📖 Chapter BN02 · NDA Class 11–12 Level🎯 NDA Level : High Priority
Cell Biology is the engine room of Biology in NDA. Questions consistently appear on cell organelle functions, prokaryote vs eukaryote differences, types of tissues, and stages of cell division. The organelle “powerhouse” and “suicide bag” nicknames, mitosis vs meiosis differences, and the cell cycle phases are directly tested — often with statement-based or match-the-column formats. Master the function of each organelle and the visible changes in each division stage.
📌 What to expect in NDA (based on 2022–2025 pattern): (1) Prokaryotic vs eukaryotic cells — key structural differences; (2) Cell organelle functions (especially mitochondria, chloroplast, ribosome, lysosome, nucleus); (3) Plant vs animal cell differences (cell wall, chloroplast, vacuole, centriole); (4) Animal and plant tissue types — names, subtypes, locations, and functions; (5) Mitosis stages (PMAT) and their visible events; significance; (6) Meiosis vs Mitosis key differences; significance of meiosis in sexual reproduction.
Topics at a Glance
① Cell Structure
Prokaryote vs Eukaryote; Plant vs Animal cell
② Cell Organelles
Structure & function of 9 key organelles
③ Tissues
Plant (meristematic/permanent) & Animal tissues
④ Cell Division
Cell cycle, Mitosis, Meiosis & comparison
1. Cell Structure: Prokaryotic vs Eukaryotic
1.1
Prokaryotic vs Eukaryotic Cells
The single most-tested structural concept in NDA Cell Biology — know every row of this comparison
Feature
Prokaryotic Cell
Eukaryotic Cell
Nuclear membrane
Absent (naked DNA)
Present (true nucleus)
Size
Small (1–10 µm)
Larger (10–100 µm)
Chromosome
Single circular DNA
Multiple linear chromosomes
Membrane-bound organelles
Absent (no mitochondria, ER etc.)
Present
Ribosomes
70S (smaller)
80S (larger); 70S in mitochondria/chloroplast
Cell wall
Present (peptidoglycan in bacteria)
Present in plants (cellulose); absent in animals
Plasmid
Often present
Absent
Examples
Bacteria, Cyanobacteria (Blue-green algae)
All animals, plants, fungi, protists
📌 NDA Trap: Mitochondria and Chloroplasts contain 70S ribosomes (same as prokaryotes) — this is evidence for the endosymbiotic theory (these organelles were once free-living bacteria). This is a favourite NDA statement-match trap.
1.2
Plant Cell vs Animal Cell
Differences directly tested — NDA often asks "which structure is absent in animal cells?"
Fig. 1 — Plant cell (left) vs Animal cell (right). Green ticks mark features unique to plant cells; red crosses mark features absent in animal cells. Centrioles are present only in animal cells.
🌿 Present in Plant Cell ONLY
Cell wall — made of cellulose; gives rigidity
Chloroplasts — site of photosynthesis; contain chlorophyll
Large central vacuole — maintains turgor pressure; stores water/salts
Plasmodesmata — cytoplasmic bridges between plant cells
🐘 Present in Animal Cell ONLY
Centrioles — form spindle fibres during cell division; absent in most plant cells
Lysosomes — more prominent; plant cells have vacuoles for digestion
Irregular shape — no rigid wall, flexible membrane
PYQTopic-Wise PYQs — Cell Structure
Q1. Which of the following is present in a prokaryotic cell but absent in a eukaryotic cell?
A. Ribosome
B. Cell membrane
C. Plasmid
D. DNA
Answer: C — Plasmid. Plasmids are small, circular, extrachromosomal DNA molecules found in bacteria (prokaryotes). Ribosomes, cell membranes, and DNA are present in both. This is a high-frequency NDA question — remember: plasmid = prokaryote exclusive.
Q2. Which structure is found in animal cells but NOT in plant cells?
A. Mitochondria
B. Endoplasmic reticulum
C. Centriole
D. Golgi apparatus
Answer: C — Centriole. Centrioles are present in animal cells and are essential for forming the spindle apparatus during cell division. Plant cells (except lower plants) lack centrioles but still divide using a different spindle formation mechanism.
Q3. The cell wall of bacteria is chemically composed of:
A. Cellulose
B. Chitin
C. Peptidoglycan (Murein)
D. Lignin
Answer: C — Peptidoglycan (Murein). Bacterial cell walls are made of peptidoglycan — a polymer of sugars and amino acids. Plant cells use cellulose, fungi use chitin, and lignin provides secondary wall reinforcement in woody plants. This distinction is tested frequently in NDA.
2. Cell Organelles — Structure & Functions
2.1
Key Organelles — Functions at a Glance
Nickname + one key function per organelle is the NDA minimum — aim to know all bullet points
🟡
Cell Membrane
"Gatekeeper of the cell"
Selectively permeable phospholipid bilayer
Controls entry/exit of substances (osmosis, diffusion, active transport)
Fluid mosaic model (Singer & Nicolson, 1972)
Made of phospholipids + proteins (cholesterol in animals)
🔵
Nucleus
"Control centre / Brain of the cell"
Surrounded by double membrane (nuclear envelope) with pores
Contains DNA → controls heredity and cell activities
Q4. Which organelle is known as the "powerhouse of the cell"?
A. Ribosome
B. Nucleus
C. Mitochondria
D. Golgi apparatus
Answer: C — Mitochondria. Mitochondria produce ATP (adenosine triphosphate) through cellular respiration, making them the primary energy source of the cell. The inner membrane folds (cristae) greatly increase the surface area for ATP synthesis.
Q5. The Golgi apparatus is primarily responsible for:
A. Protein synthesis
B. ATP production
C. Modification and packaging of proteins for secretion
D. DNA replication
Answer: C. The Golgi apparatus receives proteins from the rough ER, modifies them (adds sugar chains, sorts them), packages them into vesicles, and sends them to their destinations — inside the cell or for secretion. It acts like the cell's post office.
Q6. Lysosomes are also called "suicide bags" because:
A. They produce energy during cell death
B. They rupture and release digestive enzymes that destroy the cell
C. They contain genetic material that self-destructs
D. They are formed during apoptosis only
Answer: B. Lysosomes contain powerful hydrolytic (digestive) enzymes. When a cell is damaged or dying, lysosomes rupture and release these enzymes, which digest the cell's own contents — a process called autolysis. This earned them the name "suicide bag." Discovered by Christian de Duve.
Q7. Ribosomes in mitochondria are similar to ribosomes in:
A. Eukaryotic cytoplasm (80S)
B. Prokaryotic cells (70S)
C. Nucleus (40S)
D. Golgi apparatus
Answer: B — Prokaryotic cells (70S). Both mitochondria and chloroplasts contain 70S ribosomes — identical in size to bacterial ribosomes. This is the key evidence for the Endosymbiotic Theory (Lynn Margulis), which proposes that these organelles originated from ancient bacteria engulfed by early eukaryotic cells.
TRICKY🧐 Organelle Traps — Watch These
⚠️ "Chloroplast and mitochondria have their own DNA — does that make them prokaryotes?"
Key: No — they are not prokaryotes. They are organelles within eukaryotic cells. However, the fact that they possess their own circular DNA and 70S ribosomes strongly supports the Endosymbiotic Theory — that they evolved from ancient prokaryotic bacteria that were engulfed and formed a symbiotic relationship. NDA may frame this as: "Which organelle contains its own genetic material?" Answer: Both mitochondria AND chloroplast.
⚠️ "Mature human RBCs have no nucleus — are they dead?" NDA can ask this as a statement-true/false.
Key: Mature mammalian RBCs (Red Blood Cells) have no nucleus, no mitochondria, and no most organelles — they are pushed out during development to make more room for haemoglobin. They are still living cells, carrying out their function for ~120 days. This is unique to mammals; birds and reptiles have nucleated RBCs.
3. Tissues — Plant and Animal
3.1
Plant Tissues
Meristematic vs Permanent — a recurring NDA topic, especially with examples
Derived from meristematic tissue; have lost ability to divide
Parenchyma: thin-walled; storage and basic functions; forms bulk of plant body
Collenchyma: unevenly thick walls; provides flexibility and support (celery strings)
Sclerenchyma: dead at maturity; thick lignified walls; provides rigid support (coconut husk fibres)
Xylem: conducts water upward; dead cells (tracheids, vessels)
Phloem: conducts food (sugars) both ways; living cells
📌 Xylem vs Phloem — NDA Favourite Pair:
Xylem: water + minerals, upward only, dead cells, made of tracheids & vessels
Phloem: food (sugars), bidirectional, living cells, made of sieve tubes & companion cells
Together they form the vascular bundle — the plant's transport system.
3.2
Animal Tissues — Four Main Types
Location + one key function for each subtype — sufficient for NDA
️ Epithelial Tissue
Skin, gut lining, lung alveoli, kidney tubules
Covers body surfaces and lines cavities
Tightly packed cells with little intercellular substance
Glia cells (astrocytes, microglia): support neurons; DO divide
🧠 Tendon vs Ligament (NDA classic confusion): Tendon connects Muscle → Bone (T for Tight, M to B) Ligament connects Bone → Bone (L for Linking bones)
Both are dense fibrous connective tissue; both made of collagen fibres; tendons are less elastic.
PYQTopic-Wise PYQs — Tissues
Q8. Which of the following is a fluid connective tissue?
A. Cartilage
B. Blood
C. Bone
D. Adipose tissue
Answer: B — Blood. Blood is classified as a fluid connective tissue because it has a fluid matrix (plasma) and contains cells (RBCs, WBCs, platelets) that are suspended in it. Bone and cartilage are rigid or semi-rigid connective tissues. Adipose tissue stores fat.
Q9. Cardiac muscle is different from skeletal muscle in that it is:
A. Non-striated and voluntary
B. Striated and involuntary
C. Non-striated and involuntary
D. Striated and voluntary
Answer: B — Striated and involuntary. Cardiac muscle has striations (like skeletal muscle) but is NOT under voluntary control (like smooth muscle). It is unique — the only striated involuntary muscle. Its cells are branched and connected by intercalated discs for synchronised contractions.
Q10. The apical meristem in plants is responsible for:
A. Increase in girth (thickness)
B. Primary growth in length
C. Regeneration after injury
D. Formation of bark
Answer: B — Primary growth in length. Apical meristem is located at the tips of roots and shoots and is responsible for elongation (primary growth). Lateral meristem (cambium) causes secondary growth (increase in girth). Intercalary meristem (at leaf base/internodes) helps regrowth after grazing.
TRICKY🧐 Tissue Traps
⚠️ "Neurons cannot divide — so are they permanent tissue?" NDA can twist this concept.
Key: Neurons (nerve cells) are highly specialised and do NOT divide after birth in most regions of the brain. This is why brain injuries are often permanent. However, glial cells (supporting cells of the nervous system) CAN divide. NDA may ask: "Which cells of the nervous system can undergo mitosis?" — Answer: Glial cells / Schwann cells, NOT neurons.
⚠️ "Xylem conducts water only upward — true or false?"
Key: TRUE for xylem — water and dissolved minerals travel from root → stem → leaves via xylem, always upward (unidirectional). Phloem is bidirectional — it carries sucrose from leaves downward to roots and also upward to growing tips. This phloem bidirectionality is a repeated NDA concept. Mechanism: xylem = transpiration pull; phloem = pressure flow.
4. Cell Division — Cell Cycle, Mitosis & Meiosis
4.1
The Cell Cycle
Interphase (longest phase) + M phase (mitosis/meiosis) — know the sub-phases of interphase
Fig. 2 — The Cell Cycle: Interphase (G1 → S → G2) occupies ~90% of the cycle. G0 is the resting phase (e.g. neurons). M phase = actual division (Karyokinesis + Cytokinesis).
📚 G1 Phase (Gap 1)
Cell grows in size
Proteins and RNA synthesised
Organelles replicate
Longest sub-phase in most cells
Cell checks if ready to divide (checkpoint)
📚 S Phase (Synthesis)
DNA replication occurs
DNA content doubles (2N → 4N)
Histone proteins synthesised
Chromosome number stays same
Each chromosome becomes 2 chromatids
📚 G2 Phase (Gap 2)
Cell continues to grow
Proteins for spindle synthesised
Final preparations for division
DNA repair if errors found
Shortest of the three interphase sub-phases
4.2
Mitosis — Phases and Events (PMAT)
Each phase has a visible event — NDA tests which event occurs in which phase
INTERPHASE (Prep)
DNA replication complete. Chromosomes not yet visible (chromatin form). Centrioles duplicate. Cell at maximum metabolic activity. Key: chromosomes are NOT visible in interphase — a common NDA trap.
PROPHASE (P)
Chromosomes condense and become visible. Spindle fibres begin to form. Nuclear envelope starts to dissolve. Nucleolus disappears. Centrioles move to opposite poles. Mnemonic: P = Prepare and Pack chromosomes.
METAPHASE (M)
Chromosomes align at the cell equator (metaphase plate). Spindle fibres attach to centromeres. This is the best phase to count and study chromosomes (most condensed, clearly visible). Mnemonic: M = Middle.
ANAPHASE (A)
Sister chromatids separate and move to opposite poles. Centromeres split. Cell elongates. Chromatids are pulled by spindle fibres. Mnemonic: A = Apart — chromatids move Apart. Chromosome number briefly appears doubled.
TELOPHASE (T)
Chromosomes reach poles; nuclear envelope reforms. Chromosomes decondense. Nucleolus reappears. Spindle fibres disappear. Cytokinesis begins (cleavage furrow in animal; cell plate in plant). Mnemonic: T = Two nuclei formed.
Before mitosis: 2N (diploid)
→
After mitosis: 2 × 2N (two diploid cells)
Human: 46 → 46 + 46
🍀 Significance of Mitosis:
(1) Growth of multicellular organisms from a single cell (zygote)
(2) Repair and regeneration of worn-out cells (skin, gut lining)
(3) Vegetative reproduction in plants (clones)
(4) Maintains chromosome number (daughter cells = parent cell in chromosome number)
(5) Asexual reproduction in unicellular organisms
4.3
Meiosis — Reductive Division
Two successive divisions; produces haploid gametes — NDA asks about Meiosis I crossing over
MEIOSIS I (Reductive)
Homologous chromosome pairs separate. During Prophase I: crossing over occurs at chiasmata — exchange of segments between homologous chromosomes → genetic variation. Result: 2 haploid cells (N). This is the most complex and most tested division in meiosis.
MEIOSIS II (Equational)
Similar to mitosis. Sister chromatids separate. No DNA replication between Meiosis I and II. Result: 4 haploid cells (N). In humans: 4 cells each with 23 chromosomes. In males → 4 sperms; in females → 1 egg + 3 polar bodies.
Before Meiosis: 2N (46 in human)
→
After Meiosis: 4 × N (23 each)
Total DNA: halved
📚 Key Events of Meiosis I Prophase
Leptotene: chromosomes start condensing
Zygotene: homologous chromosomes pair (synapsis); bivalent forms
Pachytene: crossing over occurs at chiasmata
Diplotene: homologs begin to separate; chiasmata visible
Q11. In which phase of mitosis are chromosomes best studied (most clearly visible)?
A. Prophase
B. Anaphase
C. Metaphase
D. Telophase
Answer: C — Metaphase. During Metaphase, chromosomes are maximally condensed (shortest and thickest) and aligned at the equatorial plate (metaphase plate). This makes them clearly visible and easy to count. This is why karyotyping (chromosome study) uses metaphase spreads.
Q12. Crossing over during meiosis is significant because it:
A. Reduces chromosome number by half
B. Creates genetic variation by exchanging segments between homologous chromosomes
C. Produces four identical gametes
D. Occurs during S phase of interphase
Answer: B. Crossing over (recombination) occurs during Prophase I of Meiosis at structures called chiasmata. Segments of maternal and paternal chromosomes are exchanged, creating new combinations of alleles. This is the primary source of genetic variation — essential for evolution and was identified by Morgan and Bridges.
Q13. DNA replication occurs during which phase of the cell cycle?
A. G1 phase
B. G2 phase
C. S phase
D. M phase
Answer: C — S phase (Synthesis phase). The S phase is specifically dedicated to DNA synthesis (replication). The DNA content doubles during this phase (2C → 4C), even though the chromosome number does not change. Each chromosome now consists of two identical sister chromatids joined at the centromere.
Q14. The number of chromosomes in a human sperm cell is:
A. 46
B. 23
C. 92
D. 48
Answer: B — 23. Human somatic (body) cells are diploid (2N = 46 chromosomes). Gametes (sperm and egg) are haploid (N = 23), formed by meiosis. At fertilisation, sperm (23) + egg (23) = zygote (46). Chimpanzees have 48 chromosomes — another NDA trap.
TRICKY🧐 Cell Division Traps
⚠️ "After S phase, each chromosome has 2 chromatids — so the chromosome number has doubled." True or False?
False. After S phase, the DNA content doubles (2C → 4C) but the chromosome NUMBER stays the same. Each chromosome now consists of two sister chromatids joined at the centromere — they are still counted as ONE chromosome. The chromosome number only doubles transiently during Anaphase (when centromeres split), and then returns to normal when daughter cells are formed. NDA often tests: "After S phase, how many chromosomes does the cell have?" Answer: Same as before (e.g., 46 in human).
⚠️ "Meiosis II is similar to mitosis" — In what way? NDA asked this as a match-the-statement question.
Key: Meiosis II is called the equational division because, like mitosis, sister chromatids separate to opposite poles (not homologous chromosomes like in Meiosis I). However, the key difference is that Meiosis II starts with haploid cells (N), not diploid cells. No DNA replication occurs between Meiosis I and II. So it resembles mitosis in mechanism but is NOT the same in ploidy level of starting/ending cells.
📄 Quick-Reference Fact Sheet — BN02
🔵 Prokaryote vs Eukaryote Keys
Prokaryote: NO nuclear membrane, NO membrane-bound organelles, 70S ribosomes
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