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WEEK 6: Cellular Respiration and the ADP-ATP Cycle. LEARNING OUTCOMES By the end of these lessons, you should be able to: Explain the series of reactions that result in th e transfer of chemical energy to ATP. Describe ATP and explain how it is used by cells. Describe aerobic respiration.
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WEEK 6: Cellular Respiration and the ADP-ATP Cycle LEARNING OUTCOMES By the end of these lessons, you should be able to: • Explain the series of reactions that result in the transfer of chemical energy to ATP. • Describe ATP and explain how it is used by cells. • Describe aerobic respiration. • Describe anaerobic respiration.
What’s it all about? • Takes place in the inner membrane of mitochondria. • Breaking down (catalysing) glucose to release energy. • Exergonic reactions. • Redox reactions relocating electrons. • This energy is then used to synthesise ATP (a process called phosphorylation). • Three steps: • Glycolysis • Krebs cycle • Electron transport
Stage 1: Glycolysis • Occurs in the cell cytosol. • A 10-step process catalysed by enzymes. • Begins the process of breaking glucose down: • one molecule of glucose is broken down into two molecules of pyruvate (a 3-C sugar). • A small amount of energy is released. • Two molecules of ATP and two of NADH are released. • NADH is an acceptor molecule carrying two hydrogen ions and two electrons. • No CO2 is released in this stage.
CYTOSOL C C C C C C 1 glucose 2 ADP + Pi 2 ATP 2 NAD+ 2 NADH C C C C C C 2 pyruvate
Stage 2: The Krebs cycle • Occurs in the mitochondrial matrix. • Many enzymes are involved in this 8-step cycle. • Completes the job of breaking pyruvate down into carbon dioxide. Each pyruvate molecule yields: • 3 carbon dioxide molecules • Four loaded NADH molecules (carrying H+ and electrons) • One loaded FADH2 molecule (another acceptor molecule carrying H+ and electrons) • One ATP molecule.
Krebs cycle summary INPUTS: (per half glucose) • 1 pyruvate • 4 NAD+ • 1 FAD • 1 ADP + Pi OUTPUTS: • 3 CO2 • 4 NADH • 1 FADH2 • 1 ATP (Most of the energy is shuttled as electrons to the next stage.)
Stage 3: Electron transport chain • Takes place on the inner membranes of the mitochondria. • Electrons from the NADH and FADH2 are passed along a chain of components called cytochromes; mainly proteins through the membrane. • The last component of the chain passes its electrons to oxygen, which also picks up a pair of hydrogen ions from solution. • This forms water. • The chain eases the fall of electrons from food to oxygen to release energy is smaller amounts.
Not there yet?! Where is the ATP? • An enzyme at the end of the process called ATP synthase (what do you think it does...?) is used. • It puts the ADP + Pi together to form ATP.
