STEM Readiness: Biology Readiness - Energy Metabolism: Fermentation - Lactic Acid and Alcoholic, Conditions, Purpose, Products

Must?Know

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• 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, supporting their bacterial origin.
• Prokaryotic cell walls contain peptidoglycan (e.g., Escherichia coli); exceptions include Mycoplasma, which lacks a cell wall.
• Eukaryotic cell walls: plants have cellulose, fungi have chitin, animals lack cell walls.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; only eukaryotes have internal membrane systems (e.g., ER, Golgi).
• Eukaryotes possess membrane-bound organelles: nucleus, mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus, peroxisomes.
• Prokaryotes lack mitochondria and generate ATP via the plasma membrane; eukaryotes use mitochondria for aerobic respiration.
• Archaea are prokaryotes but have no peptidoglycan in their cell walls; some have pseudopeptidoglycan or other polymers.
• Plant cells have chloroplasts for photosynthesis; animal cells do not.
• Lysosomes are membrane-bound organelles in animal cells that contain hydrolytic enzymes; absent in most plant cells.
• Nuclear envelope in eukaryotes consists of two lipid bilayers with nuclear pores that regulate transport.
• Nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome assembly.
• Endoplasmic reticulum (ER): rough ER has ribosomes and synthesizes proteins; smooth ER lacks ribosomes and synthesizes lipids and detoxifies.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles.
• Mitochondria have double membranes, circular DNA, and 70S ribosomes—evidence for endosymbiotic theory.
• Chloroplasts (in plants and algae) have double membranes, thylakoids, circular DNA, and 70S ribosomes—also support endosymbiosis.
• Cytoskeleton (microtubules, actin filaments, intermediate filaments) is present in eukaryotes; prokaryotes have homologs (e.g., FtsZ, MreB) but no true cytoskeleton.
• Flagella differ: prokaryotic flagella are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and bend.
• 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 hemoglobin capacity.
• Peroxisomes contain enzymes like catalase that break down hydrogen peroxide; present in eukaryotes only.
• Vacuoles: large central vacuole in plant cells maintains turgor pressure; small or absent in animal cells.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having own DNA, ribosomes, and ability to replicate independently.

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on comparative structure and functional implications.

Common Traps

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacterial prokaryotes have peptidoglycan; archaea lack it, and eukaryotes use cellulose or chitin.
Trap: Ribosome size correlates with cell complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, not 80S, due to bacterial ancestry.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and sometimes protein-based microcompartments, but no membrane-bound organelles.
Trap: Eukaryotes are always larger than prokaryotes – Fact: While generally true, some giant bacteria (e.g., Thiomargarita namibiensis, up to 750 ?m) exceed typical eukaryotic cells.
Trap: The nucleus is the only site of DNA in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA, separate from nuclear DNA.

Practice MCQs

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Peptidoglycan in the cell wall
D) Plasma membrane with phospholipid bilayer
Answer: C
Explanation: Peptidoglycan is unique to bacterial prokaryotes; eukaryotes lack it.
Why the top distractor is wrong: 70S ribosomes (B) are also found in mitochondria and chloroplasts of eukaryotes, so not exclusive to prokaryotes.

Question: Which structure is present in plant cells but absent in animal cells?
A) Mitochondria
B) Rough endoplasmic reticulum
C) Central vacuole
D) Golgi apparatus
Answer: C
Explanation: The large central vacuole is a defining feature of plant cells.
Why the top distractor is wrong: Mitochondria (A) are present in both plant and animal cells.

Question: What is the primary evidence supporting the endosymbiotic theory for mitochondria?
A) Presence of a double membrane and 80S ribosomes
B) Presence of circular DNA and 70S ribosomes
C) Ability to synthesize proteins using nuclear DNA
D) Connection to the endoplasmic reticulum
Answer: B
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria.
Why the top distractor is wrong: Mitochondria have 70S, not 80S, ribosomes (A is incorrect).

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human neuron
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 (A) is a eukaryotic fungus and has a nucleus.

Question: Which component is found in both prokaryotes and eukaryotes?
A) Mitochondria
B) 80S ribosomes
C) Phospholipid bilayer membrane
D) Nuclear envelope
Answer: C
Explanation: Both cell types have a plasma membrane made of phospholipid bilayer.
Why the top distractor is wrong: Mitochondria (A) are only in eukaryotes.

Question: Which organism has a cell wall made of chitin?
A) Pine tree
B) Streptococcus pneumoniae
C) Saccharomyces cerevisiae
D) Human skin cell
Answer: C
Explanation: Saccharomyces cerevisiae (baker’s yeast) is a fungus with chitin in its cell wall.
Why the top distractor is wrong: Streptococcus (B) is a bacterium with peptidoglycan, not chitin.

Question: Where is ribosomal RNA synthesized in eukaryotic cells?
A) Cytoplasm
B) Nucleus
C) Nucleolus
D) Rough ER
Answer: C
Explanation: The nucleolus is the site of rRNA transcription and ribosome subunit assembly.
Why the top distractor is wrong: The nucleus (B) contains the nucleolus, but rRNA synthesis specifically occurs within the nucleolus.

Last?Minute Revision

• Prokaryotic size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: DNA in nucleoid; no nuclear membrane.
• Eukaryotes: DNA in nucleus with double membrane and pores.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Bacterial cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both cell types have phospholipid bilayer plasma membrane.
• Only eukaryotes have membrane-bound organelles (ER, Golgi, lysosomes, etc.).
• Mycoplasma lacks a cell wall – only prokaryote without one.
• Mammalian red blood cells lack a nucleus and mitochondria.
• Nuclear envelope has two lipid bilayers and nuclear pores.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Rough ER: ribosomes attached, protein synthesis.
• Smooth ER: lipid synthesis, detoxification, Ca²? storage.
• Golgi apparatus: modifies, sorts, packages proteins.
• Mitochondria: double membrane, ATP production, own DNA.
• Chloroplasts: thylakoids, photosynthesis, own DNA.
• Endosymbiotic theory supported by organelle DNA, ribosomes, and binary fission.
• Eukaryotic flagellum: 9+2 microtubule arrangement; bends.
• Prokaryotic flagellum: flagellin, rotates like a propeller.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Plant vacuole: turgor pressure, storage; animal vacuoles small.
• Cytoskeleton: microtubules, actin, intermediate filaments – eukaryote-specific.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• FtsZ protein in prokaryotes is homologous to tubulin in eukaryotes.