Photosynthesis (8.3)
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For Basics: See Topic 2.9
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What is meant by an excited electron?
Electrons can be moved out of their orbital shells into a higher orbit (excited) but they are highly unstable. When they return to their orbital shell they release energy that can be utilized by the cell for transport. (video: 0.00~1.00)
Why is this important?
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Overview of Photosystems I & II
Photosystems I & II from the Light Dependent Reactions which are the first stage in Photosynthesis.
Each system contains two main parts:
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Photosystem overview - CAPP
Light dependent reactions - Khan Academy Photosynthesis Overview - Boundless |
Each system has some minor differences:
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Photoactivation in Photosystems I & II
The absorption of light by the chlorophyll of either photosystem generates excited electrons = photoactivation
- Chlorophyll molecules absorb light energy causing electrons to become excited.
- The “excited” electrons have a higher energy state (See 'What is meant by an excited electron' above)
Plastoquinone (Photosystem II)
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Ferredoxin (Photosystem I)
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Electron Transport Chains in Photosystems I & II
Photosystem II
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Photosystem I
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Oxidation and Reduction Reactions in Biochemistry
This plays out in different ways throughout biochemistry:
Note that the addition and removal of a H+ ion is generally going to be directly related to oxidation and reduction. H+ ions are synoynomous with protons due to the loss of their one election and only having a proton. Generally the creation of a H+ ion also releases a free electron into the electrion transport chain. |
Oxidation: the loss of electrons, or the change in state/ charge is increased
Reduction: the gaining of electrons, or the change in state/ charge is decreased |
Calvin Cycle
The Calvin Cycle (Stage 2: Light Independent Stage) occurs in the stroma of the chloroplast It consists of three basic stages:
Calvin Cycle Simplified- National Geographic Calvin Cycle- Khan Academy |
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Carbon Fixation
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Reduction
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Regeneration
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Glucose Creation:
- G3Ps from the reduction stage and a variety of other 3 Carbon molecules are reduced (rearranged) to form triose phosphates
- this process is very complicated and more than is needed here.
- This step is technically outside of the Calvin Cycle but is an important piece
- 1 G3P molecule is siphoned off to become triose phosphate and ultimately a carbohydrate
- Examples: Glucose, fructose, galactose, starch etc
- Requires 2 G3Ps to form a single (6-carbon) carbohydrate molecule
- Hence the cycle has to repeat twice to make a single glucose molecule.
- The other 5 G3Ps are recycled to form RuBP to allow the Calvin cycle to continue
Calvin's Lollipop Experiment
Melvin Calvin’s experiment set the ground work for understanding the Calvin Cycle. The 1945 discovery of radioactive Carbon-14 molecules, and their resultant half life, made this experiment possible. Previously no method had been discovered to be able to track carbon molecules.
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Figure 11 (on side) shows a graph of various compounds in a plant containing radioactive Carbon-14 over time. Observations from graphs such as this helped Calvin determine the what became known as the Calvin Cycle.
Observations:
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Chloroplast Structure-Function Relationships
Chloroplasts can look very different from different angles, but certain structures are highly visable:
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STRUCTURE-FUNCTION RELATIONSHIPS
There is a clear relationship between the structure and function of a chloroplast. Structure is with reference to the above diagrams, function is with reference to the processes and the roles they play.
There is a clear relationship between the structure and function of a chloroplast. Structure is with reference to the above diagrams, function is with reference to the processes and the roles they play.
- Absorption of light: The chlorophyll found in the thylakoids absorbs light to allow for the excitation of electrons. Without the chlorophyll in the thylakoids the process would be much slower and loose energy by having to move the energy elsewhere.
- Production of ATP by photophosphorylation: The ATP synthase found in the thylakoid membrane of the is arranged in such a way as to create a space (the lumen) to capture the high concentration of H+ ions in order to allow photophosphorylation to occur through the ATP synthase found in the membrane. The ATP is generated in the stroma to allow for easy access into the Calvin Cycle.
- Carry out the many chemical reactions of the Calvin Cycle: Due to the vast number of chemical reactions and enzymes needed for the Calvin Cycle to occur it is generally indicated that the stroma allows for close proximity of enzymes and reactants for the cycle to occur.
- The double membrane: The double membrane ensure that ions, enzymes and other molecules needed for the reactions stay within the chloroplast and are not utilized by the rest of the cell. Due to the high energy needs of the Calvin Cycle, the double membrane helps keep H+ ions, ATP, and NADPH close at hand to ensure the reactions continuation unhindered.