CUET UG Biology Cell Biology Cell Structure Prokaryotic vs Eukaryotic Organelles and Functions

Must-Know (15–20 detailed bullets)

• Prokaryotic cells lack a nucleus; genetic material lies in the nucleoid region—example: Escherichia coli.
• Eukaryotic cells have membrane-bound nucleus containing linear DNA associated with histone proteins—example: human liver cell.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); in eukaryotes, they are 80S (60S + 40S)—S = Svedberg units.
• Mitochondria are double-membrane organelles with own circular DNA and 70S ribosomes—site of aerobic respiration.
• Chloroplasts are double-membrane organelles in plant cells containing thylakoids, grana, and stroma; perform photosynthesis.
• Endoplasmic reticulum (ER) is of two types: rough ER (with ribosomes) for protein synthesis; smooth ER (without ribosomes) for lipid synthesis.
• Golgi apparatus consists of cisternae, vesicles, and tubules; modifies, sorts, and packages proteins for secretion—present in all eukaryotes except mature RBCs and spermatozoa.
• Lysosomes contain hydrolytic enzymes (e.g., lipases, proteases) and function in intracellular digestion; formed by Golgi apparatus.
• Peroxisomes contain catalase enzyme that breaks down hydrogen peroxide (H₂O₂) into water and oxygen—abundant in liver and kidney cells.
• Centrosomes, containing two centrioles, organize spindle fibers during cell division in animal cells; absent in higher plants.
• Vacuoles in plant cells are large, central, and maintain turgor pressure; in amoeba, contractile vacuoles regulate osmoregulation.
• Cell wall in prokaryotes is made of peptidoglycan; in plants, it is composed of cellulose; in fungi, chitin.
• Flagella in prokaryotes are structurally simple (made of flagellin) and rotate; eukaryotic flagella are complex (9+2 microtubule arrangement) and whip-like.
• Nuclear envelope is double-membraned with nuclear pores (about 100 nm diameter) allowing RNA and protein transport.
• Nucleolus synthesizes ribosomal RNA (rRNA) and assembles ribosomal subunits—visible during interphase.
• Cytoskeleton consists of microtubules (tubulin), microfilaments (actin), and intermediate filaments—provides structural support and enables cell motility.
• Plasmids are small, circular, extrachromosomal DNA in prokaryotes; used in genetic engineering—e.g., pBR322.
• Mesosomes are infoldings of plasma membrane in prokaryotes, involved in respiration—verify from NCERT.
• Chromatin in eukaryotic nucleus condenses into chromosomes during cell division; composed of DNA and histone proteins.
• Mitochondria and chloroplasts are semi-autonomous due to their own DNA, RNA, and ribosomes—support endosymbiotic theory.

Difficulty Level

Intermediate — requires understanding of structural and functional differences between organelles and cell types, with precise terminology and examples.

Common CUET Traps

• Trap: Assuming lysosomes are present in all eukaryotic cells.
  Avoid: Lysosomes are absent or reduced in some cells like mature mammalian RBCs and certain plant cells.

• Trap: Believing centrioles are present in all eukaryotic cells.
  Avoid: Centrioles are absent in higher plants and some protists; spindle forms via other microtubule organizing centers.

• Trap: Confusing 70S ribosomes as exclusive to prokaryotes.
  Avoid: 70S ribosomes are also found in mitochondria and chloroplasts of eukaryotic cells—supports endosymbiotic origin.

Practice MCQs

1. Which of the following structures is present in prokaryotic cells but lacks in eukaryotic cells?
   
   A. Ribosomes
   
   B. Plasma membrane
   
   C. Nucleoid
   
   D. Cytoplasm
   Answer: C
   Explanation: Nucleoid is the region where prokaryotes store their genetic material without a nuclear envelope.
   Why others fail: Ribosomes, plasma membrane, and cytoplasm are common to both cell types.

2. The organelle responsible for packaging and modification of proteins for secretion is:
   
   A. Lysosome
   
   B. Mitochondria
   
   C. Golgi apparatus
   
   D. Peroxisome
   Answer: C
   Explanation: Golgi apparatus modifies, sorts, and packages proteins into vesicles for secretion.
   Why others fail: Lysosomes digest materials, mitochondria produce ATP, peroxisomes detoxify H₂O₂.

3. Which of the following is NOT a feature of mitochondria?
   
   A. Contains 70S ribosomes
   
   B. Double-membrane bound
   
   C. Involved in lipid synthesis
   
   D. Has its own circular DNA
   Answer: C
   Explanation: Smooth ER, not mitochondria, is primarily involved in lipid synthesis.
   Why others fail: Mitochondria have 70S ribosomes, double membrane, and circular DNA—making them semi-autonomous.

4. In which of the following organisms would you expect to find 80S ribosomes?
   
   A. Mycoplasma
   
   B. Saccharomyces cerevisiae
   
   C. Lactobacillus
   
   D. Cyanobacteria
   Answer: B
   Explanation: Saccharomyces cerevisiae (yeast) is a eukaryote and contains 80S ribosomes.
   Why others fail: Mycoplasma, Lactobacillus, and cyanobacteria are prokaryotes with 70S ribosomes.

5. Which pair correctly matches an organelle with its function and structural feature?
   
   A. Lysosome – protein synthesis – single membrane
   
   B. Chloroplast – photosynthesis – 70S ribosomes
   
   C. Centrosome – DNA replication – double membrane
   
   D. Nucleolus – ATP production – no membrane
   Answer: B
   Explanation: Chloroplasts perform photosynthesis and contain 70S ribosomes, similar to prokaryotes.
   Why others fail: Lysosomes digest but don’t synthesize proteins; centrosomes organize spindles but don’t replicate DNA; nucleolus makes rRNA, not ATP.

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ Prokaryotes: no nucleus, 70S ribosomes, peptidoglycan cell wall.
• ⚠️ Eukaryotes: true nucleus, 80S ribosomes, membrane-bound organelles.
• ⚠️ Mitochondria: power house, double membrane, 70S ribosomes, circular DNA.
• ⚠️ Chloroplasts: site of photosynthesis, contain chlorophyll, have grana and stroma.
• ⚠️ Nucleolus → rRNA synthesis; nucleus → DNA storage.
• ⚠️ SER → lipid synthesis; RER → protein synthesis (due to ribosomes).
• ⚠️ Golgi: cisternae + vesicles → packaging center.
• ⚠️ Lysosome: hydrolytic enzymes → intracellular digestion.
• ⚠️ Peroxisome: catalase → breaks H₂O₂ → H₂O + O₂.
• ⚠️ Vacuole: large in plant cells → turgor pressure.
• ⚠️ Centrioles: 9+0 microtubule triplets → form spindle in animal cells.
• ⚠️ Flagella: prokaryotic = rotation; eukaryotic = 9+2 sliding.
• ⚠️ Cell wall: bacteria = peptidoglycan; fungi = chitin; plants = cellulose.
• ⚠️ Plasmids: extra-chromosomal DNA in bacteria → used in cloning.
• ⚠️ Mesosome: infolding of plasma membrane in bacteria → respiration (verify from NCERT).
• ⚠️ Cytoskeleton: microtubules (tubulin), microfilaments (actin), support & movement.
• ⚠️ Nuclear pore: allows transport of RNA/proteins; part of nuclear envelope.
• ⚠️ Chromatin → condenses into chromosomes during prophase.
• ⚠️ Semi-autonomous organelles: mitochondria & chloroplasts (own DNA & ribosomes).
• ⚠️ Mnemonic: “Mitochondria Makes Money” → main site of ATP production.