Mitochondria: Structure, Cristae and ATP Production
Biology · Cell: The Unit of Life · NEET
A mitochondrion is a double membrane organelle. The outer membrane is smooth, and the inner membrane folds inward to make finger-like folds called cristae. These cristae give more surface area to make ATP, so mitochondria are called the "power house" of the cell. Memory hook: Cristae = "Crease" of the INNER membrane that makes energy.
A mitochondrion has two membranes. The smooth outer membrane surrounds the folded inner membrane, whose cristae hold the electron transport enzymes that make ATP. The inner membrane encloses the matrix, which contains circular DNA, 70S ribosomes and Krebs-cycle enzymes.
Your doubts, answered
Are cristae part of the inner or outer membrane?
Cristae are folds of the INNER membrane only. The outer membrane stays smooth and does not fold. The inner membrane pushes inward toward the matrix and makes these folds. NEET repeats this fact almost every year, so remember: Cristae = inner membrane infoldings.
Why do cristae exist? What is their job?
Cristae increase the surface area of the inner membrane. More surface area means more space to hold the enzymes that make ATP. So more cristae means the cell can make more energy. Cells that need lots of energy (like muscle cells) have mitochondria with many cristae.
Does a mitochondrion have one membrane or two?
It has TWO membranes, so it is called double membrane bound. NEET loves to trap students here. Lysosomes and vacuoles have ONE membrane, but mitochondria and chloroplasts have TWO. If a question says 'mitochondrion is single membrane bound', that statement is WRONG.
What is the matrix of mitochondria?
The matrix is the thick fluid inside, enclosed by the inner membrane. It holds the enzymes of the citric acid cycle (Krebs cycle), a single circular DNA molecule, and 70S ribosomes. Because it has its own DNA and ribosomes, the mitochondrion can make some of its own proteins.
Where are the electron transport enzymes found?
The enzymes of the electron transport chain (ETC) sit in the INNER membrane, not the outer membrane. This is a classic NEET trap. The outer membrane is permeable to small molecules; the inner membrane holds the respiration enzymes and is less permeable.
Why is the mitochondrion called the power house of the cell?
Because it makes ATP, the energy currency of the cell. Mitochondria are the sites of aerobic respiration, where energy from carbohydrates (and fats) is released and stored as ATP. ATP powers almost every activity in the cell, so the mitochondrion is the 'power house'.
Why is the mitochondrion called semi-autonomous?
Because it has its own circular DNA, its own 70S ribosomes, and divides by fission from a pre-existing mitochondrion. So it is partly independent. But it still needs many proteins coded by the nuclear DNA, so it is only 'semi' (partly) autonomous, not fully independent.
⚠️ The NEET trap ✗ Cristae and the electron transport enzymes are part of the OUTER mitochondrial membrane, and the mitochondrion is single membrane bound. ✓ Cristae are infoldings of the INNER membrane, the electron transport enzymes are embedded in the INNER membrane, and the mitochondrion is DOUBLE membrane bound. 🧠 Everything active in respiration (cristae + ETC enzymes) is on the INNER membrane. The outer membrane is just a smooth, permeable bag.
Real NEET questions
NEET 2019
Which of the following statements regarding mitochondria is incorrect?
A · Outer membrane is permeable to monomers of carbohydrates, fats and proteins.
B · Enzymes of electron transport are embedded in outer membrane. ✓
C · Inner membrane is convoluted with infoldings.
D · Mitochondrial matrix contains single circular DNA molecule and ribosomes.
Solution: The electron transport (respiratory) enzymes are embedded in the INNER membrane, whose cristae increase surface area, NOT the outer membrane. So statement B is incorrect and is the answer. The other three are correct: the outer membrane is permeable to small monomers, the inner membrane is convoluted into cristae, and the matrix holds a single circular DNA and 70S ribosomes.
NEET 2021
Match List-I with List-II: (a) Cristae (b) Thylakoid (c) Centromere (d) Cisternae | (i) Primary constriction in chromosome (ii) Disc-shaped sacs in Golgi apparatus (iii) Infoldings in mitochondria (iv) Flattened membranous sacs in stroma of plastids
A · (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii) ✓
B · (a)-(ii), (b)-(iii), (c)-(iv), (d)-(i)
C · (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)
D · (a)-(i), (b)-(iv), (c)-(iii), (d)-(ii)
Solution: Cristae are the infoldings of the inner mitochondrial membrane (iii). Thylakoids are flattened membranous sacs in the stroma of plastids (iv). The centromere is the primary constriction of a chromosome (i). Cisternae are the disc-shaped sacs of the Golgi apparatus (ii). So the match is (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii).
NEET 2017
Which of the following cell organelles is responsible for extracting energy from carbohydrates to form ATP?
A · Lysosome
B · Ribosome
C · Chloroplast
D · Mitochondrion ✓
Solution: Mitochondria are the sites of aerobic respiration where energy from carbohydrates is released and stored as ATP, so they are called the 'power houses' of the cell. Lysosomes do intracellular digestion, ribosomes make proteins, and chloroplasts capture light energy for photosynthesis, so none of them is the chief ATP-maker from carbohydrate oxidation.
Solved Cell: The Unit of Life NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
