STEM Readiness: Chemistry Readiness Stoichiometry Balancing Chemical Equations Conservation of Mass and Charge

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, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, lysosomes, and the Golgi apparatus.
• 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, similar to prokaryotes, supporting the endosymbiotic theory.
• Most prokaryotes have a cell wall made of peptidoglycan; archaea lack peptidoglycan but have other polysaccharide walls.
• Plant cells have a cell wall composed of cellulose; fungal cells have chitin; animal cells lack a cell wall.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to penicillin, which targets peptidoglycan synthesis.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport.
• Eukaryotes have internal membrane systems including the endoplasmic reticulum (ER), Golgi apparatus, and nuclear envelope.
• The nucleus contains chromatin (DNA + proteins) and the nucleolus, where ribosomal RNA is synthesized.
• Nuclear pores in the nuclear envelope allow selective transport of RNA and proteins between nucleus and cytoplasm.
• Mitochondria are the site of aerobic respiration and have double membranes; the inner membrane is highly folded into cristae.
• Chloroplasts, found in plants and algae, contain thylakoids and chlorophyll and are the site of photosynthesis.
• Lysosomes in animal cells contain hydrolytic enzymes for breaking down macromolecules and cellular debris.
• Vacuoles in plant cells are large and central, maintaining turgor pressure; animal cells have small or temporary vacuoles.
• Peroxisomes contain enzymes like catalase that break down hydrogen peroxide into water and oxygen.
• Cytoskeleton elements (microtubules, microfilaments, intermediate filaments) maintain cell shape and aid in intracellular transport in eukaryotes.
• Prokaryotes have circular DNA as their main chromosome; eukaryotes have linear chromosomes associated with histone proteins.
• Plasmids, small circular DNA molecules, are common in prokaryotes and can carry antibiotic resistance genes.
• Red blood cells in mammals lack a nucleus and organelles, maximizing space for hemoglobin.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own circular DNA, 70S ribosomes, and ability to divide independently.
• Archaea resemble prokaryotes in size and structure but have eukaryote-like transcription and translation machinery.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative bacteria have thin peptidoglycan and an outer membrane with lipopolysaccharides.

Difficulty Level

Intermediate – this content forms a foundational component of introductory biology courses and requires precise differentiation between similar structures across domains.

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 (pseudopeptidoglycan or other) do not.
• Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes in the cytoplasm, but 70S in mitochondria and chloroplasts.
• Trap: The nucleus is the only organelle that contains DNA in eukaryotes – Fact: Mitochondria and chloroplasts also contain their own DNA.
• Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have ribosomes, nucleoid, and sometimes inclusions or protein-based microcompartments.
• Trap: Eukaryotic cells are always larger and more complex than prokaryotic cells – Fact: While generally true, some eukaryotic cells (e.g., yeast) are small (~3–4 μm), and some prokaryotes (e.g., Thiomargarita namibiensis) can exceed 100 μm.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid region
D) Peptidoglycan in the cell wall
Answer: C
Explanation: The nucleoid is a membrane-free region in prokaryotes where DNA is located; eukaryotes have a membrane-bound nucleus.
Why the top distractor is wrong: While peptidoglycan (D) is mostly in prokaryotes, some archaea lack it, and it is never found in eukaryotes, but the nucleoid is universally absent in eukaryotes.

Question: Which structure is present in plant cells but not in animal cells?
A) Mitochondria
B) Rough endoplasmic reticulum
C) Lysosome
D) Chloroplast
Answer: D
Explanation: Chloroplasts are organelles for photosynthesis and are found only in plants and algae.
Why the top distractor is wrong: Lysosomes (C) are common in animal cells and sometimes in plant cells, though less prominent.

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 proteins using 80S ribosomes
C) Linear DNA molecules associated with histones
D) Division by binary fission
Answer: A
Explanation: The double membrane and independent division support mitochondrial origin from engulfed prokaryotes.
Why the top distractor is wrong: Mitochondria use 70S ribosomes, not 80S (B); they have circular DNA, not linear (C).

Question: A cell is observed to have a nucleus, mitochondria, and a cell wall made of cellulose. It is most likely:
A) Fungal cell
B) Bacterial cell
C) Plant cell
D) Animal cell
Answer: C
Explanation: Cellulose cell walls and chloroplasts (implied by context) are diagnostic of plant cells.
Why the top distractor is wrong: Fungal cells (A) have chitin, not cellulose, in their cell walls.

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human liver cell
C) Escherichia coli
D) Amoeba
Answer: C
Explanation: E. coli is a prokaryote and lacks a membrane-bound nucleus.
Why the top distractor is wrong: Yeast (A) is a eukaryotic fungus and has a nucleus.

Question: What is the primary function of the Golgi apparatus in eukaryotic cells?
A) ATP synthesis
B) Protein synthesis
C) Protein modification and sorting
D) DNA replication
Answer: C
Explanation: The Golgi modifies, sorts, and packages proteins for secretion or delivery to organelles.
Why the top distractor is wrong: Protein synthesis occurs on ribosomes (B), not in the Golgi.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All eukaryotic ribosomes are 80S, including those in mitochondria
B) Cytoplasmic ribosomes are 80S, but mitochondrial ribosomes are 70S
C) Eukaryotic ribosomes are smaller than prokaryotic ribosomes
D) 80S ribosomes are found in the nucleolus
Answer: B
Explanation: Mitochondrial and chloroplast ribosomes are 70S, resembling prokaryotic ribosomes.
Why the top distractor is wrong: Option A is false because mitochondrial ribosomes are 70S, not 80S.

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

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ Prokaryotes: DNA in nucleoid; eukaryotes: DNA in 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 = no cell wall.
• ⚠️ Mycoplasma has no cell wall – smallest known cell.
• ⚠️ Red blood cells lack a nucleus and mitochondria.
• Both cell types have phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear envelope has nuclear pores for RNA/protein transport.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Mitochondria: double membrane, cristae, site of ATP production.
• Chloroplasts: contain thylakoids, grana, and stroma.
• Lysosomes contain acid hydrolases; active at low pH.
• ⚠️ Peroxisomes break down fatty acids and detoxify H₂O₂.
• Large central vacuole in plant cells maintains turgor pressure.
• Cytoskeleton: microtubules (tubulin), microfilaments (actin), intermediate filaments.
• Prokaryotic DNA: single circular chromosome; eukaryotic: multiple linear chromosomes.
• Plasmids = small, circular, extrachromosomal DNA in prokaryotes.
• ⚠️ Endosymbiotic theory: mitochondria from alpha-proteobacteria; chloroplasts from cyanobacteria.
• Archaea lack peptidoglycan but may have pseudopeptidoglycan or other wall types.
• Gram-positive: thick peptidoglycan, one membrane; Gram-negative: thin peptidoglycan, outer LPS membrane.
• ⚠️ Yeast is a unicellular eukaryotic fungus with chitin in cell wall.
• Verify from standard textbook: exact ribosomal subunit sizes (50S/30S for 70S; 60S/40S for 80S).