Active Transport Presentation PDF

Active Transport: Protein-Mediated Movement Up the Gradient  Facilitated diffusion is important but accounts only for movement of molecules down a concentration gradient, toward equilibrium  Active transport is used to move solutes up a concentration gradient, away from equilibrium  Diffusi...

Active Transport: Protein-Mediated Movement Up the Gradient  Facilitated diffusion is important but accounts only for movement of molecules down a concentration gradient, toward equilibrium  Active transport is used to move solutes up a concentration gradient, away from equilibrium  Diffusion is nondirectional and active transport has an intrinsic directionality The Coupling of Active Transport to an Energy Source May Be Direct or Indirect Direct Active Transport Depends on Four Types of Transport ATPases  Four types of transport ATPases have been identified  P-type – (Na+/K+ pump)  V-type – (“vacuole” H+ pumps)  F-type – (also H+ pumps, includes ATP synthases) *Reversible  ABC-type  They differ in structure, mechanism, location, and roles V-Type ATPases  V-type ATPases pump protons into organelles such as vacuoles, vesicles, lysosomes, endosomes, and the Golgi complex  They have two multisubunit components: an integral component embedded in the membrane and a peripheral component that juts out from the membrane surface Direct Active Transport: The Na+/K+ Pump Maintains Electrochemical Ion Gradients  In a mammalian neuron, [K+] inside/[K+] outside is about 30:1, and [Na+] inside/[Na+] outside is around 0.08:1  The electrochemical potentials for sodium and potassium are essential as a driving force for coupled transport and for transmission of nerve impulses  In a mammalian neuron, [K+] inside/[K+] outside is about 30:1, and [Na+] inside/[Na+] outside is around 0.08:1 What is the purpose of having the [K+] gradient be 2.5 times as strong as the [Na+] gradient? a) Because Na+ is bigger than K+ b) To overcome of the membrane potential (Vm) of the cell c) To adjust for the difference in charge on the ions d) Because the Na+/K+ pump moves three Na+ for every two K+ e) Actually the [Na+] gradient is stronger than the [K+] gradient Requirement for Energy  The pumping of both Na+ and K+ ions against their gradients requires energy  The pump that is responsible, the Na+/K+ ATPase (or pump), uses the exergonic hydrolysis of ATP to drive the transport of both ions  It is primarily responsible for the asymmetric distribution of ions across the plasma membrane of animal cells Structure of the Na+/K+ ATPase The Na+/K+ Pump Is an Allosteric Protein  The Na+/K+ pump has two alternative conformational states, E1 and E2  The E1 conformation is open to the inside of the cell and has high affinity for Na+ ions  The E2 conformation is open to the outside of the cell and has high affinity for K+ ions Which of the following is true regarding the E2 conformation (open to the outside of the cell) of the Na+/K+ pump? A) It has strong binding affinity for K+ and Na+ B) It binds K+ strongly and Na+ weakly C) It binds Na+ strongly and K+ weakly D) It does not bind well to K+ or Na+ Indirect Active Transport Is Driven by Ion Gradients  Indirect active transport (secondary active transport) is not powered by ATP hydrolysis  The inward transport of molecules up their electrochemical gradients is often coupled to and driven by simultaneous inward movement of Na+ (animals) or protons (plants, fungi, bacteria) down their gradients Symport Mechanisms of Indirect Active Transport  Most cells continuously pump either sodium ions or protons out of the cell (for example, the Na+/K+ pump in animals)  The resulting high extracellular concentration of Na+ is a driving force for the uptake of sugars and amino acids  This is indirectly related to ATP because the pump that maintains the sodium ion gradient is driven by ATP Indirect Active Transport: Sodium Symport Drives the Uptake of Glucose  Although most glucose transport into and out of our cells occurs by facilitated diffusion, some cells use a Na+/glucose symporter  For example, the cells lining the intestine take up glucose and some amino acids even when their concentrations are much lower outside than inside the cells The Energetics of Transport  Every transport event in the cell is an energy transaction  For uncharged solutes, the only variable is the concentration gradient across the membrane  For charged solutes, both concentration and electrical potential are relevant For Uncharged Solutes, the ΔG of Transport Depends Only on the Concentration Gradient  For solutes with no net charge, we are concerned only with their concentration gradient across the membrane  Because of this, the ΔG for transport of uncharged solutes is relatively easy to calculate R = gas constant constant, positive value T = absolute temperature ΔG of Inward Transport  The ΔG of inward transport simplifies to  If [S] inside < [S] outside, ΔG is negative and the transport is exergonic  But, if [S] inside > [S] outside, ΔG is positive and transport is against the concentration gradient An Example: The Uptake of Lactose  Suppose that the lactose concentration inside a bacterium must be kept at 10 mM and that the external lactose concentration is 0.20 mM  We can determine the energy requirement for inward transport of lactose Energy Requirement for Lactose Uptake   For outward transport of lactose, [S] inside and [S] outside must be interchanged For Charged Solutes, the ΔG of Transport Depends on the Electrochemical Potential  For charged solutes, we must consider both concentration gradient and the membrane potential, Vm For charged solutes, we calculate the effect of electrical potential on ΔG separately from the effect of concentration gradient. We use zFVm to calculate this effect. z = charge of solute F = Faraday constant (positive) Vm = membrane potential How would membrane potential (negative for most cells) affect ΔG for the import of calcium ions (Ca2+)? A) It would make ΔG more negative B) It would make ΔG more positive C) It would have no effect For Charged Solutes, the ΔG of Transport Depends on the Electrochemical Potential  For charged solutes, we must consider both concentration gradient and the membrane potential, Vm  Because Vm is nearly always negative, it usually favors the inward movement of cations and opposes their outward movement Calculating ΔG for the Transport of Ions  If Sz is a solute with charge z, then F = Faraday constant  For a typical cell Vm is negative, so a positive ion will give a negative ΔG for inward transport, and a negative ion gives a positive ΔG An Example: The Uptake of Chloride Ions  Consider a nerve cell with [Cl–] inside = 50 mM, in a solution with [Cl–] = 100 mM and membrane potential of –60 mV  The inward movement of Cl– ions is down the concentration gradient but up the charge gradient  To determine the direction of transfer, we need to take both into account Calculate ΔG for Cl– Intake   So, though Cl– concentration is higher outside the cell, energy is still required for import