Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 7 Active Transport

The Need for Energy in Active Transport Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes Animation: Active Transport Animation: Active Transport Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

2 EXTRACELLULAR FLUID [Na + ] high [K + ] low [Na + ] low [K + ] high Na + CYTOPLASM ATP ADP P Na + P 3 K+K+ K+K+ 6 K+K+ K+K+ 5 4 K+K+ K+K+ P P 1 Fig

How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane – Voltage is created by differences in the distribution of positive and negative ions Cytoplasm is negatively charged compared to outside the cell because of there being more anions inside the cell Membrane potential favors passive transport of cations into cell and anions out of cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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) Positive ions are attracted to negative side of the membrane and vice versa Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

An electrogenic pump is a transport protein that generates voltage across a membrane – The sodium-potassium pump is the major electrogenic pump of animal cells – Moves 3 Na ions out and 2 K ions in each time – Results in a net transfer of one cation out of the cell The main electrogenic pump of plants, fungi, and bacteria is a proton pump – Transports H cations out of the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig EXTRACELLULAR FLUID H+H+ H+H+ H+H+ H+H+ Proton pump H+H+ H+H+ + + H+H+ – – – – ATP CYTOPLASM –

Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of another solute Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell – Hydrogen ions that were pumped out of the cell naturally move back down their concentration gradient into the cell – the energy released from this is used to bring other substances in at the same time Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Proton pump – – – – – – ATP H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Diffusion of H + Sucrose-H + cotransporter Sucrose

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Animation: Exocytosis Animation: Exocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: – Phagocytosis (“cellular eating”) – Pinocytosis (“cellular drinking”) – Receptor-mediated endocytosis Animation: Exocytosis and Endocytosis Introduction Animation: Exocytosis and Endocytosis Introduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle Animation: Phagocytosis Animation: Phagocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-20a PHAGOCYTOSIS CYTOPLASM EXTRACELLULAR FLUID Pseudopodium “Food” or other particle Food vacuole Food vacuole Bacterium An amoeba engulfing a bacterium via phagocytosis (TEM) Pseudopodium of amoeba 1 µm

In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Animation: Pinocytosis Animation: Pinocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-20b PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM)

In receptor-mediated endocytosis, binding of ligands, signal molecules, to receptors triggers vesicle formation Animation: Receptor-Mediated Endocytosis Animation: Receptor-Mediated Endocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings