BSCI207 – Nutrient Assimilation I – IIINUTRIENT ASSIMILATION ILet’s discuss how nutrient molecules cross cell membrane We must conduct anexperiment!- Examine how solute molecule X enters a cello We must be able to measure [X]- Protocolo Begin with cells in a medium suitable for survival and growtho Now add the solute of interest (X) and know the final [X] in the medium;repeat experiment using different [X]o Let cells incubate with X for various amounts of time and then measurethe amount of X inside the cells )or measure amount still in mediumsolution)o Now plot the results, one plot for each initial [X] - Why would X in cells reach some constant amount?o Equal concentrations in and out Probably by diffusiono Cells have some feedback mechanism to stop transport Probably by some transportmechanismo PROBLEM: It is difficult to measureintracellular concentrations of solutes!- Can you find the RATE of entry of X into thecells?o Rate = slope = umoles/mino Rate of uptake as a function of [Solute]Diffusion across Cell Membranes!Oil/Water Partition Coefficient- Overviewo Add solute X into oil/water mixtureo Shake well and determine how solute partitions between oil and water layers- O/W PC = [S]oil/[S]watero What is the relationship of polarity to O/W PC? As the polarity of a molecule is increased, the oil-water partitioncoefficient decreases!- Simple Diffusion Differential Permeability – the relative abilities of different molecules to diffuse acrossmembranes- Conceptso Diffusion of Nonpolar solutes and gases occurs by simple diffusiono No specific transport siteso Depends upon the ΔC and/or ΔE of the solute (electrochemicalgradient)o Goes in both directions across membranePassage of Water- Aquaporinso Water pores that allow only water to pass throughConcepts:- The need to transport different polar or large solutes across membranes requires the evolution of different transportmechanismsNUTRIENT ASSIMILATION IILet’s now plot the rate for each initial [X] but now with a very high [X]- So what do we know about such transport mechanisms?o The rate begins to level off eventuallyo Hence, specific transport sites are limiting the maximum rateFacilitated Diffusion- Depends upon ΔC (concentration gradient) and/or ΔE (electrochemical gradient)- Works in both directions- Specific sites for each solute!o Evolutionary considerations?- Exhibits a measurable affinity for the solute and a maximum rate of transportYou can easily see the maximum rate of transport, TmaxWhat determines Tmax?- Membrane protein channelso Some solute molecules can diffuse without any assistanceo Certain solute molecules can diffuse though the aqueous pore of a specific channel proteino Reciprocating transporters can convey selected solute molecules across the bilayer by means of a fluctuating change in their shapeo The energy released by the breakdown of ATP molecules can be coupled by a transport protein to pumping specific solute molecules against their concentration gradientFor affinity, use Ka = [X] that gives ½ of Tmax-For a given transport molecule, the value of K a is inversely proportional to the affinity o The higher the Ka, the SLOWER ? it will have a lower rate at any given [solute] below that which yields the TmaxSummary of Facilitated Diffusion- Down ΔC; works both directions; each solute transported by a specify site (therefore competition between similar solutes);maximum rate (Tmax); affinity measure as KaFacilitated Diffusion Glucose via GLUT-1Other Examples of Facilitated Diffusion- Glucose into and out of your red blood cells- Ion channels(K+ channel)- Artificialchannelso Ex. antibioticssuch as GramicidinGramicidin(FacilitatedDiffusion)K+ Channel: Voltageregulated open-closed (Facilitated Diffusion)Active Transport- Independent of ΔC (concentration gradient) and/or ΔE (electrochemical gradient)o Thus can concentrate solutes inside cells- Specificity & Competition- Affinity (Ka)- Tmax- Requires some form of energy!o Positive ΔG!Energy for Active Transport- Direct use of ATPo Phosphorylated intermediateo Ex. Na+- K+ ATPase in cell membranes- Indirecto Use ΔC and/or ΔE of a co-transported iono Ex. Na+ gradient for transport of glucose and amino acidsNa+- K+ ATPase Direct use of ATP energy; AntiportNa+ gradient Symport- Why establish Na+ gradient?o Na+ flows?down?their concentration gradient o Glucose molecules are pumped?up?theirs. o Later the sodium is pumped back out of the cell by the Na+/K+?ATPase- Why use Na+ gradient?o When the concentration of sodium ions is very high on the outside and very low inside, which provides energy forthe?transport- Na+-dependent symporto Hexoseso Amino acidso Some ionsSUMMARYNUTRIENT ASSIMILATION IIIUnicellular Organisms- Small and non-polar molecules transported or diffuse through plasma- Larger molecules ingested with aid of specialized regions of the membraneo PhagocytosisCichlids- Located in the East African Rift Lakes- >300 species- Food sources include leafy algae, crust forming algae, plankton, fish, plants,snails, fish eggs, and plantso Each species evolved specialized pharyngeal jaws o Teeth on pharyngeal jaws vary among species of cichlids and correlatewith the type of food eatenDarwin Finches of the Galapagos Islands Digestion:Meat vs. Plant Material; Variations of Teeth Enzymes to match the meal!What if you don’t have the enzymes?- Termites eat wood but lack cellulose- So they rely on a protozoan, Mixotricha, which in turn relies upon at least 4 types of bacteriaMixotricha: Bacterial Symbionts- Treponema attached to cell surface- Rod shaped bacteria on the surface o Protection?- Spherical bacteria as internal endosymbionts to function as mitochondria!o Mixotricha lacks
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