Photosynthesis and the Calvin Cycle

• Plants capture light using:
• Chlorophyll A: the main photosynthetic pigment found in all autotrophic organisms.
• Accessory chlorophyll pigments: absorb different wavelengths of light and transfer energy to chlorophyll A.
• Some wavelengths cannot be absorbed by chlorophyll A, so accessory pigments act as "assistant" chlorophyll.

Chlorophyll Types

• Chlorophyll B: Present in all true plants.
• Chlorophyll C: Present in golden brown/brown algae.
• Chlorophyll D: Present in red algae.

Pigments

• Chlorophyll A has a CH3 group.
• Chlorophyll B has a CHO group and functions as an accessory pigment.
• Accessory pigments broaden the spectrum used for photosynthesis.
• Differences in absorption spectrum arise from slight structural differences between pigment molecules.
• Carotenoids are accessory pigments to absorb excess light that would damage chlorophyll.

Porphyrin Ring

• Not the same as chlorophylls BCD.
• Porphyrin ring is the light-absorbing head of a molecule with a magnesium atom at the center.
• Hydrocarbon tail is hydrophobic and interacts with hydrophobic regions of proteins inside thylakoid membranes.

Eukaryotic Organisms

• Mosses, ferns, seed plants, green algae, and euglenoids contain Chlorophyll A & B, and carotenoids.
• Diatoms, dinoflagellates, and brown algae contain Chlorophyll A & C, and carotenoids.
• Red algae contain Chlorophyll A & D, carotenoids, and phycobiliproteins.

Prokaryotic Organisms

• Cyanobacteria contain Chlorophyll A & D, carotenoids, and phycobiliproteins.
• Prochlorophytes contain Chlorophyll A + B, and carotenoids.

Absorption

• Absorption refers to a graph plotting a spectrum's light absorption versus wavelength.
• The absorption spectrum of chlorophyll A shows that violet-blue and red light are most effective for photosynthesis.
• Action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis.

Reaction Center/Chlorophyll A

• Reaction center/chlorophyll A absorbs photons
• Light-harvesting pigments (antenna molecules) pass photons from one molecule to another until it reaches the reaction center/chlorophyll A molecules.
• Chlorophyll A molecules transfer the photon to the primary electron acceptor.
• Reaction center/chlorophyll A is important in light reactions.

Location of Photosynthetic Pigments

• Photosynthetic pigments are located inside the thylakoid in:
• Cyanobacteria
• Chloroplasts of true plants and algae.
• Chloroplasts absorb red and blue-purple light but reflect green light (the reason plants are green).

Chloroplast Structure

• Mesophyll contains:
• Stroma: Fluid inside the chloroplast
• Granum: Stack of thylakoids
• Thylakoid: Contains thylakoid space inside
• Inner membrane
• Intermembrane space: Space between inner and outer membrane
• Outer membrane

Photosystems

• Are made up of:
• Major reaction center/chlorophyll A
• Accessory pigments
• Primary electron receptor
• Photosystem 1 (P700)
• Absorbs 700nm wavelength (red light)
• Primary acceptor is chlorophyll A
• Photosystem 2 (P680)
• Absorbs 680nm wavelength
• Primary acceptor is pheophytin
• H2O is split into 1/2O2 + 2H+, which is suspended in the thylakoid lumen

Light and Atoms

• A photon is absorbed by an excitable electron, moving it to a higher energy level (farther from the nucleus).

End in Two Ways

• Cyclic electron flow:
• The electron returns to its ground state by emitting a less energetic photon.
• Non-cyclic/linear electron flow:
• The electron is accepted by an electron acceptor molecule.
• Involves both P700 and P680.
• Produces NADPH, ATP, and oxygen.
• Oxygen is released as a byproduct into the atmosphere.
• ATP provides chemical energy and NADPH reduces power.
• This process proceeds to the Calvin cycle, a carbon fixation process.

Electron Transport Chain (ETC)

• When light hits PSII, it hits PSI.
• then there is an electron transfer and then it travels to ETC.
• ETC is located between PSI and PSII.
• Electrons pass through PSII, ETC, and then go to PSI to produce NADPH.
• PSII produces oxygen.
• PSII triggers ATP synthase and produces ATP.

Non-Cyclic Electron Flow

• Photon hits a pigment, and its energy is passed among pigment molecules until it excites PSII/680
• An excited electron is transferred to the primary electron acceptor/PSII+
• Enzymes split H2O, and the electrons are transferred to PSII+, reducing it to PSII.
• PSII+ is the strongest known biological oxidizing agent.
• O2 is released as a byproduct.
• Plants produce O2 gas by splitting H2O.
• O2 liberated by photosynthesis comes from the oxygen in water (H+ and e-).
• Each electron falls down an ETC from the primary electron acceptor of PSII to PSI:
• Accepted by plastoquinone (Pq) and then by plastocyanin (Pc).
• Energy released drives the creation of a proton gradient across the thylakoid membrane.
• Diffusion of H+ (protons) across the membrane drives ATP synthesis.
• In PSI, like PSII, transferred light energy excites PSI, which loses an electron to an electron acceptor.
• PSi+ (PSI missing an electron) accepts an electron passed down from PSII via ETC.
• Each electron falls down an ETC from the primary electron of PSI to the protein ferredoxin (Fd).
• The electrons are then transferred to NADP+ and reduce to NADPH.
• The electrons of NADPH are then available for the reactions of the Calvin cycle.
• This process also removes H+ from the stroma.

Cyclic Electron Flow

• Only occurs in PSI.
• Electrons circle from PSI, primary acceptor, ferredoxin, cytochrome complex, and plastocyanin.
• Electrons cycle back from Fd to the PSI reaction center.
• This flow only uses PSI and only produces ATP.
• No oxygen is released because water was not produced.
• Believed to have evolved before noncyclic electron flow.
• May protect cells from light-induced damage.

Summary of Photosynthesis

• Light-dependent reaction:
• Increases potential energy of electrons by moving them from H2O to NADPH.
• Uses the ETC.
• Needs water and light.
• Produces ATP, NADPH, and O2 (byproduct).
• Carbon-fixation process (Calvin process):
• Needs ATP, NADPH, and CO2.
• Produces carbohydrates/glucose.

Chemiosmosis

• Is when ions move by diffusion across a semi-permeable membrane (e.g., a membrane in mitochondria).
• Ions move from high to low concentration.
• Ions move out to balance electrical charge.
• Ions are molecules with a net electric charge.
• Ions move down an electrochemical gradient, a form of potential energy.
• A type of diffusion
• Both chloroplasts and mitochondria generate ATP by chemiosmosis but use different sources of energy.
• Mitochondria transfers chemical energy from food to ATP.
• Chloroplasts transform light energy into chemical energy of ATP.
• Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but has similarities.

Mitochondria vs Chloroplasts

• In mitochondria:
• Protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix.
• In chloroplasts:
• Protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma.

Calvin Cycle

• Also called "carbon fixation".
• Requires CO2 for fixing carbon.
• Also called "dark reaction".
• Complements "light reaction".
• Plant groups (C3, C4, and CAM) differ in how they fix carbon.
• Must adjust to the environment they are planted in/facing environmental stressors.
• Occurs in the stroma (the side facing the stroma).
• Six Calvin cycles are needed to produce one glucose molecule.
• Uses 18 ATP and 12 NADPH per cycle.
• The steps are:

1. CO2 uptake

• Creates 6 molecules of ribulose bisphosphate (RuBP).
• 6 molecules of CO2
• CO2 molecules are captured by RuBP, which results in an unstable intermediate that immediately breaks apart into 2 PGA.
• 12 molecules of phosphoglycerate (PGA) will proceed with the process needing 12 molecules of ATP.

2. Carbon reduction

• Needs 12 ATP to start.
• Needs 12 NADPH to continue.
• PGA is phosphorylated by ATP and reduced by NADPH.
• Removal of phosphate results in G3P formation.
• G3P that's created is rearranged into a new RuBP molecule or another sugar via a series of reactions
• 12 G3P is created

3. RuBP regeneration

• 6 molecules of ribulose phosphate or RP will need 6 ATP
• Produces 6 molecules of RuBP.
• Restarts cycle from CO2 uptake phase.

Most Notable Features of the Calvin Cycle

• Their large nitrogen requirement for rubisco and other photosynthetic enzymes:
• Rubisco is the enzyme driving the whole Calvin cycle.
• It accounts for about 25% of the nitrogen in photosynthetic cells.
• Their dependence on the product of the light reaction (ATP and NADPH), depends on irradiance (light received by the photosynthetic cell).
• Calvin Cycle’s frequent limitation by CO2 supply to the chloroplast.

If There is Limited CO2

• The Calvin cycle will not proceed often.
• More ATP and NADPH will become concentrated in the stroma as they aren't being used.
• Less glucose will be made.
• Plants will not survive.

Rubisco Functions as Both a Carboxylase and Oxygenase

• As a carboxylase, it initiates the Calvin cycle.
• As an oxygenase, it catalyzes a reaction between rubisco and oxygen under conditions of CO2 limitations.
• This initiates the breakdown of CO2 sugars
• Is known as photorespiration
• Occurs in the light (photo) and consumes O2 while producing CO2 (respiration) and using ATP but no sugar molecules.
• Reduces the photosynthetic efficiency of the Calvin cycle by as much as 50%.
• Thus is the opposite of photosynthesis

Alternative Mechanisms of Carbon Fixation

• Certain plants minimize cost of photorespiration.
• Through the incorporation of CO2 into 4 carbon compounds in mesophyll cells.
• Alternative pathways:
• Hatch-stack pathway/C4 pathway
• Crassulacean acid metabolism/CAM

C3 and C4 Comparison

1. C3:

• The Calvin cycle takes place in mesophyll cells.
• Bundle sheath cells are nonphotosynthetic.

2. C4:

• Reactions that fix CO2 into 4 carbon compounds take place in mesophyll cells
• Then transferred to the photosynthetic sheath cells (where the Calvin cycle happens).

C4 Cycle

• Requires mesophyll and bundle sheath cells.
• CO2 enters mesophyll cells via stomata, where it will be fixed by PEP carboxylase, producing oxaloacetate (a 4-carbon molecule).
• Oxaloacetate turns into malate (another 4-carbon molecule).
• Malate goes into bundle sheath cells -> 3 carbons from the malate are reduced to pyruvate (3-carbon molecules), and another molecule turns into CO2.
• The Calvin cycle will start, and sugar travels into vascular tissue.
• Pyruvate goes back to mesophyll to become ATP, is reduced into ADP, and then the Calvin cycle restarts.

C4 Plants: Thrive in hot and moist conditions

• 15% of plants (e.g., grass, corn, and sugarcane).
• Divides photosynthesis spatially.
• Light reaction: Mesophyll cells
• Calvin cycle: Bundle sheath cells