Glyceraldehyde 3-phosphate
Commonly abbreviated to GALP, or occasionally PGAL, this molecule is the 3-carbon end-product of the Calvin Cycle which takes place in the absence of light in the stroma of any photosynthetic cell of a plant. The stroma can be considered by analogy to be the cytoplasm of a chloroplast, but this is a huge simplification of course.

At the end of the light dependent reaction small quantities of adenosine triphosphate (ATP) and larger quantities of reduced hydrogen ion accepting NADPH have been produced.

The Calvin Cycle proceeds as follow (in only enough detail to illustrate GALP production):
  1. CO2 fixation with respect to ribulose biphosphate
  2. Cleaving of the RUBP molecule to form two 3-carbon gylcerate-3-phosphate
  3. Reduction of GP to GALP
It is this reduction step that requires the NADPH. A redox reaction is undertaken and the co-enzyme is oxidised to it's original form, thereby replenishing it's original supply. If supplies of NADP (the non-reduced form of NADPH) are exhausted, then the light dependent reaction will grind to a halt, as otherwise H+ produced would lower pH and cause enzyme denaturation.

Experimentaly, 83% of the GALP is used in the reformation of RuBP - allowing cycling to continue. The remainder is used in the synthesis of whatever the plant further requires. Usually the reaction pathway proceeds to fructose, but additional methods are possible for synthesis of other compounds.

GALP and it's key importance in the biochemistry of life was discovered in the famous Calvins lollipop experiment, before which biologists had little idea as to what chemical was fulfilling such a vital role.

Biochemistry of photosynethsis and respiration is of course ridiculously complex and unlike Chemistry, Biology has yet to obtain a standardised chemical naming scheme (think IUPAC), so YMMV when comparing this accross textbooks. If you want the full story in all it's glory, you're looking in the wrong place.

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