# Cell: The Unit of Life — Part B (NEET Biology Class 11): Organelles & the Nucleus Source: https://tracknation.in/story/cell-the-unit-of-life-part-b-neet-biology-class-11-organelles-the-nucleus (TrackNation — free UPSC notes with PYQs) > NEET Class 11 Cell: The Unit of Life, Part B, mined from NCERT: the endomembrane system (ER, Golgi apparatus, lysosomes, vacuoles), mitochondria and plastids (chloroplast/chromoplast/leucoplast structure), ribosomes (80S vs 70S), the cytoskeleton, cilia/flagella (9+2 array), the centrosome, and the nucleus and chromosome structure (centromere, kinetochore, metacentric/sub-metacentric/acrocentric/telocentric). With 12 MCQs, 6 NEET PYQs and an Active Recall prompt. ## Endomembrane System: ER, Golgi, Lysosomes & Vacuoles 🎯 **NEET priority: High-yield.** *5–7 questions every year — this is Part B, the organelle-heavy half of the chapter. Mitochondria, plastids, ribosomes and the nucleus are among the most repeated single-fact questions in all of NEET Biology.* The **endomembrane system** is the set of organelles whose functions are coordinated together: **endoplasmic reticulum (ER), Golgi apparatus, lysosomes and vacuoles**. Mitochondria, plastids and peroxisomes are **NOT** part of the endomembrane system — their functions aren't coordinated with it. ### Endoplasmic Reticulum & Golgi apparatus - ER is a network of tubules dividing the cell into a **luminal** (inside ER) and **extra-luminal** (cytoplasm) space. **Rough ER (RER)** bears ribosomes and is abundant in cells active in protein synthesis/secretion; **smooth ER (SER)** lacks ribosomes and is the major site of **lipid synthesis** (and, in animal cells, steroidal hormones). - **Camillo Golgi (1898)** first described the Golgi apparatus: stacked, flat disc-shaped **cisternae** near the nucleus, with a convex **cis (forming) face** and a concave **trans (maturing) face**. Vesicles from the ER fuse at the cis face; the Golgi **packages** material (and modifies ER-made proteins) and is the chief site of **glycoprotein and glycolipid** formation. ### Lysosomes & Vacuoles - **Lysosomes** are membrane-bound vesicles packaged by the Golgi, rich in **hydrolytic enzymes** (lipases, proteases, carbohydrases) that work best at **acidic pH** and can digest carbohydrates, proteins, lipids and nucleic acids. - A **vacuole** is bound by a single membrane called the **tonoplast**; in plant cells it can occupy **up to 90 per cent** of cell volume, and the tonoplast actively concentrates ions/materials inside against a gradient. In **Amoeba**, the **contractile vacuole** handles osmoregulation and excretion; **food vacuoles** form in protists by engulfing food particles. ## Mitochondria & Plastids — the Semi-Autonomous Organelles ### Mitochondria — the "power house" - Mitochondria are **double membrane-bound**: the outer membrane is the smooth continuous boundary; the inner membrane folds inward into **cristae**, which increase surface area. The inner compartment is filled with a dense **matrix**. - Mitochondria are the site of **aerobic respiration** and produce cellular energy as **ATP** — hence "power houses of the cell." The matrix has its own **single circular DNA**, some RNA, and **70S ribosomes** — mitochondria can make some of their own proteins and **divide by fission**, independent of the cell cycle. ### Plastids — chloroplast, chromoplast, leucoplast - Plastids occur in **all plant cells and euglenoids**, classified by pigment into three types: **chloroplasts** (chlorophyll + carotenoids, trap light for photosynthesis), **chromoplasts** (fat-soluble carotenoids — carotene, xanthophylls — give yellow/orange/red colour), and **leucoplasts** (colourless, store nutrients): **amyloplasts** store starch (e.g. potato), **elaioplasts** store oils and fats, **aleuroplasts** store proteins. - Like mitochondria, chloroplasts are **double membrane-bound**. The space inside the inner membrane is the **stroma**, containing flattened sacs called **thylakoids**, stacked like coins into **grana** (singular granum) and linked between grana by **stroma lamellae**. Chlorophyll sits in the thylakoids; the stroma holds enzymes for carbohydrate/protein synthesis plus its own **circular DNA and 70S ribosomes** — smaller than the **80S** ribosomes in the surrounding cytoplasm. ### Mitochondria vs chloroplast — the semi-autonomous organelles, side by side | **Feature** | **Mitochondria** | **Chloroplast** | | Membrane | Double — outer smooth, inner folded into cristae | Double — inner encloses the stroma | | Inner compartment | Matrix | Stroma, with stacked thylakoids (grana) | | Own DNA/ribosomes | Circular DNA + 70S ribosomes | Circular DNA + 70S ribosomes | | Function | Aerobic respiration, produces ATP | Photosynthesis, traps light energy | | Found in | Almost all eukaryotic cells | Plant cells and euglenoids only | ## Ribosomes, Cytoskeleton, Cilia/Flagella & Centrosome ### Ribosomes, cytoskeleton, cilia/flagella & centrosome - Ribosomes were first seen by **George Palade (1953)** under the electron microscope; they are made of **rRNA and protein**, and are **NOT membrane-bound**. **Eukaryotic** ribosomes are **80S (60S + 40S subunits)**; **prokaryotic** ribosomes (and those inside mitochondria/chloroplasts) are **70S (50S + 30S)**. "S" is the **Svedberg unit** — a sedimentation coefficient, not a simple size measure (so 80S ≠ 70S + 10S). - The **cytoskeleton** — microtubules, microfilaments and intermediate filaments — gives mechanical support, motility and shape. **Cilia** (short, oar-like) and **flagella** (longer, for cell movement) are membrane-covered outgrowths whose core, the **axoneme**, has a **9+2 array**: **9 peripheral microtubule doublets** around **2 central microtubules**, linked by radial spokes; both emerge from a **basal body**. - The **centrosome** usually has **two centrioles** lying perpendicular to each other, each a cartwheel of **nine peripheral triplet fibrils** of tubulin. Centrioles form the **basal body** of cilia/flagella and, in animal cells, the **spindle apparatus** during cell division. Centrioles are absent in almost all plant cells. ## The Nucleus & Chromosomes, and Why This Matters for NEET ### Organelle quick-reference — the match-the-pair table | **Organelle** | **Membrane** | **Signature function** | **The NEET hook** | | Rough ER | Single | Protein synthesis and transport | Bears ribosomes on its surface | | Smooth ER | Single | Lipid and steroid synthesis | NO ribosomes; abundant in liver cells | | Golgi apparatus | Single | Packaging, glycosylation, secretion | Cis face receives, trans face ships out | | Lysosome | Single | Intracellular digestion | 'Suicide bag'; hydrolases work at acidic pH | | Mitochondrion | Double | Aerobic respiration, ATP | Cristae + 70S ribosomes + circular DNA | | Chloroplast | Double | Photosynthesis | Grana + stroma + 70S ribosomes + circular DNA | | Ribosome | None | Protein synthesis | 80S in cytoplasm, 70S in mitochondria/plastids | | Peroxisome | Single | Breaks down H₂O₂ via catalase | Photorespiration in plants | | Glyoxysome | Single | Converts fat to carbohydrate | Only in fat-storing plant seeds | | Centriole | None | Forms spindle and basal bodies | 9 + 0 cartwheel — NOT 9 + 2 | **Exam trap.** **Centriole is 9 + 0; cilium and flagellum are 9 + 2.** NEET swaps these two arrays constantly. The other perennial swap is the ribosome pair: the cytoplasm has **80S**, but mitochondria and chloroplasts carry **70S** — the evidence for their bacterial ancestry. ### Nucleus & chromosomes - **Robert Brown (1831)** first described the nucleus; its stained material was later named **chromatin** by Flemming. A double-membraned **nuclear envelope** (with a **perinuclear space** between the two membranes, continuous with the ER and bearing ribosomes) separates the nucleus from the cytoplasm, interrupted by **nuclear pores** that let RNA and proteins pass both ways. **Mature RBCs** (in many mammals) and **sieve tube cells** lack a nucleus entirely. - Inside: **chromatin** (loose nucleoprotein fibres of DNA + histone and non-histone proteins + RNA) and one or more **nucleoli** — NOT membrane-bound, and the site of **ribosomal RNA synthesis**. A single human cell packs about **2 metre** of DNA into **46 chromosomes (23 pairs)**. - During division, chromatin condenses into visible **chromosomes**, each with a primary constriction, the **centromere**, flanked by disc-shaped **kinetochores**. By centromere position, chromosomes are **metacentric** (middle, equal arms), **sub-metacentric** (slightly off centre, unequal arms), **acrocentric** (near one end, one very long + one very short arm) or **telocentric** (terminal centromere). A non-staining secondary constriction can appear as a small fragment called a **satellite**. ### Why this matters for NEET - High-value one-liners: mitochondria & chloroplasts are **double membrane-bound, semi-autonomous** (own circular DNA + 70S ribosomes); ER divides into **RER (protein) vs SER (lipid)**; Golgi has a **cis (forming) and trans (maturing)** face; nucleolus is **not membrane-bound**; four chromosome types by centromere position. - **Trap:** mitochondria/plastids/peroxisomes are **NOT part of the endomembrane system** — only ER, Golgi, lysosomes and vacuoles are. Eukaryotic cytoplasmic ribosomes are **80S**, but the ribosomes inside a mitochondrion or chloroplast are **70S**, same as a bacterium's — a classic "which organelle is this describing" trap. Cilia/flagella use a **9+2** array, NOT 9+0 (that's the sperm's basal body/centriole pattern in some other contexts) — don't confuse the axoneme count with the centriole's **nine triplets** (no central pair in a centriole). ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** Which of the following is NOT part of the endomembrane system? - (a) Lysosomes - (b) Golgi apparatus - (c) Mitochondria - (d) Endoplasmic reticulum **Answer: (c)** — Mitochondria, plastids and peroxisomes are excluded from the endomembrane system since their functions aren't coordinated with ER/Golgi/lysosomes/vacuoles. **Q2.** Rough endoplasmic reticulum is abundant in cells that are actively involved in: - (a) Photosynthesis - (b) Digestion - (c) Protein synthesis and secretion - (d) Lipid synthesis only **Answer: (c)** — RER bears ribosomes and is extensive in cells actively making and secreting proteins. **Q3.** The Golgi apparatus was first described by: - (a) Camillo Golgi - (b) Rudolf Virchow - (c) Robert Brown - (d) George Palade **Answer: (a)** — Camillo Golgi (1898) first observed the densely stained reticular structures later named after him. **Q4.** Hydrolytic enzymes inside a lysosome work best at: - (a) Alkaline pH - (b) Neutral pH - (c) Any pH equally - (d) Acidic pH **Answer: (d)** — Lysosomal hydrolases (lipases, proteases, carbohydrases) are optimally active at acidic pH. **Q5.** The single membrane bounding a plant cell's vacuole is called the: - (a) Sarcolemma - (b) Plasmalemma - (c) Tonoplast - (d) Nuclear envelope **Answer: (c)** — The vacuole is bound by a single membrane, the tonoplast, which can also concentrate solutes against a gradient. **Q6.** Mitochondria are called the 'power house of the cell' because they: - (a) Synthesise lipids - (b) Store starch - (c) Digest macromolecules - (d) Produce ATP through aerobic respiration **Answer: (d)** — Mitochondria are the site of aerobic respiration, generating ATP for cellular energy. **Q7.** The infoldings of the inner mitochondrial membrane are called: - (a) Cisternae - (b) Cristae - (c) Thylakoids - (d) Grana **Answer: (b)** — The inner mitochondrial membrane folds inward to form cristae, increasing surface area. **Q8.** A leucoplast that stores starch is specifically called a/an: - (a) Amyloplast - (b) Elaioplast - (c) Aleuroplast - (d) Chromoplast **Answer: (a)** — Amyloplasts store carbohydrates as starch, e.g. in potato; elaioplasts store oils/fats and aleuroplasts store proteins. **Q9.** Grana in a chloroplast are stacks of: - (a) Ribosomes - (b) Cisternae - (c) Cristae - (d) Thylakoids **Answer: (d)** — Thylakoids, flattened membranous sacs in the stroma, are stacked like coins into grana. **Q10.** Eukaryotic cytoplasmic ribosomes are: - (a) 50S only - (b) 60S only - (c) 70S, made of 50S and 30S - (d) 80S, made of 60S and 40S **Answer: (d)** — Eukaryotic ribosomes are 80S (60S+40S); prokaryotic ribosomes, and those inside mitochondria/chloroplasts, are 70S (50S+30S). **Q11.** The axoneme of a cilium or flagellum shows which arrangement of microtubules? - (a) 9 + 0 - (b) 9 + 2 - (c) 11 + 0 - (d) 2 + 9 **Answer: (b)** — The axoneme has 9 peripheral microtubule doublets surrounding 2 central microtubules — the 9+2 array. **Q12.** A chromosome with a centromere positioned near one end, giving one very long and one very short arm, is: - (a) Acrocentric - (b) Sub-metacentric - (c) Telocentric - (d) Metacentric **Answer: (a)** — Acrocentric chromosomes have the centromere close to one end; telocentric has a strictly terminal centromere. **Q13.** Which organelle is called the 'suicide bag' of the cell, and why? - (a) Golgi body — it destroys damaged proteins - (b) Peroxisome — it accumulates hydrogen peroxide - (c) Lysosome — it is packed with hydrolytic enzymes that can digest the cell itself - (d) Mitochondrion — it releases heat **Answer: (c)** — Lysosomes are formed by the Golgi and are filled with hydrolases — lipases, proteases, carbohydrases — that work at acidic pH. If the membrane ruptures, these enzymes digest the cell's own contents, hence the name. **Q14.** The cis face and trans face of the Golgi apparatus are respectively: - (a) The maturing face and the forming face - (b) The forming face receiving vesicles, and the maturing face shipping them out - (c) Both faces receive vesicles from the ER - (d) Both faces are attached to the nuclear envelope **Answer: (b)** — Material enters at the convex cis (forming) face nearest the ER and leaves from the concave trans (maturing) face. The two faces are entirely different in composition — the Golgi is the cell's dispatch department. **Q15.** Which of the following is NOT part of the endomembrane system? - (a) Lysosome - (b) Endoplasmic reticulum - (c) Golgi complex - (d) Mitochondrion **Answer: (d)** — The endomembrane system comprises the ER, Golgi complex, lysosomes and vacuoles — organelles whose functions are coordinated. Mitochondria, chloroplasts and peroxisomes are excluded because their functions are not coordinated with this system. **Q16.** The enzyme catalase, which breaks down hydrogen peroxide, is characteristic of: - (a) Lysosomes - (b) Glyoxysomes - (c) Peroxisomes - (d) Ribosomes **Answer: (c)** — Peroxisomes contain catalase to detoxify hydrogen peroxide, a toxic by-product of oxidative metabolism. In plants they also carry out photorespiration. Glyoxysomes are the separate fat-to-carbohydrate bodies of germinating oilseeds. **Q17.** Smooth endoplasmic reticulum differs from rough ER in that it: - (a) Lacks ribosomes and is the site of lipid and steroid synthesis - (b) Bears ribosomes and synthesises proteins - (c) Is found only in prokaryotes - (d) Is bounded by a double membrane **Answer: (a)** — Smooth ER has no ribosomes on its surface and specialises in lipid and steroid synthesis — which is why it is abundant in liver cells and in steroid-secreting glands. Rough ER, studded with ribosomes, handles protein synthesis and transport. **Q18.** Which structures in a chloroplast contain the chlorophyll pigment? - (a) The intermembrane space - (b) Thylakoid membranes stacked as grana - (c) The stroma - (d) The outer membrane **Answer: (b)** — Chlorophyll sits in the thylakoid membranes, which stack into grana — this is where the light reactions occur. The stroma is the surrounding fluid where the dark (carbon-fixing) reactions take place, and it also holds circular DNA and 70S ribosomes. **Q19.** The theory that mitochondria and chloroplasts evolved from free-living prokaryotes is supported by all of the following EXCEPT: - (a) Their enclosure by a single membrane - (b) Their ability to divide independently of the cell - (c) The presence of 70S ribosomes - (d) The presence of circular naked DNA **Answer: (a)** — Both organelles are bounded by a DOUBLE membrane, not a single one — and that double membrane is itself part of the endosymbiotic evidence. Circular DNA, 70S ribosomes and independent division are the other three classic supports. **Q20.** In a metacentric chromosome, the centromere is located: - (a) Slightly off centre, forming unequal arms - (b) In the middle, forming two equal arms - (c) Near one end, forming one long and one short arm - (d) At the very terminal end **Answer: (b)** — Metacentric means a middle centromere and two equal arms. Sub-metacentric is slightly off centre giving unequal arms, acrocentric places it near one end, and telocentric puts it strictly at the terminal end. **Q21.** In the mitochondrion shown, the enzymes of the Krebs (TCA) cycle are located in the region marked - (a) A - (b) E - (c) D - (d) B **Answer: (c)** — D is the matrix, which contains the enzymes of the Krebs cycle, the mitochondrial DNA, ribosomes and the enzymes for fatty-acid oxidation. The electron transport chain sits on the inner membrane (B and its folds). **Q22.** The space marked E in the figure, lying between the two membranes, is called the - (a) Matrix - (b) Intermembrane (perimitochondrial) space - (c) Stroma - (d) Lumen of the cristae **Answer: (b)** — A mitochondrion has two membranes separated by an intermembrane space. The matrix lies inside the inner membrane; the stroma is a chloroplast term. **Q23.** The finger-like structures marked C, which increase the surface area for ATP synthesis, are folds of the membrane marked - (a) D - (b) E - (c) B - (d) A **Answer: (c)** — Cristae [C] are infoldings of the inner mitochondrial membrane [B]. They carry the electron transport chain and ATP synthase, so folding increases the area available. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q24.** The organelle that is NOT considered part of the endomembrane system is: (NEET PYQ) - (a) Endoplasmic reticulum - (b) Peroxisome - (c) Lysosome - (d) Golgi apparatus **Answer: (b)** — Peroxisomes (like mitochondria and plastids) are excluded from the endomembrane system. **Q25.** Mitochondria and chloroplasts both possess: (NEET PYQ) - (a) A single membrane and 80S ribosomes - (b) Only a single membrane - (c) No DNA of their own - (d) A double membrane, their own circular DNA and 70S ribosomes **Answer: (d)** — Both are double membrane-bound, semi-autonomous organelles with their own circular DNA and 70S ribosomes. **Q26.** The site of light reactions within a chloroplast is the: (NEET PYQ) - (a) Stroma - (b) Matrix - (c) Outer membrane - (d) Grana (thylakoids) **Answer: (d)** — Light reactions occur in the thylakoid membranes stacked as grana; dark reactions (Calvin cycle) occur in the stroma. **Q27.** Nucleolus is best described as: (NEET PYQ) - (a) A type of chromatin only found during division - (b) The outer layer of the nucleus - (c) A membrane-bound organelle for lipid storage - (d) A non-membrane bound site of ribosomal RNA synthesis **Answer: (d)** — The nucleolus is not membrane-bound and is the site of active ribosomal RNA synthesis. **Q28.** A chromosome with the centromere exactly in the middle, forming two equal arms, is called: (NEET PYQ) - (a) Sub-metacentric - (b) Acrocentric - (c) Metacentric - (d) Telocentric **Answer: (c)** — Metacentric chromosomes have a middle centromere giving two equal arms. **Q29.** Centrioles in an animal cell give rise to which structure during cell division? (NEET PYQ) - (a) Golgi apparatus - (b) Nucleolus - (c) Cell wall - (d) Spindle apparatus **Answer: (d)** — Centrioles form the spindle apparatus that helps segregate chromosomes during animal cell division. ### Active Recall Prompt Write everything you can recall about **Cell: The Unit of Life (Part B)**, naming each part first: the **endomembrane system** (which organelles are IN it and which are NOT); **ER** (RER vs SER) and the **Golgi apparatus** (cis/trans face); **lysosomes** and **vacuoles** (tonoplast, contractile/food vacuoles); **mitochondria** (structure, function, own DNA/ribosomes); **plastids** (the three types, chloroplast structure — grana, stroma, thylakoids); **ribosomes** (80S vs 70S, subunits); the **cytoskeleton**, **cilia/flagella** (9+2 array) and the **centrosome** (centrioles); and the **nucleus** (envelope, pores, nucleolus, chromatin, and the four chromosome types by centromere position). Begin each fact with its topic and end it with a full stop.