Cellular Respiration

1. Cellular Respiration Introduction – all forms of life depend directly or indirectly on light energy captured during photosynthesis – glucose molecules are broken down back into carbon dioxide and water (molecules the plant started with)

2. ATP – Adenosine triphosphate • most common energy carrier in cells • nucleotide composed of adenine, the sugar ribose, and three phosphate groups • synthesized from adenosine diphosphate (ADP) and inorganic phosphate – process is called phosphorylation • during glucose breakdown, energy is release and stored in bonds of ATP

4. Summary of complete glucose metabolism: • Photosynthesis: 6CO2 + 6H2O + sunlight energy  C6H12O6 + 6O2 • Complete glucose metabolism: C6H12O6 + 6O2 6CO2 + 6H2O + chemical and heat energies

5. Glycolysis • first stage of aerobic respiration • does not require O2 (anaerobic) and proceeds in exactly the same way under both aerobic (with oxygen) and anaerobic (without oxygen) conditions • Splits apart a single glucose molecule (6 carbon) into two molecules of pyruvate (3 carbon) • under anaerobic conditions, pyruvate converted by fermentation to lactic acid or ethanol • occurs in cytoplasm • pyruvate may enter mitochondria if oxygen available – breaks pyruvate down completely to CO2 and water generating an additional 34 to 36 ATP – aerobic respiration • each step (reaction) is catalyzed by an enzyme

6. products are 2 molecules of ATP and 2 molecules of NADH • nicotinamide adenine dinucleotide – an electron carrier that transports energy in form of energetic electrons - Coenzyme • electrons are held in high-energy outer electron shells – NAD+ NADH • donates the electrons and their energy to other molecules • hydrogen ions are often picked up simultaneously

7. formed through oxidation/reduction reactions – involves two complementary reactions • oxidation – liberates energy from the oxidation substance; results from the removal of one more electrons, alone or with H+ • reduction – stores energy in a reduced compound; reduction results from addition of one or more electrons, alone or with H+

8. Glycolysis consists of two major sets of reactions: • Step 1 - glucose activation • 2 ATP are used to convert stable glucose into highly unstable fructose bisphosphate (6 C)

9. Step 2 – Energy harvest • fructose bisphosphate splits into two 3 C molecules of glyceraldehyde 3-phosphate (G3P or PGAL) • each G3P molecule goes through series of reactions that convert it into pyruvate (pyruvic acid) • 2 ATPs are made per G3P for a total of 4 – however, net gain is only 2 ATPs • During these reactions, 2 high energy electrons and a H+ are added to NAD+ to form “energized” carrier NADH – 2 NADH are made (one from each G3P)

11. Fermentation • In the absence of oxygen, pyruvate acts as electron acceptor from NADH producing ethanol or lactic acid – this process is called fermentation • NADH production is not used as method to capture energy – used to get rid of hydrogen ions and electrons made when glucose is broken down • NAD+ is regenerated by pyruvate acting as final electron acceptor - pyruvate may be converted to lactic acid (lactic acid fermentation – occurs in human muscles during strenuous exercise) or ethanol and CO2 (alcoholic fermentation)

13. Aerobic Respiration • In presence of oxygen, oxygen is the electron acceptor (in the electron transport system) allowing pyruvate to be fully broken down (back into CO2 and water) to make even more ATP • Aerobic Cellular Respiration – series of reactions, occurring under aerobic conditions, in which large amounts of ATP are produced – pyruvate is broken down into carbon dioxide and water – oxygen serves as final electron acceptor – each step catalyzed by enzymes

14. Aerobic Respiration • occurs in the mitochondria • double membrane – inner folds are called cristae • inner compartment contains fluid matrix • intermembrane compartment separates the two membranes • Mitochondria have their own DNA (circular chromosome) and ribosomes (70s – smaller than eukaryotic ribosomes)

15. Aerobic Respiration • Step 1 – Glycolysis • Step 2 – Oxidative Decarboxylation • two molecules of pyruvate produced by glycolysis are transported across both mitochondrial membranes into matrix • each pyruvate is split into CO2 and a 2 C acetyl group which immediately attaches to coenzyme A to form acetyl CoA – during this reaction NADH is produced

17. Step 3 – Krebs Cycle (Citric Acid Cycle) • the acetyl CoA enter Krebs cycle by briefly combining with oxaloacetate to form citrate – coenzyme A is released to be reused • Kreb’s cycle rearranges citrate to regenerate oxaloacetate giving off 2 CO2, 1 ATP and four electron carriers (1 FADH2 and 3 NADH) per pyruvate molecule (x2 per glucose molecule)

20. Electron Transport System (Oxidative Phosphorylation) • energetic electrons from NADH and FADH2 are used to generate more ATP (3 ATP are generated per NADH and 2 ATP per FADH2) • located in inner mitochondrial membrane • electrons move from molecule to molecule along transport system – energy released by electrons is used to pump hydrogen ions from the matrix across the inner membrane into the intermembrane compartment (used for chemiosmosis) • at the end of the ETS, oxygen and hydrogen ions accept the electrons to form water – clears out transport system for more electrons to run through

23. Chemiosmosis • hydrogen ions pumped across the inner membrane generate a large H+ concentration gradient (high concentration in intermembrane compartment and low concentration in matrix) • inner membrane is impermeable to hydrogen ions except at protein channels that are part of ATP-synthesizing enzymes (ATP synthase) - whole thing is called the F1 complex • during chemiosmosis, hydrogen ions move down the concentration gradient from intermembrane compartment to matrix by means of the F1 complex • the flow of hydrogen ions provides energy to synthesize 32 – 34 ATPs from ADP

25. Coupled Reactions • Many steps involved in respiration are coupled - reactions in which exergonic reactions drive endergonic reactions • Some reactions occur together with two reactions sharing a common intermediate molecule

26. Metabolism of Fats and Proteins • cells can extract energy from fats and proteins • breakdown of fat and proteins creates products that can be fed into the enzyme pathways of respiration • Fats • starts with hydrolysis into glycerol and fatty acids • glycerol is converted into G3P and enters pathway • fatty acids are converted into acetyl-CoA and enter pathway • Proteins • amino acids are broken down in a number of ways • the amino group is first removed (deamination) • some amino acids are converted into pyruvic acid, some into acetyl-CoA, and some into other compounds in the Krebs cycle

27. G3P

28. Body Temperature and Metabolism • cellular respiration captures energy in the bonds of ATP however, much of the energy is lost as heat (approx 60% of energy available is lost) • the majority of animals and plants quickly lose this thermal energy to the environment – referred to as poikilothermic (“of variable heat”) or ectothermic (“externally heated”) • body heat comes from external sources, body temp fluctuates with environmental temp • metabolic rate (organism’s rate of oxygen consumption or release of CO2) increases with temp (enzymes more active at higher temps) • ectotherms are more active with higher temps and sluggish with lower temps • many have behavioral adaptations to assist with temp control (basking)

29. Mammals and birds (and some other organisms) make use of heat produced during metabolism – have evolved mechanisms to conserve heat (insulation with fat, hair, feathers) • endothermic (“internally heated”) – have fairly high body temps (higher than environment) • tend to maintain fairly constant body temp – homeothermic even when environmental temp fluctuates • metabolic rate stays fairly constant

30. metabolic rate is inversely related to body size in both endo and ectotherms • endothermic, smaller animals have a higher surface area-to- volume ratio and therefore a larger relative heat loss to the environment • must have faster metabolism to replace heat – have to consume relatively large amounts of food • metabolic rate is higher in smaller ectotherms too but has never been fully explained as to why