STEM Readiness: Biology Readiness - Chemistry of Life: Protein Structure - Primary through, Quaternary Forces, Stabilising Each, Level

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• Prokaryotic DNA is located in the nucleoid, a region without a membrane; eukaryotic DNA is enclosed within a membrane-bound nucleus.
• Prokaryotes have 70S ribosomes (30S + 50S subunits); eukaryotes have 80S ribosomes (40S + 60S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes.
• Prokaryotic cell walls (in bacteria) contain peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cell walls are made of cellulose; fungal cell walls are made of chitin; animal cells lack cell walls.
• 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 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; have double membranes, circular DNA, and 70S ribosomes.
• Chloroplasts perform photosynthesis in plants and algae; contain thylakoids, chlorophyll, circular DNA, and 70S ribosomes.
• Lysosomes contain hydrolytic enzymes for digestion; found in animal cells, rare in plant cells (vacuoles perform similar function).
• Vacuoles in plant cells are large and central, maintaining turgor pressure; animal cells have small or transient vacuoles.
• Cytoskeleton (microtubules, microfilaments, intermediate filaments) is present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ) but no true cytoskeleton.
• Nuclear envelope is a double membrane with nuclear pores regulating transport; absent in prokaryotes.
• Nucleolus within the nucleus synthesizes ribosomal RNA and assembles ribosomal subunits.
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide by mitosis and cytokinesis.
• Red blood cells in mammals lack a nucleus and organelles, maximizing space for hemoglobin.
• Sperm cells are eukaryotic and contain mitochondria in the midpiece for energy.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and ability to replicate independently.
• Archaea lack peptidoglycan and have unique membrane lipids (ether linkages); some live in extreme environments.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.

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 bacterial cell walls contain peptidoglycan; plant (cellulose), fungal (chitin), and archaeal (pseudopeptidoglycan or other) walls differ.

Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S in cytoplasm, but mitochondria and chloroplasts have 70S.

Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes; nucleus, mitochondria, and chloroplasts are the three double-membrane-bound organelles.

Trap: Prokaryotes have no internal membrane structures – Fact: Some prokaryotes have protein-bound compartments (e.g., carboxysomes), but lack membrane-bound organelles like ER or Golgi.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in eukaryotic cells but NOT in prokaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Membrane-bound nucleus
D) Plasma membrane
Answer: C
Explanation: A membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: Circular DNA is present in prokaryotes and also in mitochondria and chloroplasts.

Question: Which structure is present in plant cells but NOT in animal cells?
A) Mitochondria
B) Lysosomes
C) Cell wall
D) Plasma membrane
Answer: C
Explanation: Plant cells have a cellulose-based cell wall; animal cells do not.
Why the top distractor is wrong: Lysosomes are primarily found in animal cells, though plant vacuoles can perform similar functions.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Use of oxygen in metabolism
C) Contain 70S ribosomes and circular DNA
D) Ability to synthesize ATP
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic features, supporting endosymbiotic theory.
Why the top distractor is wrong: Double membranes are also found in the nucleus, which is not endosymbiotic in origin.

Question: A bacterial cell is placed in a hypotonic solution. Which component prevents lysis due to osmotic pressure?
A) Capsule
B) Plasma membrane
C) Peptidoglycan cell wall
D) Cytoplasm
Answer: C
Explanation: The peptidoglycan cell wall provides structural rigidity to resist osmotic lysis.
Why the top distractor is wrong: The plasma membrane regulates transport but does not prevent physical bursting.

Question: Which of the following organisms lacks a nucleus and has 70S ribosomes?
A) Saccharomyces cerevisiae (yeast)
B) Escherichia coli
C) Homo sapiens
D) Arabidopsis thaliana
Answer: B
Explanation: Escherichia coli is a prokaryote with no nucleus and 70S ribosomes.
Why the top distractor is wrong: Yeast is a eukaryotic fungus with a nucleus and 80S ribosomes.

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Nucleus
B) Rough endoplasmic reticulum
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies, sorts, and packages proteins into vesicles for secretion.
Why the top distractor is wrong: The rough ER synthesizes proteins but does not package them for secretion.

Question: Which of the following cells lacks mitochondria?
A) Liver cell
B) Leaf mesophyll cell
C) Mature human red blood cell
D) Fungal hypha
Answer: C
Explanation: Mature human red blood cells lack a nucleus and mitochondria, relying on anaerobic glycolysis.
Why the top distractor is wrong: Leaf cells contain mitochondria despite having chloroplasts; both produce ATP.

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

• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Prokaryotes: 0.1–5.0 ?m; eukaryotes: 10–100 ?m.
• DNA in prokaryotes is in the nucleoid; in eukaryotes, it’s in the nucleus.
• Bacterial cell walls contain peptidoglycan; archaea do not.
• Plant cell walls: cellulose; fungal: chitin; animal: no cell wall.
• Mycoplasma has no cell wall – resistant to penicillin.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have membrane-bound organelles; prokaryotes do not.
• Rough ER: protein synthesis; smooth ER: lipid synthesis, detoxification.
• Golgi apparatus: modifies, sorts, packages proteins.
• Lysosomes: contain hydrolytic enzymes; found in animal cells.
• Plant vacuole: maintains turgor pressure; stores ions and metabolites.
• Mitochondria: double membrane, produce ATP via aerobic respiration.
• Chloroplasts: contain thylakoids, site of photosynthesis.
• Cytoskeleton: microtubules, microfilaments, intermediate filaments – eukaryote-specific.
• Nuclear envelope: double membrane with nuclear pores.
• Nucleolus: site of rRNA synthesis and ribosome assembly.
• Mature human red blood cells lack a nucleus and organelles.
• Sperm cells have mitochondria in the midpiece for motility energy.
• Endosymbiotic theory: mitochondria and chloroplasts replicate independently of the cell.
• Archaea have ether linkages in membrane lipids; bacteria have ester linkages.
• Gram-positive bacteria: thick peptidoglycan; Gram-negative: thin peptidoglycan + outer membrane.
• Binary fission: prokaryotic division; no mitosis.
• Verify from standard textbook: exact size ranges may vary slightly by source.