NEET Cell: The Unit of Life Previous Year Questions with Solutions

1. Matthias Schleiden (1838), a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. 2. Theodor Schwann (1839), a British zoologist, studied different types of animal cells and reported that cells had a thin outer layer (plasma membrane). He also concluded that the presence of a cell wall is a unique character of plant cells. 3. Based on these unified observations, Schleiden and Schwann jointly formulated the Cell Theory: all living organisms are composed of cells and products of cells.

• • Their theory originally did not explain how new cells were formed.

1. Robert Brown (1831): Discovered and named the cell nucleus. 2. Singer and Nicolson (1972): Proposed the Fluid Mosaic Model of the cell membrane. 3. Antonie van Leeuwenhoek (1674): First saw and described live cells (bacteria, protozoa, sperm) under a simple microscope.

• • Option (1) is the unique correct match.

Cell theory was formulated by Schleiden and Schwann.

• • Matches Option (1).

Prokaryotic ribosomes are of the 70S type. They are composed of two distinct subunits: a large 50S subunit and a small 30S subunit, which together sediment at 70S.

• • Statement (a) is correct.

In addition to the genomic DNA (single circular chromosome in nucleoid), many bacteria possess small circular self-replicating extrachromosomal double-stranded DNA molecules called plasmids. Plasmids confer unique phenotypic characters like antibiotic resistance.

• • Statement (b) is correct.

A characteristic feature of prokaryotes is the mesosome, formed by the extension and infolding of the plasma membrane into the cell (in the form of vesicles, tubules, and lamellae). Mesosomes function in cell wall formation, DNA replication, and cellular respiration.

• • Statement (c) is correct.

Peroxisomes are single-membrane-bound microbodies containing catalase and urate oxidase, involved in lipid metabolism and photorespiration. Prokaryotes lack all membrane-bound cell organelles, including peroxisomes.

• • Statement (d) is incorrect.
• • Therefore, (a), (b), and (c) only are characteristics of prokaryotic cells.

Each mitochondrion is a double-membrane-bound structure with an outer membrane and an inner membrane dividing its lumen into two aqueous compartments: 1. The intermembrane space: The narrow region situated between the outer and inner membranes. 2. The inner compartment: The central space enclosed by the convoluted inner membrane, which is packed with a dense homogeneous proteinaceous ground substance known as the MATRIX.

• • Matrix contains enzymes of the TCA cycle (citric acid cycle), mitochondrial circular DNA, RNA molecules, and 70S ribosomes.

1. Cytosol: The soluble fluid portion of the cell cytoplasm outside all organelles. 2. Mucus: Viscous slippery secretion produced by goblet cells and mucous membranes in animal tissues. 3. Aqueous humor: The clear fluid filling the space in the front of the eyeball between the lens and the cornea.

• • Option (1) is the only scientifically valid organelle term.

Mitochondrial inner membrane encloses the matrix.

• • Matches Option (1).

The inner membrane of mitochondria forms a number of infoldings called cristae (sing. crista) towards the matrix. Cristae increase the surface area available for ATP synthesis and electron transport chain complexes.

• • Match: A -> II.

The Golgi apparatus consists of many flat, disc-shaped sacs or cisternae of 0.5 µm to 1.0 µm diameter, stacked parallel to each other with distinct cis (forming) and trans (maturing) faces.

• • Match: B -> IV.

In the stroma of chloroplasts, a number of organized flattened membranous sacs are present, called thylakoids. Thylakoids are arranged in stacks like piles of coins called grana.

• • Match: C -> I.

Phospholipids form the foundational continuous bimolecular lipid bilayer of the cell membrane, with hydrophilic polar heads pointing outward and hydrophobic fatty acid tails pointing inward.

• • Match: D -> III.
• • Full Match: A-II, B-IV, C-I, D-III (Option 2).

Plastids are found in all plant cells and euglenoids. Based on the presence or absence of specific pigments, Schimper classified plastids into three types: 1. Chloroplasts: Contain chlorophyll and carotenoid pigments for photosynthesis. 2. Chromoplasts: Contain fat-soluble carotenoid pigments such as carotene and xanthophylls, imparting yellow, orange, or red color to petals, pericarp, and ripening fruits. 3. Leucoplasts: Colourless plastids of varied shapes and sizes that store nutrients.

• • Xanthophyll is an oxygenated carotenoid (yellow pigment).

Leucoplasts store reserve foods and do NOT store pigments: - Amyloplasts: Store carbohydrates (starch, e.g., potato tuber). - Elaioplasts: Store oils and fats. - Aleuroplasts: Store proteins (aleurone grains).

• • Amyloplast and aleuroplast are non-pigmented nutrient storage plastids.

The plastid that stores xanthophyll is known as chromoplast.

• • Matches Option (2).

While each membranous organelle in a eukaryotic cell is distinct in structure and biochemistry, several organelles function in close coordination in the synthesis, processing, packaging, and vesicular transport of biomolecules. Together, these form the endomembrane system.

• • Endoplasmic reticulum: Synthesizes proteins and lipids.
• • Golgi complex: Modifies, sorts, and packages materials into vesicles.
• • Lysosomes: Carry out intracellular digestion using hydrolytic enzymes.
• • Vacuoles: Store water, sap, excretory products, and osmotic solutes.

Organelles such as mitochondria and chloroplasts (which are semiautonomous and have their own circular DNA and 70S ribosomes) and peroxisomes carry out metabolic reactions (cellular respiration, photosynthesis, photorespiration/peroxide degradation) that are not coordinated with vesicular secretory transport. Hence, they are excluded.

• • Options (2), (3), and (4) all contain chloroplasts and peroxisomes, invalidating them immediately.

Option (1) contains solely the four canonical members: endoplasmic reticulum, Golgi complex, lysosomes, and vacuole.

• • Matches official NTA final answer key Option A / (1).

The endoplasmic reticulum (ER) forms an extensive interconnected network of branching tubules and flattened cisternae in the eukaryotic cytoplasm. When ribosomes are studded along the cytoplasmic surface, it is termed Rough ER (RER); when ribosomes are absent, it is termed Smooth ER (SER).

• • Continuous with the outer nuclear membrane via RER.

The biochemical machinery embedded within SER membranes catalyzes phospholipid, cholesterol, and fatty acid biosynthesis. In animal endocrine cells (e.g. adrenal cortex, gonads), SER synthesizes lipid-like steroid hormones (androgens, estrogens, progesterone). Furthermore, in liver hepatocytes, SER mediates xenobiotic detoxification.

• • NCERT verbatim: "The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER."

Option (1) describes RER. Option (2) accurately describes SER. Option (3) describes RER. Option (4) is incorrect (carbohydrate polymers are synthesized in Golgi or plastids). Option (2) is correct.

• • Matches official NTA final answer key Option B / (2).

Prokaryotes lack a membrane-delimited nucleus (their genetic material is an unclad circular naked DNA nucleoid) and lack membrane-bound cytoplasmic organelles (mitochondria, chloroplasts, Golgi, ER). In contrast, eukaryotes possess a double-membraned nucleus and compartmentalized membrane-bound organelles.

• • All prokaryotes belong to Kingdom Monera.

Bacteria, Blue-green algae (cyanobacteria, like Nostoc and Anabaena), and Mycoplasma (PPLO) are all definitive members of Kingdom Monera and are typical prokaryotic organisms.

• • All three are prokaryotes.

Fungi (e.g. Yeast, Rhizopus, Agaricus) constitute a separate eukaryotic kingdom of heterotrophic organisms characterized by chitinous cell walls, true nuclei, linear chromatin organized with histones, and 80S cytosolic ribosomes.

• • Matches official NTA final answer key Option D / (4).

In the introductory overview to Unit 3 (Cell: Structure and Functions), the philosophy of modern biological research is described:

• • Living processes are analyzed in terms of the properties of individual molecules, reaction kinetics, chemical thermodynamics, and atomic interactions.

Contrast reductionism with other biological frameworks:

• • Physiological approaches study functioning at the tissue, organ, or systemic level (holistic/integrative biology).
• • Behavioural approaches study ethology and organismal interactions.
• • The canonical definition of reductionist biology specifically combines physical and chemical methodologies: "Physico-chemical approach".
• • Matches Option (1) verbatim.

Ribosomes are granular ribonucleoprotein particles discovered by George Palade (1953). The sedimentation coefficient 'S' (Svedberg unit) measures sedimentation rate during ultracentrifugation, reflecting density and size:

• • Statement A is completely TRUE.

Each functional ribosome consists of two non-identical subunits that associate in the presence of magnesium ions (Mg²⁺):

• • Statement B is TRUE (each ribosome has two subunits).
• • Statement C is TRUE (80S has 60S + 40S; 70S has 50S + 30S).
• • Svedberg units are not additive because sedimentation rate depends on surface area and molecular friction as well as mass.

Statements D and E propose incorrect subunit values:

• • Statements D and E are unequivocally FALSE.
• • Therefore, A, B, C are true, matching Option (1).

Examine each cellular component and its definitive physiological/structural hallmark:

• • A. Centromere: Every chromosome essentially has a primary constriction called the centromere on the sides of which disc-shaped structures called kinetochores are present. These serve as the sites of attachment of spindle fibres during cell division (mitosis and meiosis) to direct chromosome segregation. Hence, A matches II.
• • B. Cilium: Cilia are small hair-like structures which work like oars, causing the movement of either the cell or the surrounding fluid (cell movement/motility). Hence, B matches III.
• • C. Cristae: The inner membrane of a mitochondrion forms a number of infoldings called cristae towards the matrix, which increase the surface area for ATP-generating electron transport complexes. Hence, C matches I.
• • D. Cell membrane: The basic framework of the plasma membrane is a quasi-fluid bimolecular layer of amphipathic phospholipids (phospholipid bilayer) with embedded proteins. Hence, D matches IV.

Combining the verified associations:

• • A -> II (Cell division)
• • B -> III (Cell movement)
• • C -> I (Mitochondrion)
• • D -> IV (Phospholipid Bilayer)

The correct matching option is A-II, B-III, C-I, D-IV, exactly matching Option (4).

In prokaryotes, membrane-bound organelles (like mitochondria, Golgi apparatus, ER) are absent. Instead, extensions of the plasma membrane invaginate into the cytoplasm to form specialised membranous structures called mesosomes:

• • These extensions can be in the form of vesicles, tubules, and lamellae.
• • They are characteristic features of prokaryotes (predominantly Gram-positive bacteria).

According to NCERT, mesosomes perform vital coordinated cellular roles:

• • 1. Cell wall synthesis: Assist in synthesizing and assembling peptidoglycan layers.
• • 2. DNA replication and segregation: Help in anchoring the bacterial chromosome during binary fission and distributing replicated DNA to daughter cells.
• • 3. Respiration: Carry respiratory electron transport chains and dehydrogenases (functioning analogous to mitochondrial cristae).
• • 4. Secretion processes and surface area enhancement for enzymatic activities.

Evaluating the other options:

• • Chromatophores: Pigment-containing membrane extensions in photosynthetic prokaryotes (e.g. cyanobacteria).
• • Cristae: Infoldings of the inner membrane of mitochondria in eukaryotic cells.
• • Endoplasmic Reticulum: A eukaryotic endomembrane network, completely absent in prokaryotes.

The prokaryotic membranous structure responsible for cell wall synthesis, DNA replication, and respiration is the Mesosome.

The endomembrane system functions as an integrated secretory conduit:

• • Proteins and lipids synthesized in the rough and smooth endoplasmic reticulum (RER and SER) must be post-translationally modified, sorted, and routed.
• • The Golgi apparatus principally performs the function of packaging these materials into membrane-bound vesicles, targeting them either to specific intracellular destinations (e.g., lysosomes) or secreting them extracellularly via exocytosis.
• • Therefore, Assertion (A) is completely TRUE.

Structural and functional polarity of Golgi cisternae:

• • Golgi cisternae are concentrically arranged near the nucleus with distinct polarity: a convex forming face (*cis* face) and a concave maturing face (*trans* face).
• • Vesicles transport synthesized proteins/lipids from the ER and fuse with the *cis* face.
• • As these cargo molecules transit through the medial cisternae, they undergo crucial biochemical modifications (e.g., glycosylation into glycoproteins and glycolipids).
• • Finally, the modified molecules are pinched off in secretory vesicles from the *trans* face.
• • Therefore, Reason (R) is completely TRUE.

Reason (R) describes the exact mechanistic basis of how the Golgi apparatus functions as the cell's packaging and distribution center for ER products. Hence, Reason (R) is the correct explanation of Assertion (A).

The nucleolus is a dense membraneless region in the nucleoplasm that serves as the site of active ribosomal RNA (rRNA) synthesis (A -> III).

Centrioles are cylindrical structures made of nine evenly spaced peripheral triplets of tubulin fibrils, arranged like a cartwheel (9+0 pattern) (B -> II).

Leucoplasts are colourless plastids specialized for nutrient storage (C -> IV). Golgi apparatus packages and modifies proteins, serving as the critical site for the formation of glycoproteins and glycolipids (D -> I).

The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes.

Plastids and mitochondria evolved from endosymbiotic prokaryotes. As in bacteria, their genetic material consists of closed circular double-stranded DNA (dsDNA), distinct from the linear chromatin of the eukaryotic nucleus.

Chloroplast DNA is circular and double stranded, matching Option (4).

The core of cilium or flagellum is called axoneme, possessing nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules ($9+2$ array) (A -> II). The central hub and radial spokes of a centriole give it a cartwheel appearance (B -> I).

The inner membrane of a mitochondrion forms a number of infoldings called cristae towards the matrix to increase surface area (C -> IV). A few chromosomes have non-staining secondary constrictions at a constant location, giving the appearance of a small fragment called satellite (SAT chromosome) (D -> III).

A-II, B-I, C-IV, D-III matches Option (2).

Both mitochondria and chloroplasts are semi-autonomous cell organelles enclosed by a double-membrane envelope consisting of an outer membrane and an inner membrane separated by an intermembrane space. Statement I is correct.

In NCERT under Chloroplasts, the text states: "Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable." This comparison is between the inner and outer membranes of the chloroplast itself. Statement II incorrectly frames this as a cross-organellar comparison between mitochondria and chloroplasts, making it factually incorrect.

Statement I is correct but Statement II is incorrect, matching Option (1).

The cytoskeleton is an elaborate eukaryotic network of proteinaceous filaments extending throughout the cytoplasm:

NCERT states verbatim: "The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell."

Transportation across membranes is handled by carrier proteins and vesicles; nuclear division (karyokinesis) is a broad cell division process of which spindle assembly is just one tool; protein synthesis is carried out by ribosomes, not the cytoskeleton.

The endomembrane system comprises membrane-bound organelles whose metabolic pathways and vesicular transport functions are coordinated in a continuum:

NCERT states verbatim: "While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated. The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system."

Mitochondria (A), Chloroplasts (C), and Peroxisomes (E) are not part of the system. This corresponds uniquely to Option (3).

Based on the relative position of the primary constriction (centromere), eukaryotic chromosomes are classified into four distinct types:

During anaphase separation, centromeric spindle attachment creates distinctive shapes: - Metacentric: V-shaped. - Sub-metacentric: L-shaped. - Acrocentric: J-shaped. - Telocentric: I-shaped. Since the question specifies two equal arms with a central centromere, it corresponds unequivocally to metacentric.

Sieve tube elements are elongated, conducting cells of the phloem in angiosperms. To minimize resistance to the bulk flow of organic sap (sucrose), they undergo selective autophagy during differentiation:

At functional maturity: - They lack a nucleus (enucleated). - They possess a thin peripheral layer of cytoplasm and a large central vacuole. - Their metabolic functions and pressure gradients are controlled by the prominent nucleus of adjacent companion cells connected via plasmodesmata.

Evaluating statements 2, 3, and 4 confirms they are all textbook-accurate statements: - Microbodies occur in both plants and animals. - The space between the two nuclear membranes is the perinuclear space ($10-50\text{ nm}$). - Nuclear pores are octagonal protein complexes providing bidirectional movement of macromolecules.

Analyzing the specific cytological descriptions defined in cell biology:

Matching (a)-(iii) immediately eliminates Options (1), (2), and (4). Verifying (b)-(iv), (c)-(i), and (d)-(ii) confirms Option (3) is 100% uniquely valid.

While each membranous organelle in a eukaryotic cell is morphologically and biochemically distinct, four organelles are grouped together as the endomembrane system because their physiological activities are tightly coordinated along the biosynthetic-secretory pathway: - Endoplasmic Reticulum (ER): Synthesizes polypeptides (RER) and lipids/steroids (SER), packaged into transport vesicles. - Golgi Complex: Receives transport vesicles on its *cis* face, modifies/glycosylates proteins, and sorts them from the *trans* face. - Lysosomes: Bud off from Golgi cisternae containing acid hydrolases for intracellular degradation. - Vacuoles: Membrane-bound storage chambers surrounded by the tonoplast that maintain cellular turgidity and sequester waste products.

Mitochondria, chloroplasts, and peroxisomes are membrane-bound, but their biogenesis, protein targeting, and metabolic functions are not coordinated with the ER-Golgi vesicular transport network. Furthermore, mitochondria and chloroplasts are semi-autonomous endosymbiotic organelles possessing their own circular DNA and 70S ribosomes. Ribosomes are non-membrane bound ribonucleoprotein complexes.

Prokaryotic cells (bacteria and cyanobacteria) lack membrane-delimited organelles. Metabolic reserves are stockpiled in specialized dense aggregations known as inclusion bodies: - Reserve Storage: They represent non-living deposits of reserve materials accumulated during nutrient excess to support the cell during starvation. - *Phosphate granules (volutin):* Store inorganic polyphosphate. - *Cyanophycean granules:* Store protein reserves (arginine-aspartate polymers). - *Glycogen granules:* Store carbohydrates. - Membrane-less Structure: They are not surrounded by any unit lipid bilayer membrane; they lie free in the cytoplasmic cytosol.

Prokaryotes are enveloped by a rigid peptidoglycan cell wall and absorb nutrients strictly via transmembrane transport systems (permeases, active transport, group translocation). Phagocytosis or particulate food ingestion does not occur in prokaryotes; food vacuoles are eukaryotic adaptations (e.g. *Amoeba*).

First observed by Camillo Golgi in 1898, the Golgi complex consists of stacked, curved cisternae arranged concentrically near the nucleus with a convex *cis* (forming) face and a concave *trans* (maturing) face: - Post-Translational Glycosylation: Proteins translated on rough endoplasmic reticulum (RER) ribosomes and lipids synthesized in smooth endoplasmic reticulum (SER) bud off as transport vesicles that fuse with the *cis* cisternae. - As these precursors migrate through medial to *trans* cisternae, resident glycosyltransferase enzymes attach carbohydrate oligosaccharides to: 1. Polypeptides via N-linked and O-linked glycosylation, forming Glycoproteins. 2. Sphingolipids and glycerol-lipids, forming Glycolipids. - NCERT explicitly designates the Golgi apparatus as the important site of formation of glycoproteins and glycolipids.

- Endoplasmic reticulum: Site of initial protein synthesis (RER) and steroid/phospholipid synthesis (SER), but the terminal completion of glycoproteins and glycolipids is the hallmark role of the Golgi. - Peroxisomes: Microbodies performing hydrogen peroxide metabolism (catalase) and photorespiration in plants. - Polysomes: Clusters of ribosomes simultaneously translating a single mRNA strand into polypeptides.