Question

Foldings of inner mitochondrial membrane are called
A. Grana
B. Thylakoids
C. Cristae
D. $F_0$-$F_1$ structures

Answer 1

Hint: Mitochondria are the powerhouse of the cell which is present in all eukaryotic cells which. They are double-membrane bound structures. Their inner membrane is folded into finger-like projections called cristae.

Complete answer:
To start with, we should know that some of the cell organelles are bound by the unit membrane. Here, the unit membrane refers to the lipid bilayer with embedded proteins as is present in the cell membrane.
However, some organelles such as mitochondria and chloroplasts are bound by two membranes and therefore, are called double membrane-bound organelles.

If we talk about the structure of chloroplasts, in short, Thylakoids are the third membrane system of the chloroplasts. Thylakoids are the interconnected network of the disc-like sacs present in the stroma. The stacks of thylakoids are called grana.

Now, let's talk about mitochondria. In mitochondria, the outer membrane is smooth and continuous as it is in the chloroplasts. However, the inner mitochondrial membrane is folded into the finger-like structures called cristae. The compartment of the mitochondria enclosed by the cristae of the inner membrane is called the matrix. But what's the significance? Cristae serve to increase the surface area of the inner mitochondrial membrane.

The electron transport chain (ETC) is embedded in the inner mitochondrial membrane. The protein complexes of the ETC and the ATP synthase are the sites for the final stage of the aerobic respiration.

ATP synthase, its $F_1$ component has the ATP synthase activity to drive the phosphorylation of ADP into ATP. The $F_0$ channel is an integral membrane protein that transports the protons from the intermembrane space towards the matrix.

Hence, The correct answer is option (C).

Note: The inner chloroplast membrane is not folded into cristae but the photosynthetic pigments are embedded in the unique membrane system called thylakoids. It is to remember that during the electron transport chain, the protons are pumped from the matrix to the intermembrane space as the electrons move from reducing powers to the molecular oxygen through protein complexes I-IV. The resultant proton concentration gradient drives the ATP synthesis during oxidative phosphorylation.