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Photosynthesis Capturing solar energy (sunlight) to convert it to chemical energy stored in food
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?
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?
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?
Light Energy
Chloroplast
CO2 + H2O Sugars + O2
6CO2 + 6H2O -----------------------------> C6H12O6 +6O2 + enzymes, chlorophyll
I. What is photosynthesis? Photosynthesis is… Exergonic or Endergonic?
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)
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?
What exactly is ATP?
• When the cells store energy, they make ATP • ADP + P + Energy ATP
I. What is photosynthesis?
• 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
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
What is NADPH?
• NADPH acts as a shuttle for high-energy electrons
• Think of it as an energy carrier • More on NADPH later…
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?
II. Where does photosynthesis occur? • Leaves!
Why broad?
Fun fact! ½ million chloroplasts/mm2
(singular = stoma)
II. Where does photosynthesis occur?
Zoom in of leaf
Cuticle
Guard cell
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?
Plant Stomata
Close-up View of a Stoma
(guard cells are mostly hidden)
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
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?
III. What is the structure of a chloroplast?
/ space
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
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?
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)
Plants Produce O2 by Splitting H2O
Experiment 2
Reactants:
Products:
Experiment 1
Not labeled
Experiment 2
Labeled
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]
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
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
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?
Light
Chloroplast
Reflected light
Absorbed light
Transmitted light
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
Primary electron acceptor
Photon
Reaction center
PHOTOSYSTEM
Pigment molecules of antenna
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)
Primary electron acceptor
Other compounds
Chlorophyll molecule
Photon
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
Where do the excited electrons go?
• Primary electron acceptor passes electrons (e-) to an electron transport chain (ETC)
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
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
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
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
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
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
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
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)
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
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
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
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
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