Photosynthese: Lichtreaktion

Photosynthesis - Light Reaction

• Occurs within the thylakoid membrane.
• Light (photons) is absorbed by antenna complexes of chlorophyll.
• Energy is transferred to an electron which becomes excited.
• Excited electron is transported via an electron transport chain.
• Photolysis splits H₂O into H⁺, electrons, and O.
• O₂ leaves the plant (the source of O₂ in the equation of photosynthesis).
• H⁺ and electrons replenish the electron vacancies in Photosystem II (P680).
• Electrons are taken up by an acceptor molecule and transported via an electron transport chain to Photosystem I (P700).
• Energy from the electron transport chain drives a proton pump in the cytochrome complex.
• Protons (H⁺) accumulate from the stroma to inside the thylakoid lumen to establish an electrochemical gradient.
• Photosystem I (P700) absorbs photons via antenna complexes, causing electrons to become excited.
• Electrons are transported via another electron transport chain to NADP⁺ reductase. -NADP⁺ + H⁺ is reduced to NADPH + H⁺.
• The electron vacancy in Photosystem I is filled by electrons from Photosystem II.
• ATP Synthase facilitates photophosphorylation.
• Driven by a charge/concentration gradient (H⁺).
• H⁺ flows from the thylakoid lumen into stroma.
• Rotation in the F₀ subunit of ATP synthase causes a spatial alteration.
• Alteration enables binding to form ATP.
• ADP + Phosphate forms ATP; converts kinetic energy into chemical energy.

Calvin Cycle

• Carboxylation: CO₂ is absorbed from the stroma and chemically bound to ribulose-1,5-bisphosphate by the enzyme RuBisCO.
• Forms an unstable C-6 body that breaks down into two C-3 bodies (PGA).
• Reduction: ATP is split into ADP + P.
• Phosphorylation occurs.
• NADPH + H⁺ transfers two electrons + H⁺.
• GAP (glyceraldehyde-3-phosphate) is produced.
• ⅙ of GAP goes to make glucose.
• ⁵/₆ is recycled for RuBP regeneration

Cellular Respiration - Glycolysis

• Occurs in the cytoplasm.
• Glucose (C₆H₁₂O₆) is split into two pyruvate molecules, requiring two ATP molecules (phosphorylation).
• Produces two pyruvate molecules, two ATP, and two NADH + H⁺.

Pyruvate Oxidation

• Occurs in the mitochondria.
• Pyruvate molecules are converted into Acetyl-CoA.
• Produces Acetyl-CoA, CO₂, and NADH per pyruvate molecule.

Citric Acid Cycle (Krebs Cycle)

• Occurs in the mitochondria.
• Acetyl-CoA undergoes chemical reactions to generate electrons.
• Per Acetyl-CoA: Produces 2CO₂, 1 ATP, 3 NADH + 1 FADH₂ which store energy, and GTP to produce ATP

Respiratory Chain (Electron Transport Chain)

• Occurs in the inner mitochondrial membrane.
• Collects electrons from NADH + FADH₂ and allows energy to be released.
• Uses the released energy for ATP synthase function and produces up to 34 ATP per glucose molecule.
• Electrons + hydrogen ions + oxygen form H₂O.

Glycolysis - Overview

• Glucose, NAD⁺, ADP + P are converted into Pyruvate, ATP, NADH + H⁺.

Activation (Pyruvate Oxidation) - Overview

• Pyruvate, CoA-SH, NAD⁺ are converted into CO₂, Acetyl-CoA, NADH + H⁺

Citric Acid Cycle - Overview

• Acetyl-CoA, NADH + H⁺ are converted into CO₂, CoA-SH, NADH + H⁺, FADH₂, GTP.

Respiratory Chain - Overview

• NADH + H⁺, FADH₂, ADP+P, O₂ are converted into H₂O, NAD⁺, FAD, ATP.

Fermentation

• Glycolysis: NADH cannot release electrons to the electron transport chain as there is no NAD⁺.
• NADH transfers electrons directly to pyruvate.
• Generate Lactate and NAD⁺.
• Lactic acid fermentation is carried out even if there is not enough O₂ to convert acetyl-CoA.

Photosystems

• Consist of light-harvesting complexes of pigment molecules and a chlorophyll-a molecule pair as the reaction center.
• Reaction centers are bound to different proteins, resulting in different absorption maxima.
• Photosystem II (P680): Absorbs light energy to facilitate hydrolysis, electron donation, and oxygen release.
• Photosystem I (P700): absorbs light energy to transfer electrons to NADP⁺ and reduces it to NADPH.

C-3 Plants

• Non-specialized photosynthetic pathway; most plants are C3 plants.
• Name originates from the first product of CO2 fixation, a 3-carbon compound (phosphoglycerate).
• CO2-fixing enzyme is RuBisCO (ribulose carboxylase).
• Mesophyll accounts for a large portion of the leaf mass with many chloroplasts, thus has high activity.
• Under hot/dry conditions, the stomata close, reducing water loss.
• O2 accumulates in the plant, RuBisCO binds O2 as a substrate.
• Photosynthesis declines by up to 50%.
• Higher temperatures reduce photosynthesis.

C-4 Plants

• Spatial separation.
• Adapted to warm or hot environments.
• First product of CO2 fixation is a C4-body (oxaloacetate).
• Close stomata under hot days.
• CO2 is fixed by mesophyll cells and transferred to bundle sheath cells.
• RuBisCO can be used in the Calvin cycle inside the bundle sheath cells.
• PEP-carboxylase is in the mesophyll: fixes CO2, even at lower CO2 concentrations.
• Oxaloacetate is converted to malate.
• Malate diffuses into bundle sheath cells.

CAM Plants (Succulents)

• Temporal separation.
• Adapted to high temperatures and water scarcity.
• Have similar initial CO₂ fixation steps as C4 plants.
• Stomata open at night (low water loss).
• CO2 in mesophyll cells is bound to PEP Carboxylase.
• Formation in malate, the salt of apple acid.
• Lowers the pH value at night.
• The next day the CO₂ from malate is released and fed into the Calvin cycle.

Sample Exam

Calvin Cycle

• In the Calvin cycle, there are three steps.
• In the first step, CO₂ is fixed by the enzyme RuBisCO and converted to PGA.
• During the reduction of PGA, ATP and NADPH are required to produce glucose.
• Turning ATP into ADP and NADPH into NADP.
• This process generates ADP and NADP, which are used in the light-dependent reactions for ATP and NADPH synthesis.
• In the last step, the rest - 5/6 - will be converted to Rubisco using ATP to regenerate it.

Disadvantages - Calvin Cycle

• The light-dependent reactions require ADP for ATP synthase.
• However, this restores the concentration gradient for hydrogen.

C3 Plants - Advantages

• Non-specific, widely distributed.
• Presence of many mesophyll cells with chloroplasts, thus resulting in high photosynthetic rate.

C3 Plants - Disadvantages

• There is not always enough CO₂.

Photosystem II

• Photosystem II P680 absorbs light at a wavelength of 680 nm by using energy for this hydrolysis.

Aerobic Respiration

• Aerobic and anaerobic respiration are types of cell respiration.
• Glycolysis and the citric acid cycle occur in three plant species.
• The first is the C3 plant since they are high in CO carbon.
• C4 is in regions and can also act as The first carbon-fixing species in those plants will then often generate C4 for a living.
• C-4 Plants have a spatial separation of CO(2) fixation, which happens in the mesophyll
• Calvin Cycle - this will require bundle sheath cells, resulting in a unique form of oxygen and RuBisCo in CAT Plants.