STEM Readiness: Biology Readiness Chemistry of Life Macromolecules Carbohydrates Lipids Proteins Nucleic Acids Structure and Function

Must‑Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 μm in diameter; eukaryotic cells range from 10–100 μm.
• Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, lysosomes, and peroxisomes.
• Ribosomes in prokaryotes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport.
• Eukaryotes have an endomembrane system including rough ER, smooth ER, Golgi apparatus, and vesicles; prokaryotes do not.
• The nucleus in eukaryotes contains nuclear pores for RNA and protein transport; prokaryotes lack nuclear pores.
• Chromosomal DNA in prokaryotes is circular and not associated with histones (except in some archaea); eukaryotes have linear chromosomes bound to histone proteins.
• Binary fission is the primary mode of reproduction in prokaryotes; mitosis and meiosis occur in eukaryotes.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin.
• Human red blood cells lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Plants are eukaryotes but lack centrosomes; animal cells have centrosomes with centrioles for mitosis.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own DNA, 70S ribosomes, and double membranes.
• Lysosomes are membrane-bound organelles in animal cells that break down macromolecules; plants and fungi use vacuoles for similar functions.
• The nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome subunit assembly.
• Peroxisomes contain enzymes like catalase that break down hydrogen peroxide; present in both plant and animal eukaryotic cells.
• Prokaryotes can have plasmids (small circular DNA); eukaryotes rarely have plasmids (except in some yeast).
• Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane; Gram-negative have a thin peptidoglycan layer and an outer membrane with lipopolysaccharides.
• Cytoskeleton elements (microfilaments, intermediate filaments, microtubules) are present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ, MreB) but no true cytoskeleton.
• Chloroplasts are double-membrane organelles in plant and algal cells that perform photosynthesis; absent in prokaryotes and animal cells.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and recognizing exceptions.

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 is the same across all cells – Fact: Prokaryotes use 70S ribosomes; eukaryotes use 80S in cytoplasm, but mitochondria and chloroplasts retain 70S.
Trap: Eukaryotes are always multicellular – Fact: Many eukaryotes (e.g., yeast, amoeba, paramecium) are unicellular.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Prokaryotes have no internal membrane structures – Fact: Some prokaryotes have protein-bound compartments (e.g., carboxysomes), but no membrane-bound organelles like eukaryotes.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Mitochondria
D) Circular DNA
Answer: C
Explanation: Mitochondria are membrane-bound organelles found only in eukaryotes.
Why the top distractor is wrong: Circular DNA is present in prokaryotes and also in mitochondria and chloroplasts of eukaryotes.

Question: A cell is observed to have a nucleus, 80S ribosomes, and a cell wall made of cellulose. This cell is most likely from a:
A) Fungus
B) Bacterium
C) Plant
D) Animal
Answer: C
Explanation: Cellulose cell walls are characteristic of plant cells; nucleus and 80S ribosomes confirm eukaryotic origin.
Why the top distractor is wrong: Fungi have chitin in their cell walls, not cellulose.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Nucleolus
B) Golgi apparatus
C) Plasma membrane
D) Mitochondria
Answer: C
Explanation: The phospholipid bilayer plasma membrane is universal to all cells.
Why the top distractor is wrong: The nucleolus is only present in eukaryotic nuclei.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to synthesize ATP
C) Containing 70S ribosomes and circular DNA
D) Involvement in cellular respiration
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic traits retained in mitochondria.
Why the top distractor is wrong: Double membranes can arise by other mechanisms; circular DNA and ribosomes are more specific evidence.

Question: Which organism lacks a cell wall and a nucleus?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Homo sapiens red blood cell
Answer: D
Explanation: Mature human red blood cells lack both a nucleus and a cell wall.
Why the top distractor is wrong: Mycoplasma lacks a cell wall but is a prokaryote with a nucleoid (has DNA, no nucleus).

Question: Where is DNA located in a prokaryotic cell?
A) Nucleus
B) Nucleolus
C) Nucleoid
D) Mitochondria
Answer: C
Explanation: Prokaryotes store DNA in the nucleoid, a region not enclosed by a membrane.
Why the top distractor is wrong: A nucleus is only present in eukaryotes.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 70S
B) Cytoplasmic ribosomes are 80S, mitochondrial ribosomes are 70S
C) Ribosomes are enclosed in a membrane
D) Ribosomes are only found in the nucleus
Answer: B
Explanation: Eukaryotic cytoplasmic ribosomes are 80S; mitochondrial and chloroplast ribosomes are 70S.
Why the top distractor is wrong: 70S ribosomes are only in organelles and prokaryotes, not in eukaryotic cytoplasm.

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

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ Prokaryotes have nucleoid; eukaryotes have membrane-bound nucleus.
• ⚠️ Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• ⚠️ Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal cells = no cell wall.
• ⚠️ Mycoplasma – smallest known bacteria, lacks cell wall.
• ⚠️ Mature human red blood cells lack nucleus and mitochondria.
• Both prokaryotes and eukaryotes have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, lysosomes); prokaryotes do not.
• Nuclear pores regulate transport in and out of the eukaryotic nucleus.
• Prokaryotic DNA is circular and not bound to histones (mostly).
• Eukaryotic chromosomes are linear and associated with histones.
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis/meiosis.
• Prokaryotic flagella = flagellin, rotating; eukaryotic = microtubules (9+2), bending.
• Lysosomes are in animal cells; vacuoles perform similar roles in plants and fungi.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Peroxisomes break down fatty acids and detoxify H₂O₂ using catalase.
• Plasmids are common in bacteria; rare in eukaryotes (e.g., yeast 2-micron plasmid).
• ⚠️ Gram-positive bacteria: thick peptidoglycan, no outer membrane.
• ⚠️ Gram-negative bacteria: thin peptidoglycan, outer membrane with LPS.
• Cytoskeleton (microtubules, microfilaments) is eukaryotic feature; prokaryotes have analogs (FtsZ, MreB).
• Chloroplasts = double membrane, 70S ribosomes, circular DNA, in plants and algae.
• All eukaryotes have a nucleus; not all are multicellular (e.g., Amoeba, Paramecium).
• Verify from standard textbook: exact composition of archaeal cell walls varies widely.