?v=moPJkCbKjBs Phospholipid bilayer Phospholipid contains phosphorylated glycerol head with tail of two fatty acids Hydrophilic head, attracted to aqueous environment of cytoplasm and ECF Hydrophobic tails, repelled by the aqueous environment of cytoplasm and ECF, attracted to other lipid tail Proteins in the cell membrane Integral membrane proteins are embedded in the cell membrane – Anchored to lipid bilayer by hydrophobic interactions – Some span entire membrane and are in contact with ECF and ICF Transmembrane proteins – e.g. ligand-binding receptors, transport proteins etc. Peripheral membrane proteins – Loosely associated with membrane by hydrostatic interactions Cell membrane acts as a barrier Selectively permeable membrane Separates ICF from ECF – ECF and ICF have different compositions – There is a need to exchange substances between ECF and ICF Delivery of O2, nutrients Removal of waste Selective ion movement Release of hormones, neurotransmitters etc. Nonpolar (lipid soluble) and small polar molecules can cross by diffusion – O2, CO2, water, lipids Large polar molecules (non-lipid soluble) cannot cross without transport proteins – Glucose, ions, proteins Factors affecting the rate of diffusion Steepness of the concentration gradient – The bigger the difference between the two sides of the membrane the quicker the rate of diffusion. Temperature – Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster. Surface area – The greater the surface area the faster the diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment. Type of molecule or ion diffusing – Large molecules require more energy to move across so tend to diffuse more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar phospholipid tails. How do substances cross the membrane? Mechanisms are either passive or active – Passive mechanisms do not require energy – Active mechanisms require energy Vesicular transport Passive transport Downhill – down a concentration gradient a) Simple diffusion – Lipid soluble substances diffuse through the membrane – Ions diffuse through ion channels – Rate of diffusion depends on permeability of membrane to the substance and the concentration gradient b) Facilitated diffusion – Most solutes are too large or too polar to cross at reasonable rates by simple diffusion – Substance is carried by a carrier protein e.g. GLUT4 glucose transporter Passive transport A) Diffusion through membrane B) Ion channels C) Facilitated diffusion Diffusion Molecules have kinetic energy, which makes them move about randomly Diffusion Ion channels Composed of protein subunits Specific for particular ions Open or closed states – a) Voltage gated – b) Ligand gated – c) Mechanosensitive Respond to mechanical deformation of the membrane (numerous examples of receptors such as heat sensation, taste, smell, touch, and osmotic and volume regulation) Ion channels - diffusion Facilitated diffusion Facilitated diffusion Large polar molecules such as glucose and amino acids, cannot diffuse across the phospholipid bilayer. Also ions such as Na+ or Cl- cannot pass. Downhill transport requiring a membrane transporter – e.g. GLUT4 glucose transporter Still a passive process Active transport Uphill – against a concentration gradient - hence energy req’d Uses a transport protein Primary active transport – Direct use of ATP Secondary active transport – Couples uphill transport of one substance to downhill transport of another Primary active transport - Direct ATP use Primary active transport Sodium-potassium pump – Na+/K+-ATPase – ATP hydrolysed to ADP Transport powered by energy released from breaking the 3rd phosphate bond – Moves both sodium and potassium against their concentration gradients – 3 Na+ out: 2 K+ in Other examples – Ca2+-ATPase – H+-ATPase Secondary active transport Secondary active transport Uphill transport of one solute is coupled to the downhill transport of sodium – i.e. harnesses the sodium gradient established by Na/K-ATPase. >Na+ outside; < Na+ inside – Na+ binding causes allosteric alteration – Does not directly use ATP e.g. SGLT1 sodium glucose co- transporter in gut and kidney Can be co-transport (symport) or counter-transport (antiport) Glucose reabsorption in kidney - SGLT1 Na+-dependent secondary active transport mechanism Na+ actively pumped out of the cell into the blood Draws Na+ into the cell Glucose is co-transported with Na+ GLUT2 Glucose leaves the cell into the blood by facilitated diffusion (GLUT2) Membrane Transport Proteins Fig. 4-15 28 Endocytosis & Exocytosis Fig. 4-20 29  Endocytosis Movement of molecules into the cell via vesicles. There are three general types of endocytosis that may occur in a cell: 1. Fluid endocytosis (pinocytosis) 2. Phagocytosis 3. Receptor-mediated endocytosis 30 Forms of Endocytosis Fig. 4-21 31 Exocytosis Movement of molecules out of the cell via vesicles. Exocytosis performs several functions for cells: 1. Provides a way to replace portions of the plasma membrane that endocytosis has removed 2. Adds new membrane components to the membrane 3. Provides a route by which membrane-impermeable molecules (such as protein hormones) the cell synthesizes can be secreted into the extracellular fluid 32 Objectives Understand the structure of the cell membrane Understand concept of fluid mosaic model Understand the factors which influence the transport of a substance across the cell membrane Describe the different mechanisms by which substances cross the cell membrane