1306FA23_MembraneTransport.pdf

Learning Goals 1. Differentiate between and Describe the following terms: • diffusion, osmosis, facilitated diffusion, passive transport, active transport, secondary active transport. 2. Describe the different types of proteins responsible for solute transport and how they work 3. Predict the permea...

Learning Goals 1. Differentiate between and Describe the following terms: • diffusion, osmosis, facilitated diffusion, passive transport, active transport, secondary active transport. 2. Describe the different types of proteins responsible for solute transport and how they work 3. Predict the permeability/transport requirements of a molecules across a membrane 4. Identify solutions as hypertonic, hypotonic, or isotonic and predict the behavior of cells placed in each type of solution 5. Describe the mechanisms of bulk transport 6. Describe the attachments adjacent cells have that allows transport of molecules between cells Membrane Transport BIOL 1306: Biology 1 for Majors Jenifer Gifford, Ph.D. [email protected] 1 2 The Nature of Science The Plasma Membrane of a Cell is FLUID Previously, we discussed the basics of the plasma membrane Phospholipids Fibers of extracellular matrix (ECM) 1. Boundary between the inside and the outside of the cell primarily composed of phospholipids and proteins. 2. Modeled as a Fluid Mosaic of lipids and proteins (and some carbohydrates) 3. Selectively Permeable and controls transport of molecules between the extracellular and intracellular environment • Small, uncharged, nonpolar molecules are capable of moving across the membrane unaided. • As size, polarity, and charge increases, molecules need additional assistance via transport proteins to traverse the plasma membrane. 3 Water Molecules Protein Channel Water Molecules Cell Membranes are not static! They are not a solid shell that surrounds the cells. Think of the overall consistency as being salad oil at room temperature 4 4 1 Significance of Membrane Permeability in Cellular Evolution Components of the Plasma Membrane Facilitate Transport ro l 5 s te 2. Peripheral Proteins are bound to the surface of the membrane o le 1. Integral Proteins (aka transmembrane proteins)penetrate the hydrophobic core of the membrane. Integral Protein Ch 1. Fibers Membranes are of primarily made up of extracellular matrix which phospholipids, (ECM) are amphipathic molecules Biological membranes allow for cell’s internal environment to differ from it’s outside environment due to • Selective permeability of lipid bilayers • Specificity of proteins • Passive and active transport Example: Neurons Membranes Phospholipids Peripheral Protein 6 How substances (NOT WATER) move (PASSIVELY) across the membrane • Dissolved solutes are in constant random motion due to their thermal energy • Diffusion—Spontaneous movement of molecules and ions: • Concentration gradient created by difference in solute concentrations • Net movement from high-concentration regions to lowconcentration regions 7 8 2 Osmosis refers to the movement of WATER across a membrane How substances move (passively) across the membrane • Passive transport occurs when substances diffuse across membrane in absence of an outside energy source • Water moves quickly across lipid bilayers: • Special case of diffusion called osmosis • Only occurs across selectively permeable membranes • Equilibrium occurs when molecules or ions are randomly distributed throughout solution: • Molecules are still moving randomly • But there is no more net movement • Water moves from regions of low solute concentration to regions of high solute concentration: • Dilutes higher concentration of solute • Equalizes SOLUTE concentration on both sides of bilayer 9 WATER FOLLOWS SOLUTES 10 Describing Tonicity of Solutions Tonicity and Water Movement Isotonic Solution Concentration of solution outside cell may differ from the concentration inside cell. • Hypertonic Solutions have a higher concentration of solute compared to the inside of cell • Hypotonic Solutions: Outside solution has lower concentration of solute than inside of cell • Isotonic Solutions have an equal concentration of solute compared to the cell Tonicity: The ability of a surrounding solution to cause a cell to gain or lose water 11 Hypertonic Solution Hypotonic Solution WATER FOLLOWS SOLUTES 12 3 Tonicity and Water Balance Tonicity and Water Balance If a cell has a concentration of 0.2M of any solute, and the environment has the below concentrations of the same solute, how will the cell respond? How would you describe each solution relative to the inside of the cell (hyper- hypo- iso- tonic)? Environment/Solution ‘‘Cell” 0.1 M 0.5 M 0.2 M 0.2 M 13 16 Concept Mapping Osmosis • • • • • • • • • • • Areas of high concentration Areas of low concentration Solute Concentration gradients Diffusion Hypertonic Solutions Hypotonic Solutions Isotonic solutions Osmosis Selectively permeable membranes Solute molecules Water molecules B Involves movement of… Involves movement of… Across H D Away From Across According to I E C Away From F And toward J But there is no net movement between.. And toward G K 17 19 4 Proteins Aid in the Function of Membrane Transport • Phospholipids provide basic membrane structure • Plasma membranes contain as much protein as phospholipids • Proteins can insert into membrane: – Amphipathic: • Side chains can be polar, charged, or nonpolar – Secondary and tertiary structures allow proteins to form openings and function as passageway across lipid bilayer 20 Review of the Plasma Membrane 2. Peripheral Proteins are bound to the surface of the membrane Phospholipids Peripheral Protein 21 Studying Membrane Proteins Studying Membrane Proteins • Membrane proteins may be separated from membrane with detergents: – Small amphipathic molecules that can form micelles • Detergents burrow into the plasma membrane and associate with the phospholipids and proteins. • The nonpolar components of the detergent associate with the nonpolar components of lipids/proteins in membrane. • The polar components are repelled by hydrophobic interactions in the core of the membrane, which causes proteins to be lifted from the membrane, isolated, and studied 22 Integral Protein Fibers of 1. extraIntegral Proteins (aka cellular matrix transmembrane (ECM) proteins) span the hydrophobic core of the membrane. Once proteins are isolated by detergents, they can be incorporated into synthetic membranes. This allowed identification of membrane proteins that affect permeability (Channels, Carriers, and Pumps) Each type of protein could be studied to figure out how they allow molecules to pass through the membrane 23 5 Channel Proteins Facilitate Diffusion Channel Proteins Facilitate Diffusion • Movement of substances through channel proteins is passive. Ions and small polar molecules diffuse across lipid bilayer without the requirement of energy • Facilitated diffusion occurs when transmembrane proteins assist passive transport • Cells have different types of pore-like channel proteins in their membranes: – Channel proteins are selective – Residues facing inside pore are hydrophilic – Each channel protein permits only particular type of ion or small molecule to pass through it • Aquaporins (“water-pores”): • Permit water to cross plasma membrane 24 Channel Proteins Facilitate Diffusion of Ions 25 Solutes can move based on both a concentration and electrochemical gradient • Ion channels are specialized membrane proteins that form pores, or openings, in the membrane. They allow ions to cross membranes • Electrochemical gradients occur when ions build up on one side of plasma membrane: – Establish both concentration gradient and charge gradient – Ions diffuse down their electrochemical gradients 26 27 6 Channel Proteins Facilitate Diffusion Gated channels: • Open or close in response to signal • Binding of particular molecule • Change in electrical voltage across membrane Carrier Proteins Move Large Molecules Across the Membrane Carrier protein: • Facilitated diffusion can occur via specialized membrane proteins • Mechanism of transport differs from channel proteins because it involves selective binding and a conformational change • Best studied carrier protein is GLUT-1 which transports glucose Glucose O2 • Flow of ions and small molecules through membrane channels carefully controlled 28 30 Facilitated Diffusion Differs from Active Transport • Facilitated diffusion through channels or carriers is passive transport: • Moves substances with their concentration gradient • Does not require input of energy • Active transport: • Moves substances against their gradient • Requires input of energy • ATP often provides energy in cells: • Phosphate group transferred via active transport protein (“pump”) 31 Sodium/Potassium pumps are an example of Active Transport Sodium Potassium Pumps transport both sodium and potassium against their concentration gradient using ATP and a transmembrane protein. Na+ High/K low Na+ low/ K High Na+ High/K low Na+ low/ K High 32 7 Secondary Active Transport Summary of Membrane Transport • Cells can harvest the electrochemical gradient of one molecule to drive the transport of another molecule against its gradient. • ATP is not directly involved in this form of active transport. 34 35 What happens when vesicles leave the Golgi? GOLGI LUMEN CYTOSOL LYSOSOME 40 Transport out of the cell is exocytosis 41 8 TheEXOCYTOSIS Nature of Science The Nature of Autophagy Science Endocytosis and Endocytosis: The process of bringing in material from the outside of the cell 42 43 Cell- Matrix and Cell-Cell Interactions • Unicellular organisms must contend with constant shifts in environmental conditions • Cells in multicellular organisms must communicate and cooperate with each other as an interdependent community of cells 44 45 9 Gap Junctions Connect Adjacent Cells and Aid in Transport Gap Junctions Connect Adjacent Cells and Aid in Transport • In animal tissues, gap junctions: • Connect adjacent cells by forming channels • Allow flow of small molecules between cells • In plant tissues, plasmodesmata: • Connect adjacent cytosols (and smooth ER) by forming channels • Allow flow of small molecules between cells • Both are communication portals: • Help adjacent cells coordinate activities • Allow rapid passage of regulatory ions and small molecules 46 47 Chapter Recap The plasma membrane serves as a selective barrier through which substances may pass How do small substances pass through a membrane? Diffusion and Osmosis How does large cargo move across the cell membrane? Membrane Transport Proteins Cells can communicate by passing molecules across their adjacent membranes Endo and Exocytosis 48 10