Chapter 10 Note
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Date Created: 03/24/16
Chapter 10: Photosynthesis • Plants aren’t the only things that go through photosynthesis and it’s not only eukaryotes • Plants are photoautotrophs: use light energy to drive synthesis of organic molecules from CO2 and H2O • In aquatic environments, photosynthetic organisms include: Multicellular algae, unicellular protists, cyanobacteria • Autotrophs: “self feeders” • Do not consume other living organisms • Produce organic molecules from CO2 and other inorganic molecules obtained from the environment • Ultimate source of organic compounds for all nonautotrophs • Referred to as Producers • Plants are photoautotrophs- use light as a source of energy to synthesize organic substances • Heterotrophs: "other eaters" • Do not make their own food, but live on compounds produced for other organisms / eat autotrophs or animals that have • May eat plants, or animals, or decompose organic litter • Ultimately dependent on photoautotrophs • Referred to as Consumers Photosynthesis converts light energy to the chemical energy of food • The enzymes that regulate photosynthesis are found on membranes • photosynthesis may have originated in bacteria that contained infolded regions within their plasma membrane • Functions similarly to the inner membrane of chloroplast - stroma: dense fluid of the chloroplast - Granum: column of thylakoid sacs • Thylakoid: sacs • Thylakoid space: space between sacs - All green plants are capable of photosynthesis, major region of the leaves • Leaves are the major sites of photosynthesis • Mesophyll is tissue in the interior of a leaf • Vein: delivers water and minerals from soil • Stomata: CO2 enter, O2 exits • Mesophylle cell contains ~30 to 40 chloroplasts • Color of leaf comes from green pigment- chlorophyll located inthe thylakoid membrane found in chloroplast - Summary equation of photosynthesis: 6CO2 + 12H2O + light energy -----> C6H12O6 + 6O2 + 6H2O • Glucose is used to simplify the relationship between photosynthesis and respiraton • However, the direct product of photosynthesis is actually a 3-carbon sugar used to make glucose • 12 H2O molecules are comsumed and 6 H2O molecules are made 6CO2 + 6H2O + light energy ------> C6H12O6 + 6O2 • Reverse reaction of cellular respiration • Both processes occur in plant cells • Divide summary equation by six to get the simplest one" CO2 + H2O + light energy ---------> [CH2O] + O2 Photosynthesis involves the splitting of water molecules: Key Points • Atoms are shuffled during photosynthesis • hydrogen is extracted from water and incorporated into sugar • oxygen is a waste product that is released into the atmosphere Photosynthesis is the reverse of cellular respiraton • Water is split and the electrons (e-) are tranferred along with protons (H+) from water to carbon dioxide, reducing it to sugar • Electrons increase in potential energy as they move from water to sugar the overall photosynthetic reaction is anabolic and endergonic (becuase it requires the use of energy) Two stages of photosynthesis: 1. Light reactions- photo part- converts solar energy into chemical energy 2. Calvin Cycle- synthesis part- incorporates CO2 into organic molecules which are converted to sugar • make ATP in light reaction and use it in the Calvin Cycle so no net ATP is made • photophosphorylation: light reaction that generates ATP The nature of sunlight • Electromagnetic energy: light energy • Travels in electromegnetic waves • Wavelengh is the distance between crests • Electromagnetic spectrum: entire range of radiation • only visible light is important to photosynthesis Key points • visible light is most important for life • it is the radiation that drives photosynthesis • violet light contains more energy than red light • Photon: discrete particle of electronmagnetic energy Photosynthesis pigments: light receptors that function to absorb light • different pigments absorb light of different wavelenghts • wavelengths that are absorbed disappear • we see wavelengths that are most reflected or transmitted by the pigment • plants look green because chlorophyll: absorbs violent-blue and red light; transmits and reflects green light Chlorophyll a: one of the major pigments that participates in the light reaction of photosynthesis What colors of light are best for photosynthesis? violet-blue(~430) and red light (6800 What color of light is worst for photosynthesis? green light How do the accessory pigments (chlorophyll b and carotenoids) contribute to photosynthesis? • these pigments absorb light at slightly different wavelengths and broden the spectrum of colors that are useful for photosynthesis Carotenoids may also provide photoprotection: • absorb and dissipate excessive light energy that could damage chlorophyll • prevents excess light energy from interacting with oxygen and forming oxidative molecules that are dangerous to cellls In animals: • the macula of the human eye contains carotenoids • these accessory pigments also funtion in photoprotection • carotenoids are found in health food products as phytochemicals that have antioxidents that they require • Animals must obtain them from their diet Key points: • vegetables and fruit have unique brilliant colors due to specific carotenoids • these chemicals provide photoprotection which may decrease the risk of some diseases, particuarly certain cancers and eye diseases Excitation of isolated chorophyll molecules: closed system A particular chlorophyll pigment (molecule) will absorb only a photon whose energy is exactly equal to the difference beween the ground state and the excited state Different pigments of chlorophyll will absorb photons corresponding to particular wavelenths Example: • Chlorophyll a pigment in photosystem II- absorbs at a wavelengths of 680nm- called P680 • Chlorophyll a pigment in photosystem I- absorbs light at a wavelenght of 700nm- called P700 proteins surrounding it causes shift in wavelenght Remember: a plants leaves exist in an open system. The energy released when chlorophyll electrons are excited by photons absorption is used to power the reactions of photosynthesis Excitation of chlorophyll within a leaf: open system Photosystem contains: 1. reaction center: proteins complex that contains special chlorophyll a molecules 2. several light harvesting complexes: consists of various pigment molecules bound to proteins Thylakoid membrane has two types of photosystems 1. photosystem II: Chlorophyll a absorbs light best at 680 nm (red part of the spectrum) 2. Photosystem I: Chlororphyll a absorbs light best at 700 nm (far red part of the spectrum) Note: photosystem I, the transfer of electrons down the electron transport chain does not create proton gradient and does not generate ATP; we never make net ATP in photosynthesis Summary of electron flow during light reaction: • photosynthsis is an endergonic reaction • light reaction uses solar power to generate ATP and NADPH • these molecules provide the chemical energy (ATP) and reducing power (NADPH) needed for carbohydrate synthesis by the calvin cycle Light reaction and chemiosmosis • Three steps in light reaction contribute to the proton movement force (PMF) 1. Water is spilt by PSII on the side of the membrane facing the thylakoid space 2. Protons are translocated across the membrane in to the thylakoid space when electrons are transferred from Pq to the cytochrome complex 3. Hydrogen ion is removed from the stroma when it is take up by NADP+ The Calvin Cycle: Anaboloic process that builds carbohydrates from small molecules Note: To synthesize one output molecule (Glye4raldehyde-3-phosphate, G3P), the cycle must take place 3 times Phase I- Carbon fixation: • 3 CO2 molecules enter the calvin cycle (1C) • Combine with 3 RuBP molecules (5C) • Generate 3 short-lived intermediates (6C) • Breakdown to form 6 3-phosphoglycerate molecules (3C) • Rubisco: is the most abundant protein in chlorplast and on earth. Functions to catalyze ; very inefficient enzyme Phase II- Reduction: requires 6 ATP and 6 NADPH Phase III- regeneration of RuBP: a complex series of reactions where 5 3C molecules of G3P is rearranged to 3 5C molecules of RuBP • Requires 3 more molecules of ATP • How much chemical energy is needed to net one molecule of G3P? 9 ATP and 6 NADPH • how does carbon enter the Calvin cycle? CO2 • How does carbon leave the Calvin cycle? Phase III G3P Photosynthsis: A review Light reaction: • Carried out by molecules in the thalikoid membrane of chloroplast • Convert light energy to the chemical energy of ATP and NADPH • Splits H2O adn released O2 to the atmosphere Calvin cycle: • Takes place in stroma of chloroplast • Uses ATP and NADPH to convert CO2 to the sugar G3P • Returns ADP, inorganic phosphate, and NADP+ to the light reaction Importance of photosynthesis: • Sugar made in the chloroplast supplies the entire plant with chemical energy and the carbon skeleton for the synthesis of all the major organic molecules of plant cells • About 50% of the organic material made by photosynthesis is consumed as fuel for cellular respiration in the mitochondria of plant cells • Carbohydrates are exported out of the leaves in the form of sucrose Cellulose is made in plant cells that are growing and maturing • It is the main ingredient in the cell wall • It is also the most abundant orgainc molecule on the surface of the planet • Plants make more organic molecules than they need as respiratory fuel and precursors for biosynthesis • Plants stockpile extra sugar as starch in cells of roots, tubers, seeds, and fruit Big Picture: • Photosynthesis is responsible for the presence of oxygen in the atmosphere • Makes ~160 billion metric tons of carbohydrates per year • No other chemical process can match the output of photosynthesis • No other process is more important than photosynthesis to the welfare of life on Earth
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