Lichtunabhängige Reaktion / Calvin-Zyklus / Dunkelreaktion [Fotosynthese, 2/2] -[Biologie,Oberstufe]

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This section introduces the molecular process of the light-independent reaction in photosynthesis, also known as the dark reaction or Calvin cycle. It explains the significance of this process in synthesizing glucose and other essential organic molecules for plants.

Molecular Process of Light-Independent Reaction

• The light-independent reaction is crucial for all photosynthetically active organisms, including higher plants, algae, and some bacteria.

• It aims to synthesize glucose as a vital compound for building other necessary organic molecules.

• The secondary reaction, also termed as the dark reaction, follows the primary step of photosynthesis involving the light reaction.

• In the light reaction, light energy converts into chemical energy stored in ATP and NADPH.

• The Calvin cycle occurs within chloroplasts and consists of three main stages: carbon fixation, reduction, sugar synthesis, and CO2 regeneration.

• Atmospheric CO2 is essential for carbohydrate synthesis and needs to be chemically bound through a series of reactions.

• Enzymes catalyze metabolic reactions like CO2 fixation by ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase (Rubisco).

• CO2 is initially transferred to RuBP forming an unstable C6 compound that quickly splits into two C3 molecules.

• Reduction involves electron uptake requiring ATP generated during the light reaction. This step transforms C3 compounds into carbohydrates like glyceraldehyde-3-phosphate (GAP).

• Electrons from NADPH are transferred to GAP along with hydrogen protons leading to chemical modifications.

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This segment delves into how free energy from ATP drives phosphorylation reactions transferring phosphate groups onto molecules like GAP. It elucidates how these processes contribute to carbohydrate synthesis in photosynthesis.

Energy Transfer through Phosphorylation Reactions

• ATP provides free energy through hydrolysis releasing phosphate groups that can be transferred onto target molecules via phosphorylation reactions.

• Molecules synthesized during the light reaction act as electron carriers transferring electrons and hydrogen protons during reduction steps.

• In each round of reduction, a phosphate group is cleaved from GAP forming a C3 compound called glyceraldehyde-3-phosphate (GAP).

• This process involves oxidation of NADPH to NAD+ alongside ADP conversion back to ATP for sustaining further reactions.

Carbohydrate Synthesis and Regeneration

• Glyceraldehyde-3-phosphate (GAP), also known as triose phosphate, can follow two pathways: sugar synthesis or regeneration of RuBP receptors.

• A significant portion of GAP contributes to regenerating RuBP receptors crucial for sustaining the Calvin cycle's continuity.

• Sugar synthesis involves combining multiple triose phosphates like GAP to form larger sugars such as glucose or fructose with varying carbon atom numbers.

Detailed Explanation of Calvin Cycle

In this section, the speaker delves into the intricacies of the Calvin Cycle, highlighting its ATP-dependent nature and the balance of molecules involved in CO2 fixation.

Calvin Cycle Process

• The conversion to 3-phosphoglycerate (3-PG) from ribulose bisphosphate (RuBP) involves the splitting of HTP NADP&P and transferring the phosphate group to the molecule. This process is ATP-dependent.

• For each complete turn of the cycle, one molecule of RuBP is used for CO2 fixation while simultaneously regenerating another molecule.

Molecule Balance in Calvin Cycle

• A glucose molecule (C6H12O6) comprising six carbon atoms necessitates six molecules of carbon dioxide for entry.

• Six turns of the Calvin Cycle are essential to incorporate six CO2 molecules, making it an energy-intensive process requiring a total of 18 ATP molecules and twelve DTH molecules.

Interconnection Between Light Reactions and Calvin Cycle

This part emphasizes how light reactions and Calvin Cycle are interdependent despite being termed as light-independent reactions.

Interdependence between Reactions

• The term "light-independent reaction" can be misleading as these reactions rely on products generated in light reactions like ATP and NADPH.

• Absence of light reactions halts both Calvin Cycle and other processes swiftly due to their reliance on products from light reactions.

Light Influence on Calvin Cycle

• Light influences various enzymes involved in the Calvin Cycle by altering pH levels upon exposure, activating enzymes crucial for photosynthesis.

Evolutionary Adaptations in Photosynthesis

Evolutionary adaptations have led to variations in photosynthetic processes among plant species based on environmental conditions.

Plant Adaptations

• Plants have evolved variants or additional reactions alongside the Calvin Cycle based on specific climates, enhancing their adaptability and competitive edge.

• Plants face challenges such as regulating stomata for CO2 intake while preventing excessive water loss through transpiration under varying temperatures.