2 kinds of secondary active transport Ion and solute move in the same direction = symport Example: Na + and glucose in the kidney 2 kinds of secondary

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1 Chapter 4 The Cell: The Fundamental Unit of Life Transport Across Cell Membranes We ve talked about how cells move solutes across membranes Simple diffusion Channel-mediated diffusion Carrier-mediated diffusion Osmosis Lecture 5: Energy-Dependent Membrane Transport and Transport Across Cell Membranes What happens when these methods don t work? What if we need to move solutes AGAINST a concentration gradient? Requires energy Active transport Moves substances through carrier proteins specific to solute Energy provided by ATP Works AGAINST concentration gradient Active transport Used to Concentrate substances in the cell or in organelles Get rid of unneeded things Create gradients that drive other processes Example: sodium-potassium pump Secondary active transport Also called ion-coupled transport Uses energy provided by ion concentration gradients to move solutes across membrane ATP is not used to drive process, but is later used to restore homeostasis 1

2 2 kinds of secondary active transport Ion and solute move in the same direction = symport Example: Na + and glucose in the kidney 2 kinds of secondary active transport Ion and solute move in opposite directions = antiport Example: Na + /Ca 2+ exchanger in kidney Filtration Hydrostatic pressure forces water and solute across a porous membrane Process based on pore size of membrane Example: cardiovascular system This process is later connected to symport and diffusion in the kidney Transport of Large Solutes Some substances are too large to pass through a membrane protein or pore Examples: whole proteins, bulk transport Must be moved by vesicular transport Membrane reorganizes to move substances through the membrane in vesicles REQUIRES ATP Substances moving into the cell = endocytosis Clathrin Transport of Large Solutes Organizing protein on inside of membrane Helps pinch off a portion of the membrane with solutes trapped inside Involved in sorting vesicles for multiple pathways Rapidly recycled 2

3 Receptor-Mediated Endocytosis Specific molecules outside cells bind to specific receptors on cell surface Bound receptors cluster Membrane folds inward to form a pocket Vesicle pinches off and is released into the cell Fuses with endosome, and eventually lysosome Molecules released from receptors Vesicle membrane and receptors recycled Cholesterol, iron brought into the cell this way Cell eating Phagocytosis Endocytosis of very large objects Examples: bacteria, cell debris Specialized process for specific cells Example: macrophages, neutrophils 3

4 Phagocytosis Process is very effective but sometimes it goes wrong Cell drinking Endocytosis of extracellular fluid containing small solutes Small vesicles Not very selective Random sampling Pinocytosis Exocytosis Essentially the opposite of endocytosis Substances moved through the membrane to the outside of the cell Energy-Dependent Membrane Transport Recall: all of these types of transport require energy ATP Pressure gradient Solute gradient Review Ions are charged particles with predictable behavior Opposites attract Likes repel Opposite charges separated = still attracted to each other Strength of attraction = potential or voltage Vast majority of particles in the body are uncharged Cells intentionally concentrate ions inside and outside cells Na + /K + pump moves Na + out and K + in Creates 2 diffusion gradients K + tries to get out Na + tries to get in 4

5 Cells have open K + channels, closed Na + channels K + leaks out, leaves a negative charge balance inside the cell K + and Na + become attracted to the negative charge on the other side of the membrane and create potential Cells use potential to Monitor environment Communicate with other cells 5