Cell Respiration - Metabolism

CELL RESPIRATION ENERGY Living organisms need chemical energy to power the metabolic reacCons that sustain life processes. These processes include the biosynthesis of organic molecules (via anabolism), the ac>ve transport of materials across a cell membrane and the overall movement of cells or cellular components (such as chromosomes). ADENOSINE TRIPHOSPHATE A coenzyme is a complex organic molecule that is required for an enzyme’s metabolic acCvity. Coenzymes cycle between two states: a loaded form capable of assisCng enzyme acCvity and an unloaded form (similar to a charged and expended baPery). Adenosine triphosphate (ATP) is a coenzyme that is used to transport chemical energy to metabolic reacCons. It funcCons as an immediate energy source for a cell (i.e. energy currency). ATP possesses three negaCvely charged phosphates that need increasing amounts of energy to hold together. When the ATP is hydrolysed to adenosine diphosphate (ADP) and an inorganic phosphate, the released energy can then be uClised by the cell. COENZYMES UNLOADED LOADED ATP Adenine Energy Ribose P P P CELL RESPIRATION Cellular respiraCon 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-exisCng 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) When the organic compounds are broken down in cell respiraCon, the chemical energy that is released is used to generate ATP (from ADP + Pi). In this respect, the organic compounds funcCon like a bank (storing high amounts of chemical energy within their covalent bonds), while the ATP is usable money. The funcCon of cell respiraCon is to convert the inaccessible chemical energy into an available form that cells can use. TYPES OF CELL RESPIRATION Cellular respiraCon can involve one of two reacCon pathways: anaerobic respiraCon or aerobic respiraCon ANAEROBIC RESPIRATION AEROBIC RESPIRATION Par>al breakdown of glucose Oxygen is not required for a small ATP yield Occurs enCrely in the cytosol Involves glycolysis and fermentaCon Products: LacCc 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 respiraCon involves the par.al breakdown of carbohydrates (glucose) in the absence of oxygen. It occurs in the cytosol and results in a low yield of ATP (net producCon = 2 ATP). The glucose can either be converted into lac>c acid (animals) or ethanol and carbon dioxide (plants and yeast). The producCon of lacCc acid in humans is used to maximise the power of muscle contracCons when levels of oxygen are low. Glucose LacCc Acid 2 ATP AEROBIC RESPIRATION Aerobic respiraCon involves the complete breakdown of carbohydrates (glucose) in the presence of oxygen. It occurs in the mitochondria and results in a high yield of ATP (net producCon = 30 ATP). The oxygen is used to complete the digesCon of glucose, resulCng in the formaCon of carbon dioxide and water. While sugars like glucose can be digested via either respiratory pathway, triglycerides can only be digested aerobically. Glucose Oxygen Carbon Dioxide Water 30 ATP MITOCHONDRIA The mitochondrion is an organelle in eukaryoCc cells that is responsible for aerobic respiraCon. It is believed to have evolved via endosymbiosis, when an aerobic bacterium was engulfed by another prokaryoCc cell. Evidence for this endosymbioCc 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, while the inner membrane is highly folded into cristae. By folding this membrane, the SA:Vol raCo is increased. This opCmises the structure of the mitochondria to allow aerobic respiraCon to occur more rapidly and in greater amounts (maximising total ATP yield). Mitochondrion FACTORS AFFECTING RESPIRATION RATE The rate of respiraCon can be measured by either the consumpCon of inputs (glucose and oxygen) or the formaCon of product (carbon dioxide). However, these condiCons may be affected by the pathway used. Factors that affect aerobic respiraCon include temperature and pH (which will alter enzyme funcConality), as well as glucose concentraCon and oxygen availability (both of which funcCon as respiratory substrates).