Which letters are broken down to produce energy for cells?

C and D

Explanation:

All living organisms need energy, which is produced by the breakdown of the food ingested in a process called CELLULAR RESPIRATION. Cellular respiration, which is the process whereby energy in form of ATP is synthesized, occurs in the MITOCHONDRIA.

However, the products of cellular respiration (CO2 and H2O) are used as reactants by plants to produce glucose and oxygen in their CHLOROPLASTS. The glucose and oxygen are broken down in the MITOCHONDRIA to yield energy by cells.

In this image, letters C and D represents glucose and oxygen, which are products of photosynthesis in the chloroplast. Glucose and oxygen represented by C and D are broken down to produce energy for cells in the mitochondria.

it is a limiting nutrient limited by a single nutrient in the ecosystem, & the single goes very slow.