This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and
animations.
To run the animations you must be in Slideshow View. Use the buttons on the animation to play, pause, and turn
audio/text on or off.
Please Note: Once you have used any of the animation functions (such as Play or Pause), you must first click on the slide’s background before you can advance to the next slide.
Photosynthesis
Chapter 8
2
3
Photosynthesis Overview
• Energy for all life on Earth ultimately comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
• Oxygenic photosynthesis is carried out by– Cyanobacteria– 7 groups of algae– All land plants – chloroplasts
Chloroplast
• Thylakoid membrane – internal membrane– Contains chlorophyll and other photosynthetic
pigments– Pigments clustered into photosystems
• Grana – stacks of flattened sacs of thylakoid membrane
• Antenna complex– Hundreds of accessory pigment molecules– Gather photons and feed the captured light
energy to the reaction center
• Reaction center– 1 or more chlorophyll a molecules
– Passes excited electrons out of the photosystem
Antenna complex
• Also called light-harvesting complex
• Captures photons from sunlight and channels them to the reaction center chlorophylls
• In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins
3. A pair of chlorophylls in the reaction center absorb two photons. This
excites two electrons that are passed to NADP+, reducing it to NADPH. Electron transport from photosystem II replaces these electrons.
H2O
H+PC
Fd
2H+ + 1/2O2
NADP+ + H+
2
2
2
2
2
1. A pair of chlorophylls in the reaction center absorb two photons of light. This excites two electrons that are transferred to plastoquinone (PQ). Loss of electrons from the reaction center produces an oxidation potential capable of oxidizing water.
Reactioncenter
Proton gradient formedfor ATP synthesis
Reactioncenter
e–
e–
PQ
e–
NADPreductase
NADPHe–
2. The electrons pass through the b6-f complex, which uses the energy released to pump protons across the thylakoid membrane. The proton gradient is used to produce ATP by chemiosmosis.
1. Photosystem II absorbs photons, exciting electrons that are passed to plastoquinone (PQ). Electrons lost from photosystem II are replaced by the oxidation of water, producing O2
2. The b6-f complex receives electrons from PQ and passes them to plastocyanin (PC). This provides energy for the b6-f complex to pump protons into the thylakoid.
3. Photosystem I absorbs photons, exciting electrons that are passed through a carrier to reduce NADP+ to NADPH. These electrons are replaced by electron transport from photosystem II.
4. ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi
enzyme acts as a channel for protons to diffuse back into the stroma using this energy to drive the synthesis of ATP.
NADPreductase
ATPsynthase
1/2O2 2H+
Water-splittingenzyme
Thylakoidspace
AntennacomplexThylakoid
membrane
Light-DependentReactions
H+
H+
e–22 22
22
22
Chemiosmosis
• Electrochemical gradient can be used to synthesize ATP
• Chloroplast has ATP synthase enzymes in the thylakoid membrane– Allows protons back into stroma
• Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions
36
Production of additional ATP
• Noncyclic photophosphorylation generates– NADPH– ATP
• Building organic molecules takes more energy than that alone
• Cyclic photophosphorylation used to produce additional ATP– Short-circuit photosystem I to make a larger
proton gradient to make more ATP37
38
Carbon Fixation – Calvin Cycle
• To build carbohydrates cells use
• Energy– ATP from light-dependent reactions– Cyclic and noncyclic photophosphorylation– Drives endergonic reaction
• Reduction potential– NADPH from photosystem I– Source of protons and energetic electrons
39
Calvin cycle
• Named after Melvin Calvin (1911–1997)
• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP to form PGA
• Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco
40
3 phases
1. Carbon fixation– RuBP + CO2 → PGA
2. Reduction– PGA is reduced to G3P
3. Regeneration of RuBP– PGA is used to regenerate RuBP
• 3 turns incorporate enough carbon to produce a new G3P
Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
43
Output of Calvin cycle
• Glucose is not a direct product of the Calvin cycle
• G3P is a 3 carbon sugar– Used to form sucrose
• Major transport sugar in plants• Disaccharide made of fructose and glucose
– Used to make starch• Insoluble glucose polymer• Stored for later use
44
Energy cycle
• Photosynthesis uses the products of respiration as starting substrates
• Respiration uses the products of photosynthesis as starting substrates
• Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse
• Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria
• C4 pathway, although it overcomes the problems of photorespiration, does have a cost
• To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone
• C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone
52
53
CAM plants
• Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups
• Stomata open during the night and close during the day– Reverse of that in most plants
• Fix CO2 using PEP carboxylase during the night and store in vacuole
• When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
• High levels of CO2 drive the Calvin cycle and minimize photorespiration