6. Photosynthesis and cellular respiration.pdf

BIOL S103F ESSENTIAL BIOLOGY PHOTOSYNTHESIS AND CELLULAR RESPIRATION Week 6 Autotrophs vs Heterotrophs 2 Autotrophs    Sustain themselves without eating anything derived from other organisms Producers of the biosphere -- Produce organic molecules from CO2 & other inorganic molecules Almost all pl...

BIOL S103F ESSENTIAL BIOLOGY PHOTOSYNTHESIS AND CELLULAR RESPIRATION Week 6 Autotrophs vs Heterotrophs 2 Autotrophs    Sustain themselves without eating anything derived from other organisms Producers of the biosphere -- Produce organic molecules from CO2 & other inorganic molecules Almost all plants are photoautotrophs -- Use the energy of sunlight to make organic molecules by photosynthesis Heterotrophs    Obtain organic material from other organisms through ingestion Consumers of the biosphere Almost all heterotrophs, incl. humans -- Depend on photoautotrophs for food & O2 Similarity  Organic molecules are oxidized to generate utilizable chemical energy through the process of cellular respiration Photosynthesis 3  Anabolic process  Converts solar energy into chemical energy  Inorganic molecules (CO2 + H2O)  Organic molecules (sugars) + O2 (by-product)  Contribute to the availability of atmospheric O2 that is essential for other metabolic processes @2015 Pearson Education, Inc.  Occurs in plants, algae, certain other unicellular eukaryotes & some prokaryotes  Feed not only themselves  But also most of the living world Scientists Traced the Process of Photosynthesis using Isotopes 4  Plants produce O2 (1800s)    Does this O2 come from CO2 or H2O ? For many years, it was assumed that O2 was extracted from CO2 taken into the plant However, later research using 18O (a heavy isotope of oxygen) confirmed that O2 produced by photosynthesis comes from H2O  Experiment 1: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2  Experiment 2: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2 6 CO2 Reactants: Products: C6H12O6 12 H2O 6 H2O 6 O2 Photosynthesis Occurs in Chloroplasts in Plant Cells 5     Leaf Cross Section Chlorophyll  Important light-absorbing pigment in chloroplasts  Green color of plants  Central role in converting solar energy  chemical energy Mesophyll Leaf Vein Mesophyll Cell CO2 Chloroplasts are concentrated in mesophyll cells (the green tissue in the interior of the leaf) Stomata  Tiny pores in the leaf that allow  CO2 to enter  O2 to exit Veins  Deliver water absorbed by roots O2 Stoma Chloroplast Inner and outer membranes Granum Stroma Thylakoid Thylakoid space @2015 Pearson Education, Inc. Chloroplasts 6   Consist of an envelope of 2 membranes  Encloses an inner compartment filled with stroma (a thick fluid) that contains thylakoids (a system of interconnected membranous sacs) Thylakoids  Concentrated in stacks (grana; singular: granum)  Thylakoid membranes house much of the machinery that converts light energy  chemical energy  Chlorophyll molecules  Built into the thylakoid membrane  Capture light energy @2015 Pearson Education, Inc. 2 Stages of Photosynthesis 7 Light Reactions Convert solar energy  chemical energy The 2 stages are linked by ATP & NADPH Calvin Cycle Reduces CO2  sugar @2015 Pearson Education, Inc. Light Reactions 8  The photo part of photosynthesis  Occur in the thylakoid membranes  Light-dependent reaction   Water is split  Providing a source of e- & protons (H+)  Giving off O2 as a by-product Convert solar energy  chemical energy  Adenosine triphosphate (ATP)  An energy supplier  Generated from ADP & a phosphate group @2008 Pearson Benjamin Cummings. Calvin Cycle 9  The synthesis part of photosynthesis  Occurs in the stroma of the chloroplast  A cyclic series of reactions that assembles sugar molecules using CO2 & energy-rich products (ATP & NADPH) of the light reactions    @2015 Pearson Education, Inc. Convert solar energy  chemical energy (in food) Carbon fixation  CO2 is incorporated into organic compounds  Carbon compounds are reduced to sugars after carbon fixation Dark reactions / light-independent reactions  None of the steps requires light directly  Enzymes are activated by light @2015 Pearson Education, Inc. The Nature of Sunlight 10  Sunlight contains energy (electromagnetic energy or radiation)  Visible light is only a small part of the electromagnetic spectrum (the full range of electromagnetic wavelengths)  Electromagnetic energy travels in waves   Wavelength : distance between the crests of 2 adjacent waves Light behaves as discrete packets of energy (photons)  Photon: fixed quantity of light energy  Shorter wavelength  greater energy @2015 Pearson Education, Inc. Visible Radiation Absorbed by Pigments Drives the Light Reactions 11   Pigments  Substances that absorb visible light  Different pigments absorb light of different wavelengths  Wavelengths that are not absorbed are reflected / transmitted  We see the color of the wavelengths that are transmitted Chloroplast  e.g. Chlorophyll transmits green Thylakoid wavelengths Plant pigments  Built into the thylakoid membrane  Chlorophyll absorbs violet-blue & red light Light Reflected light Absorbed light Transmitted light @2015 Pearson Education, Inc. 3 Major Types of Pigments in Chloroplasts 12  Absorb light of different wavelengths 1. Chlorophyll a (main photosynthetic pigment)  Absorbs mainly blue-violet & red light  blue-green under visible light  Participates directly in the light reactions 2. Chlorophyll b (accessory pigment)  Absorbs mainly blue & orange  olive-green under visible light  Broadens the spectrum used for photosynthesis 3. Carotenoids (accessory pigment)  Yellow / orange  May broadens the spectrum used for photosynthesis  Provide photoprotection  Absorbs & dissipates excessive light energy that would otherwise damage chlorophyll or interact with O2, forming reactive oxidative molecules that are dangerous to the cell Light Reactions: Summary 13   Converts light energy  chemical energy  Increase potential energy of electrons by moving them from H2O  NADPH  Light-driven electron flow generates ATP  O2 as by-product Photosynthesis is an emergent property of the structural organization of a chloroplast, which integrates the 2 stages of photosynthesis  ATP & NADPH are produced on the side of thylakoid membrane facing the stroma, where the Calvin cycle takes place H2O CO2 Light NADP+ ADP + P Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 Sugar @2015 Pearson Education, Inc. Calvin Cycle (C3 Cycle) Reduces CO2 to Sugar 14  Sugar synthesis within a chloroplast powered by ATP & NADPH  Regenerates its starting material after molecules enter & leave the cycle  Coupled closely with the light reactions  Ingredients:  Atmospheric CO2 (carbon in the form of CO2)  ATP & NADPH generated by the light reactions  Product:  Glyceraldehyde 3-phosphate (G3P)  An energy-rich, 3-C sugar  A plant cell uses G3P to make  Glucose  Sucrose (disaccharide)  Other organic molecules as needed @2015 Pearson Education, Inc. Importance of Photosynthesis 15      Solar energy entering chloroplasts stored as chemical energy in organic compounds Sugars serve as starting material for making other organic molecules e.g. proteins, lipids, cellulose Plants store excess sugar as starch in structures e.g. roots, tubers, seeds & fruits Plants (& other photosynthesizers) are the ultimate source of food for virtually all other organisms Photosynthesis is an essential process not only for plants but for all life forms of the biosphere  Major contributor of O2 in the atmosphere Becomes oxidized C6H12O6 Becomes reduced 6 CO2 + 6 H2O + 6 O2 6 CO2 + 6 H2O Becomes reduced C6H12O6 + 6 O2 Becomes oxidized 16 Cellular respiration    Uses redox reactions to harvest the chemical energy stored in a glucose molecule By oxidizing the sugar & reducing O2  H2O e- lose potential as they travel down the electron transport chain (ETC) to O2 The overall chemical change (NOT steps) during photosynthesis is the reverse of the one that occurs during cellular respiration Photosynthesis      Food-producing redox reaction requiring energy CO2 is reduced to sugar (gain e-, along with H+ from H2O) H2O molecules are oxidized (lose e- along with H+) Light energy is captured by chlorophyll molecules to boost the energy of eLight energy is converted to chemical energy stored in the chemical bonds of sugars Photosynthesis & Cellular Respiration Provide Energy for Life 17  Life requires energy  In almost all ecosystems, energy ultimately comes from the sun   Photosynthesis  Some of the energy in sunlight is captured by chloroplasts  Atoms of CO2 & H2O are rearranged  Sugar & O2 are produced Sunlight energy ECOSYSTEM Photosynthesis in chloroplastsOrganic + O CO2 + H2O 2 Cellular molecules respiration in mitochondria Cellular respiration  Takes place in the mitochondria of eukaryotic cells   Sugar is broken down  CO2 & H2O Cell captures some of the released energy to make ATP  Some energy is lost as heat ATP ATP powers most cellular work Heat energy @2015 Pearson Education, Inc. Three Steps of Cellular Respiration 18  Glycolysis (anaerobic)  Breakdown of glucose  Prepare carbon sources for entering Krebs Cycle  Krebs Cycle (aerobic)    Harvest electrons by oxidising glycolysis product Central hub of different metabolic pathway Electron Transport Chain (aerobic)   Collect electrons from Krebs Cycle Convert energy stored in electrons (orbital) into ATP 19 Cellular Respiration Occurs in 3 Main Stages 20 The cell makes a small amount of ATP by substrate-level phosphorylation Main function: Supply the 3rd stage with e– Oxidative phosphorylation produces most of the ATP in cellular respiration @2015 Pearson Education Ltd Pyruvate Oxidation 21 Krebs Cycle 22 Cellular Respiration Occurs in 3 Main Stages 23  Cellular respiration consists of a sequence of steps  Stage 1: Glycolysis  Occurs in the cytosol  Begins cellular respiration  Breaks down glucose into 2 molecules of pyruvate (3-C compound)  Stage 2: Pyruvate oxidation & the citric acid cycle  Take place in mitochondria  Oxidize pyruvate to a 2-C compound  CO2 produced in the citric acid cycle  Stage 3: Oxidative phosphorylation The cell makes a small amount of ATP by substrate-level phosphorylation Main function: supply the 3rd stage with e– Produces most of the ATP produced in cellular respiration Each Molecule of Glucose Yields Many Molecules of ATP 24    Energy payoff stage of cellular respiration involves 1. Glycolysis 2. Pyruvate Oxidation 3. Citric acid cycle 4. Oxidative phosphorylation Total yield per glucose molecule: 30 - 32 ATP molecules Number of ATP molecules cannot be stated exactly  NADH produced in glycolysis passes its e– across mitochondrial membrane to either NAD+ or FAD  FADH2 adds its e– later in the chain (where energy is lower)  Generates less ATP  Some of the energy of the H+ gradient may be used for work other than ATP production, e.g. active transport of pyruvate into the mitochondrion The ATP Yield Varies Slightly NADH  2.5 ATP FADH2  1.5 ATP 25 @2015 Pearson Education Ltd Fermentation -- Anaerobic Harvesting of Energy 26   Fermentation  Enables cells to produce ATP without O2  Uses glycolysis  Produces 2 ATP molecules per glucose  Reduces NAD+  NADH Provides an anaerobic path for recycling NADH back to NAD+ @2015 Pearson Education Ltd Lactic Acid Fermentation 27    Muscle cells & certain bacteria can regenerate NAD+ through lactic acid fermentation  NADH is oxidized back to NAD+  Pyruvate is reduced to lactate 2 ADP +2 P 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH Lactate is carried by the blood to the liver, where it is converted back to pyruvate & oxidized in the mitochondria of liver cells Industrial application  Dairy industry uses lactic acid fermentation by bacteria to make cheese & yogurt  Other types of microbial fermentation turn soybeans into soy sauce & cabbage into sauerkraut 2 Pyruvate 2 NADH 2 NAD+ 2 Lactate @2015 Pearson Education Ltd Alcohol Fermentation 28  Industrial application  Baking & winemaking industries have used alcohol fermentation for thousands of years  Yeast (single-celled fungi)  Oxidize NADH back to NAD+  Convert pyruvate to CO2 & ethanol 2 ADP +2 P 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 NADH 2 CO2 2 NAD+ 2 Ethanol @2015 Pearson Education Ltd Connections between Metabolic Pathways 29  Cells use many kinds of organic molecules as fuel for cellular respiration  Carbohydrates  Glucose  Primary source of sugar for respiration & fermentation  Fats  Excellent cellular fuel  Yield > 2X ATP per gram as a gram of carbohydrate  Contain many hydrogen atoms  many energy-rich e–  Proteins  Can also be used for fuel  Our body preferentially burns sugars & fats first @2015 Pearson Education Ltd Organic Molecules from Food Provide Raw Materials for Biosynthesis 30    A cell must be able to make its own molecules to  Build its structures  Perform its functions Food provides raw materials our cells use for biosynthesis (production of organic molecules) using energy-requiring metabolic pathways Metabolic pathways are often regulated by feedback inhibition  Accumulation of product suppresses the process that produces the product @2015 Pearson Education Ltd