Active Transport

1. Active Transport

2. Active Transport • Cellular energy is used to to transport substances across the membrane against a concentration gradient • Energy is derived from splitting ATP

3. Pumps • A transporter protein that uses energy from splitting ATP to change shape and carry a substance across a cellular membrane against its concentration gradient

4. Substances Transported • Na+ • K+ • H+ • Ca++ • I- • Cl-

5. Sodium-Potassium Pump • Expels Na+ from the cell, brings K+ into the cell • Acts as an enzyme to split ATP • Used to maintain a low [Na+] concentration in the cytosol by pumping Na+ into the extracellular fluid

6. Continuous Operation • Na+ and K+ slowly leak back across the membrane down their gradients, so the pumps must continuously move these ions back across the membrane to maintain the concentration gradient.

7. Concentration Gradient • The differing concentrations are crucial for osmotic balance of the 2 fluids and also for the ability of some cells to generate electrical signals

10. Operation of the Sodium Potassium Pump • 1. Three Na+ in the cytosol bind to the pump protein • 2. Na+ binding triggers the splitting of ATP into ADP plus a phosphate group which attaches to the pump protein. This causes a change in the shape of the pump protein • 3. Three Na+ are expelled into the extracellular fluid

11. Operation of the Sodium Potassium Pump • 4. The changed shape allows 2 K+ in the extracellular fluid to bind to the protein • 5. The binding of the K+ causes the phosphate group to be released which causes the pump protein to return to its original shape • 6. As the pump protein returns to its original shape, it releases the 2 K+ into the cytosol. The pump protein is then ready to bind Na+ and the cycle can repeat

12. Malfunctions of Transporters • Drugs – some turn off ATP production which would stop active transport of substances throughout the body • Cystic Fibrosis – a defective gene causes an abnormal Cl- transporter. There is a failure to secrete adequate amounts of Cl- resulting in extremely thick mucus secretions that result in obstruction and infection.

13. Transport in Vesicles • Vesicle – a small sac formed by the budding off from an existing membrane • Movement of vesicles requires energy supplied by ATP.

14. ENDOCYTOSIS • Materials move into a cell is a vesicle formed from the plasma membrane • Phagocytosis • Bulk Phase Endocytosis

15. Phagocytosis • Large solid particles (like whole cells) are taken in by the cell. • The particle binds to the plasma membrane receptor, the cell extends projections called pseudopods that surround the particle, then the membranes fuse to create a vesicle in the cytoplasm.

17. Any undigested material remain indefinitely in a vesicle called a residual body • Phagocytosis occurs only in special cells called phagocytes • They are specialized cells used to engulf and destroy bacterial and other foreign substances • These cells include certain white blood cells and macrophages. • Phagocytosis is one of the body’s defense mechanisms against disease

18. Bulk Phase Endocytosis • Cells take up vesicles containing tiny droplets of extracellular fluid and any solutes dissolved in that fluid • The vesicle detaches from the membrane and enters the cytosol • The vesicle fuses with a lysosome where enzymes digest the solutes into smaller molecules

20. Exocytosis • Results in secretion – materials exiting the cell • Secretory cells give off digestive enzymes, hormones, mucus or other secretions • Nerve cells give off neurotransmitters

21. Secretory Vesicle • Vesicle containing substance to be secreted forms in the cytosol, fuses with the plasma membrane then releases the contents into the extracellular fluid

23. CHECKPOINT • What is the key difference between active and passive transport? • Briefly compare/contrast facilitated diffusion and active transport through a membrane pump • Describe the process of phagocytosis. • Name one way phagocytosis helps to maintain homeostasis.