Homeostasis And Cell Transport

1. More efficient than Japanese railways Homeostasis and Cell Transport

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. The Fluid Mosaic Model Did it hurt? Did what hurt? When you fell from heaven and started selectively permeating my heart. <3

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 2 μ m/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. 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. 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 (CO 2 , 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. 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. 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. Putting the membrane to use Traffic Across Cell Membranes Yeah, it’s that complex. Not really.

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. 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. 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. 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. 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. 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. 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. 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) YEAH!

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. 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. When proteins and diffusion can’t get the job done. The Transport of Macromolecules

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. 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. The Three Types of Endocytosis In receptor mediated endocytosis , coated pits form vesicles in which the particles are taken into the membrane. In pinocytosis , extracellular fluid is engulfed by a food vesicle that takes the solute(s) into the cell. In phagocytosis , solid particles are engulfed by pseudopodia that form a food vacuole called a phagosome .

24. I Hope that you had Ex- CELL - ent experience. Fin.