(a) A channel protein Channel protein Solute Carrier protein Solute Figure 7.17 EXTRACELLULAR FLUID (a) A channel protein Channel protein Solute CYTOPLASM Figure 7.17 Two types of transport proteins that carry out facilitated diffusion. Carrier protein Solute (b) A carrier protein 1

Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane © 2011 Pearson Education, Inc.

Active transport uses energy to move solutes against gradients Facilitated diffusion is still passive because solute moves down its concentration gradient, and transport requires no energy Some transport proteins, however, can move solutes against concentration gradients © 2011 Pearson Education, Inc.

The Need for Energy in Active Transport Active transport moves substances against their concentration gradients Active transport requires energy usually in form of ATP Active transport is performed by specific proteins embedded in membranes © 2011 Pearson Education, Inc.

Sodium-potassium pump is one type of active transport system Active transport allows cells to maintain concentration gradients that differ from their surroundings Sodium-potassium pump is one type of active transport system For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files. © 2011 Pearson Education, Inc.

EXTRACELLULAR FLUID [Na] high [K] low Na Na [Na] low CYTOPLASM Figure 7.18-1 EXTRACELLULAR FLUID [Na] high [K] low Na Na CYTOPLASM [Na] low Na 1 [K] high Figure 7.18 The sodium-potassium pump: a specific case of active transport. 6

EXTRACELLULAR FLUID [Na] high [K] low Na Na Na Na Na [Na] low Figure 7.18-2 EXTRACELLULAR FLUID [Na] high [K] low Na Na Na Na Na [Na] low ATP CYTOPLASM Na P 1 [K] high 2 ADP Figure 7.18 The sodium-potassium pump: a specific case of active transport. 7

EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Figure 7.18-3 EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Na ATP CYTOPLASM [Na] low Na P P 1 [K] high 2 ADP 3 Figure 7.18 The sodium-potassium pump: a specific case of active transport. 8

EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Figure 7.18-4 EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Na CYTOPLASM [Na] low ATP Na P P 1 [K] high 2 ADP 3 K Figure 7.18 The sodium-potassium pump: a specific case of active transport. K P 4 P i 9

EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Figure 7.18-5 EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Na ATP CYTOPLASM [Na] low Na P P 1 [K] high 2 ADP 3 K Figure 7.18 The sodium-potassium pump: a specific case of active transport. K K K P 5 4 P i 10

EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Figure 7.18-6 EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na Na CYTOPLASM [Na] low ATP Na P P 1 [K] high 2 ADP 3 K Figure 7.18 The sodium-potassium pump: a specific case of active transport. K K K K P 6 K 5 4 P i 11

Facilitated diffusion ATP Figure 7.19 Passive transport Active transport Figure 7.19 Review: passive and active transport. Diffusion Facilitated diffusion ATP 12

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 across a membrane © 2011 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) © 2011 Pearson Education, Inc.

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 The main electrogenic pump of plants, fungi, and bacteria is a proton pump Electrogenic pumps help store energy that can be used for cellular work © 2011 Pearson Education, Inc.

ATP   EXTRACELLULAR FLUID   H Proton pump H H H   H   H Figure 7.20 ATP   EXTRACELLULAR FLUID   H Proton pump H H H   H   H Figure 7.20 A proton pump. CYTOPLASM 16

Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell © 2011 Pearson Education, Inc.

Sucrose-H cotransporter Figure 7.21 ATP H   H Proton pump H H   H H H   H Sucrose-H cotransporter Diffusion of H Figure 7.21 Cotransport: active transport driven by a concentration gradient. Sucrose   Sucrose 18

Bulk transport across plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files. © 2011 Pearson Education, Inc.

In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles © 2011 Pearson Education, Inc.

In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule © 2011 Pearson Education, Inc.

Receptor-Mediated Endocytosis Figure 7.22 Phagocytosis Pinocytosis Receptor-Mediated Endocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Receptor Plasma membrane Ligand Coat proteins Coated pit “Food” or other particle Coated vesicle Figure 7.22 Exploring: Endocytosis in Animal Cells Vesicle Food vacuole CYTOPLASM 25

Pseudopodium of amoeba Figure 7.22a Phagocytosis EXTRACELLULAR FLUID Solutes Pseudopodium of amoeba Pseudopodium Bacterium 1 m Food vacuole “Food” or other particle An amoeba engulfing a bacterium via phagocytosis (TEM). Figure 7.22 Exploring: Endocytosis in Animal Cells Food vacuole CYTOPLASM 26

Pinocytosis 0.5 m Plasma membrane Figure 7.22b Pinocytosis 0.5 m Plasma membrane Pinocytosis vesicles forming in a cell lining a small blood vessel (TEM). Figure 7.22 Exploring: Endocytosis in Animal Cells Vesicle 27

Receptor-Mediated Endocytosis Figure 7.22c Receptor-Mediated Endocytosis Receptor Plasma membrane Coat proteins Ligand Coat proteins Coated pit 0.25 m Figure 7.22 Exploring: Endocytosis in Animal Cells Coated vesicle Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs). 28