A simplified metabolic pathway 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 glycerate 3-phosphate (GP).
• GP is then reduced to Molecule B (a 3-carbon triose phosphate).
• Molecule B is used to regenerate Molecule A, and also serves as the building block for larger carbohydrates. For example, molecules of B can be converted into hexose sugars, which are then polymerized to form the insoluble storage polysaccharide, Molecule C, in the stroma.

Identify molecules A, B, and C.

A student investigated the effect of carbon dioxide concentration on the rate of photosynthesis in the unicellular green alga, Chlorella vulgaris, at a constant light intensity.

The experimental setup is outlined below:

• A $200{cm}^3$ culture of Chlorella vulgaris was placed in a bioreactor flask inside a water bath maintained at a constant temperature of $22{}^{\circ }C$.
• A constant LED light source was positioned at a fixed distance of $15cm$ from the flask.
• The carbon dioxide concentration in the medium was varied by preparing the suspension with different concentrations of sodium hydrogencarbonate (${NaHCO}_3$) solution: $0.1\%$, $0.2\%$, $0.3\%$, $0.4\%$, and $0.5\%$.
• After a $10-minute$ equilibration period at each concentration, an oxygen sensor connected to a data logger was used to record the rate of dissolved oxygen accumulation in the suspension over a period of $10minutes$.
• The experiment was repeated three times at each carbon dioxide concentration.

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.