NCERT grounding
NCERT Class 11 Biology, Chapter 8, opens Section 8.5 with a single defining sentence: “The eukaryotes include all the protists, plants, animals and fungi.” The same paragraph commits to the three features that NEET papers test repeatedly — extensive cytoplasmic compartmentalisation by membrane-bound organelles, an organised nucleus with a nuclear envelope, and genetic material organised into chromosomes. NIOS Senior Secondary Biology, Chapter 4 (Cell Structure and Function), expands the same idea under §4.2 “Components of the Cell” and §4.3 “The Cytoplasm and the Cell Organelles,” which is where most numerical/dimensional details (organelle sizes, ribosome S-values) come from.
What defines a eukaryotic cell
A eukaryotic cell is best defined not by its size or its shape — both vary enormously, from a 10 µm yeast bud to an ostrich egg — but by the presence of a true nucleus enclosed within a double-layered nuclear envelope, and by the partitioning of the cytoplasm into membrane-bound compartments each housing a specialised chemistry. Wherever you see a nuclear envelope with pores, an endoplasmic reticulum continuous with that envelope, a Golgi stack, and mitochondria with cristae, you are looking at a eukaryote.
The NCERT text formalises this by listing the four kingdoms that are eukaryotic — protists, plants, animals and fungi — and immediately contrasting them with the prokaryotic clade (bacteria, blue-green algae, mycoplasma, PPLO). This four-kingdom inclusion is itself a frequent matrix question: monera is prokaryotic; everything above it on the five-kingdom tree is eukaryotic. Eukaryotic cells are usually ten to a hundred times larger by linear dimension than a typical prokaryote — 10–20 µm versus 1–2 µm in NCERT Figure 8.2 — but size alone cannot be used to identify a eukaryote, because some giant prokaryotes overlap the lower end of the eukaryotic range.
The single most important additional feature is the presence of a cytoskeleton — a three-dimensional protein scaffold of microfilaments, intermediate filaments and microtubules — that no prokaryote possesses in the same elaborate form. The cytoskeleton is what allows large eukaryotic cells to move organelles around, divide using a spindle apparatus, and beat cilia or flagella with the characteristic 9+2 axoneme.
Eukaryotic Kingdoms (NCERT §8.5)
All protists, plants, animals and fungi are eukaryotic. Bacteria, blue-green (cyano-)algae, mycoplasma and PPLO are prokaryotic and are not in this list — a frequent assertion–reason hook in NEET papers.
Three non-negotiable features
Whenever a NEET stem asks “which of the following is characteristic of a eukaryotic cell?” the answer is one of these three. Memorise them as a triad — they are also the features by which the very first electron-microscope photographs of cells from the 1950s identified eukaryotes definitively.
Rule of thumb. Nuclear envelope ⇒ chromosomes ⇒ membrane-bound organelles. If even one of these is missing in the option, the cell is prokaryotic.
True nucleus
Double-membrane nuclear envelope with pores; outer membrane continuous with rough ER. Encloses chromatin and one or more nucleoli (rRNA factories).
Membrane-bound organelles
Mitochondria, ER, Golgi, lysosomes, vacuoles, peroxisomes; plastids additionally in plants and euglenoids. Each organelle is a chemically distinct compartment.
Chromosomes & cytoskeleton
Linear DNA wrapped on basic histone proteins (rich in lysine and arginine) forms chromosomes; a microfilament–IF–microtubule cytoskeleton organises the cytoplasm.
Figure 1. A schematic eukaryotic cell. The double-membrane nuclear envelope encloses a nucleolus; mitochondria show cristae; rough ER is studded with ribosomes; Golgi stacks cisternae for packaging; and faint cytoskeletal lines run through the cytoplasm. None of these compartments exist in a prokaryote.
Organelle inventory and roles
NCERT §8.5.3 to §8.5.11 walks through the eukaryotic organelle catalogue in a deliberate sequence — first the endomembrane system (ER, Golgi, lysosomes, vacuoles), then the semi-autonomous double-membrane organelles (mitochondria, plastids), then the non-membrane organelles (ribosomes, cytoskeleton), and finally the nucleus and microbodies. The grouping matters because NEET frequently builds match-the-column questions on it (NEET 2021 Q.167 asked exactly which organelles belong to the endomembrane system — answer: ER, Golgi, lysosomes and vacuoles only; mitochondria are excluded because their functions are not coordinated with the others).
| Organelle | Membrane | Principal function (NCERT) |
|---|---|---|
| Nucleus | Double, with pores | Houses chromatin/chromosomes; nucleolus synthesises ribosomal RNA. |
| Rough ER | Single | Protein synthesis (ribosomes attached), folding, glycosylation, signal-peptide cleavage. |
| Smooth ER | Single | Lipid and phospholipid synthesis; steroid hormones in animal cells. |
| Golgi apparatus | Single | Packaging of materials from ER; synthesis of glycoproteins and glycolipids; secretory vesicles bud from trans face. |
| Lysosome | Single | Hydrolases (lipases, proteases, carbohydrases, nucleases) active at acidic pH; intracellular digestion. |
| Vacuole | Single (tonoplast) | Storage of water, sap, ions, pigments (anthocyanins); osmotic regulation; up to 90% of plant cell volume. |
| Mitochondrion | Double, inner cristae | Aerobic respiration; ATP synthesis; semi-autonomous (circular DNA + 70S ribosomes). |
| Plastids (plants only) | Double, internal thylakoids | Chloroplasts trap light for photosynthesis; chromoplasts colour; leucoplasts store starch/oils/proteins. |
| Ribosomes | None | Protein synthesis; 80S free or on rough ER; 70S inside mitochondria/chloroplasts. |
| Cytoskeleton | None | Microfilaments + intermediate filaments + microtubules; mechanical support, motility, shape. |
| Centrosome & centrioles | None (proteinaceous) | Cartwheel structure of nine tubulin triplets; organise spindle in animal cell division; basal body of cilia/flagella. |
| Microbodies | Single | Vesicles containing specific enzymes (e.g., peroxisomes); present in both plant and animal cells. |
Note two NCERT-anchored exam hooks that follow from this table. First, only three classes of organelle contain DNA — the nucleus, mitochondrion and chloroplast. Lysosomes, vacuoles, ribosomes and the Golgi do not (NEET 2019 Q.29). Second, only two organelles are double-membrane-bound outside the nucleus — mitochondrion and chloroplast — and they are also the only two semi-autonomous organelles, because they divide by fission, hold their own ribosomes, and synthesise some of their own proteins (NEET 2016 Q.94).
Ribosomes, chromosomes and cytoskeleton
Three eukaryote-defining structures sit outside the membrane-bound organelle list but appear in NEET papers more often than any organelle: 80S/70S ribosomes, chromosomes built on histones, and the tripartite cytoskeleton.
Ribosomes — 80S in cytoplasm, 70S inside mitochondria and chloroplasts
Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits). However, mitochondria and chloroplasts each contain their own 70S ribosomes (50S + 30S subunits) — identical in S-value to those of bacteria. NCERT explicitly notes this on page 145 of Class 11. The conventional explanation is the endosymbiotic origin of these two organelles from once free-living α-proteobacteria and cyanobacteria respectively, retained ever since with their own ribosomes and circular DNA.
Cytoplasm / rough ER
80S
60S large + 40S small subunit
- Free in cytoplasm — synthesise soluble/cytosolic proteins
- Attached to rough ER — synthesise membrane/secreted proteins
- Several may translate one mRNA together (polyribosome)
Mitochondrion / chloroplast
70S
50S large + 30S small subunit
- Built inside the organelle; identical S-value to bacteria
- Translate organelle-encoded mRNAs (circular DNA matrix)
- Evidence for the endosymbiotic theory
Chromosomes — DNA on histones
Eukaryotic genomes are linear and partitioned into multiple chromosomes. NCERT records that a single human cell contains roughly two metres of DNA distributed across forty-six chromosomes (twenty-three pairs). DNA is wrapped on a histone octamer — a set of small, positively charged basic proteins enriched in lysine and arginine (the exact pair tested in NEET 2025 Q.111). Non-histone chromosomal proteins and RNA are also present in chromatin.
Every chromosome carries a primary constriction, the centromere, on which sit disc-shaped kinetochores. The position of the centromere classifies chromosomes into four NCERT types — metacentric, sub-metacentric, acrocentric and telocentric — a direct match-list question target in NEET 2022 Q.141. A non-staining secondary constriction may give the appearance of a small fragment called the satellite.
Cytoskeleton — three filament classes
The cytoskeleton is a network of three filament classes — microfilaments (actin), intermediate filaments, and microtubules (tubulin). NCERT lists three functional roles: mechanical support, motility, and maintenance of cell shape. Microtubules are particularly NEET-relevant — they are the structural component of spindle fibres, cilia, flagella and centrioles (NEET 2016 Q.93), and they form the 9+2 axoneme of eukaryotic cilia and flagella.
Plant vs animal eukaryotic cell
Within the eukaryotic clade, plant and animal cells are not identical. NCERT Figure 8.3 contrasts the two: the plant cell carries three structures absent in animals — a cellulosic cell wall, plastids, and a large central vacuole (with a tonoplast). The animal cell, in turn, carries centrioles (and a centrosome) that are absent in almost all plant cells, and possesses distinct lysosomes as its primary intracellular digestive compartment. This contrast is the most common five-mark stem in NCERT exercises and a frequent NEET trap distractor.
| Feature | Plant eukaryote | Animal eukaryote |
|---|---|---|
| Cell wall | Present — cellulose, hemicellulose, pectin, proteins | Absent |
| Plastids | Present — chloroplasts, chromoplasts, leucoplasts | Absent |
| Central vacuole | Single large vacuole — up to 90% of cell volume | Small, numerous, transient vacuoles |
| Centrioles / centrosome | Absent in almost all plant cells | Present — organise spindle in mitosis |
| Lysosomes | Less prominent; vacuolar hydrolases substitute | Distinct, numerous lysosomes |
| Storage carbohydrate | Starch (in leucoplasts/amyloplasts) | Glycogen |
| Plasmodesmata | Cytoplasmic bridges through cell wall | Absent — gap junctions instead |
Prokaryote vs eukaryote — clean reference table
Beyond the four-kingdom inclusion list and the nuclear envelope, NEET requires you to hold a clean comparison of the prokaryotic and eukaryotic templates. Use the table below as the canonical version — every row maps to a real NCERT sentence and at least one PYQ.
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Examples | Bacteria, blue-green algae, mycoplasma, PPLO | Protists, plants, animals, fungi |
| Typical size | 1–2 µm (PPLO ≈ 0.1 µm) | 10–20 µm |
| Nucleus | Absent — naked DNA in nucleoid; no envelope | True nucleus with double-membrane envelope and pores |
| Chromosome | Single circular DNA + plasmids; no histones | Multiple linear chromosomes; DNA wrapped on histones |
| Membrane-bound organelles | Absent (mesosome is an infolding, not an organelle) | Present — ER, Golgi, lysosomes, vacuoles, mitochondria, plastids |
| Ribosomes | 70S only (50S + 30S) | 80S in cytoplasm; 70S inside mitochondria/chloroplasts |
| Endoplasmic reticulum | Absent (both RER and SER absent) | RER + SER present |
| Cell wall | Peptidoglycan (most bacteria) | Cellulose in plants; chitin in fungi; absent in animals |
| Cytoskeleton | Rudimentary | Microfilaments + intermediate filaments + microtubules |
| Cell division | Binary fission | Mitosis / meiosis with spindle |
Worked examples
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). DNA in a eukaryotic cell occurs only in the nucleus, mitochondrion and chloroplast. Lysosomes (single-membrane hydrolase vesicles from Golgi) and vacuoles (tonoplast-bound storage compartments) hold no genome. The trap in option (1) is including mitochondria — which do contain DNA — so the pair fails the criterion. This question was asked verbatim in NEET 2019 Q.29.
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 option is correct?
(1) (b) is true but (a) is false (2) (a) is true but (b) is false (3) Both false (4) Both correct
Answer: (2). Both organelles are semi-autonomous — they have their own circular DNA and 70S ribosomes, so statement (a) is true. Statement (b) is false because they do possess protein-synthesising machinery — that is exactly what those 70S ribosomes are for. This is NEET 2016 Q.94 and is the canonical trap on the "semi-autonomous" idea.
Microtubules are the constituents of —
(1) Spindle fibres, Centrioles and Cilia (2) Centrioles, spindle fibres and chromatin (3) Centrosome, Nucleosome and Centrioles (4) Cilia, Flagella and Peroxisomes
Answer: (1). Microtubules (tubulin polymers) build spindle fibres, the nine-triplet centriole barrel, and the 9+2 axoneme of cilia and flagella. They do not form chromatin (DNA + histones) or nucleosomes (DNA + histone octamer) or peroxisomes (single-membrane microbodies). NEET 2016 Q.93.
Histones are enriched with — (1) Phenylalanine & Arginine (2) Lysine & Arginine (3) Leucine & Lysine (4) Phenylalanine & Leucine
Answer: (2). Histones are basic proteins because they carry a net positive charge that lets them grip DNA's phosphate backbone. The two basic amino acids contributing that charge are lysine and arginine. Phenylalanine and leucine are non-polar/aromatic and play no histone-charge role. Asked in NEET 2025 Q.111.
Common confusion & NEET traps
Two clusters of confusion repeatedly show up in NEET stems on eukaryotic cells. The first is the "endomembrane system" list — students wrongly include mitochondria because they are membranous. The second is the "semi-autonomous" phrase — students assume semi-autonomous means "no nuclear control," when it actually means the organelle simply retains its own DNA and ribosomes alongside.