Campbell Biology Chapter 10 Photosynthesis

Campbell Biology Chapter 10: Photosynthesis A1: The Process that Feeds the Biosphere ● Photoautotrophs convert sunlight into chemical energy that is stored in organic molecules, in a process known as photosynthesis ● Photosynthesis fuels the whole world by autotrophic or heterotrophic nutrition ● Autotrophs are self feeders that sustain themselves without consuming other organisms/products ○ They produce everything from CO2 molecules and other inorganic molecules from the environment; they are known as producers, and plants are photoautotrophs ● Heterotrophs obtain organic material by living off compounds produced by other organisms ○ They are known as consumers, and decomposers eat dead organisms while other heterotrophs eat other organisms ● All organisms are dependent on photoautotrophs ● Fossil fuels are the remains of dead organisms from long ago, and it is a nonrenewable resource ● The light reactions of photosynthesis capture solar energy and turn it into chemical energy, while the Calvin cycle uses the chemical energy to make organic molecules A1: Photosynthesis Converts Light Energy to the Chemical Energy of Food ● Photosynthesis is possible due to structural organization of the cell ● Photosynthetic enzymes and other molecules are grouped together in a biological membranes so the reactions run efficiently ● Photosynthesis likely originated in bacteria with infolded regions of the plasma membrane with molecules, and then became part of a eukaryote as part of the endosymbiont theory ● Organisms other than plants have chloroplasts A2: Chloroplasts: The Sites of Photosynthesis in Plants ● All green parts of a plant have chloroplasts, but leaves have the most ● Chloroplasts are found mainly in the cell of mesophyll, which is the tissue in the interior of the leaf ● Carbon dioxide enters and oxygen leaves the leaves through microscopic pores, or stomata ● Water absorbed by the roots gets to the leaves through veins, as sugar and other molecules are exported to the roots and other nonphotosynthetic parts ● Mesophyll cells have 30-40 chloroplasts, 2-4 micrometers by 4-7 micrometers ● Chloroplasts have a double membrane surrounding a dense fluid called stroma ● Within the stroma, there are membranes called thylakoids, which separates the stroma from the thylakoid space ○ Thylakoids can also be stacked into grana ● Chlorophyll is a green pigment that gives leaves color and resides in the thylakoid membrane ● Prokaryotes also have a section of their plasma membrane called the thylakoid membrane ● Chlorophyll absorbs light energy to drive photosynthesis A3: Tracking Atoms Through Photosynthesis: Scientific Inquiry ● The equation of photosynthesis is : 6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O ○ Glucose is used for simplification, but the result is actually a triose that can make glucose ● Simplified equation: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2 ● Further simplified: CO2 + H2 O → [CH2 O] + O2 ○ Formaldehyde represents the empirical formula of glucose 1. ● 2. ● ● The Splitting of Water Van Niel and other scientists discovered that the oxygen produced gets its oxygen from the water Photosynthesis as a Redox Process Cellular respiration has the opposite equation of photosynthesis, and energy is released from sugar to produce water and carbon dioxide and energy for ATP synthesis, and electrons flow to the oxygen In photosynthesis, CO2 is reduced into sugar and H2O is oxidized into diatomic oxygen ○ The electrons increase in potential energy, so photosynthesis is endergonic and the energy is provided by light; electrons lose potential energy in electron transport chain in respiration A4: The Two Stages of Photosynthesis: A Preview ● Photosynthesis comes into two stages, with multiple steps in each ● The two stages are the light reactions and the Calvin cycle, the former is photo and the latter is synthesis ● Light reactions convert light to chemical energy by splitting water to give off oxygen, electrons, and a proton ● Chlorophyll absorbs light to transfer the electrons and protons from water to an acceptor called nicotinamide adenine dinucleotide phosphate (NADP+), and it is very similar to NAD+ in respiration ○ NADP+ is reduced to NADPH, and chemiosmosis phosphorylates a phosphate group to ADP in a process called photophosphorylation ● Light energy turns into ATP and NADPH, the latter serves as an reducing agent ● The calvin cycle incorporates CO2 from the air into organic molecules present in the chloroplasts, known as carbon fixation ● The carbon then is reduced into carbohydrates by NADPH and ATP from the light reactions ● These steps are known as the dark reactions or light-independent reactions, since no light is needed directly ● The Calvin cycle occurs during the day since NADPH and ATP can be provided at that time ● Thylakoids are the site of the light reactions, while the Calvin cycle occurs in the stroma, and NADP+ and ADP pick up electrons and phosphate on the outside of the thylakoids to be released to the stroma for their roles in the Calvin cycle A1: The Light Reactions Convert Solar Energy to the Chemical Energy of ATP and NADPH ● Thylakoids transform light energy into the chemical energy of ATP and NADPH to synthesize glucose and other molecules that can be used as energy sources A2: The Nature of Sunlight ● Light is a form of electromagnetic radiation, and it travels in rhythmic waves ● Wavelength is the distance between crests of electromagnetic waves ● The electromagnetic spectrum is a range of radiation from gamma rays to radio waves ● The radiation important to light is called visible light because humans can see it, and it has a wavelength from 380 nanometers to 750 nanometers ● Photons are discrete particles of part and they are quantized (fixed amount of energy) ● Energy is inversely proportional to wavelength; short wavelength means large amount of energy ● Visible light is the light the atmosphere lets through, and it drives photosynthesis A3: Photosynthetic Pigments: The Light Receptors ● Light can be absorbed or reflected by matter ● Pigments are substances that absorb visible light, and different pigments absorb different wavelengths ○ If a wavelength is absorbed, it is not seen, and the color seen is the one reflected the most ○ If all light is absorbed, the substance appears black ● Chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light ● A spectrophotometer is an instrument that measures the ability of a pigment to absorb various wavelengths of light ○ The machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of light transmitted at each wavelength ● Absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength ● Light can only performs work in chloroplasts if it is absorbed ● Chlorophyll a is the key light-capturing pigment that participates directly in the light reactions ● Chlorophyll b is an accessory pigment, as are carotenoids ● Green is the color least absorbed by the chlorophylls ● Action spectrums profile the relative effectiveness of different wavelengths of radiation in driving photosynthesis ○ ● ● ● ● Action spectrums illuminate chloroplasts with different colors and measure the wavelength against other photosynthetic rates Accessory pigments broaden the spectrum of colors that can be used for photosynthesis other than just chlorophyll a Both chlorophyll a and b are made up of a hydrocarbon tail, a porphyrin (light-absorbing) head with a central magnesium, but chlorophyll a contains CH3 , while chlorophyll b contains CHO ○ The chemical difference cause different absorption spectra Carotenoids are hydrocarbons that appear to be shades of yellow and orange because they absorb violet and blue-green light, and they also broaden the spectrum of photosynthesis Carotenoids serve a greater function of photoprotection, which means they absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules dangerous to cell activity ○ Carotenoids also serve a similar role in the human eye ○ Carotenoids are antioxidants, which humans must consume as we cannot produce them A4: Excitation of Chlorophyll by Light ● When chlorophyll absorbs light, the energy from the light is used to elevate an electron of the pigment molecule to a higher potential energy orbital from its ground state; this is known as an excited state ● The photon absorbed has energy equal to the difference between the ground state and excited state of the electron, so particular compounds only absorb photons according to specific wavelengths, thus the unique absorption spectra ● Due to the instability of the excited state, the electron quickly converts back to the ground state and releases the energy as heat, but chlorophyll gives off photons as well, which is called fluorescence ○ Chlorophyll fluoresces red, which is difficult to see against the green A5: A Photosystem: A Reaction-Center Complex Associated with Light-harvesting Complexes ● Chlorophyll acts differently inside a chloroplast when exposed to light ● Chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems in the thylakoid membrane ● A photosystem is composed of a reaction-center complex surrounded by several light-harvesting complexes ● The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules ● The light-harvesting complex is a group of proteins holding a special pair of chlorophyll pigment molecules, which may include chlorophylls a and b, as well as carotenoids bound to proteins ● The number and variety of pigment molecules over a large surface area allows lots of light to be absorbed over a large area of the spectrum ● After a pigment molecule absorbs a photon, the energy is transferred from molecule to molecule until it reaches the reaction-complex center ● The primary electron acceptor in the reaction-center complex accepts electrons and becomes reduced ● The special pair of chlorophyll a molecules because their environment and location enables them to use the energy from the light to excite their own electrons and transfer the electrons to the primary electron acceptor ● The electron transfer is the first step of the light reactions, and it also counts as a redox reaction ○ Independently, chlorophyll fluoresces because there is no electron acceptor and the electrons fall to their ground state ○ The potential energy of the electron is transferred to an electron acceptor in a photosystem, so no light and heat are released ● ● ● ● ● The photosystem functions as a unit in the cell and it converts light energy to chemical energy The two types of photosystems that participate in the light reactions are called photosystem II (PS II) and photosystem I (PS I) ○ Photosystem II participates in the light reactions first Each photosystem has its own kind of primary electron acceptor Photosystem II’s reaction-center chlorophyll a is known as P680 because it absorbs light best at 700 nanometers, while P700 of other chlorophyll a absorbs light best at 700 nanometers The two pigments are nearly the same, but the association with different proteins in the thylakoid membrane affects electron distribution and causes minimal differences in light absorbance A6: Linear Electron Flow ● Light energizes the photosystems in the thylakoid, resulting in ATP and NADPH synthesis ● Linear electron flow is the flow of electrons through the photosystems and other molecular components built into the membrane; it occurs during the light reactions ● Steps of Linear Electron Flow 1. Photon strikes a pigment molecule in the light harvesting complex of photosystem II, boosting an electron to a higher energy level, and as it returns to ground state, another electron in another pigment is excited, and the energy is transferred until it reaches the P680 chlorophyll a in the reaction-center complex, where the electrons in the chlorophyll are excited 2. The excited electron is transferred to the primary electron acceptor from P680, and the resultant is P680+ 3. An enzyme catalyzes the split of a water molecule into two electrons, two protons, and an oxygen atom, and the electrons are supplied to the P680+ (strongest biological oxidizing agent) to replace those given to the primary electron acceptor, and the hydrogen goes to the thylakoid space while the oxygen combines with another one to form diatomic oxygen 4. The excited electrons pass from photosystem II to photosystem I via an electron transport chain, made up of the carrier plastoquinone (Pq), a cytochrome complex, and a plastocyanin (Pc) protein 5. The return of earlier ETC electrons to ground state releases enough energy to synthesize ATP, and as electrons pass through the cytochrome complex, protons are pumped into the thylakoid space, creating a proton gradient used in chemiosmosis 6. Light energy is transferred via the light harvesting complex pigments to the photosystem I reaction-center complex, which excites the P700 chlorophyll a pair, and the electron is transferred to the primary electron acceptor, so P700+ can now act an as electron acceptor for the electrons in the electron transport chain from photosystem II 7. Redox reactions pass the photoexcited electrons of photosystem I down a second electron transport chain through the ferredoxin (Fd) protein, but no proton gradient is produced, so no ATP is produced 8. Enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+ , but since this molecule has more potential energy, it can accept the electrons easier, and this also removes a proton from the stroma ● The ATP and NADPH produced will be used in the Calvin cycle A7: Cyclic Electron Flow ● Photoexcited electrons may sometimes take an alternate path called the cyclic electron flow, which uses photosystem I and not photosystem II ● Photoexcited electrons cycle from ferredoxin (Fe) to the cytochrome complex via plastocyanin and from there to a P700 chlorophyll in the photosystem I reaction-center and back to the electron acceptor ● No NADPH is produced and no oxygen is released, but ATP is generated ● Many bacteria have a single photosystem related to both, so the cyclic electron flow is the only way to generate ATP during photosynthesis, and this is how ancestral bacteria did photosynthesis ● Cyclic electron flow also occurs in species that have both photosystems, like cyanobacteria and plants, which enables mutant plants to grow well in low-light but poorly in high levels of light A8: A Comparison of Chemiosmosis in Chloroplasts and Mitochondria ● ● Both chloroplasts and mitochondria generate ATP by chemiosmosis The electron transport chain pushes protons across the membranes as they electrons are transferred to progressively more electronegative acceptors ● Redox energy is turned into the potential energy of a proton gradient across a membrane ● An ATP synthase complex couples the diffusion of hydrogen ions down their gradient so ADP is phosphorylated into ATP ● Iron-containing cytochromes are similar in chloroplasts and mitochondria and are electron carriers ○ The ATP synthase complex is similar ● In chloroplasts, the high energy electrons come from water, while they comes from oxidized organic molecules in mitochondria ● In chloroplasts, the light energy harvested by the photosystems is used to make ATP at the top of the ETC, while mitochondria uses food molecules to make ATP ● The inner mitochondrial membrane pumps protons from the matrix out to the reservoir in the intermembrane space ● The thylakoid membrane pumps protons from the stroma into the reservoir of the thylakoid space ● The ATP complex is analogous in both organelles, although they are spatially different ● ATP is synthesized as the protons move from the intermembrane space or thylakoid space back into the matrix or the stroma, respectively ○ ATP forms in the stroma to assist in the Calvin cycle ● The proton or pH gradient creates a thousandfold difference across the membrane when there is light ● Chemiosmosis drives the synthesis of ATP through membrane potential ● Light reaction short summary 1. Electron flow pushes electrons from water to NADPH, where they are stored at a higher energy state 2. The electron flow generates ATP 3. The thylakoid converts light energy to chemical energy stored in ATP and NADPH A1: The Calvin Cycle Uses the Chemical Energy of ATP and NADPH to reduce CO2 to Sugar ● The Calvin cycle regenerates the starting material just like the citric acid cycle, but it is anabolic ● Carbon dioxide enters the cycle and leaves in the form of sugar, and ATP and NADPH is consumed to energize the compounds to make sugar ● Glyceraldehyde 3-phosphate (G3P) is the carbohydrate produced directly by the Calvin cycle ○ To produce one molecule of G3P, the Calvin cycle must occur three times, fixing three CO2 molecules 1. Carbon Fixation ● One CO2 molecule is attached at a time to a pentose named ribulose biphosphate (RuBP) ○ Reaction catalyzed by ribulose bisphosphate carboxylase-oxygenase, or rubisco ■ Rubisco is the most abundant protein in chloroplasts and on Earth ○ The six-carbon intermediate is unstable, so it splits into two molecules of 3-phosphoglycerate ■ For one molecule of CO2 , so six total molecules for three turns 2. Reduction ● Each molecule of 3-phosphoglycerate receives a phosphate from ATP to become 1, 3-bisphosphoglycerate ● NADPH donates a pair of electrons to reduce a carboxyl group on 1, 3-bisphosphoglycerate, which loses a phosphate to become glyceraldehyde 3-phosphate ● For three turns of the Calvin cycle, 6 molecules of G3P are formed, but only one is considered a net gain ● Initially, there were 15 carbons’ worth of RuBP, and 18 carbons’ worth of G3P are formed, but only one molecule exists the cycle, as the others are used to regenerate RuBP 3. Regeneration of the CO2 acceptor (RuBP) ● The carbon skeletons of five molecules of G3P are rearranged into three molecules of RuBP, which is done by spending three more molecules of ATP ● ● ● Nine molecules of ATP and six molecules of NADPH are consumed to produce one G3P molecule The G3P molecule serves as the starting molecule for glucose or other carbohydrates’ synthesis Photosynthesis is an emergent property of chloroplasts, as the two steps work together to produce sugar A1: Alternative Mechanisms of Carbon Fixation Have Evolved in hot, Arid Climates ● There is a balance between photosynthesis and the prevention of water loss ● The stomata is the site where CO2 enters, O2 leaves, and water transpires, and plants tend to close stomata on hot, dry days to conserve water ○ CO2 levels fall and O2 levels rise due to a process called photorespiration that occurs when the stomata partially or fully close A2: Photorespiration: An Evolutionary Relic? ● Most plants use rubisco to add carbon dioxide to RuBP, and they are known as C3 plants, since the first product is the three carbon compound, 3-phosphoglycerate ● C3 plants produce less sugar on hot days because the stomata close and starve the cells of CO2 ● Rubisco also has an affinity for oxygen, so it adds oxygen to the Calvin cycle instead of CO2 when its levels are low ● The product splits and a two carbon compound is sent to the peroxisomes and mitochondria, which split the compound to release CO2, which is known as photorespiration ○ ATP is used during the process, and no sugar is produced, which makes photorespiration wasteful ○ Material that would be used in the Calvin cycle is siphoned away, and the CO2 can be fixed if its concentration is high enough ● Photorespiration exists despite its energetic cost from a time when there was much more CO2 in the atmosphere than O2, meaning rubisco’s affinity for oxygen would not have had much of an effect on photosynthesis ● The chance affinity for oxygen ensures some photorespiration occurs in modern plants ● ● Photorespiration may have a photoprotective effect for plants, as it eliminates the oxygen in the cell which builds up when there is too much light and not enough CO2 Nearly half of the carbon fixed by the Calvin cycle is drained away by photorespiration in many crop plants, so eliminating it would be helpful in increasing crop yields A3: C4 Plants ● C4 plants use an alternate form of carbon fixation in which a four carbon compound is the first product ● The C4 pathway has evolved independently 45 times and is used by many different species of plants ● Sugarcane and corn are C4 plants, and C4 plants have two types of photosynthetic cells ● Bundle-sheath cells are arranged into tightly-packed sheaths around the veins of the leaf ● Mesophyll cells are more loosely arranged between the bundle-sheath cells and the leaf surface ● CO2 is incorporated into organic compounds in mesophyll cells, while the Calvin cycle occurs in the bundle-sheath cells 1. PEP carboxylase in (only) mesophyll cells adds CO2 to phosphopyruvate (PEP), forming oxaloacetate (four carbons) a. PEP carboxylase has no affinity for O2, so carbon can be fixed when rubisco cannot in hot and dry conditions 2. Mesophyll cells export the four-carbon products to bundle-sheath cells through plasmodesmata 3. The four-carbon products release CO2 , which is assimilated into organic material by rubisco and the Calvin cycle, and pyruvate is created, which is transported to the mesophyll cells to be turned into PEP with ATP phosphorylation a. Since ATP is the only cost of the process, cyclic electron flow is used by the bundle-sheath cells that lack PS II ● The CO2 is sent to the bundle-sheath by the mesophyll to ensure CO2 levels stay high to avoid photorespiration ● ATP is used to enhance sugar production in hot regions with intense sunlight where stomata partially close during the day ● Increasing temperatures and CO2 levels may change the relative abundance of C3 and C4 plants ● C3 plants would have lowered photorespiration from the higher CO2 levels, but the higher temperature would undo that effect, and C4 plants would be unaffected by each factor ● The combination of CO2 concentration and temperature will affect the balance of both types of plants and can change community structure A4: CAM Plants ● Many succulent plants open their stomata during the night and close them during the day to conserve water ● To ensure CO2 levels are high, these plants take up CO2 during the night and incorporate it into organic acids, known as crassulacean acid metabolism, or CAM ● CAM plants store their organic acids in the vacuoles of mesophyll cells until the stomata close in the morning ● The light reactions supply the ATP and NADPH for the Calvin cycle in the day, and CO2 is released from the organic acids ● The CAM and C4 pathways both fix carbon into intermediates before the Calvin cycle occurs, but C4 plants have separate cells for carbon fixation and the Calvin cycle A5: The Importance of Photosynthesis: A Review ● Enzymes in the cytosol and chloroplasts convert G3P into many other organic compounds, and half of the compounds are used for respiration ● Green cells are the only autotrophic parts of the cell, as the rest of the plant receives sucrose from the green cells ● The sucrose serves as raw material for anabolic pathways and cellular respiration, especially for cellulose synthesis ● Most plants produce more organic material than is needed, storing it as starch in storage cells and chloroplasts, and much of it goes to heterotrophs ● Photosynthesis is responsible for oxygen in the atmosphere, and produces lots of carbohydrates, thus being an important contributor to life on Earth