Biology – Communication, Homeostasis and EnergyModule 4 – Respiration 1) WHY DO LIVING ORGANISMS NEED TO RESPIRE?Respiration is the process whereby energy stored in complex organic molecules(carbohydrates, fats and proteins) is used to make ATP.Energy exists as potential energy and kinetic energy. Moving molecules have kineticenergy that allows them to diffuse down concentration gradients. Chemical potentialenergy is present in large organic molecules.We need energy to allow all of our biological processes in our cells to take place.These processes are known as metabolism collectively. Building large moleculesare described as anabolic processes and breaking them down are known ascatabolic processes.Energy comes from sources such as sun light, and it cannot be created or destroyed,just converted to a different form. Plants, and other organisms known asphotoautotrophs, can synthesis large biological molecules such as glucose. This isan example of converting light energy from sunlight to chemical energy stored in thelarge molecule. Respiration of these organic molecules releases this chemicalenergy.ATP is a phosphorylated nucleotide, which, when hydrolysed, releases 30.6kJ ofenergy per mole. It is continually being hydrolysed and resynthesised (by respiration)during cellular processes. 2) COENZYMESThe respiration of the main respiratory substrate, glucose, can be described in fourstages, all of which involve metabolic pathways involving enzymes and substrates:Glycolysis – occurring in the cytoplasm, this involves the breakdown of a glucosemolecule to 2 pyruvate molecules. It occurs in both aerobic and anaerobicrespiration.The Link Reaction – this happens in the matrix of mitochondria. Pyruvate isdehydrogenated and decarboxylated in order to convert it to acetate.Krebs Cycle – also takes place in the matrix of mitochondria. Acetate is alsodecarboxylated and dehydrogenated.Oxidative Phosphorylation – takes place on the inner membrane of the mitochondria,and ADP is phosphorylated to ATP.In all stages of respiration, apart from oxidative phosphorylation, hydrogen atoms areremoved from substrates during oxidation reactions. They are added to other
molecules in reduction reactions. Enzymes on their own are not very efficient atcarrying out these reactions, so require coenzymes to work effectively. Coenzymessuch as NAD accept the hydrogen atoms, and then carry them to the mitochondrialmatrix to be involved in the process of oxidative phosphorylation.NAD – This coenzyme is required to carry hydrogen atoms with their electrons to thecristae of the mitochondria where the NAD will be oxidised and its associatedhydrogen atoms will be lost.Coenzyme A – This coenzyme carries ethanoate or acetate groups from the linkreaction the Krebs cycle. 3) GLYCOLYSISGlycolysis occurs in both pro- and eukaryotic cells. It occurs in the cytoplasm andinvolves the breakdown of glucose into pyruvate through the process of manyenzyme-catalysed reactions of glucose and other intermediate substrates.Glycolysis: - One molecule of ATP is hydrolysed, and its inorganic phosphate group binds to glucose at carbon 6, forming glucose 6-phosphate. - This is then changed to fructose 6-phosphate. - Another ATP molecule is hydrolysed, and this time the inorganic phosphate group is bound to fructose 6-phosphate at carbon 1, forming fructose 1,6- bisphosphate. - The energy from the hydrolysis of this ATP molecule activates the hexose sugar and keeps it within the cell. - Hexose 1,6-bisphosphate is then split into two molecules of triose phosphate, a 3-carbon compound. - The oxidation of triose phosphate reduces 2 NAD molecules and two molecules of ATP are formed at this stage by substrate level phosphorylation. - Four more reactions convert each TP molecule to a molecule of pyruvate. This process causes another two molecules of ADP to become phosphorylated to ATP by substrate level phosphorylation.Glycosis can be shown in simple terms bythis equation:Glucose 2 x Pyruvate + 2NADH + 2ATP (net)
4) STRUCTURE AND FUNCTION OF MITOCHONDRIAStructure: - All mitochondria have an inner and outer membrane, known as an envelope. - The inner membrane is folded into cristae, which increases the surface area. - The mitochondrial matrix is enclosed by the inner membrane. It’s semi-rigid, gel-like and it consists of a mixture of proteins and lipids. It also contains mitochondrial DNA, mitochondrial ribosomes and associated enzymes.The matrix is where the link reaction and the Krebs cycle take place. It contains: - Enzymes necessary for these reactions. - Molecules of NAD - Oxaloacetate – a 4-carbon compound that accepts acetate from the link reaction - Mitochondrial DNA, for synthesis of enzymes and other mitochondrial proteins. - Mitochondrial ribosomes, see above.The inner membrane is impermeable to most small ions, including protons (hydrogenions). It is also folded into numerous cristae to give it a large surface area and hasembedded in it many electron carriers and ATP synthase enzymes.The outer membrane is similar to other organelles’ membranes, in that they allowmolecules to pass over it by facilitated or passive diffusion, active transport, etc. 5) THE LINK REACTION AND KREBS CYCLEDecarboxylation and dehydrogenation of pyruvate to acetate are enzyme-catalysedreactions. Pyruvate dehydrogenase removes hydrogen atoms from pyruvate andpyruvate decarboxylase removes a carboxyl group, which eventually becomes CO2,from pyruvate. The coenzyme NAD accepts the hydrogen atoms and coenzyme Aaccepts acetate, forming acetyl coenzyme A. CoA carries acetate to the Krebs cycle.Summarising the link reaction:2 x Pyruvate + 2NAD + 2CoA 2NADH + 2 x acetyl CoA + 2CO2
The Krebs cycle takes place in the mitochondrial matrix. It oxidises the acetyl groupof acetyl CoA to two molecules of CO2. It also produces one molecule of ATP bysubstrate-level phosphorylation, and reduces 3 molecules of NAD with 1 molecule ofFAD: - The acetate is offloaded from CoA and joins with a 4-carbon compound, known as oxaloacetate, forming 6-carbon citrate. - Citrate is decarboxylated and dehydrogenated to form a 5-carbon compound. This process also reduces NAD. - The 5-carbon compound is decarboxylated and dehydrogenated to form a 4- carbon compound. This process also reduces NAD. - The 4-carbon compound is changed into another 4-carbon compound. During this, ADP is phosphorylated to form ATP. - The second 4-carbon compound is changed into another 4-carbon compound. A pair of hydrogen atoms are removed and accepted by FAD, which is reduced. - The third 4-carbon compound is further dehydrogenated and regenerates oxaloacetate, reducing another molecule of NAD.For each molecules of glucose, in the link reaction and Krebs cycle, the following areproduced: - 8 NADH - 2 FADH2 - 6 CO2 - 2 ATP
6) OXIDATIVE PHOSPHORYLATION AND CHEMIOSMOSISThe final stage of aerobic respiration involves electron carriers embedded in theinner mitochondrial membranes. These membranes are folded into cristae, thusincreasing the surface area for electron carriers and ATP synthase enzymes. NADHand FADH2 are reoxidised when they donate hydrogen atoms, split into protons andelectrons, to the electron carriers. The first electron carrier to accept electrons fromreduced NAD is a protein complex, complex I, called NADH – coenzyme Qreductase. The protons are driven into the intermembrane space as electrons flowalong the electron transport chain, so a proton gradient builds up in theintermembrane space.These protons can only flow back into the matrix through an ATP synthase enzyme.As the protons flow back through, the drive the rotation of part of the enzyme andjoin ADP to inorganic phosphate to form ATP. The electrons are accepted byoxygen, the final electron acceptor, which eventually forms water using hydrogenions.Theoretically, 32 molecules of ATP should be yielded per molecule of glucoserespired, however, this is rarely achieved, as: - Some protons leak across the mitochondrial membrane, reducing the potential energy of the protons to make ATP. - Some ATP produced is used to transport pyruvate into the mitochondrial matrix. - Some ATP is used for the shuttle to bring hydrogen from reduced NAD made during glycolysis into the mitochondria.
7) ANAEROBIC RESPIRATION IN MAMMALS AND YEASTSince oxygen is the final electron acceptor in oxidative phosphorylation, in theabsence of oxygen, the electron transport chain cannot function so the Krebs cycleand the link reaction also stop. The NADH from oxidation of glucose duringglycolysis must continually be reoxidised so that ATP can continue to be producedthrough anaerobic respiration.In humans, lactate fermentation occurs under anaerobic conditions. Essentially,pyruvate and 2H from NADH react, to form lactate in an enzyme-catalysed reaction(lactate dehydrogenase).Pyruvate + NADH Lactate + NAD+The lactate diffuses out of the cell into the blood stream and is carried to the liver.When more oxygen is available (made so by “oxygen debt”) the lactate can beconverted back to pyruvate where it can enter the link reaction and Krebs cycle. Thereduction in pH due to lactate inhibits enzyme activity so muscle fatigue occurs as aresult of this.In organisms like fungi, such as yeast, alcoholic fermentation occurs. Underanaerobic conditions, pyruvate is decarboxylated to ethanal. It is a reaction catalysedby the enzyme pyruvate decarboxylase. Ethanal accepts hydrogen atoms fromreduced NAD which reduces it to ethanol.Pyruvate + NADH Ethanal +NADH + CO2 Ethanol + NAD+ + CO2 8) RESPIRITORY SUBSTRATESA respiratory substrate is any organic substance that can be used for respiration. Itneeds to provide hydrogen atoms for the reduction of NAD and FAD and forsubsequent use in oxidative phosphorylation.Carbohydrates, i.e. sugars, such as fructose or galactose, can be converted toglucose for respiration. Disaccharides can by hydrolysed to from glucose and othermonosaccharides that can then be converted to glucose. Polysaccharides (glycogenin animals, starch in plants) can be hydrolysed to glucose for respiration. All hexoseshave the same formula (C6H12O6) and so release the same amount of energy permole, i.e. they release the same amount of hydrogen atoms. - The theoretical maximum yield for glucose is 2870 kJ per mol. - It takes 30.6 kJ to produce 1 mol ATP. - So, theoretically, the respiration of 1 mol of glucose should produce nearly 90 mol ATP. - The actual yield is around 30 mol ATP, as the remaining energy is released as heat, helping to maintain a suitable temperature for enzyme activity.
Amino acids in excess from the digestion of dietary protein can be deaminated toketo acids. The keto acids can be converted to glycogen or fat as an energy store.Both of these energy stores can be broken down to glucose for respiration.Amino acids can also be directly respired. Under conditions of starvation, fasting orprolonged exercise, muscle protein is hydrolysed to amino acids. Different aminoacids can be converted to either pyruvate, acetyl CoA, or intermediates in the Krebscycle. Thus, amino acids can only be respired aerobically.The number of hydrogen atoms per mole accepted by NAD and then used inoxidative phosphorylation is slightly more than the number of hydrogen atoms permole of glucose, so proteins release slightly more energy than equivalent masses ofcarbohydrate.Lipids in the form of triacylglycerol (triglycerides) are an energy store.Triacylglycerols are formed in a condensation reaction between glycerol and threefatty acid chains.When energy is required, the triglyceride can be hydrolysed back to glycerol andthree fatty acid chains in a reaction catalysed by the enzyme lipase. The glycerol canbe converted to glucose for respiration. The fatty acids can be oxidised to releasemore energy in a process called beta oxidation.A fatty acid must first be activated by converting it to fatty acyl CoA. This requires amolecule of ATP and release of energy from hydrolysis of two bonds to form AMP, Piand Pi. Thus, the equivalent of two molecules of ATP has been used.The fatty acyl CoA then goes through a series of reactions in the mitochondrialmatrix resulting in the release of 2 carbons as acetyl CoA. Each time a bond isbroken in the fatty acid chain to release acetyl CoA, a molecule of NADH is formedand a molecule of FADH2 is formed.