Topic 8.1 Cell Respiration (HL)

8.1.1 Comparison of Oxidation and Reduction Loss of hydrogen atoms Energy Glucose Gain of hydrogen atoms OILRIG: Oxidation is losing electrons/protons; Reduction is gaining electrons/protons

8.1.1 Comparison of Oxidation and Reduction often associated with the release of energy Reduction: often associated with the gain of energy 

Reduction and Oxidation Reaction: Electron carriers Electron carriers are substances that accept and give up electrons as required. They often link oxidations and reductions in cells. Main electron carrier is NAD (nicotinamide adenine dinucleotide), it is a coenzyme It’s Reduced to NADH when it picks up two electrons and one hydrogen ion NAD+ + 2 H  NADH + H+ FAD + 2 H  FADH2 Remember that 2H = 2 electrons and 2H+

Aerobic respiration process There are four main stage in the breakdown of glucose during aerobic respiration: Glycolysis. The link reaction Krebs cycle Electron transport chain

8.1.2 Outline the process of Glycolysis Glucose (6C) Glucose phosphate (6C) Fructose bisphosphate (6C) glicerate 3-phosphate (3C) Pyruvate (3C) ATP ADP + Pi 2 ADP + Pi 2 ATP NAD+ NADH + H + PHOSPORILATION LYSIS GLYCOLYSIS TAKE PLACE IN THE CYTOPLASM OF CELLS. Glycolysis does no need oxygen. It is the first stage of anaerobic respiration and it is, in fact, the only anaerobic stage. Iniatially the glucose is phosphorilated to make glucose phosphate. The phosphate comes from a molecule of ATP. Glucose phosphate is then phosphorilated to fructose bisphosphate using up another ATP. Fructose bisphosphate split into 2 molecules of glicerate-3-phosphate (3C) and the glicerate-3-phosphate is converted to piruvate. Hydrogen is removed and transferred to the hydrogen acceptor NAD. Enough energy is released at this stage to make two molecules of ATP. Important: Since 2 molecules of glicerate-3-phosphate are formed, there will be 2 molecules of NADH2 formed and 2x2=4 molecules of ATP So from 1 molecule of glucose, glycolysis produces the following: 2 molecules of ATP (4 ATPs are produce but 2 are used up) 2 molecules of NADH2 (reduced hydrogen acceptor) 2 molecules of Piruvate, which enter the link reaction in aerobic respiration. OXIDATION & ATP FORMATION

8.1.3 Structure mitochondrion like in micrographs

8.1.4 Aerobic respiration: the link reaction Piruvate (3C) enters the matrix of the mitochondria from the cytoplasm Acetyl Coenzyme A (2C) Coenzyme A Piruvate (3C) Acetate (2C) + CO2 NAD+ NADH + H + In the presence of Oxygen 3 things happen: The piruvate is descarboxilated (a molecule of CO2 is removed) The piruvate is dehydrogenated (a molecule of hydrogen is removed). The hydrogen is transferred to the acceptor NAD+ to form NAD+ + H+ The resulting acetate (2C) combines with coenzyme A (CoA) to form the 2C-molecule acetyl-Coenzyme A, which enters Krebs cycle. Since 2 molecules of piruvate are formed form each glucose molecule, there will be also 2 acetyl CoA molecules formed. Piruvate + CoA +NAD+ acetyl-CoA + CO2 +NADH + H+

8.1.4 Aerobic respiration: Krebs Cycle Acetyl Co-A combines with a 4-carbon compound (oxalacetate) to form a six-carbon compound (citrate) A series of reactions take place where the citrate (6C) is both decarboxylated and dehydrogenated The most important role of the Krebs cycle is to provide hydrogen that can be used in the electron transport chain to provide energy for the formation of ATP. Krebs cycle take place in the matrix of the mitochondria and includes the following reactions> Acetyl Co-A combines with a 4-carbon compound (oxalacetate) to form a six-carbon compound (citrate) A series of reactions take place where the ciitrate (6C) is both decarboxylated and dehydrogenated Carbon dioxide is released as a waste product and the hydrogen atoms are picked up by the hydrogen aceptor NAD and FAD (flavine adeninde dinucleotide) As a result, oxaloacetate (4C) is regenerated to combine with more acertyl coenzyme A. So, after one turn of the Krebs cycle, we have: 3 molecules of NADH 1 molecule of FADH 1 molecule of ATP 2 molecules of CO2 But, don’t forget that 2 molecules of Acetyl-CoA enter in the Krebs cycle for each molecule of glucose. So the cycle turns twice for each glucose molecule, so giving: 6NADH, 2FADH, 4ATPs, 4CO2

The electron transport chain 8.1.4 Aerobic respiration: The electron transport chain The final stage occurs in the inner membranes of mitochondria. This stage has two parts: an electron transport chain and ATP production by ATP synthase The electron transport chain provides the means by which the energy from the hydrogen atoms removed from compounds in Krebs cycle, glycolysis and the link reaction can be used to make ATP. Oxygen is required for this final stage of aerobic respiration. The reactions take place in the inner membrane of the mitochondria. The electron transport chain involves a chain of carriers molecules along which hydrogen atoms and electrons are passed. The hydrogen atoms are passed on to other carrier molecules from the hydrogen carriers reduced NADH and FADH2.

8.1.5 Electron Transport Chain & Oxidative phosphorilation Chemiosmosis couples the electron transport chain to ATP synthesis NADH is the first carriers in the chain, it passes its hydrogen on to FAD. The hydrogen atoms split into hydrogen ions (H+) and electrons. The electrons are transferred along a series of electron carriers. The Hydrogen ions stay in solution in the space between the inner and outer membranes of the mitochondria. Finally, the electrons recombines with the hydrogen ions to form hydrogen atoms and are passed on to oxygen to form water. Oxygen is therefore the final electron acceptor. The transfer of electrons along the chain releases sufficient energy to make ATP from ADP+Pi.

8.1.5 Oxidative phosphorilation & Chemiosmotic theory Mitochondria have a double membrane and the inner membrane is folded to form cristae. The cristae are lined with stalked granules, these stalked granules cotain ATP synthetase enzyme. The chemiosmotic theory provides a model to explain the synthesis of ATP in oxidative phosphorilation. The energy released by electron transport chain os linked to pumping hydrogen ions from the matrix into the space between the 2 membranes of the mitochondrion. This results in a higher concentration of hydrogen ions in the intermembrane space than in the matrix of the mitochondrion: an electrical electrochemical gradient is set up. The hydrogen ions pass back into the matrix through the stalked granules, along the electrochemical gradient. As they do so, their electrical potential energy is used to make ATP from ADP + Pi. ATP synthetase catalyses the reaction.

8.1.6 Relationship between structure of mitochondrion and its function Cristae: Large Surface Area for the Electron Transport Chain Intermembrane Space: Accumulation of protons Matrix: containing enzymes for the Krebs Cycle

Overview Aerobic Cell Respiration

ATP balance For each NADH entering at the chain, 3 molecules of ATP are made. And for each FADH, 2 molecules of ATP are made. The formation of ATP in this way is called oxidative phosphorilation.