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PhotosynthesisLecture 08
At the end of this series of lectures, you should be able to:
Define terms.
Describe the structure of chloroplasts and their location in a leaf.
Compare the reactants and products of the light reactions and the Calvin cycle.
Objectives
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Explain how the electron transport chain and chemiosmosis generate ATP, NADPH, and oxygen in the light reactions.
Compare the mechanisms that C3, C4, and CAM plants use to obtain and use carbon dioxide.
Objectives
Autotrophs
Primary producers
Photoautotrophs
Heterotrophs
Trophic Levels
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Plants
Protists (protozoa)
Bacteria
Kelp
Diversity of Photosynthesis
Chloroplasts are located in green parts of plants.
The green color comes from the pigment chlorophyll located in the chloroplasts.
Important role in converting light to chemical energy.
Leaves are the major site of photosynthesis and contain large numbers of chloroplasts in each cell.
Within a leaf, the chloroplasts are concentrated in the mesophyll
Stomata allow CO2 to enter the leaf and O2 to exit the leaf.
Chloroplasts
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Chloroplasts have a double membrane.
Stroma is a thick fluid that fills the inner membrane
Thylakoids are a system of interconnected membranous sacs
Stack of thylakoids are called granum (grana)
Thylakoid membranes are the location of much of photosynthesis.
Chlorophyll is built into the thylakoid membrane.
Chloroplasts
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Miguelsierra, CC BY‐SA 3.0, http://en.wikibooks.org/wiki/Structural_Biochemistry/Cell_Organelles/Chloroplast#mediaviewer/File:Scheme_Chloroplast‐en.svg
C H O 6O → 6CO 6H O 32 ATP Heat
6CO 12H O Light → C H O 6O 6H O
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Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule. Glucose is oxidized and O2 is reduced to H2O.
Electrons lose potential as they travel down the electron transport chain.
In photosynthesis, Water is oxidized and CO2 is reduced.
Light is captured by chlorophyll molecules to boost the energy of electrons.
Chemical energy is stored in the chemical bonds of sugars.
Redox Reactions
Light reactions
Occur on the thylakoids
Convert light energy to high energy intermediates (ATP and NADPH).
Produces O2
Photosynthetic Reactions
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Photosynthetic Reactions
Calvin Cycle
Occurs in the stroma
Uses high energy intermediates and CO2 to synthesize glucose.
Carbon fixation
Light Reactions and Calvin Cycle are linked by the high energy intermediates.
Daniel Mayer, CC BY‐SA 3.0, http://schools‐wikipedia.org/images/2289/228935.png.htm
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Light
Light is a small part of the electromagnetic spectrum.
Wavelengths: 380‐750 nm
Photon – packet of light energy
Shorter the wavelength the more energy the photon has
Philip Ronan, Gringer, CC BY 3.0, http://commons.wikimedia.org/wiki/File_talk:EM_spectrum.svg#mediaviewer/File:EM_spectrumrevised.png
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Pigments – Light absorbing pigments
Absorb some wavelengths
Transmit or reflect other wavelengths – this is the color that they appear.
Light
Different pigments in the chloroplast absorb different wavelengths of light. Chlorophyll a – absorbs blue‐violet and red light
Reflects green light
Chlorophyll b – absorbs blue and orange light Reflects yellow‐green light
Carotenoids – Protects the chlorophyll from excess light and oxidative molecules Might broaden the spectrum of light that can be used to drive photosynthesis.
Light
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When a pigment absorbs a photon of light, an electron jumps to an energy level further away from the nucleus.
The electron has more potential energy.
The electron then falls back to its original position and the energy is released as heat.
In a chloroplast, chlorophyll absorbs light and the potential energy of the electron is increased, but the electron is passed to another molecule before it can fall back to its original position.
Chlorophyll
A photosystem is an arrangement of pigments and proteins. Light‐harvesting complex – pigments absorb light
energy and passes it from molecule to molecule until it reaches the reaction center.
Reaction center 2 special chlorophyll amolecules that utilize the energy from the light harvesting array and ejects an electron.
Primary electron acceptor – receives the electron from the chlorophyll a, becoming reduced.
Photosystems
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Two photosystems have been identified:
Photosystem I – 700 nm wavelength
Photosystem II – 680 nm wavelength
Photosystem II proceeds Photosystem I in the process.
Named for the order they were discovered.
Photosystems
Two photosystems connected by an electron transport chain generate ATP and NADPH Process has several steps
1. Photon is absorbed by a pigment in the light harvesting complex of photosystem II. Energy from the photon is passed from molecule to molecule until it reaches the reaction center.
2. Energy cause the chlorophyll a to eject an electron which is captured by the primary electron acceptor.
3. Water is split inside the thylakoid. An electron from the water is used to recharge the chlorophyll a. Produces O2(takes 2 split water) and leaves the hydrogen in the stroma.
Light Reactions
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4. The primary electron acceptor passes the electron to an electron transport chain that causes hydrogen ions to move across the membrane and become concentrated inside the thylakoid.
5. A pigment in the photosystem I absorbs a photon and passes the energy to its reaction center causing an electron to be ejected and absorbed by the photosystem I primary electron acceptor. The electron from photosystem II (which has completed its “fall” through the electron transport chain) is used to recharge chlorophyll a in photosystem I.
6. The electron is taken from the photosystem I primary electron acceptor to reduce NADP+ to NADPH.
Light Reactions
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Light Reactions
ATP is synthesized light reactions through chemiosmosis (Photophosphorylation)
Similar to oxidative phosphorylation in mitochondria
In photophosphorylation, using the initial energy input from light,
Uses the hydrogen gradient created in the electron transport to produce ATP.
H+ diffuse back through ATP synthase, producing ATP.
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Light Reactions provide ATP and NADPH for the Calvin Cycle.
Light Reactions
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Daniel Mayer, CC BY‐SA 3.0, http://schools‐wikipedia.org/images/2289/228935.png.htm
Receives the NADPH and ATP from the lights reactions.
Requires CO2.
Produces G3P which can be used to make glucose or other materials.
We are going to learn the basics as a 4‐step process.
Calvin Cycle
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1. Carbon fixation – CO2 combines with rubsico (a 5‐carbon enzyme). This combination is unstable and it breaks into 2 3‐PGA. Occurs with three CO2 molecules at a time resulting in 6 3‐PGA .
2. Reduction – 6 ATP and 6 NADPH are used to reduce the 6 3‐PGA to 6 G3P (an energy rich 3‐carbon sugar). • NADP+ and ADP are returned to the light reactions to be recharged.
3. 1 of the G3P removed from the Calvin cycle – can be used to build glucose or other compounds. 5 G3P stay in the Calvin Cycle.
4. 3 ATP are used to reconfigure the 5 G3P into 3 rubisco.
Calvin Cycle
Dr. Katherine Harris, CC BY‐NC‐SA 3.0, http://www.hartnell.edu/tutorials/biology/photosynthesis.html
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C3 Plants
C3 Photosynthesis
Carbon fixation results in 3 PGA (a 3 Carbon compound).
Lots of plants have this type of photosynthesis.
Not very efficient under dry conditions
Have to leave stomata open to allow CO2 to enter and O2 to exit.
Photorespiration
Water also is lost while stomata are open.
A method of improving carbon fixation under dry conditions.
Fix CO2 into a 4‐carbon compound.
Enzyme that binds the CO2 has a high affinity for CO2
The 4‐carbon compound is moved from the mesophyll cells into the bundle sheath cells (specialized cells for the Calvin Cycle). CO2 is released in the bundle sheath cells and photosynthesis proceeds similar to C3.
Fewer stomata have to be open to obtain sufficient CO2,so water loss is reduced.
C4 Plants
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A method of improving carbon fixation under dry conditions.
Open stomata and capture CO2 at night when it is cooler and more humid – reducing water loss.
Fix CO2 into a 4‐carbon compound. CO2 released in the plant during the day.
All photosynthesis occurs in the same cell.
CAM Plants
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