Cell - Structure and Function PYQs – Biology, SSC & Railway

Correct Answer: Option 3 — It has a single chromosome.
A prokaryotic cell is a primitive cell that has no true nucleus. Its genetic material — a single, circular chromosome of naked DNA — lies free in the cytoplasm in a region called the nucleoid, which has no nuclear membrane and no nucleolus.
Prokaryotes also lack all membrane-bound organelles such as mitochondria, plastids, Golgi apparatus, endoplasmic reticulum and lysosomes. Only 70S ribosomes are present. Bacteria, cyanobacteria (blue-green algae), mycoplasma and archaebacteria are prokaryotes.
❌ Why the other options are incorrect
✗ It has more than one chromosome — wrong; multiple chromosomes are a feature of eukaryotic cells. A bacterium has only one chromosome (plus small plasmids).
✗ Nuclear region is well defined and surrounded by nuclear membrane — wrong; this describes the true nucleus of a eukaryotic cell. A prokaryote has only an undefined nucleoid.
✗ Membrane bound cell organelles are present — wrong; membrane-bound organelles are absent in prokaryotes and are a defining feature of eukaryotes.
📌 SSC & Railway Exam Facts
• The words prokaryote and eukaryote were popularised by Édouard Chatton; karyon means nucleus.
• Prokaryotic ribosome = 70S; eukaryotic ribosome = 80S.
• Bacterial cell wall is made of peptidoglycan (murein); plant cell wall is made of cellulose.
• Infoldings of the prokaryotic plasma membrane are called mesosomes; they help in respiration and cell division.
• Mycoplasma is the smallest cell, and it is the only prokaryote without a cell wall.

Correct Answer: Option 4 — Plasma membrane, nucleus and cytoplasm
NCERT states that every cell, whether of a plant or an animal, shows three basic features that make all its internal activities and its dealings with the outside world possible:
• Plasma membrane — the outermost living, selectively permeable boundary; it separates the cell from its surroundings and controls the entry and exit of materials by diffusion, osmosis and active transport.
• Nucleus — the control centre of the cell; it holds the chromosomes (DNA), directs all cell activities and passes on hereditary characters.
• Cytoplasm — the jelly-like fluid between the membrane and the nucleus in which all organelles float and in which most metabolic reactions (like glycolysis) take place.
❌ Why the other options are incorrect
✗ Plasma membrane, nucleus and Golgi apparatus — the Golgi apparatus is only one organelle (packaging and dispatch); it is absent in prokaryotes, so it cannot be a universal feature.
✗ Mitochondria, nucleus and cytoplasm — mitochondria are absent in prokaryotes and in mammalian RBCs; the plasma membrane, not mitochondria, is the third basic feature.
✗ Mitochondria, nucleus and Golgi apparatus — both mitochondria and Golgi apparatus are membrane-bound organelles missing from prokaryotic cells.
📌 SSC & Railway Exam Facts
• The cell was discovered by Robert Hooke (1665) in a slice of cork; he saw only the dead cell walls.
• Anton van Leeuwenhoek first saw living cells; Robert Brown (1831) discovered the nucleus.
• Cell Theory was given by Schleiden and Schwann (1838–39) and completed by Rudolf Virchow — *Omnis cellula-e cellula*.
• The plasma membrane is made of lipids and proteins; the fluid mosaic model was proposed by Singer and Nicolson (1972).
• Cytoplasm + nucleoplasm = protoplasm, called the physical basis of life by J. E. Purkinje.

Correct Answer: Option 3 — cell
The cell is the structural and functional unit of life — the smallest unit that can carry out all the activities of a living being. Inside it, different cell organelles such as mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and plastids work together like the departments of a factory, each with its own duty, all enclosed by the plasma membrane and controlled by the nucleus.
The levels of organisation in the body follow a clear ladder:
Cell → Tissue → Organ → Organ system → Organism
So organelles build the cell, and only after that do cells build tissues.
❌ Why the other options are incorrect
✗ Organ system — a group of organs working together for one major function, e.g. the digestive system (mouth, stomach, intestine, liver); it is the fourth level, far above organelles.
✗ Organ — a structure made of different tissues working together, e.g. heart, stomach, leaf, root; it is formed by tissues, not directly by organelles.
✗ Tissue — a group of similar cells performing a common function, e.g. muscle tissue, xylem; it is formed by cells, which themselves are made of organelles.
📌 SSC & Railway Exam Facts
• The term cell was coined by Robert Hooke (1665) from the Latin *cellula* meaning 'a little room'.
• Ostrich egg is the largest cell; Mycoplasma the smallest; the nerve cell is the longest cell in the human body.
• Amoeba, Paramecium and Chlamydomonas are unicellular; humans and trees are multicellular.
• An organelle without a membrane is the ribosome; mitochondria and plastids are double-membraned.
• Cell organelles were clearly seen only after the invention of the electron microscope.

Correct Answer: Option 2 — cell wall
The cell wall is a rigid, non-living covering found outside the plasma membrane in plant cells, fungi and bacteria. In plants it is made mainly of cellulose, a tough carbohydrate, along with hemicellulose and pectin.
Its functions are to give the cell shape, rigidity and mechanical strength, to protect it against mechanical injury and infection, and above all to allow plant cells to survive hypotonic surroundings — when a plant cell absorbs a lot of water by osmosis and swells, the strong wall exerts an opposing turgor pressure and stops the cell from bursting. The cell wall is fully permeable, so it does not control what enters the cell.
❌ Why the other options are incorrect
✗ Nuclear membrane — the double membrane with pores around the nucleus, present inside the cell, not outside the plasma membrane.
✗ Selectively permeable membrane — this is another name for the plasma membrane itself, which lies inside the cell wall and decides which substances enter or leave.
✗ Cell envelope — a term used for the multi-layered covering of bacteria (cell wall + membrane + capsule/glycocalyx); it is not the standard NCERT name for the rigid plant covering.
📌 SSC & Railway Exam Facts
• Plant cell wall — cellulose; fungal cell wall — chitin; bacterial cell wall — peptidoglycan.
• Animal cells have no cell wall, which is why they burst (haemolysis) in very dilute solutions.
• Adjacent plant cells stay connected through fine cytoplasmic threads called plasmodesmata.
• Loss of water makes the cell contents shrink away from the wall — plasmolysis.
• The cell wall was first seen by Robert Hooke in dead cork cells.

Correct Answer: Option 3 — meiosis
Meiosis is the reductional division in which one diploid (2n) parent cell divides twice but the chromosomes replicate only once, producing four haploid (n) daughter cells. The chromosome number is therefore halved.
It takes place in the reproductive (germ) cells — in the testis to form sperms and in the ovary to form ova, and in the anther and ovule of plants to form spores. Halving is essential: when a haploid sperm fuses with a haploid ovum at fertilisation, the diploid number is restored in the zygote, so the species keeps a constant chromosome number generation after generation. Crossing over during prophase I also creates variation.
❌ Why the other options are incorrect
✗ Mitosis — equational division; one cell gives two daughter cells with the same (2n) chromosome number. It occurs in somatic (body) cells for growth and repair.
✗ Amitosis — direct or simple division in which the nucleus and cytoplasm just pinch into two, without spindle or chromosome movements; it does not halve the chromosome number.
✗ Binary fission — an asexual reproduction method of unicellular organisms like Amoeba and bacteria; the parent splits into two identical individuals with the full chromosome set.
📌 SSC & Railway Exam Facts
• Meiosis was discovered by Oscar Hertwig (1876); the term was coined by Farmer and Moore.
• Meiosis has two stages — Meiosis I (reductional) and Meiosis II (equational).
• Crossing over happens at the pachytene stage of prophase I, at points called chiasmata.
• In humans the diploid number is 46 and the gamete's haploid number is 23.
• Mitosis stages: Prophase → Metaphase → Anaphase → Telophase; cytokinesis divides the cytoplasm.

Correct Answer: Option 4 — enzymes
Enzymes are biological catalysts (bio-catalysts) — protein molecules made by living cells that greatly speed up biochemical reactions without being used up themselves. In digestion they break complex food into simple, absorbable forms.
Each enzyme is highly specific, fitting its substrate like a lock and key, and each works best at a particular pH and temperature (about 37 °C in humans). Thus salivary amylase converts starch to maltose in the mouth, pepsin breaks proteins to peptones in the acidic stomach, and trypsin, amylase and lipase of the pancreatic juice complete the job in the alkaline small intestine, finally giving amino acids, glucose, fatty acids and glycerol.
❌ Why the other options are incorrect
✗ Oil — a liquid fat, i.e. a nutrient that supplies energy; it is itself digested by lipase, it does not catalyse anything.
✗ Fat — a energy-rich nutrient (about 9 kcal per gram) stored in adipose tissue; a food substance, not a catalyst.
✗ Carbohydrates — the body's chief source of energy (starch, glucose, cellulose); like fats they are the substrates of digestion, not the agents of it.
📌 SSC & Railway Exam Facts
• Almost all enzymes are proteins; a few RNA molecules act as enzymes and are called ribozymes.
• The word enzyme was coined by Wilhelm Kühne; the first enzyme crystallised was urease by James Sumner.
• Enzymes are denatured by high temperature and by extreme pH, which is why high fever disturbs digestion.
• Bile contains no enzyme — it only emulsifies fat and makes the medium alkaline.
• Enzyme actions to remember: Ptyalin (starch), Pepsin (protein), Trypsin (protein), Lipase (fat), Lactase (lactose).

Correct Answer: Option 1 — unicellular
Organisms whose body is made up of a single cell are called unicellular (single-celled) organisms. In them that one cell carries out all the life processes — nutrition, respiration, excretion, growth, movement and reproduction. Examples are Amoeba, Paramoecium, Euglena, Chlamydomonas, bacteria and yeast.
The cell is the basic structural and functional unit of life. It was discovered by Robert Hooke in 1665 in a thin slice of cork, and the first living cell was seen by Anton van Leeuwenhoek. According to the Cell Theory of Schleiden and Schwann, all organisms are made of cells; Rudolf Virchow added that new cells arise from pre-existing cells.
❌ Why the other options are incorrect
✗ multicellular — organisms made of many cells showing division of labour among tissues and organs, e.g. human beings, the mango tree and Rhizopus.
✗ singular — not a biological term at all; it belongs to grammar and simply means "one in number".
✗ unisexual — describes an organism or flower bearing only one sex (male or female), e.g. papaya; it has nothing to do with the number of cells.
📌 SSC & Railway Exam Facts
• Robert Hooke (1665) coined the term cell while observing cork under his microscope.
• Amoeba moves and feeds with pseudopodia; Paramoecium with cilia; Euglena with a flagellum.
• The ostrich egg is the largest cell, Mycoplasma the smallest, and the neuron the longest.
• Unicellular organisms exchange gases by simple diffusion through the body surface.
• Cell theory — Schleiden (1838) and Schwann (1839), extended by Virchow (1855).

Correct Answer: Option 3 — membrane
Every living cell — plant, animal or bacterial — is surrounded by a cell membrane (plasma membrane). It is the outermost covering of an animal cell and lies just inside the cell wall in a plant cell, and it keeps the contents of the cell separate from the external environment.
The membrane is made of lipids and proteins and is selectively permeable: it allows some substances to pass and blocks others. Movement across it occurs by diffusion (gases like CO₂ and O₂), by osmosis (movement of water), and by active transport, which uses energy. Being flexible, it also allows Amoeba to form pseudopodia, and this flexibility explains endocytosis.
❌ Why the other options are incorrect
✗ Cell wall — a rigid outer layer of cellulose, present only in plant cells, fungi and bacteria; it is fully permeable, not a boundary in animal cells.
✗ Nucleus — the control centre of the cell containing chromosomes and DNA; it lies inside, not around, the cell.
✗ Mitochondria — the powerhouse organelle that releases energy as ATP; it is an organelle, not a covering.
📌 SSC & Railway Exam Facts
• The fluid mosaic model of the plasma membrane was given by Singer and Nicolson (1972).
• The plasma membrane is selectively (semi-) permeable; the cell wall is fully permeable.
• A cell placed in a hypotonic solution swells (endosmosis) and in a hypertonic solution shrinks (plasmolysis).
• The cell was discovered by Robert Hooke (1665) in cork.
• Cell theory was given by Schleiden and Schwann; Virchow added *Omnis cellula-e cellula*.

Correct Answer: Option 3 — DNA
Inside the nucleus, the hereditary material DNA (deoxyribonucleic acid) does not stay as neat rod-like chromosomes all the time. In a non-dividing (interphase) cell it lies as a fine, entangled, thread-like network called chromatin material, made of DNA and histone proteins.
When the cell is about to divide, this chromatin condenses and coils up to become visible chromosomes. Thus chromatin and chromosomes are the same material in two different states. DNA carries the genes, the functional units of heredity, which pass information from parents to offspring and also direct the making of proteins.
❌ Why the other options are incorrect
✗ RNA — a single-stranded nucleic acid made from DNA; it works mainly in the cytoplasm for protein synthesis, not as chromatin.
✗ Ribosome — the granular organelle, the site of protein synthesis, found on the rough ER and freely in the cytoplasm.
✗ Nucleus — the organelle that *contains* chromatin; chromatin is the material inside it, not the nucleus itself.
📌 SSC & Railway Exam Facts
• The double-helix model of DNA was given by Watson and Crick (1953).
• The nucleus was discovered by Robert Brown (1831).
• A human cell has 46 chromosomes (23 pairs); the gamete has 23.
• Gene is a segment of DNA that carries the information for one character.
• The nucleus is bounded by a porous nuclear membrane and contains the nucleolus.

Correct Answer: Option 3 — ribosomes
The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets inside the cytoplasm. When tiny, dense granules called ribosomes sit on its outer surface, the membrane appears granular or rough under the microscope, and it is called Rough Endoplasmic Reticulum (RER).
Ribosomes are the sites of protein synthesis. The proteins they make on the RER are sent into the ER channels and then transported to various parts of the cell or exported out of it. Smooth Endoplasmic Reticulum (SER), which has no ribosomes, makes lipids and helps detoxify poisons and drugs in liver cells.
❌ Why the other options are incorrect
✗ protein particles — proteins are the product made by ribosomes; loose protein particles are not attached to the ER and do not make it look rough.
✗ lysosomes — membrane-bound sacs of digestive enzymes that clear worn-out organelles; called the suicide bags of the cell, they float free in the cytoplasm.
✗ chromosomes — thread-like DNA–protein structures that lie inside the nucleus and carry hereditary information; they never attach to the ER.
📌 SSC & Railway Exam Facts
• Ribosomes were discovered by George Palade; they are made of RNA and protein and have no membrane.
• Ribosome types: 70S in prokaryotes and in mitochondria/chloroplasts, 80S in eukaryotic cytoplasm.
• The ER gives internal framework (cytoplasmic skeleton) and acts as a transport channel within the cell.
• Golgi apparatus (discovered by Camillo Golgi) packages and dispatches what the ER makes.
• Mitochondrion = powerhouse of the cell; plastids are found only in plant cells.

Correct Answer: Option 4 — prokaryotes
Organisms whose cells have no nuclear membrane are called prokaryotes (*pro* = primitive, *karyon* = nucleus). Their genetic material — a single circular DNA molecule — lies naked in the cytoplasm in a region called the nucleoid, without any covering membrane.
Prokaryotic cells also lack membrane-bound organelles such as mitochondria, plastids, Golgi apparatus, endoplasmic reticulum and lysosomes. They contain only 70S ribosomes. Bacteria and blue-green algae (cyanobacteria) are the standard examples. In contrast, eukaryotes have a true nucleus bounded by a double nuclear membrane with nuclear pores.
❌ Why the other options are incorrect
✗ eukaryotes — exactly the opposite; these organisms do have a well-defined nucleus with a nuclear membrane, e.g. plants, animals, fungi and Amoeba.
✗ cytoplasm — not a class of organisms at all; it is the jelly-like fluid between the plasma membrane and the nucleus in which organelles float.
✗ chromatin — the thread-like DNA–protein network inside the nucleus that condenses into chromosomes during cell division; it is a structure, not an organism.
📌 SSC & Railway Exam Facts
• Prokaryotes: bacteria, cyanobacteria (blue-green algae), mycoplasma — mycoplasma is the smallest cell.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Prokaryotes divide by binary fission; eukaryotes by mitosis and meiosis.
• Robert Hooke discovered the cell (1665); Robert Brown discovered the nucleus (1831).
• The cell theory was given by Schleiden and Schwann; Virchow added *Omnis cellula e cellula*.

Correct Answer: Option 4 — Leeuwenhoek in 1674
Anton van Leeuwenhoek, using his own improved microscope, observed free living cells in pond water for the first time in 1674. The tiny moving organisms he saw he called "animalcules"; today we know them as protozoa and bacteria, which is why he is called the Father of Microbiology.
This observation was a milestone in the history of the cell. Cells had first been seen in dead cork by Robert Hooke, but Leeuwenhoek was the first to see living cells, proving that life exists in single-celled form as well.
❌ Why the other options are incorrect
✗ Virchow in 1855 — Rudolf Virchow stated that "all cells arise from pre-existing cells", completing the cell theory; he did not observe pond-water cells.
✗ Robert Brown in 1831 — he discovered the nucleus in the cell of an orchid leaf.
✗ Robert Hooke in 1665 — he examined a thin slice of cork under his own microscope, saw tiny compartments and coined the word "cell" (from Latin *cellula*, a little room); these were dead cells.
📌 SSC & Railway Exam Facts
• Robert Hooke (1665) — coined "cell", from cork; wrote Micrographia.
• Leeuwenhoek (1674) — first saw free living cells in pond water.
• Robert Brown (1831) — discovered the nucleus.
• Purkinje (1839) — coined the term protoplasm.
• Cell theory: Schleiden (1838) and Schwann (1839), extended by Virchow (1855).

Correct Answer: Option 1 — Rudolf Virchow
Rudolf Virchow, a German scientist, stated in 1855 that "Omnis cellula e cellula" — all cells arise from pre-existing cells. This expanded and completed the cell theory, which had been proposed by Schleiden (1838) and Schwann (1839).
The modern cell theory therefore says: (i) all living organisms are composed of cells and products of cells, (ii) the cell is the basic structural and functional unit of life, and (iii) every cell comes from a pre-existing cell by division. Virchow's addition ruled out the old idea of spontaneous generation of life from non-living matter.
❌ Why the other options are incorrect
✗ Robert Brown — in 1831 he discovered the nucleus in the cells of an orchid leaf.
✗ Purkinje — in 1839 he coined the term protoplasm for the fluid substance of the cell.
✗ Robert Hooke — in 1665 he observed a thin slice of cork under his microscope and coined the word "cell".
📌 SSC & Railway Exam Facts
• Cell theory — Schleiden (plants, 1838) + Schwann (animals, 1839); extended by Virchow (1855).
• Virchow is also called the Father of Modern Pathology.
• Leeuwenhoek (1674) first saw free living cells in pond water.
• Robert Hooke described cells in his book Micrographia.
• The cell is the structural and functional unit of life; the largest human cell is the ovum, the longest is the nerve cell.

Correct Answer: Option 1 — Both proteins and lipids
The plasma membrane (cell membrane) is the outermost covering of an animal cell and lies just inside the cell wall in a plant cell. It is made of lipids and proteins — a bilayer of phospholipid molecules in which protein molecules are embedded. This arrangement is described by the fluid mosaic model of Singer and Nicolson.
The membrane is selectively permeable (semi-permeable) — it allows only some substances to pass. Gases such as O₂ and CO₂ move across it by diffusion, water moves by osmosis, and larger particles are taken in by endocytosis, the flexibility of the lipid layer allowing the membrane to fold, as in Amoeba.
❌ Why the other options are incorrect
✗ Only lipids — a purely lipid sheet could not transport substances selectively; the carrier and channel proteins are essential for that.
✗ Only proteins — proteins alone cannot form a continuous flexible barrier; the phospholipid bilayer provides the basic framework.
✗ Only cellulose — cellulose builds the cell wall of plant cells, which is rigid, freely permeable and is outside the plasma membrane.
📌 SSC & Railway Exam Facts
• Fluid mosaic model — given by Singer and Nicolson (1972).
• The plasma membrane is selectively permeable; the cell wall is fully permeable.
• Diffusion moves gases; osmosis moves water across a semi-permeable membrane.
• Plant cell wall is cellulose; fungal wall is chitin; bacterial wall is peptidoglycan.
• Endocytosis (Amoeba engulfing food) is possible only because the membrane is flexible.

Correct Answer: Option 3 — Mitochondria
Mitochondria are double-membrane organelles that possess their own DNA and ribosomes. Because of this they can synthesise some of their own proteins and are therefore called semi-autonomous organelles. Their inner membrane is deeply folded into cristae, giving a large surface area for the enzymes of respiration.
Mitochondria are the powerhouse of the cell: they carry out aerobic respiration and release energy in the form of ATP (adenosine triphosphate), the energy currency of the cell. In plants the plastids (chloroplasts) also contain their own DNA and ribosomes.
❌ Why the other options are incorrect
✗ Endoplasmic reticulum — a membrane network; RER (with ribosomes on its surface) helps in protein transport and SER in lipid synthesis and detoxification, but it has no DNA of its own.
✗ Golgi apparatus — stacks of membrane-bound cisternae that modify, pack and dispatch materials and form lysosomes; no DNA, no ribosomes.
✗ Lysosomes — membrane-bound "suicide bags" filled with powerful digestive enzymes that clear worn-out organelles; they too lack DNA.
📌 SSC & Railway Exam Facts
• Mitochondria and plastids are the only organelles with their own DNA and ribosomes.
• Mitochondria = powerhouse of the cell; ATP = energy currency.
• Lysosome = suicidal bag; Golgi body = secretory/packaging organelle.
• Ribosome = site of protein synthesis; found free or on the RER.
• Mitochondria and plastids are absent in prokaryotes such as bacteria.

Correct Answer: Option 1 — plant cells
Vacuoles are storage sacs for solid or liquid contents, bounded by a single membrane called the tonoplast. In plant cells a single large central vacuole may occupy up to 50–90% of the cell volume, while animal cells have only small vacuoles.
The plant vacuole is filled with cell sap containing water, sugars, amino acids, salts and pigments. Being full of sap it provides turgidity and rigidity to the cell, keeping the plant firm. It also stores waste products and amino acids, sugars and organic acids, keeping them away from the cytoplasm.
❌ Why the other options are incorrect
✗ Bacteria — prokaryotic cells; they lack membrane-bound organelles, so no true large vacuole (only gas vacuoles in some).
✗ Animal cells — they do have vacuoles, but these are small and numerous, never a single large central one.
✗ Both plant cells and animal cells — large vacuoles are characteristic of plant cells only, so this is wrong.
📌 SSC & Railway Exam Facts
• The membrane around the vacuole is the tonoplast.
• In Amoeba the food vacuole digests food and the contractile vacuole removes excess water.
• Plastids, cell wall and large central vacuole are present in plant cells but absent in animal cells.
• Centrosome and lysosomes in large numbers are typical of animal cells.
• Lysosome is the suicidal bag; mitochondrion is the powerhouse of the cell.

Correct Answer: Option 2 — The nuclear membrane is absent in prokaryotic cells.
Cells are of two kinds. In prokaryotic cells the genetic material is not enclosed by a nuclear membrane; the naked DNA lies free in the cytoplasm in a region called the nucleoid. In eukaryotic cells the DNA is enclosed in a well-defined nucleus bounded by a double-layered nuclear membrane with nuclear pores.
Prokaryotic cells also lack membrane-bound organelles — there is no mitochondrion, plastid, endoplasmic reticulum, Golgi apparatus or lysosome. The only organelle they possess is the ribosome (the smaller 70S type). Bacteria and blue-green algae (cyanobacteria) are prokaryotes; all plants, animals, fungi and protists are eukaryotes.
❌ Why the other options are incorrect
✗ The nuclear membrane is present in prokaryotic cells — false; the very meaning of *pro-karyon* is 'before the nucleus', i.e. no true nucleus.
✗ All cytoplasmic organelles are present in prokaryotic cells — false; membrane-bound organelles are absent, only ribosomes are present.
✗ The nuclear membrane is absent in eukaryotic cells — false; *eu-karyon* means 'true nucleus', so the nuclear membrane is definitely present.
📌 SSC & Railway Exam Facts
• The term cell was given by Robert Hooke (1665); Leeuwenhoek first saw living cells.
• Robert Brown discovered the nucleus (1831); Purkinje coined the term protoplasm.
• The Cell Theory was given by Schleiden and Schwann, and Virchow added that new cells arise from pre-existing cells.
• Prokaryotic ribosomes are 70S; eukaryotic ribosomes are 80S.
• Mycoplasma is the smallest cell; the ostrich egg is the largest cell, and the nerve cell the longest.

Correct Answer: Option 1 — Rhizopus
NCERT states that a single cell may constitute a whole organism, as in Amoeba, Chlamydomonas, Paramoecium and bacteria. Such organisms are called unicellular, and that one cell performs all the life processes — nutrition, respiration, excretion, movement and reproduction — by itself.
Rhizopus, the bread mould, is the exception. Its body is made of many thread-like hyphae that together form a mycelium, with coenocytic (multinucleate) filaments and separate sporangia for reproduction. It is therefore a multicellular (filamentous) fungus, not a single-celled organism.
❌ Why the other options are incorrect
✗ Amoeba — a unicellular protozoan with an irregular changing shape; it moves and feeds using pseudopodia and divides by binary fission.
✗ Bacteria — unicellular prokaryotes without a nuclear membrane or membrane-bound organelles; they reproduce by binary fission.
✗ Chlamydomonas — a unicellular green alga with a cup-shaped chloroplast and two flagella; it makes its own food by photosynthesis.
📌 SSC & Railway Exam Facts
• Unicellular — Amoeba, Paramoecium, Chlamydomonas, bacteria, yeast; Multicellular — Rhizopus, mushroom, plants and animals.
• In multicellular organisms there is division of labour, with different cells forming different tissues and organs.
• Yeast is a unicellular fungus, while Rhizopus and mushroom are multicellular fungi.
• Fungal cell walls are made of chitin; fungi are saprophytes or parasites.
• Mycoplasma is the smallest cell, and the nerve cell is the longest cell in the human body.

Correct Answer: Option 4 — Four
Meiosis is the reduction division that takes place only in the reproductive (germ) cells of the gonads. One parent cell passes through two successive divisions — Meiosis I and Meiosis II — but the DNA is copied only once. The result is four daughter cells, each with half (haploid, n) the chromosome number of the parent cell.
This halving is essential: when a haploid sperm (n) fuses with a haploid egg (n) at fertilisation, the diploid (2n) chromosome number of the species is restored, so it stays constant generation after generation. Crossing over during Prophase I also creates genetic variation.
❌ Why the other options are incorrect
✗ Eight — eight cells would need three successive divisions; meiosis has only two, so eight is not possible.
✗ Twenty-four — 24 is not a power of 2 at all and can never be the product of successive equal divisions.
✗ Sixteen — sixteen cells would require four rounds of division, far more than the two rounds of meiosis.
📌 SSC & Railway Exam Facts
• Mitosis gives 2 diploid (2n) daughter cells and occurs in body (somatic) cells for growth and repair.
• Meiosis gives 4 haploid (n) cells and occurs in germ cells for gamete formation.
• Stages of division: Prophase → Metaphase → Anaphase → Telophase, followed by cytokinesis.
• The term mitosis was given by Walther Flemming and meiosis by Farmer and Moore.
• Humans have 46 chromosomes (23 pairs) in body cells and 23 in each gamete.

Correct Answer: Option 4 — 1665
In 1665, the English scientist Robert Hooke examined a thin slice of cork (dead bark tissue of a tree) under his own self-designed compound microscope. He saw a honeycomb-like arrangement of small box-shaped compartments and named each of them a 'cell', from the Latin word cellula, meaning 'a little room'.
Hooke published this observation in his famous book 'Micrographia' (1665). What he actually saw were only the dead cell walls of cork cells, but the name he gave became the foundation of cell biology, and the cell is today accepted as the structural and functional unit of life.
❌ Why the other options are incorrect
✗ 1664 — one year too early; Hooke's observation and his book Micrographia both belong to 1665.
✗ 1667 — no landmark cell discovery is dated to this year; it is only a distractor.
✗ 1670 — by this date microscopy had advanced, but the discovery of the cell had already been made in 1665.
📌 SSC & Railway Exam Facts
• Anton van Leeuwenhoek (1674) first saw free-living cells — bacteria, sperms and protozoa — and is called the Father of Microbiology.
• Robert Brown (1831) discovered the nucleus; Purkinje (1839) coined the term protoplasm.
• The Cell Theory was given by Schleiden (1838) and Schwann (1839); Rudolf Virchow (1855) added that 'Omnis cellula e cellula' — all cells arise from pre-existing cells.
• The largest cell is the ostrich egg; the longest cell is the nerve cell (neuron).
• The smallest cell is that of Mycoplasma (PPLO); RBCs of mammals have no nucleus.

Correct Answer: Option 3 — Ribosomes
Ribosomes are the site of protein synthesis in every living cell. They are extremely small, dense, non-membrane-bound granules made of RNA (rRNA) and protein, which is why they are also called the protein factory of the cell.
Ribosomes may lie free in the cytoplasm (their proteins are used inside the cell) or remain attached to the Rough Endoplasmic Reticulum (RER) (their proteins are exported out of the cell). During synthesis the ribosome reads the codons of mRNA and links amino acids brought by tRNA into a polypeptide chain — a process called translation.
❌ Why the other options are incorrect
✗ Vacuole — a fluid-filled sac that stores water, cell sap, pigments and wastes and gives turgidity to plant cells; it makes nothing.
✗ Cell membrane — the selectively permeable outer boundary that controls the entry and exit of substances; it is a barrier, not a synthesis site.
✗ Plastid — a plant organelle; the chloroplast carries out photosynthesis and leucoplasts store starch, oil and protein — storage, not protein manufacture.
📌 SSC & Railway Exam Facts
• Ribosomes were discovered by George Palade; the name was given by Richard B. Roberts.
• Prokaryotic ribosome — 70S (50S + 30S); eukaryotic ribosome — 80S (60S + 40S).
• Ribosomes are also present inside mitochondria and chloroplasts, which are therefore semi-autonomous organelles.
• Organelle nicknames: mitochondrion — powerhouse; chloroplast — kitchen; lysosome — suicide bag; Golgi body — dispatching centre.
• The genetic message travels as DNA → RNA → Protein, called the central dogma of molecular biology.

Correct Answer: Option 4 — they defend plant cells from a variety of external effects
The cell wall is a rigid, non-living outer covering lying outside the plasma membrane in plants, fungi and bacteria. In plants it is made mainly of cellulose. Being tough yet fully permeable, it gives the cell definite shape, rigidity and mechanical strength and defends it from a variety of external effects — mechanical injury, invasion by pathogens and, most importantly, osmotic stress.
When a plant cell is placed in a hypotonic solution, water rushes in by endosmosis and the cell swells. The wall pushes back with an equal wall pressure against the turgor pressure, so the cell becomes turgid but never bursts. An animal cell, which has no wall, would simply burst under the same conditions.
❌ Why the other options are incorrect
✗ they are required for photosynthesis — photosynthesis occurs in the chloroplast using the green pigment chlorophyll; the cell wall carries no pigment.
✗ they do not have proper genetic material — plant cells are eukaryotic and possess a well-organised nucleus containing DNA on chromosomes.
✗ they do not have cytoplasm — every living cell, plant or animal, contains cytoplasm in which all the organelles are suspended.
📌 SSC & Railway Exam Facts
• Plant cell wall — cellulose; fungal cell wall — chitin; bacterial cell wall — peptidoglycan (murein).
• Robert Hooke (1665) saw and named the cell while examining dead cell walls in a thin slice of cork.
• The cell wall is freely permeable, while the plasma membrane is selectively (semi-) permeable.
• Adjacent plant cells stay connected by fine cytoplasmic bridges called plasmodesmata.
• Loss of water first makes the cell flaccid and then plasmolysed, which is why plants wilt in hot weather.

Correct Answer: Option 1 — Two
The Endoplasmic Reticulum (ER) is a large network of membrane-bound tubules, vesicles and sheets spread through the cytoplasm. It occurs in two types:
• Rough Endoplasmic Reticulum (RER) — its surface is studded with ribosomes, which make it look rough. It is the site of protein synthesis and of the transport of those proteins to the Golgi apparatus.
• Smooth Endoplasmic Reticulum (SER) — it carries no ribosomes. It synthesises lipids and steroids and, in liver cells, detoxifies poisons and drugs.
Because the ER also serves as a channel between the cytoplasm and the nucleus and provides a huge surface for reactions, it is called the cytoplasmic framework of the cell.
❌ Why the other options are incorrect
✗ Four — no textbook recognises four kinds of ER; only the rough and smooth forms exist.
✗ Three — a third variety of ER does not exist; the Golgi apparatus, though related, is a separate organelle.
✗ Five — far too many; the ER membrane is simply either ribosome-bearing (RER) or ribosome-free (SER).
📌 SSC & Railway Exam Facts
• The endoplasmic reticulum was discovered by Keith Porter (1945).
• ER takes part in membrane biogenesis, since its membrane resembles the plasma membrane.
• Ribosomes were discovered by George Palade; they are the protein factories of the cell.
• The Golgi apparatus packages, modifies and dispatches the material produced by the ER.
• ER is absent in prokaryotes such as bacteria, which have no membrane-bound organelles.

Correct Answer: Option 2 — Chromosomes
Chromosomes are the rod-shaped structures seen inside the nucleus only when the cell is about to divide. In a non-dividing cell the genetic material lies as a loose, thread-like network called chromatin. As division approaches, this chromatin condenses and coils tightly and becomes visible as distinct rod-like chromosomes.
Each chromosome is made of DNA and protein and carries the hereditary information in units called genes. The tight packing ensures that the DNA is distributed equally and without tangling to the two daughter cells.
❌ Why the other options are incorrect
✗ Lysosomes — membrane-bound sacs of digestive enzymes in the cytoplasm, called the 'suicide bags'; they are not in the nucleus and are not rod-shaped.
✗ Mitochondria — rod-shaped indeed, but they are cytoplasmic organelles, the powerhouse producing ATP; they stay in the cell at all times, not only at division.
✗ Chromatin — the same material, but in its uncondensed, thread-like (network) form seen in a non-dividing cell; it is not rod-shaped.
📌 SSC & Railway Exam Facts
• The term chromosome was coined by W. Waldeyer; chromatin by W. Flemming.
• Human body cells have 46 chromosomes (23 pairs); gametes have 23.
• Chromosome number — pea 14, onion 16, housefly 12, dog 78, rice 24.
• The point at which the two chromatids of a chromosome are joined is the centromere.
• Genes are the functional segments of DNA on the chromosome; DNA + protein (histone) = chromatin.

Correct Answer: Option 1 — Plastids
Plastids are large, double-membrane organelles found only in plant cells. The type that helps in photosynthesis is the chloroplast, which contains the green pigment chlorophyll.
Inside a chloroplast the inner membrane is folded into stacks of flat sacs called grana (thylakoids), suspended in a fluid stroma. Chlorophyll in the grana traps sunlight and converts it to chemical energy, which is then used in the stroma to build glucose from CO₂ and water, releasing oxygen. This is why the chloroplast is called the kitchen of the cell.
Other plastids are chromoplasts, which give colour to flowers and fruits, and leucoplasts, which store starch, oils and proteins.
❌ Why the other options are incorrect
✗ Lysosomes — membrane sacs containing powerful digestive enzymes; called the 'suicide bags', they clear worn-out organelles but make no food.
✗ Nucleus — the control centre of the cell; it holds the chromosomes (DNA) and directs all activities, but it does not carry out photosynthesis.
✗ Golgi body — the packaging and dispatching centre that modifies and sends out materials made by the ER; it forms lysosomes but has no pigment.
📌 SSC & Railway Exam Facts
• Photosynthesis — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of sunlight and chlorophyll.
• Chloroplast is the kitchen of the cell; mitochondria the powerhouse; ribosome the protein factory.
• Chloroplast and mitochondria both have their own DNA and ribosomes — they are semi-autonomous.
• Magnesium (Mg) is the central metal atom of the chlorophyll molecule.
• Gas exchange during photosynthesis takes place through the stomata of the leaf, guarded by guard cells.

Correct Answer: Option 2 — 1-b, 2-c, 3-a, 4-d
• Cell wall (1-b) — a rigid outer covering of cellulose found only in plant cells, bacteria and fungi. It gives the plant structural strength, definite shape and protection, and allows the cell to withstand hypotonic surroundings without bursting.
• Ribosomes (2-c) — tiny granules of RNA and protein without any membrane, found free in the cytoplasm or attached to the rough ER. They are the sites of protein manufacture (protein synthesis).
• Lysosomes (3-a) — membrane-bound sacs of powerful hydrolytic (digestive) enzymes that digest foreign material and worn-out cell organelles; they burst during cell injury, hence the name 'suicide bags'.
• Golgi apparatus (4-d) — a stack of membrane-bound cisternae that stores, modifies, packages and dispatches materials; lysosomes are formed from the Golgi apparatus.
❌ Why the other options are incorrect
✗ 1-b, 2-d, 3-c, 4-a — wrongly links ribosomes to lysosome formation and lysosomes to protein synthesis.
✗ 1-c, 2-a, 3-d, 4-b — makes the cell wall a protein factory and gives the Golgi the job of the cell wall.
✗ 1-a, 2-b, 3-c, 4-d — makes the cell wall a digestive organelle, which is entirely wrong.
📌 SSC & Railway Exam Facts
• Lysosome was discovered by Christian de Duve; Golgi body by Camillo Golgi.
• Ribosome was discovered by George Palade; it is the only organelle present in prokaryotes too.
• Cell wall is freely permeable; the plasma membrane is selectively permeable.
• Fungal cell wall is made of chitin, bacterial cell wall of peptidoglycan.
• Golgi apparatus is called the 'dispatching / traffic centre' of the cell.

Correct Answer: Option 2 — Vacuoles
Vacuoles are membrane-bound storage sacs filled with a watery fluid. In animal cells they are small and few, while in plant cells a single central vacuole may occupy 50–90 per cent of the cell volume. The membrane around the plant vacuole is called the tonoplast.
The large plant vacuole is filled with cell sap — water, sugars, amino acids, mineral salts, pigments and organic acids. Water stored in it creates turgor pressure, which presses the protoplasm against the cell wall and keeps soft herbaceous plants firm and erect. Vacuoles also store waste products that the plant cannot excrete.
❌ Why the other options are incorrect
✗ Golgi apparatus — present in both cell types as stacks of cisternae; it modifies, packages and dispatches materials made by the ER, and its size is not the distinguishing feature asked here.
✗ Plastids — these are found only in plant cells, so they cannot be described as "small in animal cells".
✗ Lysosomes — the suicide bags filled with digestive enzymes; they are prominent in animal cells and rare in plant cells, the opposite of what the question describes.
📌 SSC & Railway Exam Facts
• Amoeba uses a food vacuole to digest food and a contractile vacuole for osmoregulation.
• The plant vacuole membrane is the tonoplast; the fluid inside is cell sap.
• Loss of vacuolar water causes wilting; regaining it restores turgidity.
• Cell wall, plastids and a large central vacuole are the three structures found in plant cells but not in animal cells.
• Centrioles and lysosomes are typical of animal cells.

Correct Answer: Option 1 — Both contain their own DNA and ribosomes.
Plastids and mitochondria are both double-membrane organelles, and both carry their own circular DNA and their own ribosomes. Because of this they can synthesise some of their own proteins and can even divide on their own, independently of nuclear division. Organelles of this kind are called semi-autonomous organelles.
This shared feature is the basis of the endosymbiotic theory, which proposes that both organelles were once free-living prokaryotes that were engulfed by an early eukaryotic cell and stayed on as permanent partners.
❌ Why the other options are incorrect
✗ Both produce ATPs — chloroplasts do make ATP during the light reaction, but leucoplasts and chromoplasts do not; the powerhouse role of producing ATP for the whole cell belongs to the mitochondrion.
✗ Both are present only in plant cells — plastids are restricted to plant cells, but mitochondria occur in all eukaryotic cells, plant and animal alike.
✗ Both contain chlorophyll — only the chloroplast type of plastid contains chlorophyll; mitochondria never do, and neither do leucoplasts.
📌 SSC & Railway Exam Facts
• Three types of plastids — chloroplast (green, photosynthesis), chromoplast (colour of flowers and fruits), leucoplast (colourless, stores starch, oil and protein).
• Chloroplast — the kitchen of the cell; mitochondrion — the powerhouse of the cell.
• The inner membrane of the mitochondrion is folded into cristae; the chloroplast has stacks of grana in the stroma.
• Mitochondrial DNA is inherited only from the mother.
• Cell organelles with no membrane — ribosomes; with a single membrane — lysosome, vacuole, Golgi body, ER.

Correct Answer: Option 4 — Endoplasmic reticulum
The Endoplasmic Reticulum (ER) is a large network of membrane-bound tubes and sheets spread through the cytoplasm. It performs two roles at once:
• As a passageway (channel) — it transports materials, especially proteins, between different parts of the cell and between the cytoplasm and the nucleus.
• As a manufacturing surface — RER (Rough ER) carries ribosomes and makes proteins, while SER (Smooth ER) makes lipids and steroids and detoxifies poisons and drugs in the liver.
Because it provides a huge surface area for these processes, ER is also called the cytoplasmic framework of the cell.
❌ Why the other options are incorrect
✗ Mitochondria — the powerhouse of the cell; releases energy as ATP by respiration. It does not transport material across the cell.
✗ Ribosome — only the site of protein synthesis; it is a tiny granule without a membrane, so it cannot act as a passageway.
✗ Plastids — found only in plant cells; chloroplasts perform photosynthesis and leucoplasts store starch, oil and protein.
📌 SSC & Railway Exam Facts
• ER was discovered by Keith Porter (1945).
• RER looks rough because ribosomes are attached; SER has no ribosomes.
• The membrane of ER is similar to the plasma membrane, and ER also helps in membrane biogenesis.
• Golgi apparatus packages and dispatches the material made by the ER.
• Lysosomes are the suicide bags, mitochondria the powerhouse, and ribosomes the protein factory of the cell.

Correct Answer: Option 3 — Mitochondria
Mitochondria are called the powerhouse of the cell because they carry out aerobic respiration and release energy stored in food in the form of ATP (Adenosine Triphosphate). ATP is the energy currency of the cell and is used for muscle contraction, protein synthesis and all other life processes.
Mitochondria are double-membrane organelles; the inner membrane is deeply folded into cristae, which greatly increases the surface area for ATP-generating reactions. They have their own DNA and ribosomes, so they can make some of their own proteins — hence they are called semi-autonomous organelles.
❌ Why the other options are incorrect
✗ Vacuoles — storage sacs for water, sap and wastes; they give turgidity and rigidity to plant cells but produce no energy.
✗ Plastids — plant organelles; chloroplast traps sunlight for photosynthesis (it is called the kitchen of the cell, not the powerhouse).
✗ Endoplasmic reticulum — transport channel and manufacturing surface for proteins and lipids, not an energy producer.
📌 SSC & Railway Exam Facts
• Lysosome — 'suicide bag', discovered by Christian de Duve; contains powerful digestive enzymes.
• Mitochondria were discovered by Altmann; the name was given by Carl Benda.
• Chloroplast — kitchen of the cell; Golgi body — dispatching/traffic centre; Ribosome — protein factory.
• Mitochondrial DNA is inherited only from the mother.
• RBCs of mammals have no mitochondria and no nucleus, so they respire anaerobically.

Correct Answer: Option 3 — Plasmolysis
Plasmolysis is the shrinkage of the protoplasm (cell contents) away from the cell wall when a plant cell is placed in a hypertonic solution (a solution more concentrated than the cell sap). Water moves out of the cell by osmosis, the vacuole shrinks, and the plasma membrane pulls away from the rigid cell wall.
If the plasmolysed cell is put back into pure water or a hypotonic solution, water re-enters and the cell regains its shape — this reverse process is called deplasmolysis.
❌ Why the other options are incorrect
✗ Passive Transport — the general movement of substances along the concentration gradient without energy; plasmolysis is one specific result, not the name of the process asked.
✗ Diffusion — movement of particles from higher to lower concentration, but it does not require a semi-permeable membrane and does not describe shrinking of cell contents.
✗ Active Transport — movement against the concentration gradient using ATP, e.g. uptake of minerals by root hair cells.
📌 SSC & Railway Exam Facts
• Osmosis is the movement of water (solvent) through a semi-permeable membrane from a region of higher water potential to lower water potential.
• Hypotonic solution → cell swells (endosmosis, turgid); Isotonic → no change; Hypertonic → cell shrinks (exosmosis, plasmolysed).
• Plasmolysis proves that the cell is living and that the plasma membrane is semi-permeable.
• Salt sprinkled on vegetables or applied for pickling kills microbes by plasmolysis.
• Turgor pressure from water inside the vacuole keeps herbaceous plants erect; loss of it causes wilting.

Correct Answer: Option 1 — Nucleus
In eukaryotic cells the nucleus is surrounded by a double-layered nuclear membrane (nuclear envelope) that separates it from the cytoplasm. This membrane bears nuclear pores through which materials move between the nucleus and the cytoplasm.
The nucleus contains chromatin, made of DNA and protein, which condenses into chromosomes at the time of cell division. DNA carries the genes that control heredity and direct the making of proteins and enzymes. Because every activity of the cell depends on these proteins, the nucleus is called the control centre or director of the cell; it also controls cell division.
❌ Why the other options are incorrect
✗ Endoplasmic reticulum — a single-membrane network that acts as a transport channel and a surface for making proteins (RER) and lipids (SER); it does not direct the cell.
✗ Plastids — double-membrane organelles found only in plant cells; chloroplasts carry out photosynthesis and leucoplasts store food.
✗ Ribosome — a tiny membrane-less granule of RNA and protein; it is only the site of protein synthesis.
📌 SSC & Railway Exam Facts
• The nucleus was discovered by Robert Brown (1831).
• Prokaryotes (bacteria, blue-green algae) have no nuclear membrane; their DNA lies in a nucleoid.
• The nucleolus inside the nucleus makes ribosomes; it is not membrane-bound.
• Human beings have 46 chromosomes (23 pairs); mature RBCs have no nucleus.
• Mitochondria and plastids are the other double-membrane organelles and have their own DNA.

Answer: Option 2 — Meiosis
Reproductive cells (gametes) are made by meiosis, a reduction division.
It halves the chromosome number, so each gamete carries half the DNA of a body cell.
Revision note — Mitosis and meiosis
• Mitosis — in body (somatic) cells; gives 2 identical daughter cells with the same chromosome number; used for growth and repair.
• Meiosis — in reproductive organs (testes, ovaries, anthers, ovules); gives 4 daughter cells with half the chromosome number (haploid, n).
• Humans: body cells 46 chromosomes (2n); sperm and egg 23 each (n).
• At fertilisation two haploid gametes fuse, restoring 2n in the zygote — so the DNA amount stays constant across generations.
• Crossing over and random sorting in meiosis create variation among offspring.
Do not mix these up
• Cytokinesis is only the splitting of the cytoplasm at the end of a division; amitosis is direct division with no spindle.
Also asked from this topic: the number of daughter cells formed in meiosis (four); the name of reproductive cells formed by meiosis (gametes); why siblings look different (meiosis).