Cell: The Unit of Life — Part B (NEET Biology Class 11): Organelles & the Nucleus

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

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

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

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

Q3. The Golgi apparatus was first described by:

• (a) Camillo Golgi

• (b) Rudolf Virchow

• (c) Robert Brown

• (d) George Palade

Q4. Hydrolytic enzymes inside a lysosome work best at:

• (a) Alkaline pH

• (b) Neutral pH

• (c) Any pH equally

• (d) Acidic pH

Q5. The single membrane bounding a plant cell's vacuole is called the:

• (a) Sarcolemma

• (b) Plasmalemma

• (c) Tonoplast

• (d) Nuclear envelope

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

Q7. The infoldings of the inner mitochondrial membrane are called:

• (a) Cisternae

• (b) Cristae

• (c) Thylakoids

• (d) Grana

Q8. A leucoplast that stores starch is specifically called a/an:

• (a) Amyloplast

• (b) Elaioplast

• (c) Aleuroplast

• (d) Chromoplast

Q9. Grana in a chloroplast are stacks of:

• (a) Ribosomes

• (b) Cisternae

• (c) Cristae

• (d) Thylakoids

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

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

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

NEET Previous Year Questions (PYQs)

Real NEET previous-year questions on this chapter, with explanations in our own words.

Q13. The organelle that is NOT considered part of the endomembrane system is: (NEET PYQ)

• (a) Endoplasmic reticulum

• (b) Peroxisome

• (c) Lysosome

• (d) Golgi apparatus

Q14. 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

Q15. The site of light reactions within a chloroplast is the: (NEET PYQ)

• (a) Stroma

• (b) Matrix

• (c) Outer membrane

• (d) Grana (thylakoids)

Q16. 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

Q17. 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

Q18. 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

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.