Photosynthesis Chapter 16 PDF

PHOTOSYNTHESIS  THE EARTH'S EARLIEST LIFE FORMS Early life raw materials & energy from simple organic molecules dissolved in aqueous environment. A. These molecules formed abiotically (via nonbiological chemical reactions) in primeval seas. B. Such organisms (like us) that depend on an...

PHOTOSYNTHESIS  THE EARTH'S EARLIEST LIFE FORMS Early life raw materials & energy from simple organic molecules dissolved in aqueous environment. A. These molecules formed abiotically (via nonbiological chemical reactions) in primeval seas. B. Such organisms (like us) that depend on an external source of organic compounds are known as heterotrophs. C. The number of heterotrophs on primitive Earth was probably severely restricted initially because the spontaneous production of organic molecules occurs very slowly. D. Evolution of life on Earth gave a tremendous boost when organisms appeared that employed a new metabolic strategy. 1. Unlike their predecessors, they could make their own organic nutrients from the simplest types of inorganic molecules (carbon dioxide [CO2] & hydrogen sulfide [H2S]) 2. Such organisms that can survive on CO2 as their principal carbon source are autotrophs.  Life as autotroph requires much energy since it takes a large input of energy to manufacture complex molecules from CO2: 2 types of autotrophs evolved that can be distinguished by their energy source. A. Chemoautotrophs - use energy stored in inorganic molecules (ammonia, H2S, nitrites) to convert CO2 to organic compounds; all are prokaryotes/bacteria that contribute relatively little to Earth biomass B. Photoautotrophs – Sun's radiant energy is used; most of Earth’s biomass made by photosynthesis (PS) They include higher plants, eukaryotic algae, some flagellated protists & members of 5 groups of prokaryotes (heliobacteria, cyanobacteria, purple sulfur, green nonsulfur & green sulfur bacteria). They capture the energy that fuels the activities of nearly every organism on Earth. All of these organisms carry out photosynthesis, a process in which energy from sunlight is transformed into chemical energy that is stored in carbohydrates & other organic molecules. In photosynthesis, sunlight energy is converted to chemical energy that is stored in carbohydrates & other organic metabolites. A. Relatively low energy electrons are removed from a donor compound & converted to high energy electrons by energy from light absorption. B. High energy electrons reduce carbon skeletons to make reduced biomolecules, like starches & oils. Earliest photosynthetic organisms (photoautotrophs) may have dominated Earth for 2 billion years & probably used hydrogen sulfide (H2S) as electron source via this reaction: CO2 + 2H2S + light (CH2O) + H2O + 2S Many bacteria still do, but H2S is not abundant or widespread, so these organisms are restricted in their importance & distribution (limited to habitats like sulfur springs & deep sea vents). ~2.7 billion years ago, a new photosynthetic prokaryote arose; used the much more abundant H2O (water) instead as a source of electrons.  The new photosynthetic prokaryotes are cyanobacteria & they produced an important waste product (O2; molecular oxygen) of much consequence via the following reaction: CO2 + 2H2O + light (CH2O) + O2 Also able to live in much more diverse array of habitats because of water abundance. Cyanobacteria became dominant & set stage for evolution of aerobic metabolism. Chloroplasts & cyanobacteria share many basic traits, including similar photosynthetic machinery, circular chromosomes, smaller ribosomes, etc. WHY?  PHOTOSYNTHESIS Some organisms can produce their own Giant food (carbohydrate) from CO2, H2O, kelp minerals and sunlight energy. 6CO2 + 6H2O C6H12O6 + 6O2 Plants, algae (eg. giant kelp) cyanobacteria are able to carry out photosynthesis. Cyanobacteria  Plant life converts ~500 trillion kg of CO2 to carbohydrate, releases ~450 trillion kg of O2 each year. Most photosynthesis is done by phytoplankton (single celled algae in thin, upper level of ocean). WHERE DOES PHOTOSYNTHESIS TAKE PLACE IN THE CELL? In eukaryotes, the light reactions of photosynthesis take place in the thylakoid membranes of chloroplasts. A series of membrane-bound electron carriers and pigments (chlorophyll) is able to harness the light energy of the sun. Like the mitochondrion, the chloroplast has inner and outer membranes and an intermembrane space. Membrane structures in chloroplasts  THE LIGHT-DEPENDENT AND LIGHT-INDEPENDENT REACTIONS OF PHOTOSYNTHESIS Light reactions are associated with the thylakoid membranes. Light-independent reactions are associated with the stroma.  Photosynthesis: Light reactions & Dark reactions. The light reactions of photosynthesis are related to the respiratory electron transport chain of mitochondria. The dark reactions are related to both gluconeogenesis and the pentose phosphate pathway.  PHOTOSYNTHESIS 6CO2 + 6H2O C6H12O6 + 6O2 The above reaction actually represents two processes: i) The oxidation of H2O to produce O2 (accompanied by the reduction of NADP+ to NADPH), requires light energy from the sun (light reactions; consist of two parts: Photosystem I for the reduction of NADP+ to NADPH; Photosystem II for the oxidation of H2O to produce O2 ). Photosystems I&II are linked by an electron transport chain coupled to the production of ATP. A proton gradient drives the production of ATP in photosynthesis, as it does in mitochondrial respiration. (Photosystems: membrane bound protein complexes that carry out the light reactions) ii) The fixation of CO2 to give sugars, uses solar energy indirectly (dark reactions: need NADPH from the light reactions). Events of photosynthesis can be divided into 2 series of reactions A. Light-dependent (light) reactions - light energy from sunlight is absorbed, converted to chemical energy & stored as 2 key high- energy biological molecules (ATP, NADPH). [ATP is cell's primary source of chemical energy; NADPH is its primary source of reducing power] B. Light-independent (dark) reactions - carbohydrates are made from CO2 using energy stored in ATP & NADPH molecules produced by light-dependent reactions.  CHLOROPHYLL Primary event in photosynthesis is the absorption of light by chlorophyll. There are 2 principal types of chlorophyll: a) chlorophyll a b) chlorophyll b Eukaryotes (plants and algae) contain both chlorophyll a and b (but amount of a>b). Prokaryotes (cyanobacteria) contain only chlorophyll a.  BACTERIOCHLOROPHYLL Photosynthetic bacteria other than cyanobacteria have bacteriochlorophylls (usually bacteriochlorophyll a). Green and purple sulfur bacteria which contains bacteriochlorophyll, do not use H2O as the ultimate source of electrons. They also do not produce O2! H2S is used instead of H2O as electron source and elemental S is produced instead of O2. Organisms that contain bacteriochlorophyll are anaerobic and have only one photosystem.  Propionic acid side chain Structures of chlorophyll a, chlorophyll b, and bacteriochlorophyll a Structure of chlorophyll is similar to that of the heme group of myoglobin, hemoglobin, and the cytochromes in that it is based on the tetrapyrrole ring of porphyrins. Difference: In heme, iron replaces Mg(II) and cyclopentanone ring is absent.  PHOTOSYNTHETIC UNITS AND REACTION CENTERS Not all of the chlorophylls in a chloroplast are directly involved in the conversion of light energy into chemical energy. Most pigments do not participate directly in conversion of light to chemical energy, but they are responsible for light absorption. They form a light-harvesting antenna that absorbs photons of various wavelengths & transfers the energy (excitation energy) very rapidly to the pigment at the reaction center. Schematic diagram of a photosynthetic unit. The light- harvesting pigments, or antenna molecules (green), absorb and transfer light energy to the specialized chlorophyll dimer that constitutes the reaction center (orange). LIGHT ABSORPTION SPECTRA OF CHLOROPHYLL Absorption spectra of chlorophyll a differs slightly from chlorophyll b. Chlorophyll a absorbs red light (600-700 nm). Chlorophyll b absorbs blue light (400-500 nm). So, green plants and algae have more efficient photosynthesis compared to cyanobacteria because the eukaryotes have both chlorophylls a and b. Besides the chlorophylls, various accessory pigments absorb light and transfer energy to chlorophylls (carotenoids, phycoerythrin, phycocyanin). Bacteriochlorophylls absorb light at longer wavelengths (>780 nm, up to 870 or 1050 nm). PROPERTIES OF LIGHT Light wavelengths that drive photosynthesis BLUE RED (a) The absorption of visible light by chlorophylls a and b. The areas marked I, II, and III are regions of the spectrum that give rise to chloroplast activity. There is greater activity in regions I and III, which are close to major absorption peaks. There are high levels of O2 production when light from regions I and III is absorbed by chloroplasts. Lower (but measurable) activity is seen in region II, where some of the accessory pigments absorb. (b) The absorption of light by accessory pigments (superimposed on the absorption of chlorophylls a and b). The accessory pigments absorb light and transfer their energy to chlorophyll.  Photosynthesis: An Overview ◼ The net overall equation for photosynthesis is: 6 CO2 + 6 H2O light C6H12O6 + 6 O2 ◼ Photosynthesis occurs in 2 “stages”: 1. The Light Reactions (or Light-Dependent Reactions) 2. The Calvin Cycle (or Calvin-Benson Cycle or Dark Reactions or Light-Independent Reactions) 28  Photosynthesis: An Overview ◼ To follow the energy in photosynthesis, Light light Calvin Reactions Cycle Organic light ATP compounds thylakoids NADPH (carbs) stroma 29  PHASE 1: LIGHT REACTIONS (PHOTOSYSTEMS I & II ) A. Light vs. dark reactions B. Splitting of water at PS II 1. Oxygen-evolving complex 2. Electron transfers 3. Phaeophytin 4. Plastoquinone 5. Cytochrome b6-f complex Produce ATP (energy) and NADPH (reducing power) for 6. Plastocyanin the fixation of CO2 in the dark C. Reduction of NADP+ via PS I reactions 1. Reaction center electron transfers 2. Ferredoxin D. Cyclic electron transport E. Photosystem structure  Connections between Photosystems I and II in the Light Reactions of Photosynthesis Photosystem I (PSI) can be excited by light of wavelengths