43- Biología celular. Fotosíntesis II

Photosynthesis and Chloroplasts: Understanding the Biochemical Stage

Overview of Photosynthesis Stages

• The tutorial focuses on photosynthesis and chloroplasts, building upon previous discussions about their general characteristics and the light-dependent stage of photosynthesis.

The Biochemical Stage of Photosynthesis

• This section introduces the biochemical stage where ATP and NADPH produced in the light stage are used to convert carbon dioxide into glucose through a series of reactions known as the Calvin Cycle.

The Calvin Cycle Explained

• The Calvin Cycle is compared to the Krebs cycle in cellular respiration, as both cycles regenerate their starting molecules. It begins with a 5-carbon molecule (ribulose bisphosphate) that combines with carbon dioxide from the atmosphere or water for algae.

• The combination of ribulose bisphosphate and carbon dioxide forms an unstable 6-carbon compound that quickly splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA).

Importance of Rubisco Enzyme

• Rubisco (ribulose bisphosphate carboxylase/oxygenase) is highlighted as a crucial enzyme, making up about 50% of chloroplast enzymes. It has dual functions: fixing carbon dioxide during photosynthesis and acting as an oxidase in photorespiration.

Phases of the Calvin Cycle

• The cycle consists of three phases:

• Carbon Fixation: Initial phase where CO2 is fixed.

• Reduction Phase: Converts an acid into an aldehyde using ATP.

• Regeneration Phase: Recycles ribulose bisphosphate to continue the cycle.

Summary of Photosynthetic Process

• This biochemical process occurs in the stroma of chloroplasts and does not require light directly, unlike the initial phase.

• In total, six turns of the Calvin Cycle produce one glucose molecule from six carbon atoms derived from CO2 while releasing six oxygen molecules essential for life.

Key Events in Photosynthesis

• Three significant events summarize photosynthesis:

• Conversion of light energy into chemical energy (ATP & NADPH).

• Production of oxygen via photolysis.

• Reduction of CO2 to form glucose, which can be further utilized by cells for energy through cellular respiration.