Homeostasis And Cell Transport


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  • Transport: some have hydrophilic channels that allow certain molecules or ions to pass through; some hydrolyze ATP in order to actively pump substances across the membraneEnzymatic activity: embedded proteins may protrude so that its active site is exposed to substances (sometimes, in multiple, enzymes are ordered to carry out sequential steps of metabolism)Signal transduction: binding sites may have specific shapes that match with chemical messengers, causing conformational changes to relay messages to the inside of cellsIntercellular joining: adjacent cells hook together in various kinds of junctionsCell-Cell recognition: glycoproteins (proteins with oligosaccharides) serve as identification tagsAttachment to the cytoskeleton and ECM: bonds to cytoskeletal structures maintain cell shape and fixes a protein’s location
  • Facilitated diffusion is the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane. Many large molecules, such as glucose, are insoluble in lipids and too large to fit through the membrane pores. Therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane. The facilitated diffusion is a passive process, and the solutes still move down the concentration gradient.
  • Homeostasis And Cell Transport

    1. 1. Homeostasis and Cell Transport MORE EFFICIENT THAN JAPANESE RAILWAYS
    2. 2. The plasma membrane Why is it so awesome?  Chemical exchange discrimination=selective permeability  Scientists conjecture its place in evolution  It may be only 8nm thick, but it controls inter-cell traffic.  They vary according to function. Proteins determine the membranes’ specific functions. Ex: Mitochondrial membranes have a greater percentage of proteins.
    3. 3. The Fluid Mosaic Model Did it hurt? Did what When you hurt? fell from heaven and started selectively permeating my heart. <3
    4. 4. The Fluid Mosaic Model  Held together by hydrophobic interactions (weaker than covalent bonds)  Membrane=mosaic of protein molecules bobbing in fluid of phospholipids  Maximizes the contact of hydrophilic regions of phospholipids and proteins  Provides hydrophobic parts a nonaqueous environment  Lateral movement of lipids and proteins  Phospholipids move 2μm/s (rarely flip-flop across the membrane)  Proteins move more slowly (larger) Some proteins may be driven along cytoskeletal fibers by motor proteins (in its cytoplasmic regions)   Temperature  Membrane solidifies once reaching a certain cold temperature  Cholesterol  Wedged between phospholipid molecules in animal cell membranes  At warm temperatures: makes membrane less fluid by restraining phospholipid movement  At cold temperatures: hinders the close packing of phospholipids; temperature required for membrane solidification is lowered  Solidification?  Permeability changes, enzymatic proteins become inactive  To avoid: increase unsaturated phospholipids (ex: winter wheat)
    5. 5. Components of the Membrane  Amphipathic molecules + Cell Membranes= BFFs  Amphipathic molecule: both hydrophilic and hydrophobic regions  Lipids  Phospholipid bilayer (amphipathic)  Proteins  Membrane proteins(amphipathic) Integral Proteins: penetrate the hydrophobic core of the lipid bilayer  Peripheral Proteins: not embedded in the bilayer, loosely bound (often to  parts of integral proteins)  Carbohydrates  Glycoproteins  Glycolipids  Note: membranes have distinct inside and outside faces  Molecules that start out on the inside ER face end up on the outside face of the membrane, and vice versa.
    6. 6. Lipids  Most abundant lipids in membranes=phospholipids  Two lipid layers may differ in lipid composition  Why lipids? All membrane lipids are amphipathic.  Unsaturated hydrocarbon tails have kinks keeping from molecules  from packing together (enhancing membrane fluidity) Hydrophobic (nonpolar) molecules (CO2, hydrocarbons, O) can  dissolve in lipid bilayer Hydrophobic core impedes transport of ions and polar (hydrophilic)  molecules (water, sugars, charged atoms or molecules)  Cell adjusts lipid composition in changing temperatures to maintain fluidity.
    7. 7. Proteins  Has a directional orientation in the membrane  More than 50 kinds have been found so far  Functions: Transport:  some have hydrophilic channels that allow certain molecules or ions to pass through; some  hydrolyze ATP in order to actively pump substances across the membrane Enzymatic activity:  embedded proteins may protrude so that its active site is exposed to substances  (sometimes, in multiple, enzymes are ordered to carry out sequential steps of metabolism) Signal transduction:  binding sites may have specific shapes that match with chemical messengers, causing  conformational changes to relay messages to the inside of cells Intercellular joining:  adjacent cells hook together in various kinds of junctions  Cell-Cell recognition:  glycoproteins (proteins with oligosaccharides) serve as identification tags  Attachment to the cytoskeleton and ECM:  bonds to cytoskeletal structures maintain cell shape and fixes a protein’s location 
    8. 8. Carbohydrates  Only found on the exterior surface of the cell  Important for cell to cell recognition  Ex: sorting of cells into tissues and organs in embryos  Usually branched oligosaccharides (fewer than 15 monosaccharides) Oligo=few in Greek  Glycolipids=oligosaccharides covalently bonded to lipid  Glycoprotein=oligosaccharides covalently bonded to protein  Vary from species to species, individuals among a species, and  one cell type to another within an individual
    9. 9. Traffic Across Cell Membranes PUTTING THE MEMBRANE TO USE Yeah, it’s that complex. Not really.
    10. 10. Diffusion  Principles:  A substance will diffuse down its concentration gradient (where it is more to less concentrated) Imagine a group of molecules spreading out in space  Result of thermal motion (intrinsic kinetic energy)  Movement: random for individual molecules, directional for population  of molecules (ex: red dye in water) Increases entropy by producing a more random mixture  Each substance diffuses down its own concentration gradient and is not  affected by other substances’ concentration differences.  In action:  Occurs when a substance that is permeable is concentrated on one side of the membrane  Ex: uptake of oxygen by a cell performing cellular respiration; dissolved oxygen diffuses into the cell across the plasma membrane  Did you know that diffusion was the simplest type of passive transport?
    11. 11. Passive Transport  The diffusion of a substance across a biological membrane  Requires no energy Concentration gradient represents potential energy, drives  diffusion  Types:  Diffusion  Osmosis  Facilitated Diffusion  Filtration
    12. 12. Osmosis  The passive transport of water; the diffusion of water molecules across a selectively permeable membrane  Water will diffuse across the membrane from the hypotonic solution to the hypertonic solution What in the world does that mean?  Hypertonic: higher concentration of solutes   Hypotonic: lower solute concentration  Isotonic: equal solute concentration  Translation: Water will move from areas of higher (water) concentration to lower (water) concentration, depending on the amount of solute.  Direction is determined only by total solute concentration differences
    13. 13. Cell Survival and Osmosis  Osmoregulation: control of water balance  Membranes are adapted to environments, ex: paramecium  Animal/wall-less cell water balance in ___ environments:  Isotonic: Optimal!=no net movement of water,   Hypertonic: lose water to environment, shrivel, die,  Ex: animals die when lake salinity increases  Hypotonic: water enters faster than it leaves, swell, lyse (burst),    Plant cells water balance in ___ environments:  Isotonic: no net tendency for water to enter, flaccid cells (wilted plant),   Hypertonic: cell loses water to environment, shrinks (membrane pulls away from wall=plasmolysis), usually dies,   Hypotonic: wall helps maintain water balance, turgid (firm) state=healthy! 
    14. 14. Facilitated diffusion  Diffusion of polar molecules and ions across the membrane by transport (carrier) proteins  Facilitated diffusion is a passive process because the solutes still move down the concentration gradient.  Transport proteins Has specialized binding site (like enyzmatic active site) for the solute it  transports Can be inhibited by “imposters” compete with normal soutes  Some undergo subtle shape change that translocates solute-binding site  across the membrane Channel proteins: provide hydrophilic “corridors” to allow a specific  molecule/ion to cross membrane (channel proteins)=quick flowing (ex: aquaporins) Gated channels: stimuli (electrical signals or chemical signals [ex: nerve cells by  neurotransmitter molecules] or stretching of the cell membrane) cause proteins to open or close  Speeds the transport of a solute by providing an efficient passage through the membrane
    15. 15. Facilitated Diffusion: Examples  Ex: of polar molecule tranport:  Transport of Glucose: sugars are polar molecules; cannot simply diffuse across membrane  Glucose requires a specific carrier protein to cross membrane (in or out of cell)  Ex: of ion transport  Cl -, Na+, K+, Ca 2+ are moved across the membrane through carrier proteins
    16. 16. Ion Movement Across the Membrane  Movement depends on concentration gradient (chemical) and voltage differences across the membrane  Membrane potential: voltage across a membrane Ranges from -50 to -200 mmV  Acts as energy source that affects the traffic of all charged substances  across the membrane Favors the passive transport of cations in and anions out (because  cytoplasm of a cell is negative in charge compared to the extracellular fluid)  Electrochemical gradient: combination of electrical and chemical forces that affect ions during passive transport  Active ion transport Electrogenic pump: transport proteins that generate voltage across a  membrane, store energy that can be tapped for cellular work (cotransport) Proton pump: actively transports H+ out of the cell 
    17. 17. YEAH Active Transport !  The “uphill” movement of solutes up their concentration gradient across the plasma membrane  In order to pump a molecule against its [ ] gradient, a cell must expend its own metabolic energy  A major factor in the ability of a cell to maintain internal concentration of small molecules that differ from concentrations in its environment  Performed by specific embedded (integral) proteins ATP usually supplies energy for active transport by transferring its  terminal phosphate group directly to the transport protein, causing a conformational change (causing solute to translocate across membrane)
    18. 18. The Infamous Sodium-Potassium Pump  Cells maintain high internal K+ concentration (pump it in) and low internal Na+ concentration (pump it out)  For every 3 Na+ pumped in, 2K+ are pumped out.  Generates voltage (membrane potential) across membrane  Steps (fig. 8.15, pg. 149): 1. Binding of cytoplasmic Na+ to the protein stimulates  phosphorylation by ATP 2. Phosphorylation causes the protein to change its conformation  3. The conformational change expels Na+ to the outside, and  extracellular K+ binds. 4. K+ binding triggers release of a phosphate group.  5. Loss of phosphate restores original conformation.  6. K+ is released and Na+ sites are receptive again; cycle repeats 
    19. 19. Cotransport  Single ATP-powered pump transports a specific solute can indirectly drive the active transport of several other solutes  Primarily used in the transport of amino acids and sugars  How? A substance that has been pumped across can do work as it leaks  back by diffusion Transport proteins can couple the downhill diffusion of a substance  to actively transport another substance Ex: return of H+ helps to actively transport sucrose against its  concentration gradient; used by plants to load photosynthesis- produced sugars into leaf veins
    20. 20. The Transport of Macromolecules WHEN PROTEINS AND DIFFUSION CAN’T GET THE JOB DONE.
    21. 21. Exocytosis: into cell via vesicles  How do cells secrete macromolecules? Transport vesicle (that budded  from the Golgi apparatus) surrounding macromolecule fuses with plasma membrane Vesicle moves across  cytoskeleton on its way The two bilayers rearrange  themselves so that the two membranes fuse and the vesicle contents spill outside Ex: export of cell products  (pancreas cells and insulin, neuron and chemical signals to stimulate other neruons/muscle cells)
    22. 22. Endocytosis: out of cell via vesicles  Used when molecules are too large to be moved by simple diffusion or transport proteins  Pinocytosis: (drinking) “Gulps” droplets of extracellular fluid into tiny vesicles  Nondiscriminatory: any and all solutes dissolved in the droplet are taken  into the cell  Phagocytosis: (eating)  Engulfing: wrapping pseudopodia around in order to package particle in vacuole  Digestion: vacuole fuses with a lysosome with hydrolytic enzymes  Receptor-mediated: (specific)  proteins with specific receptor sites (clustered in coated pits, which form the vesicle) are exposed to extracellular fluid  Enables cell to acquire bulk quantities of specific substances (ex: cholesterol binding to low-density lipoproteins [LDLs])
    23. 23. In receptor mediated 1) endocytosis, coated pits form vesicles in which the particles are taken into the membrane. In 2) pinocytosis, extracell ular fluid is engulfed by a food vesicle that takes the solute(s) into the cell. In 3) phagocytosis, solid particles are engulfed by The Three Types of Endocytosis pseudopodia that form a food vacuole called a phagosome.