Sucrose serves as the primary transportable form of carbon and energy in most plants, synthesized primarily in the cytosol of leaf mesophyll cells. This occurs via the concerted action of sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (SPP).

Correct Answer

Cytosol.

Option Analysis

Chloroplast: Incorrect. Chloroplasts fix CO2 into triose phosphates (e.g., G3P) during photosynthesis and synthesize starch transiently, but lack the enzymes for sucrose assembly; trioses are exported to cytosol via phosphate translocators.

Cytosol: Correct. Sucrose synthesis happens here in mesophyll cytosol: triose phosphates convert to UDP-glucose and fructose-6-phosphate, then SPS forms sucrose-6-phosphate, dephosphorylated by SPP to sucrose for phloem loading.

Mitochondria: Incorrect. Mitochondria handle respiration and some sucrose synthase (SuSy) isoforms for breakdown/utilization (e.g., UDP-glucose for cell wall synthesis), but not primary synthesis.

Endoplasmic reticulum: Incorrect. ER manages protein/lipid synthesis and secretion; no role in sucrose biosynthesis, which is cytosolic.​

Sucrose transportable form synthesized cytosol acts as plants’ main phloem sugar, exporting photosynthetic carbon from source leaves to sinks.

Biosynthesis Pathway

Triose phosphates (DHAP, G3P) exit chloroplasts to cytosol, forming fructose-6-P and UDP-glucose. SPS transfers glucose to yield sucrose-6-P; SPP hydrolyzes it irreversibly to sucrose.

Key Enzymes

• Sucrose phosphate synthase (SPS): Cytosolic, regulated by phosphorylation/light.

• Sucrose phosphate phosphatase (SPP): Ensures irreversibility.

• Sucrose synthase (SuSy): Reversible, mainly for breakdown/sink metabolism.

Organelle Roles Comparison

This partitioning balances source-sink relations, preventing osmotic stress in leaves. For exams, note cytosol as the definitive site distinguishing from starch (chloroplast).​