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Botany · Cell — The Unit of Life

Mitochondria

Mitochondria are the sausage-shaped, double-membrane organelles where the cell extracts usable energy from food. NCERT Section 8.5.4 places them at the heart of the eukaryotic cytoplasm and labels them the "power houses" of the cell. For NEET, mitochondria are a perennial favourite — almost every recent paper carries one question on cristae, the inner membrane, the matrix DNA or the semi-autonomous nature of the organelle. This page goes deeper than the chapter on a single organelle.

NCERT grounding

NCERT Class XI Biology, Chapter 8 — Section 8.5.4 (Mitochondria) — opens the topic with a frank confession: "Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope." The chapter then anchors three exam-critical facts: each mitochondrion is a double membrane-bound structure, the inner membrane forms infoldings called cristae that increase surface area, and these organelles are the sites of aerobic respiration. NIOS Biology Chapter 4 reinforces the same picture, adds the phrase "energy releasers" and confirms multiplication by fission.

Structure & function deep-dive

A typical mitochondrion is a small, sausage-shaped or cylindrical organelle floating in the cytoplasm of nearly every eukaryotic cell. Its job is single-minded: take energy-rich molecules — chiefly pyruvate, fatty acids and certain amino acids — and oxidise them in a controlled way, capturing the released chemical energy in ATP. Everything about mitochondrial architecture, from the smooth outer membrane down to the studded oxysomes on the inner membrane, is shaped by this one task.

The number of mitochondria per cell is not fixed. Metabolically lazy cells (mature plant parenchyma, fat-storage cells) carry only a handful; cells with high ATP demand — flight muscle, secretory glandular tissue, the dividing tip of a root — pack hundreds to thousands. NCERT explicitly notes this variability with cellular physiology.

0.5 × 1.5 µm

Typical mitochondrial size

NCERT range: diameter 0.2–1.0 µm (average 0.5 µm); length 1.0–4.1 µm. Shape varies — sausage-shaped or cylindrical — and considerable variability is observed across cell types.

Two membranes, two compartments

Each mitochondrion is wrapped in two membranes. The two membranes together divide the lumen into two aqueous compartments — the outer compartment (intermembrane space, between the two membranes) and the inner compartment (the matrix, bounded by the inner membrane). The two membranes have their own specific enzymes associated with mitochondrial function. NEET papers repeatedly probe which enzyme sits on which membrane, so this distinction matters.

Outer vs inner membrane — at a glance

Outer membrane

Smooth · Permeable

continuous limiting boundary

  • Studded with porins — large channel proteins that let small metabolites pass freely.
  • Permeable to monomers of carbohydrates, fats and proteins (NEET 2019 statement).
  • Does NOT carry ETS enzymes.
  • Lacks the convoluted folds of the inner face.
VS

Inner membrane

Folded · Selective

infoldings = cristae

  • Highly impermeable; transport is enzyme-mediated.
  • Hosts the electron transport system (ETS).
  • Carries F0–F1 ATP synthase (oxysomes) as stalked particles facing the matrix.
  • Folds (cristae) sharply increase surface area.

The outer membrane is the organelle's contact with the cytoplasm — every fuel molecule entering the mitochondrion crosses it first. Because it is studded with porin channels, small solutes diffuse through largely unimpeded. The inner membrane is a different beast: it is intentionally tight, allowing the cell to build the steep proton gradient on which oxidative phosphorylation depends. NEET 2019 (Q.43) tested this directly with an incorrect-statement trap that claimed "Enzymes of electron transport are embedded in outer membrane" — the answer hinges on knowing the ETS is on the INNER membrane.

The matrix and its cargo

The inner compartment is filled with a "dense homogeneous substance called the matrix" — NCERT's exact phrasing. The matrix is not just a passive jelly. It is the metabolic engine room that holds four exam-critical contents.

NCERT (§8.5.4): "The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins."

Circular dsDNA

1 copy

single, naked, double-stranded

Encodes a small subset of mitochondrial proteins; resembles prokaryotic DNA — a prime piece of evidence for endosymbiotic origin.

70S ribosomes

50S + 30S

prokaryote-type, NOT 80S

Translate the proteins encoded by mitochondrial DNA. Cytoplasmic ribosomes of the host eukaryote are 80S — the contrast is a classic single-mark trap.

Krebs cycle enzymes

8 reactions

citric acid / TCA cycle

All enzymes of the citric acid cycle are dissolved in the matrix, plus pyruvate dehydrogenase that feeds it. NADH and FADH2 generated here power the ETS on the inner membrane.

tRNAs & few RNAs

complete kit

for in-organelle translation

A few mRNAs, tRNAs and rRNAs allow the matrix to translate its own proteins independently of nuclear instructions — partially.

Note the single most NEET-favoured fact in this list: the mitochondrial matrix carries one circular dsDNA molecule plus 70S ribosomes. Both of these are prokaryote-typical features inside a eukaryotic cell. This is the central piece of evidence behind the endosymbiotic theory, which proposes that mitochondria descended from a free-living aerobic bacterium engulfed by an ancestral eukaryote roughly two billion years ago.

Cristae and F1 oxysomes

If the matrix is the engine room, the cristae are the radiators that release energy at workable rates. NCERT defines them with precision: "The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area." A single mitochondrion may carry dozens of cristae; in a hard-working cell, the inner membrane folds so densely it appears as parallel sheets across the entire organelle.

On the matrix-facing side of every crista sit thousands of microscopic stalked knobs — the F1 particles, also called oxysomes or elementary particles. Each F1 particle is the catalytic head of ATP synthase; its membrane-embedded base is the F0 proton channel. The two together convert the proton-motive force generated by the ETS into a stream of ATP molecules. This is oxidative phosphorylation.

Figure 1 Mitochondrion — longitudinal section Mitochondrion (longitudinal section) Outer membrane Inner membrane Intermembrane space (outer compartment) Cristae Matrix circular DNA 70S ribosomes F₁ oxysomes

Figure 1. Longitudinal section of a mitochondrion. The smooth outer membrane (black) is permeable; the green inner membrane folds inward to form cristae bearing F1 oxysome knobs (red) on the matrix side. The matrix carries a single circular DNA molecule (purple) and 70S ribosomes (green dots). The two membranes create two compartments: the intermembrane space and the matrix.

Semi-autonomous nature & fission

Mitochondria are termed semi-autonomous organelles for a reason rooted in their unique cargo. They carry their own DNA, their own (70S) ribosomes, and the apparatus to make a subset of their own proteins. They also divide by fission — pinching in two from a pre-existing mitochondrion, much like a bacterium — rather than being assembled from scratch by the nucleus. NCERT closes Section 8.5.4 with this exact phrase: "The mitochondria divide by fission."

The "semi" is not a throwaway. Most mitochondrial proteins are still coded by nuclear DNA, translated on 80S cytoplasmic ribosomes, and imported. The organelle cannot live outside the cytoplasm for any meaningful period. NIOS Biology, Chapter 4, puts it bluntly: chloroplasts and mitochondria "are termed as semi-autonomous only because they are incapable of independent existence outside the cytoplasm for a long time" since most of their proteins are still synthesised with nuclear help.

From food to ATP — the mitochondrial workflow

Aerobic respiration · sites mapped to mitochondrial structure

  1. Step 1

    Pyruvate enters

    Pyruvate (from cytoplasmic glycolysis) crosses the outer membrane through porins, then the inner membrane via a specific transporter, into the matrix.

    Location: cytoplasm → matrix
  2. Step 2

    Krebs cycle

    Acetyl-CoA enters the citric acid cycle; the matrix-resident enzymes release CO2 and generate NADH and FADH2.

    Location: matrix
  3. Step 3

    Electron transport

    NADH/FADH2 donate electrons to ETS complexes on the inner membrane; protons are pumped into the intermembrane space.

    Location: inner membrane
  4. Step 4

    Oxidative phosphorylation

    Protons flow back through F0–F1 ATP synthase (the oxysome knobs); ATP is generated; O2 is reduced to H2O at Complex IV.

    Location: F₁ oxysomes

Notice how the four steps map neatly onto the four anatomical zones: outer membrane (porins for entry), matrix (Krebs cycle), inner membrane (ETS), and oxysomes (ATP synthase). NEET routinely tests this site-of-action mapping — knowing the geography of the organelle is half the battle.

Worked examples

Worked example 1

Q. Which of the following is the site of the citric acid cycle in a eukaryotic cell?

A. The citric acid (Krebs) cycle takes place in the mitochondrial matrix. All eight enzymes of the cycle, along with pyruvate dehydrogenase that channels pyruvate into it, are dissolved in the matrix fluid. The inner membrane carries the downstream electron transport system, but the cycle itself is matrix-located. Glycolysis is in the cytoplasm; the light reaction is in chloroplast thylakoids — these are common distractors.

Worked example 2

Q. A student writes: "Mitochondria contain their own DNA but lack a protein-synthesising machinery; this is why they are called semi-autonomous." Identify and correct the error.

A. The claim that mitochondria lack protein-synthesising machinery is wrong. The matrix contains 70S ribosomes, several tRNAs and the enzymes needed to translate the proteins encoded by mitochondrial DNA. They are semi-autonomous because most — not all — of their proteins are still encoded by nuclear DNA and imported, so they cannot live independently of the cytoplasm. (This exact distinction was tested in NEET 2016 Q.94.)

Worked example 3

Q. Match each mitochondrial component with its primary function: (i) Outer membrane (ii) Inner membrane (iii) Matrix (iv) Cristae.

A. (i) Outer membrane → smooth, permeable limiting boundary carrying porins. (ii) Inner membrane → houses the electron transport chain and F0–F1 ATP synthase; impermeable, drives proton gradient. (iii) Matrix → site of Krebs cycle, holds circular dsDNA, 70S ribosomes, RNA. (iv) Cristae → infoldings of the inner membrane that increase the surface area for ETS and oxysomes. Many NEET stems disguise this as a Match-the-Column question; remembering the four-zone map removes the guesswork.

Worked example 4

Q. Why are mitochondrial ribosomes 70S rather than 80S like the rest of the eukaryotic cytoplasm?

A. The 70S ribosomes (50S + 30S) of the mitochondrial matrix resemble those of prokaryotes, not eukaryotic cytoplasmic ribosomes (80S = 60S + 40S). This is a major piece of evidence for the endosymbiotic theory: an ancestral aerobic bacterium was engulfed by a primitive eukaryote and over evolutionary time became the mitochondrion, retaining its prokaryote-style ribosomes and circular DNA. Chloroplasts show the same pattern, supporting an analogous origin.

Common confusion & NEET traps

NEET PYQ Snapshot — Mitochondria

Real NEET papers 2016–2023 — mitochondria appears almost every year.

NEET 2019

Which of the following statements regarding mitochondria is incorrect?

  1. Outer membrane is permeable to monomers of carbohydrates, fats and proteins.
  2. Enzymes of electron transport are embedded in outer membrane.
  3. Inner membrane is convoluted with infoldings.
  4. Mitochondrial matrix contains single circular DNA molecule and ribosomes.
Answer: (2)

Why: The ETS enzymes are embedded in the inner mitochondrial membrane, not the outer. Options (1), (3) and (4) are all correctly worded NCERT statements.

NEET 2018

Which of the following cell organelles is responsible for extracting energy from carbohydrates to form ATP?

  1. Mitochondrion
  2. Lysosome
  3. Ribosome
  4. Chloroplast
Answer: (1)

Why: Mitochondria are the power house of the cell — the only organelle in this list that extracts energy from carbohydrates and stores it as ATP via the electron transport system.

NEET 2021

Match List-I with List-II.
(a) Cristae — (i) Primary constriction in chromosome
(b) Thylakoids — (ii) Disc-shaped sacs in Golgi apparatus
(c) Centromere — (iii) Infoldings in mitochondria
(d) Cisternae — (iv) Flattened membranous sacs in stroma of plastids
Choose the correct answer.

  1. (a)-(ii), (b)-(iii), (c)-(iv), (d)-(i)
  2. (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)
  3. (a)-(i), (b)-(iv), (c)-(iii), (d)-(ii)
  4. (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)
Answer: (4)

Why: Cristae = infoldings of the inner mitochondrial membrane; thylakoids = flattened sacs in chloroplast stroma; centromere = primary constriction holding sister chromatids; cisternae = disc-shaped Golgi sacs. Correct mapping is (iii)(iv)(i)(ii).

NEET 2016

Mitochondria and chloroplast are — (a) semi-autonomous organelles (b) formed by division of pre-existing organelles and they contain DNA but lack protein-synthesising machinery. Which one of the following options is correct?

  1. (b) is true but (a) is false
  2. (a) is true but (b) is false
  3. Both (a) and (b) are false
  4. Both (a) and (b) are correct
Answer: (2)

Why: Mitochondria and chloroplasts ARE semi-autonomous, formed by fission of pre-existing organelles, and DO carry their own DNA — but they also possess 70S ribosomes and a complete protein-synthesising machinery. So (a) is true, (b) is false.

NEET 2019

Which of the following pair of organelles does not contain DNA?

  1. Mitochondria and Lysosomes
  2. Chloroplast and Vacuoles
  3. Lysosomes and Vacuoles
  4. Nuclear envelope and Mitochondria
Answer: (3)

Why: Mitochondria and chloroplasts carry DNA (semi-autonomous); the nuclear envelope encloses nuclear DNA. Only lysosomes and vacuoles among these completely lack DNA.

NEET 2016

Which one of the following cell organelles is enclosed by a single membrane?

  1. Chloroplasts
  2. Lysosomes
  3. Nuclei
  4. Mitochondria
Answer: (2)

Why: Mitochondria, chloroplasts and the nucleus are all double-membrane-bound. Only the lysosome is single-membrane-bound. Memorise the double-membrane trio: mitochondria, plastids, nucleus.

FAQs — Mitochondria

Quick answers to the questions NEET aspirants ask most about mitochondria.

Why are mitochondria called the powerhouse of the cell?

Because they are the sites of aerobic respiration. The Krebs cycle in the matrix and the electron transport system on the inner membrane couple substrate oxidation to oxidative phosphorylation, generating the bulk of cellular ATP. NCERT explicitly states they produce cellular energy in the form of ATP and are therefore called the power houses of the cell.

What are cristae and why are they important?

Cristae are the infoldings of the inner mitochondrial membrane that project into the matrix. They dramatically increase the surface area available for embedding the electron transport chain complexes and the F1 (oxysome) particles bearing ATP synthase. Without cristae, the inner membrane could not accommodate enough respiratory machinery to meet the cell's ATP demand.

Why are mitochondria called semi-autonomous organelles?

The matrix carries a single circular double-stranded DNA molecule, a few RNA species and 70S ribosomes, so the organelle can transcribe and translate some of its own proteins and can divide by fission. However, most of its proteins are still encoded by nuclear DNA and imported, so it cannot live independently of the cytoplasm — hence semi-autonomous.

What is the typical shape and size of a mitochondrion?

NCERT describes mitochondria as typically sausage-shaped or cylindrical with a diameter of 0.2–1.0 µm (average 0.5 µm) and a length of 1.0–4.1 µm. For NEET, remember the average dimensions as 0.5 µm wide × about 1.5 µm long, with considerable variability depending on cell type.

Where exactly are the enzymes of the electron transport system located?

The electron transport system (ETS) enzymes are embedded in the inner mitochondrial membrane, not the outer one. NEET 2019 explicitly tested this trap. The outer membrane is smooth and permeable (porins); the inner membrane carries the ETS complexes and F0–F1 ATP synthase (oxysomes).

What does the mitochondrial matrix contain?

The matrix is a dense, homogeneous fluid filling the inner compartment. It contains a single circular double-stranded DNA molecule, a few RNA molecules, 70S ribosomes, the enzymes of the Krebs (citric acid) cycle, and the machinery for the synthesis of some mitochondrial proteins. Pyruvate oxidation and the citric acid cycle occur here.

How do mitochondria multiply inside the cell?

Mitochondria divide by fission — they pinch into two from a pre-existing mitochondrion, much like prokaryotes. Coupled with their own DNA, 70S ribosomes and double-membrane envelope, this fission mode of multiplication is one of the main pieces of evidence cited for the endosymbiotic theory of mitochondrial origin.

Related Topics
Cell — The Unit of Life Back to the full chapter notes. Plastids The other double-membrane semi-autonomous organelle. Eukaryotic Cells — Overview Mitochondria within the wider organelle map. Endomembrane System ER · Golgi · lysosomes · vacuoles — and why mitochondria are not in it. Ribosomes 70S vs 80S — contrast with the mitochondrial ribosome. Nucleus The other double-membrane organelle that carries DNA.