STEM Readiness: Chemistry Readiness - Equilibrium: Acid-Base Equilibrium - Ka, Kb, pH, Weak Acid/Base Calculations

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 eukaryotic cells contain 70S ribosomes, similar to prokaryotes.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan and have different cell wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls are made of chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; eukaryotes also have internal membrane systems (e.g., endoplasmic reticulum).
• Eukaryotic organelles include the nucleus, mitochondria, lysosomes, Golgi apparatus, endoplasmic reticulum, and (in plants) chloroplasts; prokaryotes lack these.
• Prokaryotes lack membrane-bound organelles; metabolic functions occur in the cytoplasm or at the plasma membrane.
• Eukaryotic mitochondria generate ATP via aerobic respiration; prokaryotes perform respiration at the plasma membrane.
• Chloroplasts are found in plant cells and some protists (e.g., Chlamydomonas); they perform photosynthesis and contain thylakoids.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; absent in most plant cells.
• Centrioles are present in animal cells and aid in spindle formation during mitosis; absent in most plant cells.
• Plasmids are small, circular DNA molecules found in prokaryotes (e.g., Salmonella) and some yeast (eukaryotes).
• Mycoplasma species are bacteria that lack a cell wall; they are the smallest known cells capable of independent growth.
• Human red blood cells lack a nucleus and mitochondria; they transport oxygen using hemoglobin.
• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from engulfed prokaryotes; evidence includes their circular DNA, 70S ribosomes, and ability to divide independently.
• Mitochondrial DNA is circular and lacks histones, resembling bacterial chromosomes.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport; absent in prokaryotes.
• Cytoskeleton (microtubules, microfilaments, intermediate filaments) is present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ) but no true cytoskeleton.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative bacteria have thin peptidoglycan and an outer membrane with lipopolysaccharide.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes; some live in extreme environments (e.g., Methanogens in anaerobic conditions).
• Peroxisomes are single-membrane organelles in eukaryotes that break down fatty acids and detoxify hydrogen peroxide; absent in prokaryotes.

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; plant cell walls are made of cellulose, fungal walls of chitin.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based microcompartments (e.g., carboxysomes).
Trap: Eukaryotes evolved from prokaryotes – Fact: Current evidence suggests eukaryotes and archaea share a common ancestor; eukaryotes did not directly descend from modern prokaryotes.
Trap: The nucleus is the only site of DNA in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA.

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) Circular DNA
D) Membrane-bound nucleus
Answer: D
Explanation: The membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: Circular DNA is found in prokaryotes and also in mitochondria and chloroplasts of eukaryotes.

Question: A cell is observed to have a cell wall, no nucleus, and 70S ribosomes. Which of the following is the most likely identity?
A) Fungal cell
B) Plant cell
C) Bacterial cell
D) Animal cell
Answer: C
Explanation: Bacteria have cell walls, no nucleus, and 70S ribosomes.
Why the top distractor is wrong: Fungal cells have cell walls but are eukaryotic, with 80S ribosomes and a nucleus.

Question: Which structure is present in plant cells but not in animal cells?
A) Lysosome
B) Mitochondrion
C) Chloroplast
D) Golgi apparatus
Answer: C
Explanation: Chloroplasts are organelles for photosynthesis and are found only in plants and some protists.
Why the top distractor is wrong: Lysosomes are primarily in animal cells; plants use vacuoles for degradation.

Question: Mitochondria are thought to have originated from endosymbiotic bacteria. Which of the following supports this theory?
A) Mitochondria lack DNA
B) Mitochondria have 80S ribosomes
C) Mitochondria divide by mitosis
D) Mitochondria have circular DNA and 70S ribosomes
Answer: D
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria.
Why the top distractor is wrong: Mitochondria divide by binary fission, not mitosis, and do contain DNA.

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

Question: What is the primary component of the cell wall in fungi?
A) Peptidoglycan
B) Cellulose
C) Chitin
D) Lipopolysaccharide
Answer: C
Explanation: Fungal cell walls are composed of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: Peptidoglycan is found in bacterial cell walls, not fungi.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 70S
B) Ribosomes are only found in the cytoplasm
C) 80S ribosomes are found in the cytoplasm
D) Mitochondrial ribosomes are 80S
Answer: C
Explanation: Cytoplasmic ribosomes in eukaryotes are 80S.
Why the top distractor is wrong: Mitochondrial ribosomes are 70S, not 80S.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: nucleoid; eukaryotes: nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence for endosymbiosis.
• Bacteria cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal cell wall = chitin; animal cells = no cell wall.
• Mycoplasma – smallest known cells, lack cell wall.
• Human red blood cells lack a nucleus and mitochondria.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes do not.
• Lysosomes are in animal cells, not plants.
• Centrioles present in animal cells, absent in most plants.
• Plasmids occur in bacteria and some yeast.
• Flagellin = prokaryotic flagella protein; microtubules (9+2) = eukaryotic flagella.
• Gram-negative bacteria have an outer membrane with lipopolysaccharide (LPS).
• Archaea have ether-linked membrane lipids, not ester-linked.
• Endosymbiotic theory explains origin of mitochondria and chloroplasts.
• Mitochondrial DNA is circular and lacks histones.
• Nuclear pores regulate nucleocytoplasmic transport.
• Cytoskeleton components: microtubules, microfilaments, intermediate filaments – only in eukaryotes.
• Prokaryotes use FtsZ protein for division; homologous to eukaryotic tubulin.
• Chloroplasts found in plants and some protists, not in animals or fungi.
• Peroxisomes contain catalase to break down H?O?; absent in prokaryotes.
• Verify from standard textbook: exact lipid composition in archaeal membranes.
• Verify from standard textbook: presence of microcompartments in some prokaryotes (e.g., carboxysomes).