Harvesting Energy: Glycolysis and Cellular Respiration Chapter 8
Overview of Glucose Breakdown The overall equation for the complete breakdown of glucose is: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + ATP The main stages of glucose metabolism are: Glycolysis Cellular respiration
Overview of Glucose Breakdown Glycolysis Occurs in the cytosol Does not require oxygen Breaks glucose into pyruvate Yields two molecules of ATP per molecule of glucose
Overview of Glucose Breakdown If oxygen is absent fermentation occurs pyruvate is converted into either lactate, or into ethanol and CO 2 If oxygen is present cellular respiration occurs
Overview of Glucose Cellular respiration Breakdown Occurs in mitochondria (in eukaryotes) Requires oxygen Breaks down pyruvate into carbon dioxide and water Produces an additional 32 or 34 ATP molecules, depending on the cell type
Glycolysis Glucose activation phase Glucose molecule converted to highly reactive fructose bisphosphate by two enzyme- catalyzed reactions, using 2 ATPs
Glycolysis Energy harvesting phase Fructose bisphosphate is split into two three- carbon molecules of glyceraldehyde 3- phosphate (G3P) In a series of reactions, each G3P molecule is converted into a pyruvate, generating two ATPs per conversion, for a total of four ATPs Because two ATPs were used to activate the glucose molecule there is a net gain of two ATPs per glucose molecule
Glycolysis 1. Energy harvesting phase (continued) As each G3P is converted to pyruvate, two high-energy electrons and a hydrogen ion are added to an empty electron-carrier NAD+ to make the high-energy electron-carrier molecule NADH Because two G3P molecules are produced per glucose molecule, two NADH carrier molecules are formed
Glycolysis Summary of glycolysis: Each molecule of glucose is broken down to two molecules of pyruvate A net of two ATP molecules and two NADH (high-energy electron carriers) are formed
Fermentation Pyruvate is processed differently under aerobic and anaerobic conditions Under aerobic conditions, the high energy electrons in NADH produced in glycolysis are ferried to ATP-generating reactions in the mitochondria, making NAD+ available to recycle in glycolysis
Fermentation Under anaerobic conditions, pyruvate is converted into lactate or ethanol, a process called fermentation Fermentation does not produce more ATP, but is necessary to regenerate the high-energy electron carrier molecule NAD+, which must be available for glycolysis to continue
Fermentation Some cells ferment pyruvate to form acids Human muscle cells can perform fermentation Anaerobic conditions produced when muscles use up O 2 faster than it can be delivered (e.g. while sprinting) Lactate (lactic acid) produced from pyruvate
Fermentation Some microbes ferment pyruvate to other acids (as seen in making of cheese, yogurt, sour cream) Some microbes perform fermentation exclusively (instead of aerobic respiration) Yeast cells perform alcoholic fermentation
Cellular Respiration In eukaryotic cells, cellular respiration occurs within mitochondria,, organelles with two membranes that produce two compartments The inner membrane encloses a central compartment containing the fluid matrix The outer membrane surrounds the organelle, producing an intermembrane space
Pyruvate Breakdown in Mitochondria After glycolysis, pyruvate diffuses into the mitochondrion into the matrix space Pyruvate is split into CO 2 and a 2-carbon acetyl group, generating 1 NADH per pyruvate
Pyruvate Breakdown in Mitochondria Acetyl group is carried by a helper molecule called Coenzyme A, now called Acetyl CoA Acetyl CoA enters the Krebs Cycle and is broken down into CO 2
Pyruvate Breakdown in Mitochondria Electron carriers NAD + and FAD are loaded with electrons to produce 3 NADH & 1 FADH 2 per Acetyl CoA 6. One ATP also made per Acetyl CoA in the Krebs Cycle
Electron Transport Chain Most of the energy in glucose is stored in electron carriers NADH and FADH 2 Only 4 total ATP produced per glucose after complete breakdown in the Krebs Cycle NADH and FADH 2 deposit electrons into electron transport chains in the inner mitochondrial membrane Electrons join with oxygen gas and hydrogen ions to made H 2 O at the end of the ETCs
Chemiosmosis 1. Energy is released from electrons as they are passed down the electron transport chain 2. Released energy used to pump hydrogen ions across the inner membrane Hydrogen ions accumulate in intermembrane space
Chemiosmosis 1. Hydrogen ions form a concentration gradient across the membrane, a form of stored energy Hydrogen ions flow back into the matrix through an ATP synthesizing enzyme Process is called chemiosmosis
Chemiosmosis Flow of hydrogen ions provides energy to link 32-34 molecules of ADP with phosphate, forming 32-34 ATP ATP then diffuses out of mitochondrion and used for energy-requiring activities in the cell
Influence on How Organisms Function Metabolic processes in cells are heavily dependent on ATP generation (cyanide kills by preventing this) Muscle cells switch between fermentation and aerobic cell respiration depending on O 2 availability