Photosynthesis Chapter 10 Photosynthesis What is Photosynthesis Photosynthesis
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Photosynthesis Chapter 10: Photosynthesis
What is Photosynthesis? Photosynthesis is the process that converts Directly or indirectly, photosynthesis nourishes almost the entire living world Autotrophs sustain themselves by creating Heterotrophs obtain their organic material from
Auto versus Hetero Autotrophs are the of the biosphere, producing organic molecules from CO 2 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of to make organic molecules from Heterotrophs are the of the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2
Overall Reaction What does this remind you of? Includes: 1) Light energy + H 2 O Chemical Energy (ATP & NADPH) + O 2 2) Chemical energy (ATP and NADPH) + CO 2 C 6 H 12 O 6
Photosynthesis Coenzyme NADP+ (nicotinamide adenine dinucleotide phosphate) It picks up a to become NADPH The H+ and electron move to the Calvin Cycle
Photosynthesis Reactants Products
Photosynthesis in a Leaf Leaves are the major locations of photosynthesis Their green color is from , the green pigment within chloroplasts absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast CO 2 enters and O 2 exits the leaf through microscopic pores called Chloroplasts are found mainly in cells of the , the interior tissue of the leaf A typical mesophyll cell has 30– 40 chloroplasts
Chloroplast Interconnected sacs inside chloroplasts Stacks of thylakoids are called Contain (give leaves green color) Accessory pigments: carotenoids, phycocyanins Fluid inside the chloroplasts surrounding the thylakoids
Overview in Words Photosynthesis is a process in which is oxidized and is reduced Chloroplasts split H 2 O into Electrons of hydrogen are put into sugar molecules Photosynthesis consists of the (the photo part) and (the synthesis part) The light reactions (in the thylakoids) Reduce NADP+ to NADPH Generate ATP from ADP by The Calvin cycle (in the stroma) , using ATP and NADPH The Calvin cycle begins with incorporating CO 2 into organic molecules ,
Overview in Picture
Light Energy Light is a form of electromagnetic energy, also called Light travels in rhythmic waves Wavelength determines the type of electromagnetic energy is the distance between crests of waves The is the entire range of electromagnetic energy, or radiation consists of wavelengths (including those that drive photosynthesis) that produce colors we can see
Photons & Pigments Light also behaves as though it consists of discrete particles, called are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are Leaves appear green because chlorophyll A green light measures a pigment’s ability to absorb various wavelengths
Absorption Spectrum An is a graph plotting a pigment’s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that work best for photosynthesis
Action Spectrum An profiles the relative effectiveness of different wavelengths of radiation in driving a process is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called absorb excessive light that would damage chlorophyll
Light-Dependent Reactions GOAL: To trap convert it to and NADPH and as ATP & NADPH (energy carriers) carry energy to the Calvin Cycle Takes place in Light-dependent reaction animation
Photolysis H 2 O Water O + one oxygen atom during photosynthesis 2 H+ + 2 e 2 hydrogen ions 2 electrons Electrons from photolysis replenish ones taken by NADP+ (to form NADPH) NADP+ also picks up H+ ions is freed as waste
Capturing Light When a pigment absorbs light, it goes from a ground state to an excited state, which is When excited electrons fall back to the ground state, photons are given off A consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes (pigment molecules bound to proteins) that funnel the energy of photons to the reaction center Solar-powered transfer of an electron from a molecule to a primary electron acceptor is the first step of the light reactions
Capturing Light
Photosystems There are two types of photosystems in the thylakoid membrane functions first and is best at absorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II is called functions second and is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called
Excitation , the primary pathway, involves both photosystems and produces ATP and NADPH using light energy A photon hits a pigment and its energy is passed among pigment molecules until it excites P 680 An excited electron from P 680 is transferred to the primary electron acceptor P 680+ (P 680 that is missing an electron) is a very strong oxidizing agent H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P 680+, thus reducing it to P 680 O 2 is released as a by-product of this reaction
ETC Each electron “falls” down an from the primary electron acceptor of PS II to PS I Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane Diffusion of H+ (protons) across the membrane drives
Repeat (kinda) In PS I (like PS II), transferred light energy excites P 700, which loses an electron to an electron acceptor P 700+ (P 700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein The electrons are then transferred to NADP+ and reduce it to NADPH The electrons of NADPH are available for the reactions of the
Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces , but not NADPH Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle Some organisms (such as purple sulfur bacteria) have PS I but not PS II Cyclic electron flow may protect cells from
Cyclic Electron Flow
Chemiosmosis ATP is produced in conjunction with electron transport by the process of Results from electrons flowing the concentration gradient When water is broken the H+ accumulates in the thylakoid allowing ATP synthesis to occur
Chemiosmosis Differences Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Mitochondria use to make ATP; chloroplasts use to make ATP In mitochondria, protons are pumped to the and drive ATP synthesis as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the and drive ATP synthesis as they diffuse back into the stroma
Light-Dependent Reactions
Light-Dependent Reaction: Summary Location: Reactants Products Light Dependent Reaction summary
Calvin Cycle The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3 -phosphate For net synthesis of 1 G 3 P, the cycle must take place times, fixing 3 molecules of CO 2 The Calvin cycle has three phases: (catalyzed by rubisco) (Ru. BP)
Carbon Fixation Each carbon dioxide (6 of them) is “fixed” (added) to a five carbon compound called ribulose bisphosphate This creates an unstable six-carbon compound that splits into 2 three-carbon compounds called 3 phosphoglycerate The enzyme that catalyzes this first step is called Ru. BP carboxylase
Reduction Each PGA receives a phosphate from to become 1, 2 bisphoglycerate A pair of electrons (donated from NADPH) the compound and loses a phosphate to become glyceraldehyde-3 -phosphate (G 3 P) G 3 P is a same three carbon sugar formed in G 3 P is given off and eventually produces
Regeneration of Ru. BP 5 molecules of G 3 P are rearranged into 3 molecules of Cycle spends more molecules of ATP Ru. BP can now accept another
Calvin Cycle Summary Location: Reactants Products Calvin Cycle Summary
Overview
Alternative Mechanisms Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close , which conserves H 2 O but also photosynthesis The closing of stomata reduces access to and causes to build up These conditions favor a seemingly wasteful process called In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound (C 3)
Photorespiration In photorespiration, rubisco adds instead of CO 2 in the Calvin cycle Photorespiration consumes O 2 and organic fuel and releases without producing ATP or sugar Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as of the carbon fixed by the Calvin cycle
C 4 Plants C 4 plants minimize the cost of photorespiration by incorporating into fourcarbon compounds in mesophyll cells This step requires the enzyme Has a higher affinity for CO 2 than rubisco does Can fix CO 2 even when CO 2 concentrations are These four-carbon compounds are exported to bundle-sheath cells (surround leaf veins), where they release CO 2 that is then used in the Calvin cycle Example:
CAM Plants Some plants, including succulents, use crassulacean acid metabolism to fix carbon CAM plants open their stomata at , incorporating CO 2 into organic acids Stomata close during the , and CO 2 is released from organic acids and used in the Calvin cycle Example:
Review The energy entering chloroplasts as gets stored as in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as In addition to food production, photosynthesis produces the in our atmosphere
Review Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. Define photosynthesis and state its chemical equation. Be able to list the light reactions and Calvin cycle as separate equations as well. Differentiate between autotrophs and heterotrophs. Explain the anatomy of a leaf and how its parts pertain to photosynthesis. Define the role of NADP+ in photosynthesis. Detail the parts of the chloroplast and explain the events that occur in each part. Briefly describe the electromagnetic spectrum and its influence on photosynthesis. Define photons and pigments and state their role in photosynthesis. Differentiate between the absorption spectrum and action spectrum of various pigments. State the goal, location, and steps of the light dependent reactions. Define photosystem and differentiate between the 2 kinds used in photosynthesis. Explain the events of the electron transport chain (ETC). Trace the pathway that electrons from the hydrogen atom of water take in the light dependent reactions. Explain chemiosmosis and identify the differences in function in the chloroplast versus the mitochondrion. State the goal, location, and 3 steps of the Calvin cycle. Differentiate between PGA, G 3 P, and Ru. BP. Explain photorespiration. Explain the differences between C 4 plants and CAM plants, as oppossed to C 3 plants.
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