RESPIRATION The stepwise breakdown of glucose to carbon

RESPIRATION The stepwise breakdown of glucose to carbon dioxide and water to release energy

Aerobic respiration • References: Rowland pages 120 - 132 Collins pages 11 - 17 • Aerobic respiration glucose + oxygen C 6 H 12 O 6 + 6 O 2 carbon dioxide and water + energy 6 CO 2 + 6 H 20 + 38 mols ATP • A great deal of energy is released when glucose is oxidised to carbon dioxide and water in the presence of oxygen. If all this energy was released in one go, as it is in combustion, it would damage the cell. • It is released in a stepwise catabolism of glucose to release usable amounts of energy to produce ATP

ATP • ATP - adenosine triphosphate = adenosine P - P • ADP - adenosine diphosphate = adenosine P - P ADP + P ATP ADP + energy for all cell activities

Oxidation and reduction (redox) Three ways in which oxidation and reduction occurs in respiration • Direct involvement of oxygen A + O 2 AO 2 (oxidation = addition of oxygen) AO 2 A + O 2 (reduction = removal of oxygen) • Removal/addition of hydrogen AH A + H (oxidation = removal of hydrogen) A + H AH (reduction = addition of hydrogen) • Removal/addition of electrons Ae. A + e- (oxidation = removal of electrons) A + e- Ae(reduction = addition of electrons)

Hydrogen carriers • Oxidation of substrates in respiration often involves the removal of hydrogen atoms (dehydrogenation) and is important in the synthesis of ATP. • With the help of a dehydrogenase enzyme, hydrogen atoms are removed from a compound and taken up by a hydrogen carrier (acceptor) • There are two types of hydrogen carriers in respiration: NAD FAD nicotinamide adenine dinucleotide flavine adenine dinucleotide AH 2 NAD BH 2 A NADH + H+ B

Aerobic respiration can be divide into 4 stages • • Glycolysis Link reaction Krebs cycle Electron transport chain takes place in the cytoplasm take place in the mitochondria Total yield of ATP from one molecule of glucose = 38 ATP

GLYCOLYSIS • • Takes place in cytoplasm Does not need oxygen Each glucose molecule yields 2 ATP molecules Common to both aerobic and anaerobic respiration Glycolysis represents a series of reactions in which one molecule of glucose is broken down into 2 molecules of pyruvate. 3 main steps: • Phosphorylation of glucose(6 C) to form fructose diphosphate (6 C) • Fructose diphosphate split into two molecules of 3 C sugars • Conversion of two 3 C sugars into two molecules of pyruvate (3 C) • Net gain of 2 ATP and 2 NADH + H+

Glycolysis Read textbook references • Rowland pages 122 – 125 • Collins page 11 -12 • Make notes on glycolysis

What happens next? • If no oxygen is available: Pyruvate is converted (reduced) to lactate by accepting hydrogen from NADH There is no further production of ATP, so net gain of anaerobic respiration is 2 ATP from 1 molecule of glucose • If oxygen is available: • Pyruvate enters the mitochondria and will be fully oxidised to carbon dioxide and water and 38 ATP produced • The mitochondrion is the site of the Link Reaction, the Krebs Cycle and the Electron Transport Chain

Into the mitochondrion • Need to know the structure of a mitochondrion and to be able to recognise electronmicrographs; see Core Principles • The Link Reaction and the Krebs Cycle occur in the matrix of the mitochondrion • The Electron Transport Chain takes place on the membranes of the cristae • The cristae are highly folded to increase the surface area for the reactions to take place

The Link Reaction • The Link Reaction links glycolysis to the Krebs Cycle. • Oxygen is needed for the process • It occurs in the matrix of the mitochondrion • The products are all x 2 as there are 2 x pyruvates which enter the Link Reaction

So what happens? • Pyruvate enters the mitochondria (the inner mitochondrial membrane is impermeable to glucose and other intermediates) • The 3 -carbon pyruvate combines with the carrier molecule coenzyme A (Co. A) • It forms a 2 -carbon compound called acetyl coenzyme A (acetyl. Co. A) • The extra carbon is lost as carbon dioxide. • NAD+ removes hydrogen to form reduced NAD (NADH +H+) • The process is therefore called oxidative decarboxylation

The Krebs Cycle • Oxygen is needed for the process to occur • It takes place in the matrix of the mitochondrion • The cycle goes round twice for every molecule of glucose oxidised; the products are, therefore, all x 2 • It is where most of the oxidation takes place in aerobic respiration • It results in the complete breakdown of pyruvate to carbon dioxide and water

So what happens? • Acetyl coenzyme A enters the cyclic series of reactions called the Krebs Cycle • The 2 -carbon acetyl part of the molecule joins up with a 4 -carbon acceptor molecule to form a 6 -carbon acid • There are 2 decarboxylations to regenerate the 4 -carbon acceptor molecule and to release 2 molecules of carbon dioxide • Hydrogens and electrons are removed at 4 points in the cycle; involves NAD+ at three points and FAD at one point • One molecule of ATP is produced for each turn of the cycle; called substrate level phosphorylation

Result At the end of glycolysis, the Link Reaction and Krebs Cycle for every glucose molecule oxidised: • Glycolysis 2 NADH 2 ATP • Link Reaction 2 NADH 2 CO 2 • Krebs Cycle 6 NADH 2 FADH 2 2 ATP So where are the 38 ATP molecules? Don’t miss next week’s exciting instalment! 4 CO 2

Link Reaction and Krebs Cycle • Read textbooks and make notes on Link Reaction and Krebs Cycle – Rowland page 124 onwards – Roberts page 101 onwards – Collins page 12 -13

Electron Transport Chain • Oxygen is needed • Takes place in the phospholipid bilayer of the inner mitochondrial membranes - the cristae • Reduced coenzymes NAD (NADH + H+) and FAD (FADH 2) are re-oxidised to release large amounts of ATP • 34 molecules of ATP are produced from the oxidation of the reduced coenzymes

So what happens? • Reduced NAD and FAD are reoxidised by the removal of hydrogen by dehydrogenase enzymes located on the cristae of the inner membrane of the mitochondrion. • Each hydrogen atom is split into a hydrogen ion (H+) and an electron (e-) • The electrons then pass from one electron carrier molecule to another electron carrier molecule; • Electron carrier molecules in the ETC are called cytochromes and are proteins embedded in the cristae membranes • The electron carrier molecules are at successively lower energy levels

And more …… • As the electrons are transferred from one carrier to another some of their energy is released and used to convert ADP into ATP • One molecule of ATP is formed every time an electron is transferred from one carrier to the next • The removal of hydrogen ions and electrons is oxidation and so the overall reactions of the ETC are known as oxidative phosphorylation • Finally, each electron is reunited with a hydrogen ion (H+) which immediately combines with oxygen to form water

ETC - even more detail! • As the electrons are passed from one electron carrier to another, the energy released is used to actively pump hydrogen ions (H+ - protons) from the matrix into the space between the inner and outer membranes of the mitochondrion • Hydrogen ions accumulate and this sets up a large hydrogen ion gradient with a higher concentration of H+ in the inter-membrane space than in the matrix; this makes it more positively charged. • Hydrogen ions can only diffuse back down the concentration and electrical gradients through large protein carrier molecules which span the membrane, called ATP synthases • As H+ ions pass through the enzyme, it catalyses the synthesis of ATP from ADP and Pi

And finally …. . • The passage of a pair of electrons from one reduced NAD molecule along the ETC provides enough energy to move sufficient hydrogen ions to produce 3 molecules of ATP • As the first electron carrier is FAD, the passage of electrons from reduced FAD results in the production of only 2 ATP molecules • The final carrier in the chain transfers the electrons to 2 oxygen atoms. Each of these oxygen atoms picks up two protons (H+ ions) to produce a molecule of water 2 e- + 2 H+ + O —> 2 H 2 O • The enzyme cytochrome oxidase catalyses this last reaction involving oxidation by oxygen; it is inhibited by the poison cyanide

How much ATP is produced? • Calculate the number of ATPs produced by these processes: • a) Substrate level phosphorylation – glycolysis = 2 – Krebs Cycle = • b) 2 Oxidative phosphorylation • re-oxidation of 10 NADH along ETC @ 3 ATPs a go • re-oxidation of 2 FADH 2 along ETC @ 2 ATPs a go • Total 38 30 4

Electron Transport Chain • Read and make additional notes from – Rowland pages 127 - 128 – Collins page 14 – Roberts pages 103 - 104