Photosynthesis · • Photosynthesis – Capturing solar energy to convert to chemical energy...
Transcript of Photosynthesis · • Photosynthesis – Capturing solar energy to convert to chemical energy...
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Photosynthesis Capturing solar energy (sunlight) to convert it to chemical energy stored in food
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Photosynthesis Outline
• I. What is photosynthesis? • II. Where does photosynthesis occur? • III. What is the structure of a chloroplast? • IV. What are the steps of photosynthesis?
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Photosynthesis Outline
• I. What is photosynthesis? • II. Where does photosynthesis occur? • III. What is the structure of a chloroplast? • IV. What are the steps of photosynthesis?
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Types of Reactions
• Endergonic - energy is absorbed during the reaction (energy enters)
• Exergonic - energy is released during the reaction (energy exits)
Reactants
Products
Reactants
Products
I. What is photosynthesis?
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Light Energy
Chloroplast
CO2 + H2O Sugars + O2
6CO2 + 6H2O -----------------------------> C6H12O6 +6O2 + enzymes, chlorophyll
I. What is photosynthesis? Photosynthesis is… Exergonic or Endergonic?
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I. What is photosynthesis (PS)? • Life on earth is powered by sunlight ▫ Plants, algae, and bacteria produce energy by PS
• PS occurs in the chloroplasts: light reactions in the thylakoid membranes and dark reactions in stroma
• Two stages of PS ▫ 1. Light-dependent reactions: store solar energy in
molecules of ATP and NADPH, O2 is produced from H2O ▫ 2. Calvin cycle (light-independent reactions): use ATP
and NADPH to drive synthesis of organic molecules (sugars) from CO2 in air (carbon fixation)
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What exactly is ATP?
• ATP = Adenosine triphosphate = an energy carrier • When the cells need energy, they use/break ATP • ATP ADP + P + Energy
I. What is photosynthesis?
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What exactly is ATP?
• When the cells store energy, they make ATP • ADP + P + Energy ATP
I. What is photosynthesis?
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• Phosphate groups are negatively charged • Repulsion acts as potential energy – like a compressed spring • When unstable bond between 2nd and 3rd phosphate broken, energy is released
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How is ATP used? • As ATP is broken down, it
gives off usable energy to power chemical work
• Synthesizes molecules for growth and reproduction
• Transport work – active transport, endocytosis, and exocytosis
• Mechanical work – muscle contraction, cilia and flagella movement, organelle movement
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What is NADPH?
• NADPH acts as a shuttle for high-energy electrons
• Think of it as an energy carrier • More on NADPH later…
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Photosynthesis Outline
• I. What is photosynthesis? • II. Where does photosynthesis occur? • III. What is the structure of a chloroplast? • IV. What are the steps of photosynthesis?
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II. Where does photosynthesis occur? • Leaves!
Why broad?
Fun fact! ½ million chloroplasts/mm2
(singular = stoma)
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II. Where does photosynthesis occur?
Zoom in of leaf
Cuticle
Guard cell
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II. Where does photosynthesis occur?
• Plant leaves (& green parts) ▫ Specifically, in mesophyll cells (these contain
chloroplasts)
• Stomata - openings allow gases to move in (carbon dioxide) and out (oxygen)
• Veins – xylem brings water to the leaves from the roots; phloem carries away sugar
How do other leaf parts help?
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Plant Stomata
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Close-up View of a Stoma
(guard cells are mostly hidden)
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PS Recap
• Photosynthesis – Capturing solar energy to convert to chemical energy stored in organic compounds such as sugar
• *Autotrophs make their own food, but aren’t 100% self-sufficient, need inorganic materials (CO2 and H2O)
• Site of photosynthesis: ▫ Green parts → leaf → mesophyll → chloroplast
• Helpful leaf adaptations: ▫ Broad leaf, stomata, cuticle, less packed spongy
layer, proximity of vein to mesophyll
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Photosynthesis Outline
• I. What is photosynthesis? • II. Where does photosynthesis occur? • III. What is the structure of a chloroplast? • IV. What are the steps of photosynthesis?
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III. What is the structure of a chloroplast?
/ space
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III. What is the structure of a chloroplast?
• Two membranes surround: ▫ Stroma: fluid found within the chloroplast ▫ Thylakoid Disks: (inside = thylakoid
space); membranes contain chlorophyll, the green light-capturing pigment ▫ A stack of thylakoid disks is called a
granum
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Photosynthesis Outline
• I. What is photosynthesis? • II. Where does photosynthesis occur? • III. What is the structure of a chloroplast? • IV. What are the steps of photosynthesis?
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Chloroplast H2O
O2 3-C Sugars
CO2
Light- Dependent Reactions
Calvin Cycle
NADPH ATP
ADP + P NADP+
Chloroplast Go to Section:
IV. What are the steps of photosynthesis?
Note: The Calvin Cycle is part of the light-independent reactions.
Light energy
(grana) (stroma)
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Plants Produce O2 by Splitting H2O
Experiment 2
Reactants:
Products:
Experiment 1
Not labeled
Experiment 2
Labeled
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Photosynthesis is a Redox Process • Water molecules are split apart and electrons
and H+ ions are removed, leaving O2 gas
– These electrons and H+ ions are transferred to CO2, producing sugar
Reduction
Oxidation
“LEO says GER” or OIL RIG Losing Electrons = Oxidation [oxygen] Gaining Electrons = Reduction [glucose]
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Photosynthesis: Redox Redux
• H2O molecules are split using light energy ▫ H2O is oxidized ▫ Loses high-energy electrons and H+ ions ▫ NADP+ (an electron carrier) picks up these
electrons and H+ ions and becomes NADPH • ATP powers several steps in Calvin cylce • NADPH carries electrons that are used to reduce
carbon dioxide • Electrons are gained so CO2 is reduced
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Step 1: Light Dependent Reactions (thylakoid: light energy → chemical energy)
1. Light-dependent Reactions -- require light! 2. Remember: visible light = many colors 3. Light can be reflected, transmitted, or absorbed 4. Pigments can absorb light of certain
wavelength (colors) better than others
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Step 1: Light Dependent Reactions 1. Thylakoid membrane houses photosynthetic
pigments that capture light energy to make ATP 2. Most important pigments = chlorophylls
What is being absorbed?
What is being reflected?
How can you relate this to plant color?
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Light
Chloroplast
Reflected light
Absorbed light
Transmitted light
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What’s so special about chlorophyll?
• When surrounded by other chlorophyll molecules, carotenoids, and proteins → photosystem (there are 2)
• Light energy absorbed → transferred from chlorophyll to chlorophyll
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Primary electron acceptor
Photon
Reaction center
PHOTOSYSTEM
Pigment molecules of antenna
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What’s so special about chlorophyll?
• A specially positioned chlorophyll molecule has electrons that it will “give up” to a primary electron acceptor when struck by absorbed light
• Only some wavelengths of light cause the electrons to get excited (have more energy than before)
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Primary electron acceptor
Other compounds
Chlorophyll molecule
Photon
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Photosystem Recap
• Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons
• The excited electrons are passed from the primary electron acceptor to electron transport chains ▫ Their energy ends up in ATP and NADPH
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Where do the excited electrons go?
• Primary electron acceptor passes electrons (e-) to an electron transport chain (ETC)
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How do we replace lost electrons?
• Wait, these e- need to be replaced so the ETC can keep going.
• How do e- get replaced? ▫ Get e- from water ▫ 2H2O → 4H+ +4e- + O2
▫ 4e- replace e- in chlorophyll ▫ O2 diffuse out stoma ▫ 4H+ stay in thylakoid
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What happens in the ETC?
• High energy electrons move through ETC and lose energy
• This energy is used to actively transport H+ against its concentration gradient
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How does ATP get made?
• Located in the membrane near the photosystems is an enzyme called the ATP synthase
• ATP synthase acts as a carrier protein, allowing H+ ions to diffuse down their concentration gradient, releasing energy
• ATP synthase uses energy to create ATP
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What happens in the 2nd photosystem?
• Low energy e- from photosystem II replace lost excited e- in next photosystem
• Excited e- move through another, different ETC
• e- at end of ETC is transferred to NADP+ to make NADPH
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How ATP is Made - Recap
• The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane ▫ The flow of H+ back through the membrane is
harnessed by ATP synthase to make ATP ▫ In the stroma, the H+ ions combine with NADP+ to
form NADPH
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ATP Production by Chemiosmosis
Thylakoid compartment (high H+)
Thylakoid membrane
Stroma (low H+)
Light
Antenna molecules
Light
ELECTRON TRANSPORT CHAIN
PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE
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Hydrogen Ion Movement
Photosystem II
Inner Thylakoid Space
Thylakoid Membrane
Stroma
ATP synthase
Electron Transport Chain Photosystem I ATP Formation
Chloroplast
Summary of Light Reactions
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Step 2: The Calvin Cycle (Light-independent rxns in stroma: CO2 to sugar)
• The dark reactions: the Calvin Cycle • uses 3 CO2 to build a 3-carbon carbohydrate (an
organic molecule) = G3P • Needs electrons and H+ from NADPH
• Is this energy-consuming or energy-releasing? • remember that we have ATP, a high-energy
molecule from the Light Reactions, to “power” the Calvin cycle
• So, it is energy consuming • Carbon fixation = initial incorporation of CO2
into organic molecules (sugar molecules)
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The major point of the Calvin cycle is to form new C-C bonds from piecing together CO2 molecules. Making these bonds takes energy and electrons.
1) Carbon fixation 2) Energy consumption and redox
3) Release of G3P 4) regeneration of RuBP
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Photosynthesis Summary • Light-dependent reactions – thylakoid membrane ▫ Energy absorbed from sunlight, exciting e- in
chlorophyll – lost to 1st ETC ▫ Chloroplast e- replaced by splitting H2O: e-, O2, and H+
▫ Oxygen diffuse out stomata, H+ pumped across membrane in ETC ▫ 1st ETC e- replace lost chlorophyll e- in 2nd photosystem ▫ e- move through different ETC to combine with H+ to
make NADPH • Calvin cycle (light-independent) – stroma ▫ CO2 combine to make organic 3-C compound ▫ ATP energy used to drive addition of H+ from NADPH
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Photosynthesis Summary
Light
Chloroplast
Photosystem II Electron transport
chains Photosystem I
CALVIN CYCLE Stroma
LIGHT REACTIONS CALVIN CYCLE
Cellular respiration Cellulose Starch
Other organic compounds
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Factors that affect photosynthesis • Light intensity ▫ ↑ light intensity, ↑ rate of PS ▫ More e- become excited, more rapid PS ▫ Eventually all e- excited, max rate
• Carbon dioxide levels ▫ ↑ carbon dioxide, ↑ rate of PS ▫ Max rate will eventually be reached
• Temperature ▫ ↑ temperature, ↑ rate of chemical rxns ▫ Rate peaks at point where enzymes start
to become ineffective – denaturation! ▫ Stomata close – limit H2O loss and CO2
entry
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What do plants do with sugar?
• 50% used for glucose in cellular respiration! ▫ C6H12O6 + 6O2 --> 6H2O + 6CO2 + ATP
• Made into sucrose, glucose, cellulose • Store the extra sugar in… ▫ Starch ▫ Maple syrup ▫ Sap ▫ Fruits ▫ Roots ▫ Tuber