This MCQ module is based on: Mitochondria Plastids Others
TOPIC 33 OF 90
Mitochondria Plastids Others
🎓 Class 11
Biology
CBSE
Theory
Ch 8 – Cell: The Unit of Life
⏱ ~14 min
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Mitochondria Plastids Others
8.8 Mitochondria
Unless stained, mitochondria are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, considerable variations exist. Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0 µm (average 0.5 µm) and length 1.0-4.1 µm.
Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment. The inner compartment is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area. The two membranes have their own specific enzymes associated with the mitochondrial function.
Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called 'power houses' of the cell. The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. The mitochondria divide by fission.
8.9 Plastids
Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as they are large. They bear specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
- Chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
- Chromoplasts contain fat soluble carotenoid pigments like carotene, xanthophylls and others. This gives the part of the plant a yellow, orange or red colour (e.g., petals of flowers, ripe fruits).
- Leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: amyloplasts store carbohydrates (starch, e.g., potato); elaioplasts store oils and fats; aleuroplasts store proteins.
Chloroplast Structure
Chloroplasts are found mainly in the mesophyll cells of the leaves. These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10 µm) and width (2-4 µm). Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll. Like mitochondria, chloroplast is also a double membrane bound organelle. The space limited by the inner membrane of the chloroplast is called the stroma.
A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (sing.: granum) or intergranal thylakoids. In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).
The chlorophyll pigments are present in the thylakoids. Photosynthetic prokaryotes lack chloroplasts but contain photosynthetic pigments attached to thylakoid-like membranes. Internally chloroplasts also have small chloroplast DNA, ribosomes and enzymes required to manufacture proteins.
8.10 Ribosomes
Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane. The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S. Here 'S' (Svedberg's Unit) stands for the sedimentation coefficient; it indirectly is a measure of density and size. Both 70S and 80S ribosomes are composed of two subunits.
8.11 Cytoskeleton
An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.
The cytoskeleton has three main types of filaments:
- Microfilaments (actin filaments) — thinnest; involved in cell shape changes, division (cleavage furrow), and muscle contraction.
- Intermediate filaments — provide tensile strength (e.g., keratin in epithelial cells).
- Microtubules — hollow tubes of tubulin; form spindle fibres in cell division, support cilia/flagella, and serve as tracks for vesicle transport.
8.12 Cilia and Flagella
Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement. The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella.
The electron microscopic study of a cilium or the flagellum shows that they are covered with plasma membrane. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9 + 2 array. The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by a radial spoke.
The cilia and flagella emerge from centriole-like structures called the basal bodies.
8.13 Centrosome and Centrioles
Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spoke made of proteins.
The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.
8.14 Microbodies
Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells. Examples: peroxisomes (contain catalase enzyme which breaks down H₂O₂) and glyoxysomes (found in plant seeds — convert stored fats to sugars for the germinating seed).
| Organelle | Membrane | Key role |
|---|---|---|
| Mitochondrion | Double; inner folded into cristae | ATP production via aerobic respiration |
| Chloroplast | Double; thylakoids in stacks (grana) inside | Photosynthesis (light + Calvin) |
| Ribosome (80S) | None | Protein synthesis (cytoplasmic) |
| Microfilament | None | Cell shape, contraction |
| Microtubule | None | Spindle, cilia/flagella core, vesicle highways |
| Cilium/Flagellum | Plasma membrane sheath | Movement (9+2 axoneme) |
| Centrosome (centrioles) | None (proteinaceous) | Microtubule organising centre; spindle in animal cells |
| Peroxisome | Single | Breaks down H₂O₂; β-oxidation of fatty acids |
Interactive: Mitochondrion vs Chloroplast
Pick a feature and see how the two organelles differ.
| Mitochondrion | Chloroplast |
|---|---|
| — | — |
Activity 8.3 — See Chloroplasts in Leaf Cells
Setup: A young leaf (e.g., spinach, Hydrilla, or Tradescantia); microscope slide; coverslip; water; microscope.
Predict: What shape and colour will the chloroplasts be? Will they move?
My prediction: …
- Take a small piece of a young leaf — peel off the thin lower epidermis if possible, or take a single Hydrilla leaflet (it's only one cell thick at the margin).
- Mount in water on a slide; cover with coverslip.
- Observe at 40×. Look for green, lens-shaped/disc-shaped bodies inside the cells.
- Watch one cell carefully for 2-3 minutes. Do chloroplasts move?
- Sketch a cell with 10-20 chloroplasts.
Observation: Green oval/disc chloroplasts (5-10 µm) sit near the cell periphery, often along the cell wall. They move slowly around the edge — a phenomenon called cytoplasmic streaming (cyclosis). Why? Streaming positions chloroplasts to catch maximum light at different parts of the day, and circulates metabolites. The number of chloroplasts varies from 1 (Chlamydomonas) to 20-100 per mesophyll cell.
Try shining a bright lamp directly on the slide briefly — chloroplasts may even reorient! In bright light they line up edge-on (less light absorbed, avoid damage); in dim light they spread out face-on (maximise absorption).
Try shining a bright lamp directly on the slide briefly — chloroplasts may even reorient! In bright light they line up edge-on (less light absorbed, avoid damage); in dim light they spread out face-on (maximise absorption).
8.15 Worked Examples
Worked Example 1: Endosymbiotic theory
Mitochondria and chloroplasts share several features unique to prokaryotes. List three such features and explain what they tell us about the origin of these organelles.
1. Own circular DNA — like bacteria, not like eukaryotic nuclei.
2. 70S ribosomes — bacterial size, not the 80S of eukaryotic cytoplasm.
3. Binary fission — they replicate by dividing, like bacteria.
4. Double membrane — the inner is bacterial-type; the outer is thought to come from the engulfing cell.
Conclusion: These features support the endosymbiotic theory — mitochondria evolved from an ancestral aerobic bacterium engulfed by a host eukaryotic cell, while chloroplasts evolved from an ancestral cyanobacterium engulfed later. The host benefited from energy supply; the engulfed cells got shelter and nutrients. Over time, the engulfed organisms became permanent organelles.
2. 70S ribosomes — bacterial size, not the 80S of eukaryotic cytoplasm.
3. Binary fission — they replicate by dividing, like bacteria.
4. Double membrane — the inner is bacterial-type; the outer is thought to come from the engulfing cell.
Conclusion: These features support the endosymbiotic theory — mitochondria evolved from an ancestral aerobic bacterium engulfed by a host eukaryotic cell, while chloroplasts evolved from an ancestral cyanobacterium engulfed later. The host benefited from energy supply; the engulfed cells got shelter and nutrients. Over time, the engulfed organisms became permanent organelles.
Worked Example 2: Why are cilia and flagella so similar yet so different?
A bacterium has a flagellum, and a sperm cell has a flagellum. Both are used for swimming. Are they 'the same' structure?
No — they are structurally very different:
Bacterial flagellum: Solid filament of flagellin protein; rotates like a propeller, driven by a proton-motive force motor at the base. NOT covered by plasma membrane.
Eukaryotic (sperm) flagellum: Hollow tube containing a 9+2 microtubule axoneme of tubulin; bends by ATP-driven sliding of dynein arms. Covered by plasma membrane.
This is a beautiful case of convergent evolution — two completely unrelated mechanisms solving the same problem (swim through water). The shared name 'flagellum' is misleading.
Bacterial flagellum: Solid filament of flagellin protein; rotates like a propeller, driven by a proton-motive force motor at the base. NOT covered by plasma membrane.
Eukaryotic (sperm) flagellum: Hollow tube containing a 9+2 microtubule axoneme of tubulin; bends by ATP-driven sliding of dynein arms. Covered by plasma membrane.
This is a beautiful case of convergent evolution — two completely unrelated mechanisms solving the same problem (swim through water). The shared name 'flagellum' is misleading.
Worked Example 3: Centrosome essential for cell division?
Plant cells lack centrosomes/centrioles. How do they still form a mitotic spindle and divide?
Plant cells use diffuse microtubule organising centres (MTOCs) spread around the nucleus instead of a single centrosome. The microtubules nucleate from multiple sites and self-organise into a bipolar spindle. The lack of centrioles also means plant cells use a phragmoplast (a special microtubule array that lays down a new cell plate from the middle outward) to separate the daughter cells — instead of a contractile actin ring like animal cells.
This shows that the centrosome is not universally required — it's just one solution. The fundamental requirement is some way to organise microtubules into a spindle, and plants found a different way.
This shows that the centrosome is not universally required — it's just one solution. The fundamental requirement is some way to organise microtubules into a spindle, and plants found a different way.
Competency-Based Questions
Q1. The 'powerhouse of the cell' refers to: L1 Remember
Answer: (b) Mitochondrion — generates ATP via cellular respiration.
Q2. The stacks of thylakoids in chloroplasts are called: L1 Remember
Grana (singular: granum). They greatly increase the surface area for light-trapping pigments.
Q3. Apply: A rapidly contracting muscle cell has more mitochondria than a fat-storage cell. Predict and explain why. L3 Apply
Muscle contraction is highly ATP-demanding — every contraction cycle uses ATP. So muscle cells (especially Type I 'slow-twitch' fibres) need many mitochondria to produce that ATP continuously by aerobic respiration. A heart cell can have 5000+ mitochondria; an adipocyte has very few.
A fat storage cell, by contrast, mostly stores energy as triglyceride droplets and has low metabolic activity. It does not need many mitochondria.
This is why endurance training increases mitochondrial number in muscle — a classic adaptation.
A fat storage cell, by contrast, mostly stores energy as triglyceride droplets and has low metabolic activity. It does not need many mitochondria.
This is why endurance training increases mitochondrial number in muscle — a classic adaptation.
Q4. Analyse: Compare the inner membranes of mitochondria and chloroplasts. What are the implications? L4 Analyse
Mitochondrial inner membrane: Folded into cristae that project into the matrix; bears electron transport chain and ATP synthase. Cristae → increased surface for ETC.
Chloroplast inner membrane: Relatively smooth; the analogous folding is the thylakoid system floating in the stroma — separate flattened sacs stacked into grana, NOT connected to the inner membrane.
Implication: Both organelles maximise their membrane surface to embed many copies of the energy-transducing machinery (ETC or photosystem). They use different geometries to achieve the same goal. This is functional convergence with structural divergence.
Chloroplast inner membrane: Relatively smooth; the analogous folding is the thylakoid system floating in the stroma — separate flattened sacs stacked into grana, NOT connected to the inner membrane.
Implication: Both organelles maximise their membrane surface to embed many copies of the energy-transducing machinery (ETC or photosystem). They use different geometries to achieve the same goal. This is functional convergence with structural divergence.
Q5. Create: Design a quick demonstration to teach a younger student why mitochondria are called the 'powerhouse'. L6 Create
Step 1: Show two glucose tablets — one labelled 'fuel', the other 'mitochondrial input'. Explain glucose has chemical energy but cannot be used directly by cells.
Step 2: Place a small toy battery on the desk and call it 'ATP'. Compare: just as devices use batteries (small, portable, ready energy), cells use ATP.
Step 3: Show a candle ('combustion') vs. a slow-burning charcoal in an iron stove ('controlled, contained burning'). Explain that mitochondria do controlled 'burning' of glucose (oxidation) to harvest the energy as many ATPs — 36-38 per glucose vs 2 in cytoplasm alone.
Step 4: Use a paper folding craft to show how the inner membrane is folded (cristae) — a flat paper has less area than a folded paper. More folds = more 'sockets' to make ATP.
Conclude: glucose + O₂ → 36 ATPs in mitochondria. That's the 'power plant'!
Step 2: Place a small toy battery on the desk and call it 'ATP'. Compare: just as devices use batteries (small, portable, ready energy), cells use ATP.
Step 3: Show a candle ('combustion') vs. a slow-burning charcoal in an iron stove ('controlled, contained burning'). Explain that mitochondria do controlled 'burning' of glucose (oxidation) to harvest the energy as many ATPs — 36-38 per glucose vs 2 in cytoplasm alone.
Step 4: Use a paper folding craft to show how the inner membrane is folded (cristae) — a flat paper has less area than a folded paper. More folds = more 'sockets' to make ATP.
Conclude: glucose + O₂ → 36 ATPs in mitochondria. That's the 'power plant'!
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Mitochondria are considered semi-autonomous organelles.
R: They contain their own DNA and 70S ribosomes that can synthesise some of their own proteins.
Answer: (A). Both true; R explains A. Semi-autonomous = makes some but not all of its own proteins.
A: Cilia and flagella have a 9 + 2 axoneme.
R: Bacterial flagella also have a 9 + 2 axoneme.
Answer: (C). Assertion is true (for eukaryotic cilia/flagella). Reason is false — bacterial flagella are made of a single solid filament of flagellin, NOT microtubules.
A: Centrosomes are found in all eukaryotic cells.
R: Centrosomes are essential for spindle formation during cell division.
Answer: (D). Assertion is FALSE — plant cells almost completely lack centrosomes and yet divide successfully. Reason is also FALSE in a strict sense (plants form spindles without centrosomes). So D, with reason taken in its biological sense that spindle is essential for division (true) but not the centrosome specifically.
Frequently Asked Questions - Mitochondria Plastids Others
What is the main concept covered in Mitochondria Plastids Others?
In NCERT Class 11 Biology Chapter 8 (Cell: The Unit of Life), "Mitochondria Plastids Others" covers the core biological structures, functions, and classifications students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and ecological/physiological significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Mitochondria Plastids Others useful in real-life or applied biology?
Real-life applications of "Mitochondria Plastids Others" from NCERT Class 11 Biology Chapter 8 include medical diagnostics, agriculture, food preservation, biotechnology, ecological monitoring, and public health. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems, not just textbook content.
What are the key terms students should memorize for Mitochondria Plastids Others?
Key terms in "Mitochondria Plastids Others" (NCERT Class 11 Biology Chapter 8 Cell: The Unit of Life) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of Chapter 8?
NCERT Class 11 Biology Chapter 8 (Cell: The Unit of Life) is structured so each part builds biological understanding sequentially. "Mitochondria Plastids Others" connects to neighbouring parts via shared classifications, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Mitochondria Plastids Others?
CBSE board questions from "Mitochondria Plastids Others" typically include: (1) 1-mark MCQs on definitions and classification, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining structure + function + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Mitochondria Plastids Others" lesson allows students to explore biological structures, classifications, or processes using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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