STEM Readiness: Chemistry Readiness - Equilibrium: Le Châtelier’s, Principle Predicting, Shifts, from Concentration, Pressure, Temperature Changes

Must?Know

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• Prokaryotes lack a membrane-bound nucleus; DNA is located in the nucleoid region.
• Eukaryotes have a nuclear envelope enclosing linear DNA within the nucleus.
• Prokaryotic ribosomes are 70S (30S + 50S subunits); eukaryotic cytoplasmic ribosomes are 80S (40S + 60S).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli).
• Archaea lack peptidoglycan; their cell walls are made of pseudopeptidoglycan or other polysaccharides.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin.
• Animal cells lack a cell wall entirely.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane.
• Eukaryotes have extensive internal membrane systems (e.g., endoplasmic reticulum, Golgi apparatus).
• Prokaryotes lack membrane-bound organelles (e.g., no mitochondria, lysosomes, or ER).
• Mitochondria are present in nearly all eukaryotic cells and are sites of aerobic respiration.
• Chloroplasts are found in plant and algal cells and conduct photosynthesis.
• The endoplasmic reticulum (ER) exists as rough ER (with ribosomes) and smooth ER (lipid synthesis, detoxification).
• The Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery.
• Lysosomes contain hydrolytic enzymes and are present in animal cells (rare in plant cells).
• Peroxisomes break down fatty acids and detoxify hydrogen peroxide in both animals and plants.
• Bacteria and archaea are prokaryotes; animals, plants, fungi, and protists are eukaryotes.
• Mycoplasma species are bacteria that lack a cell wall – an exception among prokaryotes.
• Mature mammalian red blood cells lack a nucleus and mitochondria – an exception among eukaryotic cells.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Cytoskeleton elements (microtubules, microfilaments) are present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ) but no true cytoskeleton.
• Binary fission is the primary mode of reproduction in prokaryotes; mitosis occurs in eukaryotes.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, circular DNA, and 70S ribosomes.

Difficulty Level

Intermediate – requires precise differentiation of structural and functional features across domains and kingdoms.

Common Traps

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; archaea, plants (cellulose), and fungi (chitin) have different wall compositions.
Trap: Ribosome size is the same across all cells – Fact: Prokaryotes have 70S ribosomes; eukaryotic cytoplasmic ribosomes are 80S, but mitochondria and chloroplasts have 70S.
Trap: Eukaryotes are always larger because they have more DNA – Fact: Genome size does not always correlate with cell size or complexity (e.g., some protists have larger genomes than humans).
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 nucleoids, ribosomes, inclusions, and some have protein-based cytoskeletal elements.

Practice MCQs

Question: Which of the following is a defining feature of prokaryotic cells?
A) Presence of mitochondria
B) Membrane-bound nucleus
C) 80S ribosomes
D) DNA in a nucleoid region
Answer: D
Explanation: Prokaryotes lack a nucleus; their DNA is located in the nucleoid.
Why the top distractor is wrong: B is incorrect because a membrane-bound nucleus is found only in eukaryotes.

Question: A cell is observed to have a cell wall containing chitin and lacks chloroplasts. It is most likely from which organism?
A) Moss
B) Bacterium
C) Mushroom
D) Alga
Answer: C
Explanation: Fungi (e.g., mushrooms) have chitin in their cell walls and lack chloroplasts.
Why the top distractor is wrong: A (moss) is a plant and has cellulose, not chitin, in its cell wall.

Question: Which structure is present in eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Golgi apparatus
D) Circular DNA
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle unique to eukaryotes.
Why the top distractor is wrong: A and B are present in both cell types; D is found in prokaryotes and also in mitochondria/chloroplasts.

Question: Mitochondria are thought to have evolved from free-living prokaryotes due to all of the following EXCEPT:
A) They have 70S ribosomes
B) They are surrounded by a single membrane
C) They contain circular DNA
D) They reproduce by binary fission
Answer: B
Explanation: Mitochondria are surrounded by a double membrane, supporting endosymbiotic origin.
Why the top distractor is wrong: B is false; the double membrane (not single) is key evidence.

Question: Which of the following cells lacks a nucleus at maturity?
A) Yeast cell
B) Neuron
C) Human red blood cell
D) Root cell in onion
Answer: C
Explanation: Mature mammalian red blood cells eject their nucleus to accommodate hemoglobin.
Why the top distractor is wrong: A, B, and D are all eukaryotic cells with nuclei.

Question: What is the primary component of the cell wall in most bacteria?
A) Cellulose
B) Chitin
C) Peptidoglycan
D) Silica
Answer: C
Explanation: Peptidoglycan is a polymer found in bacterial cell walls, absent in archaea and eukaryotes.
Why the top distractor is wrong: A is found in plants; B in fungi; D in diatoms – not typical bacteria.

Question: Which of the following is true about ribosomes in chloroplasts?
A) They are 80S, like cytoplasmic ribosomes
B) They are 70S, similar to bacterial ribosomes
C) They are synthesized entirely in the nucleus
D) They are larger than mitochondrial ribosomes
Answer: B
Explanation: Chloroplast ribosomes are 70S, supporting their endosymbiotic origin from cyanobacteria.
Why the top distractor is wrong: A is incorrect because 80S ribosomes are only in the eukaryotic cytoplasm.

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m
• Eukaryotic cell size: 10–100 ?m
• Prokaryotic ribosome = 70S (30S + 50S)
• Eukaryotic cytoplasmic ribosome = 80S (40S + 60S)
• Mitochondria and chloroplasts have 70S ribosomes
• Mitochondria and chloroplasts have circular DNA
• Bacterial cell walls contain peptidoglycan
• Archaeal cell walls lack peptidoglycan – use pseudopeptidoglycan or other polymers
• Plant cell wall = cellulose
• Fungal cell wall = chitin
• Animal cells = no cell wall
• Nucleus = membrane-bound, contains linear DNA
• Prokaryotes store DNA in nucleoid (no membrane)
• Eukaryotes have internal membranes (ER, Golgi, etc.)
• Prokaryotes lack membrane-bound organelles
• Mycoplasma – no cell wall
• Mature human RBC – no nucleus, no mitochondria
• Endosymbiotic theory supported by: double membrane, 70S ribosomes, circular DNA, binary fission
• Lysosomes are common in animal cells, rare in plants
• Peroxisomes break down fatty acids and H?O?
• Plasmids = small circular DNA, common in bacteria
• Cytoskeleton (microtubules, actin) = eukaryotic feature
• FtsZ protein in bacteria is homologous to tubulin
• Binary fission = prokaryotic division
• Mitosis = eukaryotic nuclear division