School of Sciences, Lautoka Campus BIO509 Lecture 27: Respiration

Aerobic respiration is the biological process by which reduced organic compounds are metabolized and subsequently oxidized in a controlled manner. Aerobic respiration is common to all organisms. Respiratory process in plants is similar to animals and lower eukaryotes.

glucose & oxygen carbon dioxide & ATP The equation for cellular respiration is the opposite of the equation for photosynthesis. Reactants Products glucose & oxygen carbon dioxide & ATP A redox reaction where glucose is oxidized to CO2 while O2 acts as the ultimate electron acceptor.

Loss of hydrogen atoms (oxidation) Gain of hydrogen atoms (reduction) C6H12O6 6 O2 6 CO2 6 H2O Loss of hydrogen atoms (oxidation) Gain of hydrogen atoms (reduction) Energy (ATP) Glucose +

Photosynthesis provides the fuel for cellular respiration CO2 H2O Glucose O2 ATP ECOSYSTEM Sunlight energy Photosynthesis in chloroplasts Cellular respiration in mitochondria (for cellular work) Heat energy + Photosynthetic organisms use light energy to: Produce glucose + O2 from CO2 + H2O. Cellular respiration produces chemical energy (ATP) by: Breaking down glucose & consuming O2, producing CO2 + H2O.

Mitochondria are the sites of respiration

Respiration is divided into three stages. 1. Glycolysis (occurs in cytosol). 2. Citric Acid Cycle or Tricarboxylic Acid Cycle or the Kreb’s Cycle (occurs in the mitochondrial matrix). 3. Electron Transport Chain (occurs in the inner mitochondrial membrane).

Glycolysis Glycolysis occurs in the cytosol. It does not require oxygen. Produces only about 25% of the total energy from glucose. Glucose (6-C) is first broken down to two 3-carbon sugars using ATP. These are oxidized and rearranged to produce two molecules of pyruvate. A small amount of energy in the form of ATP and NADPH is released from glycolysis.

Glycolysis 2 Produces 4 ATP molecules by substrate-level phosphorylation. When an enzyme transfers phosphate from a substrate molecule directly to ADP to form ATP.

Reduces 2 NAD+ to form 2 NADH. NADH molecules will be used to produce ATP in the 3rd stage of respiration (oxidative phosphorylation). Since 2 ATP were used in the preparatory phase of glycolysis, there is a net gain of 2 ATP molecules for each glucose molecule that enters glycolysis.

Substrate-level phosphorylation Enzyme transfers a phosphate group from a substrate molecule directly to ADP to form ATP. This produces 4 ATP molecules in glycolysis. Enzyme Adenosine Organic molecule (substrate) ADP ATP P ATP

For each glucose molecule processed, what are the net molecular products from glycolysis?

Citric Acid Cycle (Kreb’s Cycle) Occurs in the matrix of the mitochondria. It has been found that in the absence of air, cells produced ethanol and lactic acid while in the presence of air they produced CO2 and H20. The Kreb cycle was discovered by Hans Kreb in 1937.

For the Kreb cycle to function pyruvate generated in the cytosol during glycolysis needs to be transported to the mitochondria. Between glycolysis & the citric acid cycle: pyruvate is converted to acetyl Coenzyme A (CoA consist of vitamin pantothenic acid and a nucleotide).

A carbon atom is removed from pyruvate & released as CO2. The 2 carbon compound is oxidized while a molecule of NAD+ is reduced to NADH. CoA joins with the 2 carbon molecule to form acetyl coenzyme A (acetyl CoA) which then enters the Kreb cycle.

Kreb’s Cycle Takes place in the matrix of the mitochondria. Acetyl group (2 carbon) enters the cycle by combining with oxaloacetate (4 carbon), to form citrate (6 carbon). This initiates citric acid cycle.

As acetyl group passes round the cycle, the 2 carbon atoms are lost in CO2 in two decarboxylation reactions. Hydrogen is added to hydrogen carriers in four dehydrogenation reactions, resulting in a total of 3 NADH2 and 1 FADH2 molecules.

Details of the citric acid cycle 2 carbons enter cycle in the form of Acetyl CoA CoA is stripped from AcetylCoA and recycled. 2C + 4C forms a 6C compound (citric acid). 1 Step NAD- Nicotinamide adenine dinucleotide FAD - Flavin adenine dinucleotide CoA – Coenzyme A Succinate FAD FADH2 Malate NAD+ Oxaloacetate Citrate leaves cycle + H+ NADH Alpha-ketoglutarate CO2 ADP + P Acetyl CoA CITRIC ACID CYCLE 2 CoA 1 3 4 5 ATP Steps & 4 5 original 4C molecule is regenerated by oxidation to start cycle over again. this produces 1 NADH and 1 FADH2. Steps & 2 3 2 carbons are completely oxidized to CO2, producing 2 molecules of NADH. 1 ATP is produced by substrate-level phosphorylation.

The main functions of the TCA cycle Production of energy rich NADH, and CO2 Regeneration of oxalocatate. Producing NADH, FADH2, ATP

Glycolysis produces: 2 ATP So far, Overall Glycolysis produces: 2 ATP Krebs Cycle produces: 2 ATP, 10 NADH and 2 FADH2

Electron transport chain and ATP synthesis in the mitochondria Since cells only use energy in the form of ATP, the high-energy electrons present in the from of NADH and FADH2 need to be converted to ATP. This occurs in the inner membrane of the mitochondria and requires oxygen via the electron transport chain. The ETC consists of 4 large multi molecular complexes and two mobile carriers on the inner mitochondrial membrane.

Mobile carriers - ubiquione (Q) and cytochrome c (cyt c)

Electrons from NADH enters ETC at complex I (NADH –ubiquione oxidoreductase). From here the electron moves to ubiquione. It is mobile and transports the electrons to from complex I to complex II.

Complex III acts as an oxidoreductase and oxidizes reduced ubiquinone, transferring the electrons to cytochrome c. Cytochrome c is not an integral membrane protein in ETC and serves as a mobile carrier to electron transfer between complex III to complex IV. Complex IV reduces O2 (using 4 electrons ) to water.

During the movement of electrons in the ETC H+ are moved from the matrix to the intermembrane space through chemiosmosis. This creates a proton gradient. Electrons move down the gradient through the ATP synthase complex. It uses this energy to combine ADP and Pi to form ATP

Overall Aerobic respiration yields about 23-36 molecules of ATP per molecule of glucose. NADH FADH2 Cytoplasm Electron shuttle across membrane Mitochondrion GLYCOLYSIS Glucose Pyruvate by substrate-level phosphorylation by oxidative phosphorylation 2 Acetyl CoA CITRIC ACID CYCLE + 2 ATP + about 34 ATP About 38 ATP 2 6 (or 2 FADH2) OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) Maximum per glucose: ~38 ATP 34+4=38

Questions??