Mitochondria

Each mitochondrion is bounded by two membranes, of which the inner membrane is folded inward to form cristae. These infoldings, described in NCERT Class 11, Chapter 8 (Cell: The Unit of Life), greatly increase the available surface area on which the enzymes of the electron transport chain and the F1 particles for ATP synthesis are arranged, thereby maximising oxidative phosphorylation. The option about storing starch grains is incorrect, because starch is stored in plastids such as amyloplasts rather than in mitochondria. The choice describing synthesis of secretory glycoproteins is wrong, as that role belongs to the rough endoplasmic reticulum and Golgi apparatus. The statement about trapping sunlight applies to chloroplast thylakoids, not to mitochondrial cristae, since mitochondria are sites of respiration, not photosynthesis. Tiny stalked particles, the F0–F1 oxysomes, stud the cristae and physically carry out the synthesis of ATP from ADP and inorganic phosphate. This explains why cells with high energy demands, such as cardiac muscle and flight muscle, possess mitochondria packed with densely folded cristae. A functional sanity check reinforces the answer: aerobic ATP production depends on membrane-bound enzyme complexes, so any structural modification that enlarges the inner membrane surface directly enhances energy output, which is precisely the purpose served by the cristae.