Cell Respiration Note

CELL RESPIRATION Key Knowledge: • • • • • General structure of the biochemical pathways in cell respiration from initial reactant to final product The main inputs, outputs and locations of glycolysis, Krebs Cycle and electron transport chain including ATP yield (details of biochemical pathway mechanisms are not required) The location, inputs and the difference in outputs of anaerobic fermentation in animals and yeasts Uses and applications of anaerobic fermentation of biomass for biofuel production Factors affecting the rate of cell respiration (temperature, glucose availability and O2 concentration) CELL RESPIRATION Cellular respiration is the controlled release of energy from the breakdown of organic compounds. These compounds are produced by autotrophs (via photosynthesis) or can be synthesised from other pre-existing molecules within the cell (e.g. excess glucose can be converted to fats). Usable carbon compounds include: • • • Carbohydrates: The main organic molecule used in cell respiration is the monomer glucose (C6H12O6) Triglycerides: Fats produce more energy per gram than sugars, but are harder to transport and digest Proteins: Not a primary source as produces nitrogenous by-products (which are toxic if not excreted) ENERGY CONVERSIONS Organic molecules store energy in their chemical bonds – but this energy is not easily accessible for use by the cell. Cell respiration transfers this stored energy into coenzymes. Two types of coenzymes are used: • • ATP: Immediately available energy source (energy is released for use when ATP is hydrolysed to ADP) Hydrogen carriers: Transitional energy source (carries high energy electrons and protons for transfer) ATP can be produced directly from organic molecules via substrate level phosphorylation (pink arrow) or it can be indirectly synthesised by hydrogen carriers (needs O2) via oxidative phosphorylation (yellow arrow). Glucose = Piggy Bank (stored chemical energy) Hydrogen Carrier = Wallet (transitional energy source) ATP = Cash (usable energy) TYPES OF CELL RESPIRATION Cellular respiration can involve one of two reaction pathways: anaerobic respiration or aerobic respiration ANAEROBIC RESPIRATION AEROBIC RESPIRATION Partial breakdown of glucose Oxygen is not required for a small ATP yield Occurs entirely in the cytosol Involves glycolysis and fermentation Products: Lactic acid / Ethanol + CO2 Complete breakdown of glucose Oxygen is required for a large ATP yield Occurs in the mitochondria Involves glycolysis, Krebs cycle and ETC Products: Carbon dioxide and water ANAEROBIC RESPIRATION Anaerobic respiration involves the partial breakdown of carbohydrates (glucose) in the absence of oxygen. It occurs in the cytosol and results in a low yield of ATP (net production = 2 ATP). This ATP is produced via substrate level phosphorylation. The process of anaerobic respiration involves glycolysis and fermentation. GLYCOLYSIS Both anaerobic and aerobic respiration begins with the breakdown of glucose in the cytosol via glycolysis. Glycolysis splits glucose into two molecules of pyruvate in a process that consumes two molecules of ATP. However, four molecules of ATP are produced via substrate level phosphorylation, resulting in a net gain of two ATP molecules. Additionally, the coenzyme NAD is loaded with hydrogen to form molecules of NADH. C6H12O6 Glucose + 2 ATP ENERGY INVESTMENT 2 × C3H4O3 Pyruvate NADH + + Hydrogen carrier 4 ATP ENERGY PAYOFF (COENZYME LOADING) FERMENTATION In the presence of oxygen, the hydrogen carriers produced by glycolysis may be used by the mitochondria to produce large amounts of ATP (via oxidative phosphorylation). However, in the absence of oxygen the hydrogen carriers must be unloaded to allow for glycolysis to continue (NADH must be unloaded to NAD). Fermentation involves the conversion of pyruvate via a reaction that unloads hydrogen carriers to restore stocks of NAD. In plants and yeasts, pyruvate is irreversibly converted into ethanol and carbon dioxide. In animals, pyruvate is converted into lactic acid (however, this reaction can be reversed if oxygen is present). GLYCOLYSIS FERMENTATION glucose lactate (muscles) Fermentation is reversible in animals (but is irreversible in plants or yeast) This means that lactic acid can be converted back into pyruvate when exercise is over and the pyruvate can then be digested aerobically to make ATP (via oxidative phosphorylation) NAD+ ATP NADH pyruvate AEROBIC RESPIRATION Aerobic respiration completes the breakdown of glucose begun by glycolysis. This process requires oxygen and occurs within the mitochondrion. Aerobic respiration occurs via two distinct reactions: • Krebs Cycle: Pyruvate is broken down to make carbon dioxide and large amounts of hydrogen carriers • Electron Transport Chain: Hydrogen carriers are unloaded to produce ATP (oxidative phosphorylation) C6H12O6 Glucose + 6 O2 6 CO2 Oxygen Carbon dioxide + 6 H 2O Water + 30 ATP MITOCHONDRIA The mitochondrion is an organelle in eukaryotic cells that is responsible for aerobic respiration. It is believed to have evolved via endosymbiosis, when an aerobic bacterium was engulfed by another prokaryotic cell. Evidence for this endosymbiotic origin includes the fact that the mitochondrion possesses circular DNA, 70S ribosomes has a double membrane. In terms of structure, the central region is called the matrix and is the location of the Krebs cycle. Mitochondria contain an inner membrane that is highly folded into cristae. The cristae are the site of the electron transport chain. By folding the inner membrane, the SA:Vol ratio is increased, which optimises electron transport. Intermembrane space Small space to quickly accumulate protons Mitochondrion Inner membrane Contains ETC and ATP synthase for oxidative phosphorylation Cristae Highly folded to raise SA:Vol ratio Matrix Appropriate enzymes and suitable pH for Krebs cycle Outer membrane Contains transport proteins KREBS CYCLE Pyruvate from glycolysis is transported into the mitochondrion and converted into acetyl CoA. It is then completely broken down via a series of reactions collectively called the Krebs cycle (or citric acid cycle). The breakdown of one pyruvate produces three carbon dioxide molecules, as well as one ATP molecule via substrate level phosphorylation (as glucose produces two pyruvate molecules, the Krebs cycle will produce 6 × CO2 molecules and 2 × ATP per glucose). It will also result in the mass production of large quantities of hydrogen carriers (mainly NADH). These hydrogen carriers are unloaded via the electron transport chain. ELECTRON TRANSPORT CHAIN Hydrogen carriers (formed in glycolysis and the Krebs cycle) are unloaded to release protons and electrons. The high-energy electrons move through an electron transport chain, which essentially syphons this energy to synthesise ATP. This method of ATP production is known as oxidative phosphorylation as it requires the unloading of hydrogen (i.e. protons and electrons) from the hydrogen carriers. The de-energised electrons are taken up by oxygen, which combines with the protons to form water. The electron transport chain can only continue to unload hydrogen carriers if oxygen is present to accept the de-energised electrons, hence aerobic respiration will not occur in the absence of oxygen. Unloading all the hydrogen carriers produced by one molecule of glucose will result in the production of 26 molecules of ATP. If oxygen is not available, the hydrogen carriers can be unloaded anaerobically via fermentation, however this will not produce any additional ATP (this is why aerobic respiration has a much higher ATP yield than anaerobic respiration). OVERVIEW OF AEROBIC RESPIRATION CYTOSOL 2 Glucose 4 ATP Glycolysis ATP Pyruvate 2 NADH 3 ATP 2 NADH 5 ATP 2 ATP Acetyl-CoA Krebs Cycle Substrate level 6 NADH 15 ATP 2 FADH2 3 ATP Oxidative Total net ATP yield: 30 Aerobic Stages: Glycolysis Glucose → Pyruvate Substrate Level: 2 ATP Krebs Cycle Pyruvate → CO2 Substrate Level: 2 ATP Electron Transport Oxygen → Water Oxidative: 26 ATP Intermediates: NADH = NAD + H+ + e– ANAEROBIC VERSUS AEROBIC Cell respiration involves the partial (anaerobic) or complete (aerobic) digestion of glucose to produce ATP for use by the cell. Both pathways begin with the initial breakdown of glucose (by glycolysis) to form two molecules of pyruvate. In anaerobic respiration, this pyruvate is converted within the cytosol into either lactic acid (animals) or ethanol and carbon dioxide (plants and yeast). In aerobic respiration, the pyruvate is converted into carbon dioxide and water within the mitochondrion. Aerobic respiration requires oxygen to proceed and produces a larger yield of ATP (oxidative phosphorylation utilises the hydrogen carriers). ATP YIELD TYPE OF CELL RESPIRATION Glycolysis Krebs Cycle Electron Transport Chain Aerobic Cell Respiration 2 × ATP 2 × ATP 26 × ATP Anaerobic Fermentation 2 × ATP BIOFUELS Biofuels are an energy source produced from the anaerobic fermentation of biomass (i.e. organic material from plants or animals). Biofuels are a renewable resource and are typically associated with a lower carbon footprint (because biomass is typically produced via photosynthesis, which uses CO2 as an input). Biomass has historically been produced from agricultural feedstocks (edible crops), which requires large amounts of arable land and drives up local food prices (as less crops are being used as a food source). Biomass can also be produced from non-edible plant components and certain municipal wastes; however, these sources are associated with higher costs of production. More recently, algae has been used as a source of biomass. The algae can photosynthesise at low costs and does not require large quantities of land (can be maintained in a photobioreactor). Bioethanol is a common biofuel that can be used to supplement or replace traditional fossil fuels (i.e. petrol) in fuel tanks. Drawbacks of bioethanol include the fact that it has a lower energy output than fossil fuels, is harder to vaporise (more difficult to use in colder temperatures) and is more likely to corrode materials (such as car engines) upon extended exposure (i.e. higher maintenance costs). FACTORS AFFECTING RESPIRATION RATE The rate of respiration can be measured by either the consumption of inputs (glucose and oxygen) or the formation of product (carbon dioxide). However, these conditions may be affected by the pathway used: • Anaerobic respiration does not use oxygen and carbon dioxide is only produced as a by-product of yeast or plant fermentation (animal cells convert pyruvate into lactic acid via a reversible reaction) Factors that affect aerobic respiration include: temperature, glucose concentration and oxygen availability Cell respiration is catalysed by a variety of enzymes and is therefore impacted by ambient temperatures. If the temperature is too low, the activation energy threshold cannot be reached. As temperatures increase, reaction rate will also increase as more kinetic energy results in more frequent enzyme-substrate collisions. At optimal temperatures, activity will peak, as higher temperatures will denature the enzymes involved in cell respiration. Rate of Respiration TEMPERATURE Temperature (°C) Glucose is the initial substrate for both pathways of respiration (anaerobic and aerobic). Higher glucose levels will result in increased frequency of collisions with glycolytic enzymes over a given period of time. Above a certain glucose level, the rate of respiration will plateau. This is because the environment is now saturated with glucose and some other condition has become the limiting factor that determines the rate. Rate of Respiration GLUCOSE CONCENTRATION Glucose Concentration Increasing oxygen levels will result in higher rates of aerobic respiration. This is because oxygen is needed to maintain the functioning of the electron transport chain. Higher oxygen concentrations will increase the rate of respiration up to a certain point, above which the respiration rate will plateau as the environment is now saturated with oxygen and some other factor has become the rate-limiting factor for respiration. Rate of Respiration OXYGEN AVAILABILITY Oxygen Concentration EXERCISE INTENSITY One condition that will increase the rate of cellular respiration is exercise (muscles require ATP in order to contract). Strenuous physical exertion will utilise anaerobic respiration, as ATP requirements exceed levels of oxygen intake. The resultant accumulation of lactic acid within the muscles will cause fatigue, rendering high levels of exercise unsustainable. Aerobic respiration is then used as the level of activity diminishes.