A simplified diagram of metabolic pathways branching from the Calvin cycle is described below:

• Molecule A (a 5-carbon compound) combines with ${CO}_2$ to form an unstable 6-carbon intermediate, which immediately splits into two molecules of Molecule B (a 3-carbon acid).
• Molecule B is then reduced to a triose phosphate, which can be further processed into glycerol or converted via glycolysis/transamination pathways to produce Molecule C, the nitrogen-containing monomers required to build proteins.

Identify molecules A, B, and C.

A student investigated the effect of light wavelength on the rate of photosynthesis in spinach, Spinacia oleracea, using the leaf disc flotation method.

The experimental setup is outlined below:

• Ten leaf discs, each with a diameter of $5mm$, were cut from fresh spinach leaves using a cork borer.
• The leaf discs were placed in a syringe containing $10{cm}^3$ of $1.0\%$ sodium hydrogencarbonate (${NaHCO}_3$) solution.
• A vacuum was applied by pulling the syringe plunger and holding it, drawing air out of the spongy mesophyll and causing the discs to sink.
• The syringe was placed in a beaker of water kept at a constant temperature of $22{}^{\circ }C$ to act as a heat shield.
• Different coloured cellophane filters (red, green, blue, and clear/white) were placed between a constant LED light source and the syringe to vary the wavelength of light.
• The time taken for 50% of the leaf discs to float ($E{T}_{50}$, in seconds) was measured as an indicator of the rate of photosynthesis.
• The experiment was repeated three times for each light wavelength.

Identify the independent variable and the dependent variable in this investigation.

State two control variables that the student kept constant, or should have kept constant, in this investigation.

Name the enzyme responsible for fixing carbon dioxide in the light-independent stage of photosynthesis.