STEM Readiness: Chemistry Readiness - Chemical Bonding: Hybridisation - sp, sp², sp³, sp³d, sp³d² Predicting, from Structure

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• DNA in prokaryotes is located in the nucleoid, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes have 70S ribosomes (30S + 50S subunits); eukaryotes have 80S ribosomes (40S + 60S subunits).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotic cell walls typically contain peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Mycoplasma species are bacteria that lack a cell wall, making them resistant to antibiotics like penicillin.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have membrane-bound organelles (e.g., nucleus, mitochondria, lysosomes); prokaryotes do not.
• Endoplasmic reticulum (ER) is present only in eukaryotes; rough ER synthesizes proteins, smooth ER synthesizes lipids and detoxifies.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery; absent in prokaryotes.
• Mitochondria generate ATP via aerobic respiration; they have double membranes, circular DNA, and 70S ribosomes.
• Chloroplasts perform photosynthesis in plants and algae; they have thylakoids, chlorophyll, circular DNA, and 70S ribosomes.
• Lysosomes contain hydrolytic enzymes for degradation; found in animal cells, rare in plant cells (vacuoles perform similar function).
• Vacuoles in plant cells maintain turgor pressure and store nutrients; central vacuole can occupy up to 90% of cell volume.
• Cytoskeleton (microtubules, microfilaments, intermediate filaments) is present in eukaryotes; prokaryotes have homologs (e.g., FtsZ, MreB) but no true cytoskeleton.
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide by mitosis and cytokinesis.
• Bacteria and archaea are prokaryotes; animals, plants, fungi, and protists are eukaryotes.
• Red blood cells in mammals lack a nucleus and most organelles to maximize hemoglobin capacity.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport.
• Nucleolus within the nucleus synthesizes ribosomal RNA and assembles ribosomal subunits.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having independent DNA, 70S ribosomes, and ability to replicate autonomously.
• Plasma membrane fluidity is maintained by cholesterol in animal cells and hopanoids in some bacteria.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on comparison and organelle function.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (other polymers) have different wall compositions.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes have protein-bound compartments (e.g., carboxysomes), but no membrane-bound organelles like nucleus or ER.
Trap: The nucleus is the only site of DNA in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA.
Trap: Eukaryotes evolved from prokaryotes directly – Fact: Eukaryotes likely arose from archaeal ancestors with bacterial endosymbionts (mitochondria).

Practice MCQs (5–7 questions)

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Phospholipid bilayer plasma membrane
D) Mitochondria
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (mitochondria) are only in eukaryotes.

Question: A cell is observed to contain circular DNA, 70S ribosomes, and a cell wall with peptidoglycan. It lacks a nucleus. The cell is most likely:
A) A plant cell
B) A fungal cell
C) A bacterium
D) An animal cell
Answer: C
Explanation: Presence of peptidoglycan, 70S ribosomes, and nucleoid identifies it as a bacterium.
Why the top distractor is wrong: A (plant cells) have cellulose walls, 80S ribosomes, and a nucleus.

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Nucleolus
B) Smooth ER
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies and packages proteins from the ER.
Why the top distractor is wrong: B (smooth ER) synthesizes lipids and detoxifies, but does not package proteins.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Mitochondria use 80S ribosomes
B) Mitochondria have circular DNA and divide independently
C) Lysosomes contain digestive enzymes
D) The nucleus has double membrane
Answer: B
Explanation: Mitochondria having circular DNA and independent division supports bacterial origin.
Why the top distractor is wrong: A is false—mitochondria have 70S ribosomes, not 80S.

Question: Which cell type lacks a nucleus but contains hemoglobin?
A) Yeast cell
B) Escherichia coli
C) Human red blood cell
D) Onion root cell
Answer: C
Explanation: Mature mammalian red blood cells lack a nucleus and organelles but contain hemoglobin.
Why the top distractor is wrong: B (E. coli) lacks hemoglobin.

Question: Which structure is found in plant cells but not in animal cells?
A) Mitochondria
B) Central vacuole
C) Plasma membrane
D) Ribosomes
Answer: B
Explanation: The large central vacuole is a defining feature of plant cells.
Why the top distractor is wrong: A (mitochondria) are present in both plant and animal cells.

Question: What is the primary component of fungal cell walls?
A) Peptidoglycan
B) Cellulose
C) Chitin
D) Silica
Answer: C
Explanation: Fungi have cell walls made of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: A (peptidoglycan) is found only in bacteria.

Last?Minute Revision (20–25 one?liners)

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Bacteria: peptidoglycan cell wall; Archaea: no peptidoglycan.
• Plant cell wall: cellulose; Fungal cell wall: chitin; Animal: no cell wall.
• Mycoplasma – smallest known bacteria, lacks cell wall.
• Both cell types have phospholipid bilayer plasma membrane.
• Eukaryotes have membrane-bound organelles; prokaryotes do not.
• Rough ER: protein synthesis; Smooth ER: lipid synthesis, detoxification.
• Golgi apparatus: modifies, sorts, packages proteins and lipids.
• Mitochondria: double membrane, site of aerobic respiration, produces ATP.
• Chloroplasts: contain thylakoids, perform photosynthesis, have 70S ribosomes.
• Lysosomes: contain hydrolytic enzymes, break down macromolecules.
• Plant vacuole can occupy up to 90% of cell volume.
• Cytoskeleton: microtubules, microfilaments, intermediate filaments – only in eukaryotes.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• Examples: E. coli – prokaryote; Saccharomyces cerevisiae – eukaryote (fungus).
• Mature mammalian red blood cells lack nucleus and mitochondria.
• Nuclear pores regulate molecular traffic between nucleus and cytoplasm.
• Nucleolus: site of rRNA synthesis and ribosome assembly.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria, chloroplasts from cyanobacteria.
• Cholesterol in animal membranes stabilizes fluidity; hopanoids in some bacteria.
• Gram-negative bacteria have outer membrane with lipopolysaccharide (LPS).
• Prokaryotic flagellum: rotates, made of flagellin; eukaryotic: 9+2 microtubules, bends.
• Verify from standard textbook: exact size ranges and ribosomal subunit compositions.