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

• Molecule X (a 5-carbon compound) combines with ${CO}_2$ to form an unstable 6-carbon intermediate, which immediately splits into two molecules of glycerate 3-phosphate (GP).
• GP is then reduced to Molecule Y (a 3-carbon triose phosphate).
• Molecule Y is used to regenerate Molecule X, and also serves as the precursor for biological macromolecules. For example, it can be converted to glycerol and Molecule Z, which combine to form lipids.

Identify molecules X, Y, and Z.

A student investigated the effect of light intensity on the rate of photosynthesis in Canadian pondweed, Elodea canadensis, at a constant temperature.

The experimental setup is outlined below:

• A $10cm$ shoot of Elodea canadensis was placed in a beaker containing $150{cm}^3$ of $0.5\%$ sodium hydrogencarbonate (${NaHCO}_3$) solution.
• The beaker was kept in a water bath at a constant temperature of $20{}^{\circ }C$.
• An LED light source was positioned at various distances ($10cm$, $20cm$, $30cm$, $40cm$, and $50cm$) from the beaker to vary the light intensity.
• After a $5-minute$ equilibration period, an oxygen sensor connected to a data logger was used to record the change in dissolved oxygen concentration in the water over a period of $15minutes$ at each distance.
• The experiment was repeated three times at each distance.

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.