- Dr.Sanjaykumar Pawar
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| Eukaryotic cell structure — the foundation of NEET's most-tested Cell Biology chapter." |
Internal Links
CBSE Class 11 Biology Notes: Introduction to Plant Physiology — natural "explore more Class 11 notes"
Small Molecular Weight Organic Compounds in Living Tissues (Biomolecules Notes) — links cell chemistry to cell structure
Co-factors in Enzymes Class 11 Biology Notes — enzymes are discussed in relation to organelle function (e.g., lysosomal hydrolases)
Factors Affecting Enzyme Activity — same enzyme-topic cluster, good "related reading"
"Cell The Unit of Life" label page and "Golgi apparatus function," "Ribosomes," "Cilia," "Centrioles," "Nucleus" label pages — link key terms in the post directly to these existing label archives for topical internal linking
NEET-UG BIOLOGY MOCK TEST
SECTION A — Cell Theory, Overview & Prokaryotic Cell (Q1–Q10)
Which of the following correctly extends the classical cell theory to include the concept added later by Rudolf Virchow?
An E. coli cell is about 1–2 µm in length, whereas most eukaryotic cells range from 10–100 µm. Which statement best explains a functional consequence of this size difference?
Which of the following is NOT a defining feature distinguishing prokaryotic cells from eukaryotic cells?
Consider the following statements about the bacterial cell envelope: (i) The glycocalyx may exist as a loose sheath called the slime layer or a thick, tough capsule. (ii) The cell wall of Gram-positive bacteria contains a thick peptidoglycan layer. (iii) The plasma membrane is freely permeable to all substances. How many of the above statements are correct?
Mesosomes in bacterial cells are extensions of the plasma membrane. Which of the following is NOT a proposed function of mesosomes?
Which of the following statements about bacterial flagella is correct?
Pili and fimbriae in bacteria are:
Ribosomes and inclusion bodies in a bacterial cell are best described as:
Which of the following statements regarding the nucleoid of a bacterial cell is correct?
Which of the following cell types is correctly matched with its category?
SECTION B — Cell Envelope: Membrane & Cell Wall (Q11–Q16)
The Fluid Mosaic Model of the plasma membrane, proposed by Singer and Nicolson (1972), describes the membrane as:
Integral proteins of the plasma membrane are distinguished from peripheral proteins because integral proteins:
Movement of water across a selectively permeable plasma membrane from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) is termed:
Which statement about the cell wall in plants is correct?
Assertion (A): Endocytosis allows engulfment of external material into vesicles formed by invagination of the plasma membrane. Reason (R): Amoeba obtains food through phagocytosis, a form of endocytosis. Choose the correct option.
The primary function of the cell wall in a plant cell is to:
SECTION C — Endomembrane System: ER, Golgi, Lysosomes, Vacuoles (Q17–Q25)
Which of the following organelles is generally NOT considered a part of the endomembrane system, since its function is not coordinated with the others?
Which of the following is true regarding rough and smooth endoplasmic reticulum?
Regarding the polarity of the Golgi apparatus, which of the following correctly describes its two faces?
Besides packaging materials for secretion, the Golgi apparatus is also involved in:
Lysosomes are correctly described by which of the following statements?
Rupture of the lysosomal membrane within a cell can lead to:
Which of the following statements about vacuoles is INCORRECT?
Which of the following correctly traces the pathway of a secretory protein destined for export out of the cell?
Sap vacuoles in plant cells contain, in solution, various substances including:
SECTION D — Mitochondria, Plastids & Ribosomes (Q26–Q34)
Mitochondria are often called the "powerhouse of the cell" because they:
Which of the following statements about mitochondrial structure is correct?
Mitochondria are described as semi-autonomous organelles primarily because they:
Which plastid type is correctly matched with its pigment/function?
With respect to chloroplast internal organisation, which statement is correct?
Which of the following correctly compares ribosomes across different cellular locations?
The 70S ribosome is composed of a larger 50S and a smaller 30S subunit. This composition is found in:
Ribosomes attached to the endoplasmic reticulum, as well as free ribosomes in the cytoplasm, are the sites of:
Several ribosomes may attach to a single strand of mRNA, forming a chain-like structure. This structure is called a:
SECTION E — Cytoskeleton, Cilia, Flagella & Centrosome (Q35–Q39)
The cytoskeleton, found in the cytoplasm of eukaryotic cells, is made up of a network of which filamentous protein structures?
Besides mechanical support, the cytoskeleton is involved in which of the following functions?
Regarding the internal structure of a cilium or flagellum, which is correct?
Which of the following statements about centrioles is correct?
The centriole forms the basal body of cilia or flagella and also gives rise to which structure during cell division?
SECTION F — Nucleus (Q40–Q45)
The nucleus, discovered by Robert Brown, was described as the "physical basis of heredity" by which scientist, as noted in NCERT?
Which statement about the nuclear envelope is correct?
Chromatin, euchromatin and heterochromatin are best distinguished as follows:
The nucleolus, though not bound by any membrane, is the site of:
With reference to chromosome structure at metaphase, which statement is correct regarding the kinetochore and centromere position?
Which of the following best defines a nucleosome, the fundamental packaging unit of chromatin?
ANSWER KEY
DETAILED SOLUTIONS
Schleiden and Schwann proposed the cell theory stating all living organisms are composed of cells and cell products. Rudolf Virchow (1855) extended it with the crucial addition that all cells arise from pre-existing cells (Omnis cellula-e cellula), explaining cell origin through division rather than spontaneous generation.
A smaller cell has a proportionally larger surface area relative to its volume (surface area to volume ratio is inversely related to size). This favours efficient exchange of materials by diffusion, which is one reason prokaryotic and small cells function efficiently without extensive internal compartmentalisation.
Ribosomes ARE present in prokaryotic cells (as 70S ribosomes) — their absence is not a distinguishing feature. Prokaryotes lack a membrane-bound nucleus and membrane-bound organelles, and typically possess a single circular DNA molecule, but they do possess ribosomes for protein synthesis.
Statements (i) and (ii) are correct as per NCERT: the glycocalyx varies from a loose slime layer to a thick capsule, and Gram-positive bacterial walls have a thick peptidoglycan layer. Statement (iii) is false — the plasma membrane is selectively permeable, not freely permeable to all substances.
Mesosomes are folded, convoluted extensions of the plasma membrane in bacteria and help in cell wall formation, DNA replication and distribution to daughter cells, and also aid in respiration, secretion, and increasing the surface area of the plasma membrane along with enzymatic content. Option D describes mitochondrial cristae function (eukaryotic), not mesosomes — bacteria lack mitochondria.
A bacterial flagellum has three parts: filament (made of the protein flagellin, the longest portion, extending outside the cell), hook, and basal body. The filament rotates like a propeller using energy from the basal body, driving motility — this contradicts option B. Fimbriae aid attachment (not locomotion), reversing option C. Not all bacteria have flagella, so D is incorrect.
Fimbriae and pili are minute, hair-like, filamentous protein structures found on many bacteria; unlike flagella, they are not involved in locomotion. Pili typically help in conjugation (DNA transfer) between bacterial cells; fimbriae assist in attachment/adhesion.
Bacterial ribosomes are 70S, non-membrane-bound, and found in large numbers freely in the cytoplasm. Inclusion bodies such as phosphate granules, cyanophycean granules and glycogen granules also lie freely in the cytoplasm without a boundary membrane, unlike the membrane-bound reserve material seen in some eukaryotic cells.
The bacterial nucleoid is NOT membrane-bound (contradicts A). Bacteria may carry extra-chromosomal circular DNA called plasmids, which confer special phenotypic traits such as resistance to antibiotics, and are widely exploited as vectors in recombinant DNA technology — contradicting D. The nucleoid contains genomic DNA, not solely RNA, so C is wrong.
Blue-green algae (cyanobacteria), like all bacteria and mycoplasma, are prokaryotic. Mycoplasma is also prokaryotic (not eukaryotic, contradicting A), yeast is a unicellular eukaryote (not prokaryotic, contradicting C), and PPLO (Pleuropneumonia-Like Organisms, i.e., mycoplasma) are unicellular prokaryotes, not multicellular eukaryotes (contradicting D).
Singer and Nicolson's Fluid Mosaic Model describes the membrane as quasi-fluid, allowing lateral movement of lipids and most proteins within the bilayer, giving the membrane its dynamic, mosaic-like organisation — not a rigid or static structure.
Integral proteins are partially or completely embedded within the lipid bilayer and require detergents/organic solvents for isolation, unlike peripheral proteins, which lie on the surface and can be easily separated by mild treatments.
Osmosis is defined as the movement of water molecules from a region of higher water potential (i.e., a dilute solution) to a region of lower water potential (a concentrated solution) through a selectively permeable membrane — a special case of diffusion, not requiring ATP.
The middle lamella, made mainly of calcium pectate, cements together the walls of adjacent cells. The primary wall is present in young, growing cells, and the secondary wall is deposited later on the inner side — reversing option A. Animal cells lack a cell wall, so C and D are incorrect (plasmodesmata are channels connecting plant cells across walls, not present in animal cell walls, which don't exist).
Both statements are true. In endocytosis, the plasma membrane invaginates to engulf external particles or fluid, forming a vesicle. Amoeba is the classic NCERT example, engulfing food particles by phagocytosis (a type of endocytosis), which correctly explains the assertion.
The cell wall's chief roles are to give shape to the cell and to protect it from mechanical injury and infection. It also allows easy passage of molecules through it but is not selective in the way the plasma membrane is (that is a membrane function, not a wall function), so A is not the best answer.
The endomembrane system comprises organelles whose functions are coordinated — ER, Golgi complex, lysosomes and vacuoles. Mitochondria (as well as chloroplasts and peroxisomes) are excluded because their functions are not coordinated with this system.
SER lacks ribosomes on its surface and is the major site of lipid synthesis; in steroid hormone-producing cells (e.g., adrenal cortex, gonads), SER is prominent for steroidal hormone synthesis. RER, studded with ribosomes, is chiefly involved in protein synthesis and secretion, not steroid synthesis.
The cis (forming) face of the Golgi apparatus is convex and faces the endoplasmic reticulum, receiving transport vesicles from it. The trans (maturing) face is concave and faces the plasma membrane, dispatching processed, modified material from the Golgi. Option B reverses this polarity, option C is false since the two faces differ both structurally and functionally, and option D is incorrect as the Golgi does not receive material directly from mitochondria.
Beyond packaging and secretion, the Golgi apparatus plays a crucial role in forming glycoproteins and glycolipids through glycosylation of proteins and lipids received from the ER.
Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. They contain hydrolytic enzymes (lipases, proteases, carbohydrases) capable of digesting all major classes of biomolecules, and these enzymes work optimally at the acidic pH maintained inside the lysosome — not at alkaline pH.
If the lysosomal membrane ruptures, the released hydrolytic enzymes digest the cell's own organic material, a phenomenon exploited during autolysis — hence lysosomes are sometimes called "suicide bags" of the cell.
Statement D is incorrect (hence the answer, since the question asks for the incorrect statement): mature plant cells often have a large central vacuole that can occupy up to 90% of cell volume, playing an important osmotic role in maintaining cell turgidity, not less than 5% with no osmotic role. A, B and C are all correct as per NCERT.
Newly synthesised proteins on the RER are transported via transport vesicles to the cis face of the Golgi apparatus, processed and modified as they move from cis to trans cisternae, and then packaged into secretory vesicles at the trans face, which fuse with the plasma membrane to release contents outside the cell.
Sap vacuoles contain water along with various organic and inorganic substances dissolved in it, including amino acids, sugars, organic acids, and some proteins — not exclusively nucleic acids, pigments, or structural cellulose.
Mitochondria are the sites of aerobic respiration; they produce cellular energy in the form of ATP, earning them the title "powerhouse of the cell."
The mitochondrion is a double membrane-bound organelle; the outer membrane is smooth, while the inner membrane forms infoldings called cristae, which increase surface area for ATP synthesis. The mitochondrial matrix contains single circular DNA, RNA molecules, ribosomes (70S), and enzymes required for the Krebs cycle.
Mitochondria divide by fission and possess their own circular DNA and 70S ribosomes, allowing them to synthesise some of their own proteins; however, they remain dependent on nuclear-encoded proteins for many of their components, hence the term "semi-autonomous."
Chloroplasts contain chlorophyll pigments and are the sites of photosynthesis. Chromoplasts (not leucoplasts) contain carotenoid pigments (fat-soluble) giving yellow, orange, or red colour to flowers/fruits, reversing option B. Leucoplasts (not chromoplasts) are colourless plastids that store nutrients — starch (amyloplasts), oils/fats (elaioplasts), and proteins (aleuroplasts) — reversing option C. Option D is factually wrong since chloroplasts are pigmented, not colourless.
Inside the chloroplast stroma lie flattened membranous sacs called thylakoids, which stack to form grana; adjacent grana are interconnected by flat membranous tubules called stroma lamellae, maintaining structural continuity. Chloroplasts, like mitochondria, possess their own circular DNA and ribosomes (70S), contradicting D. The dark reaction enzymes are located in the stroma, not the thylakoid space, contradicting C.
Prokaryotic ribosomes, and organellar ribosomes within mitochondria and chloroplasts of eukaryotic cells, are 70S (50S + 30S subunits). Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits) — this reflects the endosymbiotic origin of mitochondria and chloroplasts.
The 70S ribosome (50S large subunit + 30S small subunit) is characteristic of prokaryotic cells and also occurs in the semi-autonomous organelles — mitochondria and chloroplasts — of eukaryotic cells, consistent with their prokaryotic ancestry.
Ribosomes, whether membrane-bound (on RER) or free in the cytoplasm, are the sites of protein synthesis, translating mRNA into polypeptide chains.
When several ribosomes attach to and translate a single mRNA strand simultaneously, the resulting chain-like assembly is called a polysome or polyribosome, which allows multiple copies of a protein to be synthesised from one mRNA at once.
The cytoskeleton network is composed of three main types of protein filaments: microtubules, microfilaments (actin filaments), and intermediate filaments, each contributing differently to cell architecture.
Besides mechanical support and maintaining cell shape, the cytoskeleton participates in cell motility (e.g., via actin-myosin interactions) and helps organise the mitotic spindle during cell division.
The axoneme of cilia and flagella has a characteristic "9 + 2" array: nine peripheral doublet microtubules arranged in a ring around two central single microtubules, all enclosed by the plasma membrane. This arrangement, together with dynein arms, enables movement.
Each centriole is a cylindrical structure made of nine evenly spaced peripheral fibrils, and unlike the axoneme's doublets, each fibril of a centriole is a triplet of tubulin. Two centrioles lie perpendicular (not parallel) to each other in the centrosome, connected by cartwheel-like structures made of protein.
The centriole forms the basal body of cilia and flagella. During cell division, centrioles help organise the spindle fibres/spindle apparatus, which is essential for chromosome segregation, particularly emphasised in animal cell division.
While Robert Brown first described the nucleus (1831), it was Strasburger who, based on further study, described the nucleus as playing a role in heredity, and this concept was later expanded (per NCERT's historical note on the nucleus and heredity).
The nuclear envelope consists of two parallel membranes with a perinuclear space in between; nuclear pores form at points where the two membranes fuse, and these pores allow the exchange of RNA and protein molecules between the nucleus and the cytoplasm.
Euchromatin is the loosely packed, lightly staining, transcriptionally active form of chromatin, while heterochromatin is densely packed, stains darkly, and is largely transcriptionally inactive. Chromatin refers to the entire DNA-protein complex in the interphase nucleus, not solely condensed metaphase chromosomes.
The nucleolus is a dense, non-membrane-bound structure within the nucleus that is the site of active synthesis of ribosomal RNA (rRNA), which combines with proteins to form ribosomal subunits.
A metacentric chromosome has the centromere positioned exactly in the middle, giving two equal arms. An acrocentric chromosome has the centromere very near one end (not the middle), and a telocentric chromosome has the centromere at the terminal end (not near the middle). Kinetochores are disc-shaped structures on the surface of the centromere (not on the arms), where spindle fibres attach during cell division.
The nucleosome is the basic unit of chromatin packaging: a segment of DNA (about 200 bp) wrapped around a core histone octamer (two molecules each of H2A, H2B, H3 and H4), giving chromatin its characteristic "beads-on-a-string" appearance under the electron microscope.
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