STEM Readiness: Biology Readiness Chemistry of Life Water Properties Hydrogen Bonding Cohesion Adhesion Specific Heat Solvent

Must‑Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 μm in diameter; eukaryotic cells range from 10–100 μm.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region lacking a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S subunits).
• Mitochondria and chloroplasts in eukaryotes 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 made of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics targeting peptidoglycan synthesis.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membrane-bound organelles (e.g., ER, Golgi, lysosomes); prokaryotes do not.
• Endoplasmic reticulum (ER) is present only in eukaryotes: rough ER has ribosomes; smooth ER synthesizes lipids.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery—found only in eukaryotes.
• Mitochondria generate ATP via aerobic respiration and have double membranes; absent in prokaryotes.
• Chloroplasts perform photosynthesis in plants and algae; contain thylakoids and chlorophyll; absent in prokaryotes.
• Lysosomes contain hydrolytic enzymes for degradation; present in animal cells but not typical in plant cells.
• Vacuoles in plant cells are large and central, maintaining turgor pressure; animal cells have small, temporary vacuoles.
• Bacteria and archaea are prokaryotes; animals, plants, fungi, and protists are eukaryotes.
• Red blood cells in mammals lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport.
• Nucleolus within the nucleus synthesizes ribosomal RNA and assembles ribosome subunits.
• Flagella differ structurally: prokaryotic flagella are rotary filaments of flagellin; eukaryotic flagella are 9+2 microtubule arrays.
• Cilia are short, numerous motile structures in some eukaryotes (e.g., Paramecium); absent in prokaryotes.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide via mitosis and cytokinesis.
• Plasmids are small, circular DNA molecules in prokaryotes; eukaryotes may have plasmids but rarely.
• Cytoskeleton (microtubules, actin filaments) is present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ) but no true cytoskeleton.

Difficulty Level

Intermediate – expected foundational knowledge in first-semester biology but requires precise differentiation of structures and exceptions.

Common Traps (3–5 factual traps)

• Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; archaea, plants, and fungi have different wall compositions.
• Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in complex eukaryotic cells.
• Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes have protein-bound compartments (e.g., carboxysomes), but no membrane-bound organelles like ER or Golgi.
• Trap: The nucleus is the only site of DNA in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA.
• Trap: Eukaryotic cells always have a nucleus – Fact: Mature red blood cells in mammals lack a nucleus.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Peptidoglycan in the cell wall
D) Endoplasmic reticulum
Answer: C
Explanation: Peptidoglycan is a key component of bacterial cell walls, a hallmark of prokaryotes.
Why the top distractor is wrong: A is incorrect because a membrane-bound nucleus is exclusive to eukaryotes.

Question: Where are 70S ribosomes found in eukaryotic cells?
A) Cytoplasm
B) Nucleus
C) Mitochondria
D) Golgi apparatus
Answer: C
Explanation: Mitochondria contain 70S ribosomes, reflecting their prokaryotic ancestry.
Why the top distractor is wrong: A is incorrect because cytoplasmic ribosomes in eukaryotes are 80S.

Question: Which structure is present in plant cells but not in animal cells?
A) Lysosome
B) Centriole
C) Large central vacuole
D) Plasma membrane
Answer: C
Explanation: A large central vacuole is a defining feature of mature plant cells.
Why the top distractor is wrong: A is incorrect because lysosomes are common in animal cells but rare in plants.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Presence of a nucleoid in bacteria
B) Double membrane of mitochondria
C) 80S ribosomes in the cytoplasm
D) Cell wall in plants
Answer: B
Explanation: The double membrane of mitochondria suggests engulfment by a host cell.
Why the top distractor is wrong: A is a general prokaryotic trait and does not support endosymbiosis.

Question: Which organism lacks a cell wall entirely?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Arabidopsis thaliana
Answer: C
Explanation: Mycoplasma is a bacterium without a cell wall, making it resistant to penicillin.
Why the top distractor is wrong: A has a peptidoglycan cell wall, typical of bacteria.

Question: What is the primary function of the Golgi apparatus?
A) ATP synthesis
B) Protein modification and sorting
C) DNA replication
D) Lipid synthesis
Answer: B
Explanation: The Golgi modifies proteins from the ER and packages them for transport.
Why the top distractor is wrong: D is the function of the smooth ER, not the Golgi.

Question: Which of the following is true about the nuclear envelope?
A) It is a single phospholipid layer
B) It lacks pores
C) It separates nucleoplasm from cytoplasm
D) It is found in prokaryotes
Answer: C
Explanation: The nuclear envelope is a double membrane that isolates the nucleus from the cytoplasm.
Why the top distractor is wrong: D is incorrect because prokaryotes lack a nucleus and nuclear envelope.

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

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ 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: chitin; animal: no cell wall.
• ⚠️ Mycoplasma has no cell wall—resistant to beta-lactam antibiotics.
• Both cell types have phospholipid bilayer plasma membrane.
• Eukaryotes have membrane-bound organelles; prokaryotes do not.
• Rough ER: ribosomes attached; smooth ER: lipid synthesis.
• Golgi apparatus: modifies and sorts proteins—cis to trans face.
• Mitochondria: double membrane, produce ATP, have own DNA.
• Chloroplasts: site of photosynthesis, contain thylakoids and grana.
• Lysosomes: contain hydrolytic enzymes, found in animal cells.
• Plant vacuole: large, central, maintains turgor pressure.
• Bacteria and archaea = prokaryotes; all others = eukaryotes.
• Mature mammalian RBCs lack nucleus and mitochondria.
• Nuclear pores regulate mRNA and protein transport.
• Nucleolus: site of rRNA synthesis and ribosome assembly.
• Prokaryotic flagella: rotary, made of flagellin.
• Eukaryotic flagella and cilia: 9+2 microtubule arrangement.
• ⚠️ Endosymbiotic theory evidence: organelle circular DNA, 70S ribosomes, double membranes.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• Plasmids: small circular DNA, common in bacteria.
• Cytoskeleton: microtubules, actin, intermediate filaments—eukaryote-specific.
• FtsZ protein in prokaryotes is homologous to eukaryotic tubulin.