Download
1 / 59
590 likes | 592 Views
Chapter 10 . Chapter 10 Photosynthesis The conversion of radiant energy into chemical energy & Converting inorganic matter into organic matter. Overview of Chapter 10. Autotrophs vs. Hetertrophs Properties and Characteristics of Light
E N D
Chapter 10 Chapter 10 Photosynthesis The conversion of radiant energy into chemical energy & Converting inorganic matter into organic matter
Overview of Chapter 10 • Autotrophs vs. Hetertrophs • Properties and Characteristics of Light • Chloroplast Structure & Function Key Pigments: Chlorophyll a & b, & Carotenoids • Light Reactions (Light Dependent)-Photosystems • Cyclic vs. Non-cyclic flow of Electrons • ‘Dark’ Reactions (Light independent)-Calvin Cycle • Photorespiration: ↓ Photosynthetic efficiency • C3, C4, and CAM Metabolic Pathways of Plants
Introductory Questions #1 • Name the three parts that make up a photosystem. • How does NADPH differ from NADH? • What does it mean when we “FIX” carbon? Does this happen in the light or ‘dark’ reactions? • What is required in order for the light reactions to proceed? • How does non-cyclic photophosphorylation differ from cyclic photophosphorylation? Which process is more common?
Introductory Questions #2 • Name the three phases of the Calvin Cycle. Which phases require ATP and how much ATP would be needed for producing on glucose molecule? 2) What are the substrates that attach to the active sites of Rubisco? • How does a C3 plant differ from a C4 plant? Give 3 examples of a C3 & C4 plant. • What happens as a result of stomata closing? • Which type of plant undergoes photorespiration? Does photorespiration occur at night or during the day? How is photorespiration different from cellular respiration seen in the mitochondria? • How are C4 and CAM plants similar and how are they different? Give an example of both.
More Introductory Questions • How is an autotroph different from a heterotroph? • Briefly explain what light is and how it is generated. • In plant tissue, where are chloroplasts highly concentrated? 4) How are chloroplasts similar to mitochondria? How are they different? 5) How do plants absorb light energy? Name some features that allow plants to absorb light. What are some differences between chlorophyll a and chlorophyll b? • What did Engelmann’s experiment measure? What organisms did he use? • Which reactant does the oxygen produced from photosynthesis directly come from? • Where specifically do the light and dark reaction take place within a plant cell?
Photosynthesis in Nature Autotrophs are biotic Producers; Ex. Photoautotrophs and chemoautotrophs; obtains organic food without eating other organisms Heterotrophs: are biotic Consumers; obtains organic food by eating other organisms or their by-products (includes decomposers)
Properties of Light • Electromagnetic Radiation • Possesses properties of a particle and a wave • Generated when electrons move from a high energy state to a lower energy state. • Small portion of the EM spectrum (pg .190) • Composed of small “packets” or quantized amount of energy called PHOTONS • Described by Max Plank (Plank’s constant-ER can be quantized ) and DeBroglie (objects move in waves)
Visible Light • Wavelength range of: 380 nm – 760 nm • Colors include: R O Y G B I V Red: Lowest energy, Longest wavelength Violet: Highest energy, smallest wavelength
Photons and Electrons • Photons interact with electrons and move electrons to higher energy levels from the “ground state” • When electrons “fall” to the lower ground state, and light is emitted as it falls. This light is called “Fluorescence”.
Leaves: The Solar Collectors for Plants • Considered to be an organ of the plant • Site for Photosynthesis (lots of chloroplasts) • Cutin-thin wax layer helps to reduce or control water loss • Other features worth noting: -Upper & Lower epidermis -Stomata & Guard cells -Xylem & Phloem (vascular bundle sheaths) -Palisade & spongy Mesophyll Cells (contain the chloroplasts) -Trichomes (hairlike structures) • High surface area: Can cause water to be lost • See a definite trade off
The Chloroplast and Light (pg. 187) • The (3) Fates of Light as it interacts with a chloroplast. • *If wavelengths are not absorbed, those wl are further transmitted or reflected back
Structure of the Chloroplast • Double membrane • Has its own DNA • Internal membrane system called Thylakoids • Contains protein pigmets: ex- chlorophyll a
Typical Pigments Found in the Thylakoid Membrane • Chlorophyll a - important in light reactions • Chlorophyll b - accessory pigment - has a yellow/green reflection • Carotenoids – are yellow & orange • Anthocyanins– are red pigments • Fucoxanthin – is a brown pigment • Xanthophylls – are typically yellow
The Chlorophyll Molecule (Pg. 192) • Porphyrin ring (absorbs light) • Central Magnesium Atom • Hydrocarbon tail • Alternating double & single bonds • Similar to hemoglobin • History of Discovering Chlorophyll: http://www.chm.bris.ac.uk/motm/chlorophyll/chlorophyll_h.htm
Engelmann’s Experiment • Obtained the first action spectrum in 1883 • Used Spirogyra w/spiral shaped chloroplasts • Exposed this alga to a color spectrum using a prism • Measured photosynthesis of algae by using certain motile bacteria to determine which segments of algae were releasing the most O2 (& p’synthesizing the most). Bacteria congregated most around algae releasing the most O2 (violet/blue and red regions).
Other experiments conducted (*doubt you need to know specific experiments or names) • Jan Van Helmont - accounted for the water (hydrate) aspect of photosynthesis • Joseph Priestly – accounted for the release of oxygen by photosynthesis using a a burning candle, glass jar and a mint leaf. • Jan Ingenhousz – same as Priestly except showed that light was required.
Photosynthesis-Overall Chemical Equation • Reactants: carbon dioxide & water • Products: Glucose and oxygen gas • Also: Light energy, enzymes, pigments (such as chlorophyll a)
Another Perplexing Question about Photosynthesis • Where does the oxygen released by photosynthesis come from directly? Does it come from the carbon dioxide or water? • First challenged by Challenged by C.B. Van Niel using photosynthetic bacteria which showed that CO2 is not split. • Isotopic Oxygen (18O) was used to trace and track the fate of oxygen. • (* many thought the O2 was coming from CO2)
Photosynthesis: an overview Redox process H2O is split into: 2e- and 4 H+ The H’s are transferred to CO2 and a sugar is produced (CH2O) 2 Major steps to Photosynthesis: • Light Reactions (“photo”) -occurs in the thylakoids • Dark Reactions (bad name! WHY?) -Also called “Carbon fixation” -occurs in the stroma -Involves the Calvin Cycle
A Photosystem • Light Harvesting Pigments • Have “antennae pigments complexes” (200-300 pigment molecules) • Chlorophyll a and chlorophyll b are present • Chlorophyll a = Reaction Center • Primary Electron Acceptor will receive the electron (reduced) and chlorophyll a will be oxidized and lose the electron.
Structure of a Photosystem • Light harvesting units of the thylakoid membrane • Composed pigment antenna complexes (including c’phyll a, b, cartenoids- all bound to protein) • Antenna pigment molecules are struck by photons • Energy is passed to reaction centers (redox locations) containing c’phyll a. • Excited e- from chlorophyll a will either undergo: Cyclic or Noncyclic electron flow.
2 ‘fates’ of electron flow • Electron flow at reaction centers involves photosystems: • Cyclic flow- only Photosystem1 (P700) • Noncyclic flow- involves Photosystems 1 and 2 (P700 and P680) • *Note: Photosystem 1 was discovered first, but then later discovered that in noncyclic flow: P’system 2 occurs before P’system 1. Also, 700 and 680 refer to wavelengths of light
Noncyclic Electron Flow • Most common light reaction pathway • Involves (2) Photosystems: Photosystem II (P680) Photosystem I (P700) • Exhibits A “Z scheme” or Zig-Zag flow of electrons • Electrons flow in one direction • ATP and NADPH are produced • Electrons do not cycle back to the ground state to chlorophyll.
Mechanical view of both Photosystems (*shows noncyclic flow)
Cyclic Flow of Electrons • Utilizes Photosystem I (P700) only • Electrons cycle back to chlorophyll • NADPH is not produced. • Helps to produce more ATP that is used in the Calvin Cycle
Cyclic Electron flow • Alternative cycle when ATP is deficient • Photosystem I utilized but not II; produces ATP but no NADPH • Why? The Calvin cycle consumes more ATP than NADPH……. • Cyclic photophosphorylation Review of Light reactions: http://web.mit.edu/esgbio/www/ps/light.html
The Calvin Cycle-C3 pathway *Overall ‘output’ for 1 glucose: 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P). When this ‘repeats’ with 3 more CO2, 1 glucose can be synthesized 3 Phases: 1- Carbon fixation~ Each CO2 is attached to RuBP by ‘rubisco’ (ribulose biphosphate carboxylase) enzyme 2- Reduction~ NADPH is oxidized and reduces to G3P; ATP is required 3- Regeneration~ Remaining G3P are rearranged to RuBP; ATP used; cycle continues
Calvin Cycle: First Phase Carbon Fixation: (1 carbon) + (5 carbon) (3 carbon) CO2 + Ribulose Bisphosphate (RuBP) →2 Phosphoglycerate (PGA) w/ help of: RUBISCO (Ribulose Bisphosphate Carboxylase- aka Rubisco)-most abundant protein on earth **Carbon is converted from an inorganic form into an organic form and thereby “FIXED”. **A Total of Six carbons must be fixed for one glucose molecule or some other hexose.
Calvin Cycle: Second Phase Reduction Phase: Phosphoglycerate (PGA) ↓ is phosphorylated (using ATP) 1,3-bisphosphoglycerate ↓ Redox Rxn w/NADPH Glyceraldehyde-3-Phosphate (G3P) *G3P is a sugar (*also seen in glycolysis) *For every 3 CO2→ 6 G3P is produced but only ONE can be counted as a gain in carbohydrate and can exit the cycle. The remaining 5 G3P are converted back to one 5-C RuBP to continue the cycle
Calvin Cycle: Third Phase Regeneration of RUBP: 5 G3P are phosphorylated (5x3) 3 RuBP (3x5) 3 ATP’s are used to do the chemical rearrangement RuBP can now accept more CO2 molecules
Calvin Cycle - Net Synthesis • For every G3P molecule produced: 3 CO2 are brought in 9 ATP’s are consumed 6 NADPH are used **G3P can then be used by the plant to make glucose and other organic compounds Website for review of the Calvin Cycle: http://web.mit.edu/esgbio/www/ps/dark.html
To Make a Six Carbon Molecule (exp- glucose): • Needed: 6 CO2 molecule (6 carbons) • 6 molecules of RuBP (30 carbons) (remain in the cycle from TEN G3P’s) • 18 ATP molecules Produced: • 12 molecules of PGA (36 carbons) • 2 molecules of G3P (6 carbons of the 36. The remaining 30 C are converted back to 6 RuBP)
C3 Metabolic Pathway in Plants • CO2 enters directly into the Calvin Cycle • The first organic compound made is a 3 carbon molecule called PGA (phosphoglycerate) • Close their stomata on hot, dry days to conserve water. • Photorespiration occurs typically in these plants. • Examples include: Rice, Wheat, and Soybeans.
Photorespiration • Observed in C3 plants when stomata are closed during hot, dry days • CO2 levels ↓ & O2 levels • Rubisco binds with O2 instead of CO2 • Drains the Calvin cycle (↓ photosynthetic output) • No food molecules (G3P) are made • Thought to be an evolutionary relic (Rubisco’s affintiy for O2 remains). Why? Think about earth’s early atmosphere. Was it an oxidizing atmosphere?? • Considered to be wasteful and no benefit known • TWO Adaptations have emerged to minimize photorespiration: They are observed in the C4 and CAM plants
C4 Metabolic Pathway in Plants • CO2 and 3-C PEP (phosphoenolpyruvate) combine to produce a 4-Carbon compound called Oxaloacetate in mesophyll. • PEP carboxylase (PEPcase) is used initially instead of Rubisco (higher affinity for CO2and doesn’t try and put O2 thru cycle) • Unique anatomy is present w/BundleSheathcells that are photosynthetic surrounding the veins of the leaf. • In mesophyll, Oxaloacetate is converted to malate and it ‘delivers’ CO2 to Bundle Sheath cells thru plasmodesmata. • This maintains a high CO2 concentration for the Calvin cycle to occur in BS cells, which minimizes photorespiration. • CO2 is continually fed into the Calvin cycle from the mesophyll cells even when the stomata are closed. • Examples include: Corn & Sugarcane (also Bermuda grass and most weeds)
