STEM Readiness: Chemistry Readiness - Equilibrium: Equilibrium Expressions - Kc, Kp, Writing from Balanced Equations

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.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region lacking 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.
• Ribosomes in prokaryotes 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.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., in bacteria); archaea lack peptidoglycan but have other polysaccharide walls.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics like penicillin that target peptidoglycan.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; eukaryotes also have internal membranes forming organelles.
• The nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate molecular transport.
• Nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome subunit assembly.
• Endoplasmic reticulum (ER): rough ER has ribosomes and synthesizes proteins; smooth ER lacks ribosomes and synthesizes lipids and detoxifies drugs.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles.
• Mitochondria are the site of aerobic respiration, generate ATP, and have double membranes; the inner membrane is highly folded into cristae.
• Chloroplasts (in plants and algae) perform photosynthesis, contain thylakoids, and have their own circular DNA and 70S ribosomes.
• Lysosomes (in animal cells) contain hydrolytic enzymes for breaking down macromolecules and cellular debris.
• Peroxisomes break down fatty acids and detoxify hydrogen peroxide (H?O?).
• Centrioles (in animal cells) organize microtubules during cell division; absent in most plant cells.
• Vacuoles: large central vacuole in plant cells maintains turgor pressure; smaller vacuoles in animal cells for storage and transport.
• Cytoskeleton components include microtubules, microfilaments (actin), and intermediate filaments; provide structure, transport, and motility.
• Red blood cells in mammals lack a nucleus and organelles, maximizing space for hemoglobin.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and the ability to replicate independently of the cell.
• Plasma membrane fluidity is maintained by cholesterol in animal cells and hopanoids in some bacteria.
• Gram-positive bacteria have thick peptidoglycan layers; gram-negative bacteria have thin peptidoglycan and an outer membrane with lipopolysaccharides.

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on comparative cell biology and organelle function.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plant (cellulose), fungal (chitin), and archaeal (pseudopeptidoglycan or other) walls differ chemically.

Trap: Ribosome size correlates directly with cell complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, not 80S, due to prokaryotic ancestry.

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 cytoskeletal elements, ribosomes, and nucleoids, though not membrane-bound organelles.

Trap: Eukaryotic cells are always larger than prokaryotic cells – Fact: While generally true, some giant bacteria (e.g., Thiomargarita namibiensis, up to 750 ?m) exceed typical eukaryotic cells.

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) Presence of nucleoid
D) Mitochondria for ATP production
Answer: C
Explanation: Prokaryotes lack a nucleus and instead have a nucleoid region containing DNA.
Why the top distractor is wrong: A is incorrect because a membrane-bound nucleus is exclusive to eukaryotes.

Question: Which structure is found in plant cells but not in animal cells?
A) Lysosome
B) Centriole
C) Large central vacuole
D) Plasma membrane
Answer: C
Explanation: Plant cells have a large central vacuole for turgor pressure; animal cells have smaller, multiple vacuoles.
Why the top distractor is wrong: B is incorrect because centrioles are found in animal cells, not most plant cells.

Question: Which organelle contains 70S ribosomes and circular DNA?
A) Nucleus
B) Lysosome
C) Golgi apparatus
D) Mitochondrion
Answer: D
Explanation: Mitochondria have 70S ribosomes and circular DNA, consistent with endosymbiotic origin.
Why the top distractor is wrong: A is incorrect because the nucleus contains 80S ribosomes (in cytoplasm) and linear DNA.

Question: Which of the following is true about the cell wall in fungi?
A) Made of peptidoglycan
B) Made of cellulose
C) Made of chitin
D) Absent
Answer: C
Explanation: Fungal cell walls are composed of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: A is incorrect because peptidoglycan is found in bacterial cell walls, not fungal.

Question: Which of the following lacks a nucleus?
A) Human liver cell
B) Yeast 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: B is incorrect because yeast is a eukaryotic fungus and has a nucleus.

Question: Which component is present in both prokaryotes and eukaryotes?
A) Mitochondria
B) Nuclear envelope
C) Plasma membrane
D) Endoplasmic reticulum
Answer: C
Explanation: Both cell types have a phospholipid bilayer plasma membrane.
Why the top distractor is wrong: A is incorrect because mitochondria are exclusive to eukaryotes.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Lysosomes digest foreign material
B) Mitochondria divide by binary fission
C) The Golgi modifies proteins
D) Ribosomes synthesize polypeptides
Answer: B
Explanation: Mitochondria replicate independently via binary fission, similar to prokaryotes.
Why the top distractor is wrong: A is incorrect because lysosomal function is not evidence of endosymbiosis.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have no nucleus; DNA in nucleoid.
• Eukaryotes have membrane-bound nucleus with nuclear pores.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Endosymbiotic theory supported by mitochondrial/chloroplast independent division.
• Bacteria cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal cells = no cell wall.
• Mycoplasma lacks a cell wall – resistant to penicillin.
• Both cell types have phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes forming organelles; prokaryotes do not.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Rough ER = ribosomes attached; synthesizes proteins for secretion.
• Smooth ER = synthesizes lipids, metabolizes carbohydrates, detoxifies.
• Golgi apparatus = modifies, sorts, packages proteins in vesicles.
• Mitochondria = ATP production; double membrane; cristae = inner folds.
• Chloroplasts = photosynthesis; contain thylakoids and grana.
• Lysosomes = contain acid hydrolases; break down macromolecules (animal cells).
• Peroxisomes = break down fatty acids, detoxify H?O?.
• Red blood cells lack nucleus and organelles.
• Centrioles = in animal cells; organize spindle fibers; absent in most plants.
• Plant vacuole = large central, maintains turgor; animal vacuoles = small, multiple.
• Cytoskeleton = microtubules, actin filaments, intermediate filaments.
• Gram-positive bacteria = thick peptidoglycan; gram-negative = thin peptidoglycan + outer membrane.
• Verify from standard textbook: Hopanoids in bacterial membranes function like cholesterol in eukaryotes.