How ells Harvest Energy hapter 7 & 8 Evolution of Metabolism A hypothetical timeline for the evolution of metabolism - all in prokaryotic cells!: 1. ability to store chemical energy in ATP 2. evolution of glycolysis 3. anaerobic photosynthesis (using H 2 S) 4. use of H 2 in photosynthesis (not H 2 S) 5. evolution of nitrogen fixation 6. aerobic respiration evolved most recently 2 Introduction rganisms can be classified based on how they obtain energy: autotrophs: are able to produce their own organic molecules through photosynthesis heterotrophs: live on organic compounds produced by other organisms All organisms use cellular respiration to extract energy from organic molecules. Introduction ellular respiration & Photosynthesis are reactions that: 1. Are oxidations loss of electrons A. -are also dehydrogenations lost electrons are accompanied by hydrogen B. Therefore, what is actually lost is a hydrogen atom (1 electron, 1 proton). 2. Also are reductions - gain of electrons (& hydrogens) 3 4 Introduction During redox reactions, electrons carry energy from one molecule to another. NAD + and NADP are electron carriers. NAD accepts 2 electrons and 1 proton to become NADH NADP becomes NADPH -the reaction is reversible 5 6 1
Respiration During respiration, electrons are shuttled through electron carriers to a final electron acceptor. 1. anaerobic respiration: final electron acceptor is an inorganic molecule (not 2 ) 2. fermentation: final electron acceptor is an organic molecule 3. aerobic respiration: final electron receptor is oxygen ( 2 ) 7 8 The goal of respiration is to produce ATP ells are able to make ATP via: 1. substrate-level phosphorylation A. Transferring a phosphate directly to ADP from another molecule 2. oxidative phosphorylation A. Use of ATP synthase and energy derived from a proton (H + ) gradient to make ATP B. Requires 2 9 10 xidation of Glucose The complete oxidation of glucose proceeds in stages: 1. glycolysis 2. pyruvate oxidation 3. Krebs cycle 4. electron transport chain & chemiosmosis 11 12 2
xidation Without 2 Respiration occurs without 2 via either: 1. anaerobic respiration -use of inorganic molecules (other than 2 ) as final electron acceptor 2. fermentation -use of organic molecules as final electron acceptor 13 14 xidation Without 2 Anaerobic respiration by methanogens -methanogens use 2-2 is reduced to H 4 (methane) Anaerobic respiration by sulfur bacteria -inorganic sulphate (S 4 ) is reduced to hydrogen sulfide (H 2 S) xidation Without 2 Fermentation reduces organic molecules in order to regenerate NAD + 1. ethanol fermentation occurs in yeast A. 2, ethanol, and NAD + are produced 2. lactic acid fermentation A. occurs in animal cells (especially muscles) B. electrons are transferred from NADH to pyruvate to produce lactic acid 15 16 Glycolysis Glycolysis converts glucose to pyruvate. 1. a 10-step biochemical pathway 2. occurs in the cytoplasm 3. 2 molecules of pyruvate are formed 4. net production of 2 ATP molecules by substrate-level phosphorylation 5. 2 NADH produced by the reduction of NAD + 17 18 3
Glycolysis For glycolysis to continue, NADH must be recycled to NAD +. NADH is very costly (energy) to produce. 1. fermentation occurs when oxygen is not available; an organic molecule is the final electron acceptor 2. aerobic respiration occurs when oxygen is available as the final electron acceptor 19 20 opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display. 2 ADP 2 ATP Alcohol Fermentation in Yeast H 3 Glucose G L Y L Y S I S 2 Pyruvate 2 NAD + 2 NADH 2 H H H H 3 2 Ethanol H H 3 2 Acetaldehyde Lactic Acid Fermentation in Muscle ells 2 ADP 2 ATP H 3 Glucose G L Y L Y S I S 2 Pyruvate 2 NAD + 2 NADH H H H 3 2 Lactate 21 22 Glycolysis The fate of pyruvate depends on oxygen availability. Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD + and there is a net gain of only 2 ATP per glucose. Photosynthesis hapter 8 When oxygen is present, pyruvate is oxidized to acetyl-oa which enters the Krebs cycle and the ATP yield is many times greater, BUT NT WITHUT 2 23 4
Photosynthesis verview Energy for all life on Earth ultimately comes from photosynthesis. 6 2 + 12H 2 6 H 12 6 + 6H 2 + 6 2 xygenic photosynthesis is carried out by: 1. cyanobacteria, 7 groups of algae, 2. all land plants 3. 2 is a waste product of this reaction. Photosynthesis verview Photosynthesis is divided into: 1. light-dependent reactions A. capture energy from sunlight B. make ATP and reduce NADP + to NADPH 2. carbon fixation reactions A. use ATP and NADPH to synthesize organic molecules from 2 25 26 Discovery of Photosynthesis Discovery of Photosynthesis The work of many scientists led to the discovery of how photosynthesis works.. B. van Niel, 1930 s -proposed a general formula: 2 +H 2 A + light energy H 2 + H 2 + 2A Jan Baptista van Helmont (1580-1644) Joseph Priestly (1733-1804) Jan Ingen-Housz (1730-1799) F. F. Blackman (1866-1947) 27 where H 2 A is the electron donor 1. Earliest organisms used H 2 S as the electron donor 2. van Niel identified water as the source of the 2 released from photosynthesis today. 28 Fig. 8.3 Light Energy photon: a particle of light -acts as a discrete bundle of energy -energy content of a photon is inversely proportional to the wavelength of the light photoelectric effect: removal of an electron from a molecule by light -occurs when photons transfer energy to electrons 29 30 5
Light Energy Pigments Pigments: molecules that absorb visible light Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. 31 32 Fig. 8.5.b 33 34 Pigments chlorophyll a primary pigment in plants and cyanobacteria -absorbs violet-blue and red light chlorophyll b secondary pigment absorbing light wavelengths that chlorophyll a does not absorb Structure of pigments: Pigments porphyrin ring: complex ring structure with alternating double and single bonds -magnesium ion at the center of the ring -photons excite electrons in the ring -electrons are shuttled away from the ring 35 36 6
Pigments accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a -increase the range of light wavelengths that can be used in photosynthesis -include: chlorophyll b, carotenoids, phycobiloproteins -carotenoids also act as antioxidants Photosynthesis verview Photosynthesis takes place in chloroplasts in eukaryotic cells. thylakoid membrane internal membrane arranged in flattened sacs -contain chlorophyll and other pigments grana stacks of thylakoid membranes stroma semiliquid substance surrounding thylakoid membranes 37 38 39 40 Photosystem rganization A photosystem consists of 1. an antenna complex of hundreds of accessory pigment molecules 2. a reaction center of one or more chlorophyll a molecules Energy of electrons is transferred through the antenna complex to the reaction center. 41 42 7
Photosystem rganization At the reaction center, the energy from the antenna complex is transferred to chlorophyll a. This energy causes an electron from chlorophyll to become excited. The excited electron is transferred from chlorophyll a to an electron acceptor. Water donates an electron to chlorophyll a to replace the excited electron. 43 44 Light-dependent reactions occur in 4 stages: 1. primary photoevent a photon of light is captured by a pigment molecule 2. charge separation energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule 3. electron transport electrons move through carriers to reduce NADP + 4. chemiosmosis produces ATP 45 46 In sulfur bacteria, only one photosystem is used for cyclic photophosphorylation 1. an electron joins a proton to produce hydrogen 2. an electron is recycled to chlorophyll -this process drives the chemiosmotic synthesis of ATP 47 48 8
In chloroplasts, two linked photosystems are used in noncyclic photophosphorylation 1. photosystem I -reaction center pigment (P 700 ) with a peak absorption at 700nm 2. photosystem II -reaction center pigment (P 680 ) has a peak absorption at 680nm 49 50 Photosystem II acts first: -accessory pigments shuttle energy to the P 680 reaction center -excited electrons from P 680 are transferred to b 6 -f complex -electron lost from P 680 is replaced by an electron released from the splitting of water The b 6 -f complex is a series of electron carriers. -electron carrier molecules are embedded in the thylakoid membrane -protons are pumped into the thylakoid space to form a proton gradient 51 52 Photosystem I -receives energy from an antenna complex -energy is shuttled to P 700 reaction center -excited electron is transferred to a membrane-bound electron carrier -electrons are used to reduce NADP + to NADPH -electrons lost from P 700 are replaced from ATP is produced via chemiosmosis. - ATP synthase is embedded in the thylakoid membrane -protons have accumulated in the thylakoid space -protons move into the stroma only through ATP synthase -ATP is produced from ADP + P i the b 6 -f complex 53 54 9
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