state that Calvin cycle intermediates are used to produce other molecules, limited to GP to produce some amino acids and TP to produce carbohydrates, lipids and amino acids

Photosynthesis as an Energy Transfer Process

Learning Objective

State that Calvin‑cycle intermediates are used to produce other molecules, limited to glyceraldehyde‑3‑phosphate (GP) to produce some amino acids and triose phosphate (TP) to produce carbohydrates, lipids and amino acids.

1. Overview of the Calvin Cycle

The Calvin cycle fixes atmospheric CO₂ into organic compounds in the stroma of chloroplasts. The key steps are:

1. Carbon fixation – CO₂ combines with ribulose‑1,5‑bisphosphate (RuBP) to form two molecules of 3‑phosphoglycerate (3‑PGA).
2. Reduction – ATP and NADPH from the light reactions convert 3‑PGA into glyceraldehyde‑3‑phosphate (GP, also called G3P).
3. Regeneration – A portion of GP is used to regenerate RuBP, allowing the cycle to continue.

2. Calvin‑Cycle Intermediates as Precursors

Two central intermediates are:

• Glyceraldehyde‑3‑phosphate (GP, G3P) – a three‑carbon sugar phosphate.
• Triose phosphate (TP) – includes both GP and dihydroxyacetone phosphate (DHAP), which interconvert.

These intermediates serve as carbon skeletons for the synthesis of a limited range of biomolecules.

3. Use of Glyceraldehyde‑3‑phosphate (GP)

GP can be diverted from the cycle to form certain amino acids. The main pathways are:

• Conversion to serine via the phosphorylated pathway:

  \$\text{GP} \xrightarrow{\text{phosphoglycerate dehydrogenase}} 3\text{-phosphoglycerate} \rightarrow \text{phosphohydroxypyruvate} \rightarrow \text{phosphoserine} \rightarrow \text{serine}\$

• Formation of cysteine after incorporation of sulfide:

  \$\text{Serine} + \text{S}^{2-} \rightarrow \text{Cysteine}\$

Only a small proportion of GP is allocated to these amino‑acid pathways; the majority returns to RuBP regeneration.

4. Use of Triose Phosphate (TP) for Carbohydrates, Lipids and Amino Acids

TP (GP + DHAP) is the gateway to larger biosynthetic routes:

1. Carbohydrate synthesis – TP can be polymerised to form starch or sucrose.

   • Starch synthesis in the chloroplast:

     \$\text{TP} \xrightarrow{\text{ADP‑glucose pyrophosphorylase}} \text{ADP‑glucose} \rightarrow \text{Starch}\$

   • Sucrose synthesis in the cytosol:

     \$2\,\text{TP} \xrightarrow{\text{Sucrose‑phosphate synthase}} \text{Sucrose‑6‑phosphate} \rightarrow \text{Sucrose}\$

2. Lipid synthesis – TP is converted to acetyl‑CoA, the building block of fatty acids.

   Key steps:

   \$\text{TP} \rightarrow \text{Pyruvate} \xrightarrow{\text{pyruvate dehydrogenase}} \text{Acetyl‑CoA} \rightarrow \text{Fatty acids}\$

3. Amino‑acid synthesis beyond serine and cysteine – TP provides carbon skeletons for the synthesis of alanine, glycine and others via transamination reactions.

   Example for alanine:

   \$\text{Pyruvate} + \text{Glutamate} \xrightarrow{\text{alanine aminotransferase}} \text{Alanine} + \text{α‑ketoglutarate}\$

5. Summary of Pathways from Calvin‑Cycle Intermediates

6. Key Points to Remember

• The Calvin cycle fixes carbon but only a limited fraction of its intermediates is diverted to biosynthetic pathways.
• GP is primarily used for the synthesis of serine and cysteine.
• TP (including DHAP) supplies the carbon skeletons for the majority of carbohydrate, lipid and additional amino‑acid synthesis.
• All these pathways depend on the ATP and NADPH generated by the light reactions, linking the light‑dependent and light‑independent phases of photosynthesis.