2.A.2- Capture and Storage of Free Energy

Transcription

1 2.A.2- Capture and Storage of Free Energy Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. EU 2.A- Growth, reproduction and maintenance of the organization of living systems require free energy and matter. EU 2.B- Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. EU 2.C- Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. EU 2.D- Growth and dynamic homeostasis of a biological system are influenced by changes in the system s environment. EU 2.E- Many biological involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination. A. Autotrophs capture free energy from physical sources in the environment. 1. Photosynthetic organisms capture free energy present in sunlight. 2. Chemosynthetic organisms capture free energy from small inorganic molecules present in their environment, and this process can occur in the absence of oxygen. B. Heterotrophs capture free energy present in carbon compounds produced by other organisms. 1. Heterotrophs may metabolize carbohydrates, lipids and proteins by hydrolysis as sources of free energy. 2. Fermentation produces organic molecules, including alcohol and lactic acid, and it occurs in the absence of oxygen. C. Different energy-capturing use different types of electron acceptors. 1. NADP + in photosynthesis 2. NAD+ and Oxygen in cellular respiration D. The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture free energy present in light to yield ATP and NADPH, which power the production of organic molecules (light-independent reactions). 1

2 E. Photosynthesis first evolved in prokaryotic organisms F. Scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere G. Photosynthetic pathways were the foundation of eukaryotic photosynthesis. (Photorespiration, C 4 and CAM Plants) End Photosynthesis H. Cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions that harvest free energy from simple carbohydrates. 1. Glycolysis 2. Pyruvate is transported from the cytoplasm to the mitochondrion, where further oxidation occurs. 3. Krebs cycle a. Carbon dioxide is released from organic intermediates b. ATP is synthesized from ADP and inorganic phosphate via substrate level phosphorylation c. Electrons are captured by coenzymes. 4. Electrons that are extracted in the series of Krebs cycle reactions are carried by NADH and FADH 2 to the electron transport chain. I. The electron transport chain captures free energy from electrons in a series of coupled reactions that establish an electrochemical gradient across membranes. 1. In cellular respiration the terminal electron acceptor is oxygen. 2. In photosynthesis, the terminal electron acceptor is NADP+. 3. The passage of electrons is accompanied by the formation of a proton gradient across the inner mitochondrial membrane or the thylakoid membrane of chloroplasts 4. In prokaryotes, the passage of electrons is accompanied by the outward movement of protons across the plasma membrane. 5. Chemiosmosis 6. In cellular respiration, decoupling oxidative phosphorylation from electron transport is involved in thermoregulation. 7. Uncoupling Protein-1 (UCP1) a. Shown to increase energy expenditure by uncoupling a step in cellular metabolism. b. Leaks protons across the mitochondrial inner membrane. c. The energy is released in the form of heat. 2

3 J. Free energy becomes available for metabolism by the conversion of ATP ADP, which is coupled to many steps in metabolic pathways. A. Requires two light-gathering units; photosystem I (PS I) and photosystem II (PS II) B. Occur in the thylakoid membranes. D. Photosystems 1. A pigment complex and electron acceptor. 2. Pigment complex composed of chlorophyll a and b molecules, and carotenoids 3. Chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystems I and II. [See also 4.A.2] 4. Energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a. E. Noncyclic Electron Pathway 1. The PS II pigment complex absorbs solar energy 2. High-energy electrons (e-) leave the reaction-center chlorophyll a molecule(p680) 3. PS II takes replacement electrons from H 2 O, which splits, releasing O 2 and H + 4. The H + ions temporarily stay within the thylakoid space. 5. High-energy electrons that leave PS II are captured by an electron acceptor, which sends them to an electron transport system. 6. ATP Production a) The thylakoid space acts as a reservoir for H+ ions; each time H 2 O is split, two H + remain. b) As electrons move carrier-to-carrier, they give up energy to pump H + from stroma into thylakoid space. c) Chemiosmosis occurs forming ATP in the stroma. 7. Low-energy electrons enter PS I. 8. High-energy electrons leave reaction-center chlorophyll a(p700) and are captured by an electron acceptor. 9. The electron acceptor passes them on to NADP NADP + takes on 2H + to become NADPH + H NADPH and ATP are used by enzymes in stroma during lightindependent reactions. 3

4 F. Cyclic Electron Pathway 1. PS I (P700) pigment complex absorbs solar energy. 2. High-energy electrons leave PS I reaction-center chlorophyll a molecule. 3. Electrons enter and travel down electron transport system. 4. Energy released is stored in form of a hydrogen (H + ) gradient. 5. Chemiosmosis occurs forming ATP in the stroma 6. Some photosynthetic bacteria utilize cyclic electron pathway only; pathway probably evolved early. 7. Need more ATP than NADPH for Calvin Cycle. 8. NADPH concetration may influence which way electrons flow. 9. When CO 2 is in limited supply, carbohydrate is not being produced, no need for additional NADPH A. Take place in the stroma B. Occur in either the light or the dark. C. Use NADPH and ATP to reduce CO 2. D. Called the Calvin Cycle (C3 Cycle) after Melvin Calvin Light-independent Reactions Light-independent Reactions E. Calvin Cycle Has Three Stages 1. Fixing Carbon Dioxide a) The attachment of CO 2 to an organic compound. b) RuBP (ribulose bisphosphate) is a fivecarbon molecule that combines with carbon dioxide. c) Enzyme RuBP carboxylase(rubisco) speeds reaction d) Rubisco is 20-50% protein in chloroplasts. Light-independent Reactions 2. Reducing PGA a) Six-carbon molecule immediately breaks down b) Forms two PGA (C3). Molecules (phosphoglycerate) c) Each PGA molecule undergoes reduction to PGAL (glyceraldehyde phosphate). d) Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce PGA to PGAL. Light-independent Reactions 3. Regenerating RuBP a) Every three turns of Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP. b) Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five PGAL regenerate RuBP. c) First molecule identified by Calvin was PGA [C3], a three-carbon product; Calvin cycle is also known as C3 cycle 4

5 Photorespiration A. In hot weather, stomates close to save water; CO 2 concentration decreases in leaves; O 2 increases. B. In C 3 plants, O 2 competes with CO 2 for the active site of rubisco. C. Called "photorespiration" since oxygen is taken up and CO 2 is produced. D. No sugar or ATP is produced. E. Relic of evolution when O 2 was in short supply. C 4 Plants A. Sugarcane, Corn, Grasses B. Fix CO 2 by first forming a C 4 molecule C. Shuttle C 4 into Bundle sheath cells D. CO 2 is released and used in Calvin Cycle. E. In hot, dry climates, net photosynthetic rate of C 4 plants (e.g., corn) is 2-3 times that of C 3 plants. A. Succulent desert plants, cacti, pineapple B. CAM plants open stomates only at night C. Store CO 2 as ac 4 molecule D. Released during the day in Calvin Cycle CAM (crassulacean-acid metabolism) Plants Glycolysis A. The breakdown of glucose to two molecules of pyruvate. B. Occurs in the cytosol C. Doesn t require oxygen D.Universal in organisms (most likely evolved before Krebs cycle and electron transport system) Glycolysis E. Energy Investment Steps 1. Two ATP molecules phosphorylate Glucose 2. Glucose splits into two C3 molecules (PGAL), each with a phosphate group. F. Energy Harvesting Steps 1. Reduction of 2NAD + produces 2 NADH. 2. Further steps generate 4 ATP molecules by substratelevel phosphorylation 3. Two H 2 O molecules are produced 4. There is a net gain of two ATP from glycolysis. G. Summary 1. Two Pyruvate molecules are the final products 2. No CO 2 is released 3. If O 2 is present, pyruvate enters mitochondria. 4. If no O 2, fermentation follows 5. Net energy yield per glucose molecule = 2 ATP plus 2 NADH (Video) Glycolysis 5

6 A. Consists of glycolysis plus reduction of pyruvate to either lactate or alcohol and CO 2. B. NADH passes its electrons to pyruvate C. Regenerates NAD + for glycolysis D. Two Types 1. Lactic Acid Fermentation 2. Alcohol Fermentation E. Fermentation results in a net gain of only two ATP per glucose molecule F. Lactic acid and alcohol are toxic to cells. Fermentation Fermentation G. Examples 1. Anaerobic bacteria produce lactic acid when we manufacture some cheeses. 2. Anaerobic bacteria produce industrial chemicals: isopropanol, butyric acid, propionic acid, and acetic acid. 3. Yeasts use CO 2 to make bread rise, produce alcohol in winemaking 4. Animals reduce pyruvate to lactate when it is produced faster than it can be oxidized by Krebs cycle. Transition Reaction A. Pyruvate is transported from the cytoplasm to the mitochondrion, where further oxidation occurs. [See also 4.A.2] B. Each pyruvate loses a CO 2 (becoming acetate) C. NAD + is reduced to NADH D. Coenzyme A( B vitamin derivative) attaches to acetate making it very reactive Kreb s Cycle A. Sir Hans Krebs (1930 s) B. Also called Citric Acid Cycle C. Occurs in mitochondrial matrix if O 2 is present D. Acetyl CoA combines with Oxaloacetate (forming citric acid) E. For each turn of cycle 1. Two CO 2 are released 2. Three 3 NADH produce 3. One FAD is reduced to FADH 2 4. GTP accepts a phosphate group and passes it on to convert ADP to ATP. F. Oxaloacetate is regenerated G. Summary 1. NADH and FADH 2 carry electrons to electron transport system 2. Krebs cycle turns twice for each original glucose molecule 3.Products of the Krebs cycle per glucose molecule include 4CO 2, 2ATP, 6NADH and 2FADH 2 Kreb s Cycle Electron Transport Chain A. Series of electron carrier molecules in mitochondrial cristae B. Cristae are the inner folds of membrane that jut out into matrix C. Carrier Molecules 1. Most are proteins (heme and cytochromes) 2. Ubiquinone is a lipid 3. Electrons pass from higher to lower energy states 6

7 Electron Transport Chain D. NADH and FADH 2 give electrons to carriers E. Oxygen is final acceptor and combines with hydrogen ions to form H 2 O F. Energy released from flow of electrons down electron transport chain is used to pump H + ions into the mitochondrial intermembrane space. Electron Transport Chain G. Electrons from NADH pump 3 H + into intermembrane space. H. Electrons from FADH 2 pump 2 H + into intermembrane space I. H + ions in this intermembrane space creates an electrochemical gradient. (proton-motive force) (ETC Video) Chemiosmosis A. Proposed by Peter Mitchell(1961) B. H + ions flow from high to low concentration through ATP synthase C. ATP Synthase Complexes 1. Channel proteins that also serve as enzymes for ATP synthesis 2. Found in mitochondrial and chloroplast membranes. 3. Found in Prokaryote cell membranes D. Because O 2 must be present for system to work, it is also called oxidative phosphorylation. (ATP Synthase Video) ATP A. Coupling Reactions 1. Energy released by an exergonic reaction is used to drive an endergonic reaction. 2. Hydrolysis of ATP (adenosine triphospate) a. Energy from ATP ADP + P i is used to fuel reactions. b. P i phosphoraletes an intermediate molecule making it less stable ATP B. Structure of ATP 1. Nucleotides a. Nitrogen base adenine b. Ribose c. Three phosphates. 2. ATP is called a "high-energy molecule a. Three negative phosphates repel b. ADP is more stable c. Some energy is lost as heat d. Overall reaction is exergonic. e. ATP is constantly recycled from ADP + P i f. Muscle Cell= 10 million used and recycled per second C. Function of ATP 1. Chemical work 2. Transport work 3. Mechanical work ATP 7

8 A. Ribosomes B. Endoplasmic Reticulum (see 2.b.3) C. Golgi Complex (see 2.b.3 D. Mitochondria (see 2.b.3) E. Lysosomes F. Vacuoles G. Peroxisomes H. Nucleus (see 2.b.3) I. Chloroplasts (see 2.b.3) J. Cell Wall K. Cytoskeleton L. ECM M. Intercellular Junctions Stop Back A. Have a double membrane that allows compartmentalization B. Inner membrane is highly convoluted, forming folds called cristae. 1. Cristae contain enzymes important to ATP production 2. Cristae also increase the surface area for ATP production C. Sites of cellular respiration. D. Contain ribosomes and their own DNA E. Specialize in energy capture and transformation. 8