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Active Transport CONCEPT 5.4: Active transport uses energy to move solutes against their concentration gradients • Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane but does not alter the direction of transport • Some transport proteins, however, can move solutes against their concentration gradients. These are used for active transport. © 2014 Pearson Education, Inc. Figure 5.15: Passive vs. Active Transport Passive transport Diffusion © 2014 Pearson Education, Inc. Active transport Facilitated diffusion The Need for Energy in Active Transport • Active transport moves substances against their concentration gradients • Active transport requires energy, usually in the form of ATP © 2014 Pearson Education, Inc. • Active transport allows cells to maintain concentration gradients that differ from their surroundings • The sodium-potassium pump is one type of active transport system • Na/K Pump used in muscle contraction and to send signals in nerve cells © 2014 Pearson Education, Inc. EXTRACELLULAR FLUID [Na] high [K] low Figure 5.14: the sodium potassium pump: a specific case of active transport 1 © 2014 Pearson Education, Inc. CYTOPLASM [Na] low [K] high 2 6 3 5 4 ADP How Ion Pumps Maintain Membrane Potential • All cells have voltages across their plasma membranes • Voltage: electrical potential energy • The cytoplasmic side of the membrane is negative; the outside of the cell is positive • Membrane potential is the voltage across a membrane • Serves as an energy source that controls the movement of substances across the membrane • Voltage is created by differences in the distribution of positive and negative ions across a membrane © 2014 Pearson Education, Inc. • Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane • A chemical force (the ion’s concentration gradient) • An electrical force (the effect of the membrane potential on the ion’s movement) • An ion diffuses down its concentration gradient and its electrochemical gradient. Active transport is necessary to move a particle whose concentration gradient is opposite of its electrochemical gradient. © 2014 Pearson Education, Inc. • An electrogenic pump is a transport protein that generates voltage across a membrane by transporting ions • The sodium-potassium pump is the major electrogenic pump of animal cells • The main electrogenic pump of plants, fungi, and bacteria is a proton pump (transports H+) • Electrogenic pumps help store energy that can be used for cellular work © 2014 Pearson Education, Inc. Figure 5.16 EXTRACELLULAR FLUID Proton pump CYTOPLASM © 2014 Pearson Education, Inc. Cotransport: Coupled Transport by a Membrane Protein • Cotransport occurs when active transport of a solute indirectly drives transport of other solutes • Plant cells use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell © 2014 Pearson Education, Inc. Figure 5.17 Proton pump Sucrose-H cotransporter Diffusion of H Sucrose Sucrose © 2014 Pearson Education, Inc.