Biology Practice Test 2026: Energy, Enzymes, and Metabolism Essentials

C4 plants minimize photorespiration by concentrating CO2 where the Calvin cycle operates, using a four-carbon shuttle to transport CO2 from one cell type to another. In the mesophyll cells, CO2 is first fixed by PE conserves carboxylase to form a four-carbon compound (like oxaloacetate, which becomes malate). This shuttle carries CO2 to the bundle-sheath cells, where the four-carbon compound releases CO2 for fixation by RuBisCO in the Calvin cycle. This keeps CO2 levels high around RuBisCO, so the enzyme is less likely to fix O2 and perform photorespiration, especially under high light and heat. The Calvin cycle is not carried out directly in the mesophyll, which is why the other statements aren’t true. CAM is a different strategy where carbon is fixed at night in some plants, and saying there’s no protection against photorespiration contradicts how C4 anatomy and biochemistry work.