RESPIRATION

1 EJC H2 Biology T3W7 Respiration Respiration 1. Overview of Topic Cellular respiration is defined as ‘the process by which chemical energy in organic molecules is released by oxidation’ i.e. it involves metabolic processes which allow organisms to obtain energy from organic molecules. Under this broad definition, there are 2 main energy-producing pathways: (1) aerobic respiration which occurs in the presence of oxygen. (2) anaerobic respiration which occurs in the absence of oxygen. Respiration is an energy-releasing process. As ATP is the primary energy currency of the cell, main function of respiration is production of ATP. Part of respiration occurs in cytosol and part of it in mitochondria of the cell. 2. Learning Outcomes a. outline the process of glycolysis, highlighting the location, raw materials used and products formed (knowledge of details of the intermediate compounds and isomerisation is not required) b. outline the processes of the link reaction and Krebs cycle, highlighting the location, raw materials used and products formed (in terms of dehydrogenation and decarboxylation) c. outline the process of oxidative phosphorylation including the role of oxygen and the electron transport chain in aerobic respiration (names of complexes in the ETC are not required) d. explain the production of a small yield of ATP from respiration in anaerobic conditions in yeast and in mammalian muscle tissue e. explain the significance of the formation of ethanol in yeast and lactate in mammals in the regeneration of NAD f. investigate the effect of factors such as substrate concentration, type of substrate and temperature on the rate of respiration g. outline chemiosmosis in photosynthesis and respiration (names of complexes in the ETC are not required). Use the knowledge gained in this section in new situations or to solve related problems. 3. References Main references:  Campbell, Reece et al. (2015) Biology, 10th edition. Benjamin-Cummings Publishing Co.  Raven, Johnson, Losos, Mason and Singer (2008). Biology, 8th edition. McGraw-Hill.

2 EJC H2 Biology T3W7 Respiration Contents 1. Overview of Topic ...................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Respiration - Introduction ......................................................................................... 3 5. Aerobic Respiration ................................ ................................ ................................ ... 4 6. Anaerobic Respiration ................................................ Error! Bookmark not defined. 7. Other Substrates Used in Respiration ……………………………………………………..………....20

3 EJC H2 Biology T3W7 Respiration Inner membrane Outer membrane Cristae: infoldings of inner membrane of mitochondrion Mitochondrial matrix 70S ribosome Circular DNA Figure 1. Diagram of a mitochondrion Intermembrane space 4. Respiration - Introduction Cellular respiration is defined as ‘the process by which chemical energy in organic molecules is released by oxidation’ i.e. it involves metabolic processes which allow organisms to obtain energy from organic molecules. Under this broad definition, there are 2 main energy-producing pathways: (3) aerobic respiration which occurs in the presence of oxygen. (4) anaerobic respiration which occurs in the absence of oxygen. Functions of respiration: Respiration is an energy-releasing process. As ATP is the primary ene rgy currency of the cell, main function of respiration is production of ATP. Some examples where ATP is required: (1) muscle contraction, beating of cilia (e.g. on cells lining trachea), beating of flagella (e.g. in sperm, bacteria) (2) active transport of substances into or out of cells (e.g. Na +-K+ pump in cell membranes) (3) synthesis of substances for growth and repair (e.g. translation) (4) electrical transmission of nerve impulses (5) maintenance of constant body temperature i.e. in homoeothermic/ warm-blooded animals (6) bioluminescence by fireflies, glow-worms and some deep sea animals Locations of respiration: Part of respiration occurs in cytosol and part of it in mitochondria of the cell. Notes to self

4 EJC H2 Biology T3W7 Respiration Glycolysis Cytosol ATP NADH NADH FADH2 CO2 Glucose Krebs cycle Link reaction Pyruvate Oxidative phosphory- lation Figure 2. Respiration summary. Grey boxes indicate stages of aerobic respiration. White boxes indicate substrates and products. Mitochondrion 5. Aerobic Respiration  In a eukaryotic cell, aerobic respiration involves 4 main stages (see Fig. 2 for overview): (1) glycolysis in cytosol (2) link reaction in mitochondrial matrix (3) Krebs cycle in mitochondrial matrix (4) oxidative phosphorylation involving electron transport chain on cristae  All 4 stages require many different enzymes. Figure 3. Structure of ATP. Loss of terminal phosphate group results in an ADP molecule, as occurs during the first stage of glycolysis. Loss of 2nd phosphate group will produce AMP. Note: ATP is a ribonucleotide. Suitability of ATP as an energy source:  Universally used – all cells and organisms use it as an energy source.  Soluble, highly mobile and can be transported readily/ diffuses readily to point of need.  Easy for terminal phosphate group to be lost  Easy interconversion: ADP + Pi forms ATP, and ATP can be hydrolysed to ADP + Pi, leading to energy release.

5 EJC H2 Biology T3W7 Respiration  The first stage of respiration involves glycolysis - oxidation of glucose (6C) to 2 pyruvate (3C). In effect, a 6 carbon sugar is split into two 3 -carbon sugars. (Glycolysis = splitting of sugar)  It does not require O2 (it occurs whether or not O2 is present) and does not release CO2.  It can be divided into 4 stages: (1) Phosphorylation of glucose First, 2 phosphate groups are added to glucose to produce fructose 1,6-bisphosphate. The 2 phosphate groups are donated to the sugar by 2 ATP molecules, which form 2 ADP molecules. This phosphorylation activates the sugar, making it more reactive and committing it to the glycolytic pathway. Phosphorylation also confers a negative charge to glucose, making it impermeable, cannot diffuse across cell membrane hence it is trapped within the cytosol. Therefore, phosphorylation of sugar involves initial investment of 2 ATP molecules - no ATP has been produced as yet; instead, 2 ATP molecules have been used up! Phosphofructokinase (PFK) catalyses addition of the 2nd phosphate group. This enzyme is inhibited by excess ATP and/or citrate in the cell. Since the ultimate goal of glycolysis is to produce ATP and too much of this end product (ATP) inhibits the enzyme, this is an example of end product inhibition (allosteric inhibition). Conversely, PFK is stimulated by AMP and ADP (allosteric activators). Thus, rate of glycolysis is regulated according to the energy demands of the cells. (Note: Hexokinase catalyses addition of the first phosphate) 5.1 Glycolysis Glycolysis occurs in cytosol Glucose Fructose 1,6-bisphosphate 2ATP 2ADP Phosphofructokinase (PFK) p p

6 EJC H2 Biology T3W7 Respiration (2) Lysis Next, phosphorylated 6C sugar (fructose 1,6 -bisphosphate) is split into two 3C sugar phosphates: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate. G3P is also known as triose phosphate (TP) and phosphoglyceraldehyde (PGAL). The 2 sugar phosphates (G3P and dihydroxyacetone) a