Cell Structure — Set 5

Correct Answer: A. Mature Red blood cell

• **Mature Red Blood Cell (RBC)** = The only human cell that deliberately ejects its nucleus during differentiation in the bone marrow, becoming a non-nucleated biconcave disc at maturity • Removing the nucleus frees up approximately one-third of the cell volume, allowing it to pack ~270 million hemoglobin molecules per cell and maximise oxygen-carrying capacity; the lifespan of an RBC is ~120 days • Nucleated RBCs in the bloodstream of adults can signal disease — conditions like bone marrow stress or severe anaemia may release immature nucleated RBCs prematurely • 💡 Option B (Muscle cell) retains multiple nuclei (skeletal muscle is multinucleated) to manage its large cytoplasmic volume; Option C (Nerve cell) retains a nucleus to continuously synthesise proteins needed for long axons; Option D (White blood cell) must have a nucleus to carry out immune functions like producing antibodies and dividing

Correct Answer: A. Protein synthesis

• **Protein synthesis** = The primary function of ribosomes; they read messenger RNA (mRNA) sequences and link amino acids together via peptide bonds to build polypeptide chains — a process called translation • Ribosomes consist of two subunits (large and small): in eukaryotes they are 80S (60S + 40S); in prokaryotes 70S (50S + 30S). Free ribosomes make cytoplasmic proteins; membrane-bound ribosomes (on rough ER) make secretory proteins • Antibiotics like streptomycin and erythromycin target bacterial 70S ribosomes specifically, killing bacteria without harming human 80S ribosomes • 💡 Option B (Lipid storage) is the function of the vacuole or fat droplets, not ribosomes; Option C (ATP production) is the function of mitochondria; Option D (DNA replication) occurs in the nucleus using DNA polymerase enzymes, not ribosomes

Correct Answer: A. Mitochondria

• **Mitochondria** = Double-membraned organelle that produces the bulk of a cell's ATP through aerobic cellular respiration, earning it the nickname 'powerhouse of the cell' • Mitochondria contain their own circular DNA, 70S ribosomes, and can self-replicate — evidence supporting the endosymbiotic theory that they evolved from ancient aerobic bacteria engulfed by a host cell • A single liver cell can contain 1,000–2,000 mitochondria; cells with higher energy demands (e.g., cardiac muscle) contain proportionally more • 💡 Option B (Nucleus) controls all cellular activities but is called the 'control centre', not powerhouse; Option C (Golgi body) packages and dispatches proteins, not energy production; Option D (Chloroplast) is present only in plant cells and captures solar energy — it is called the 'kitchen of the cell', not the powerhouse

Correct Answer: B. Cellulose

• **Cellulose** = A long-chain polysaccharide made of β-1,4-glycosidic bonds between glucose units, forming rigid microfibrils that give plant cell walls their structural strength • Cellulose is the most abundant organic polymer on Earth; humans cannot digest it (no cellulase enzyme) so it acts as dietary fibre, while microorganisms in ruminant guts break it down via cellulase • The plant cell wall has three layers: middle lamella (pectin), primary wall (cellulose + pectin), and secondary wall (cellulose + lignin in woody plants) • 💡 Option A (Chitin) is the structural polysaccharide in fungal cell walls and insect exoskeletons, not plant cell walls; Option C (Glycogen) is the animal/fungal storage carbohydrate made of α-glycosidic bonds; Option D (Starch) is the plant storage polysaccharide, not a structural component of the cell wall

Correct Answer: A. Lysosome

• **Lysosome** = A membrane-bound organelle containing over 50 types of hydrolytic (digestive) enzymes that operate optimally at pH ~5, earning it the nickname 'suicide bag' because rupture of lysosomes can digest the cell from within • Lysosomes are formed when vesicles from the Golgi apparatus fuse with early endosomes; they digest worn-out organelles (autophagy), foreign bacteria (phagocytosis), and cellular debris • In Tay-Sachs disease, a lysosomal enzyme (hexosaminidase A) is absent, causing toxic accumulation of gangliosides in nerve cells — a classic lysosomal storage disorder • 💡 Option B (Vacuole) is a storage organelle for water and nutrients, not a digestion centre with hydrolytic enzymes; Option C (Ribosome) synthesises proteins and has no digestive function; Option D (Mitochondria) produces ATP and is called the powerhouse, not the suicide bag

Correct Answer: C. Nucleus

• **Nucleus** = The membrane-bound organelle containing the cell's entire genetic blueprint (DNA packaged as chromatin), which it uses to direct all cellular activities through transcription and gene regulation • The nucleus is bounded by a double-membrane nuclear envelope perforated by nuclear pores; it contains the nucleolus where ribosomal RNA is made, and the nucleoplasm fluid • Often called the 'control centre' of the cell — removal of the nucleus (enucleation) causes the cell to cease directed functions and eventually die • 💡 Option A (Cytoplasm) is the gel-like substance filling the cell outside the nucleus — it suspends organelles but does not contain the genetic blueprint; Option B (Mitochondria) produces ATP but takes its instructions from the nucleus; Option D (Cell Membrane) regulates what enters and exits the cell but does not control cellular activities or house DNA

Correct Answer: C. Cytoplasm

• **Cytoplasm** = The gel-like, semi-fluid substance that fills the interior of the cell between the plasma membrane and the nucleus, serving as the medium in which all organelles are suspended • Cytoplasm consists of the cytosol (aqueous solution of salts, proteins, and metabolites), organelles, and the cytoskeletal network; many metabolic reactions (glycolysis, protein synthesis by free ribosomes) occur directly in the cytosol • The term cytoplasm includes everything inside the plasma membrane excluding the nucleus; the nucleus contains nucleoplasm, not cytoplasm • 💡 Option A (Nucleoplasm) is the fluid specifically inside the nucleus, not outside it; Option B (Plasma) refers to blood plasma, not a cell component; Option D (Cell sap) is the fluid inside the vacuole of plant cells, not the general cell filler

Correct Answer: A. Plasma Membrane

• **Plasma Membrane** = A fluid mosaic of phospholipid bilayer embedded with proteins that is selectively permeable — it allows certain molecules to pass freely while restricting or actively controlling the movement of others • The selective permeability arises from the hydrophobic core of the lipid bilayer (blocks water-soluble ions and large molecules) combined with specific channel and carrier proteins that create regulated pathways • This property is essential for maintaining the cell's internal environment (homeostasis) — it allows nutrients in, exports waste, and maintains ion gradients vital for nerve and muscle function • 💡 Option B (Nuclear Membrane) is a boundary within the cell that separates the nucleus — it is not responsible for the cell's selective permeability to the external environment; Option C (Cell Wall) is fully permeable and provides structural support only — it does not control molecular passage; Option D (Tonoplast) is the membrane of the vacuole in plant cells, not the outer selective barrier of the cell

Correct Answer: B. Chloroplast

• **Chloroplast** = A double-membraned organelle found exclusively in plant and algal cells (never in animal cells) that contains chlorophyll and performs photosynthesis to convert light energy into chemical energy (glucose) • Chloroplasts are absent in animal cells because animals are heterotrophs — they obtain energy by consuming other organisms rather than making their own food through photosynthesis • Like mitochondria, chloroplasts contain their own DNA and ribosomes (evidence for endosymbiotic origin from ancient cyanobacteria) • 💡 Option A (Ribosome) is found in both plant and animal cells (and even prokaryotes); Option C (Mitochondria) is present in both plant and animal cells for ATP production; Option D (Golgi body) is present in both plant and animal cells for protein processing and secretion

Correct Answer: A. Cell

• **Cell** = The smallest structural and functional unit of life that can carry out all basic life processes independently — including metabolism, growth, response to stimuli, and reproduction • Proposed by Robert Hooke (1665) who first observed cells in cork, and formalized in the Cell Theory by Schleiden (1838, plants), Schwann (1839, animals), and Virchow (1855, cells from cells) • All living organisms are made of one or more cells; viruses are not considered 'alive' because they cannot replicate independently and lack cellular structure • 💡 Option B (Organelle) is a sub-cellular structure that performs specific functions but cannot exist independently; Option C (Tissue) is a group of similar cells working together, not the smallest unit; Option D (Organ) is made of multiple tissues and is far larger than the smallest unit of life