STEM Readiness: Biology Readiness - Physiology Preview: Nervous System - Action Potential, Synapse, Neurotransmitters, Reflex Arc

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 contain 70S ribosomes, supporting their origin via endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls are made of chitin.
• Animal cells lack cell walls; some protists (e.g., algae) have cell walls, others (e.g., amoeba) do not.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems including the endoplasmic reticulum, Golgi apparatus, and lysosomes; prokaryotes do not.
• Mitochondria are present in nearly all eukaryotic cells but absent in mature red blood cells of mammals.
• Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• Mycoplasma species are bacteria that lack a cell wall, making them resistant to penicillin.
• The nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport.
• Chromosomes in prokaryotes are typically a single circular DNA molecule; eukaryotes have multiple linear chromosomes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes, but rare in eukaryotes (e.g., 2-micron plasmid in yeast).
• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
• Evidence for endosymbiosis includes: mitochondria and chloroplasts divide by binary fission, have their own DNA, and possess 70S ribosomes.
• Chloroplasts are found in plants and some protists (e.g., algae), but not in fungi or animals.
• Lysosomes are membrane-bound organelles containing hydrolytic enzymes; they are present in animal cells, but rare in plant cells (vacuoles perform similar function).
• Prokaryotes lack a cytoskeleton; eukaryotes have a dynamic cytoskeleton (microtubules, actin filaments, intermediate filaments).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Pili are present in some prokaryotes for attachment and conjugation; eukaryotes lack pili.
• Nucleolus is located within the nucleus and is the site of ribosomal RNA synthesis and ribosome assembly.
• Smooth endoplasmic reticulum (SER) synthesizes lipids and detoxifies drugs; rough ER (RER) has ribosomes and synthesizes proteins for secretion.
• Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport to lysosomes, plasma membrane, or secretion.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and organelles, with attention to exceptions and evolutionary evidence.

Common Traps

Trap: All cells with cell walls have peptidoglycan. – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (no peptidoglycan) differ.
Trap: Ribosome size correlates directly with cell complexity. – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, like prokaryotes, due to endosymbiotic origin.
Trap: Prokaryotes have no internal structure. – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based microcompartments (e.g., carboxysomes).
Trap: The nucleus is the only organelle with a double membrane. – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Eukaryotes evolved from archaea. – Fact: Current evidence suggests eukaryotes share a common ancestor with archaea, but are a distinct domain (eocyte hypothesis).

Practice MCQs

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Phospholipid bilayer plasma membrane
D) Mitochondria
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (mitochondria) are only found in eukaryotes; prokaryotes generate ATP on the plasma membrane.

Question: A cell is observed to contain circular DNA, 70S ribosomes, and peptidoglycan in its cell wall. It lacks a nucleus. This cell is most likely:
A) A plant cell
B) A fungal cell
C) A bacterial cell
D) An animal cell
Answer: C
Explanation: Presence of peptidoglycan and 70S ribosomes with no nucleus identifies it as a bacterial (prokaryotic) cell.
Why the top distractor is wrong: A (plant cells) are eukaryotic, have cellulose walls, 80S ribosomes, and a nucleus.

Question: Which structure is found in eukaryotic cells but absent in prokaryotic cells?
A) Ribosomes
B) Plasma membrane
C) Golgi apparatus
D) DNA
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle unique to eukaryotes.
Why the top distractor is wrong: A (ribosomes) are present in both, though differing in size (70S vs 80S).

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Mitochondria are surrounded by a single membrane
B) Mitochondria have 70S ribosomes and circular DNA
C) Mitochondria are larger than most bacteria
D) Mitochondria depend on nuclear genes for some proteins
Answer: B
Explanation: 70S ribosomes and circular DNA are prokaryotic traits, supporting mitochondrial origin from bacteria.
Why the top distractor is wrong: A is false—mitochondria have a double membrane, consistent with engulfment.

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human liver cell
C) Mature human red blood cell
D) Onion root cell
Answer: C
Explanation: Mature mammalian red blood cells eject their nucleus to accommodate hemoglobin.
Why the top distractor is wrong: A (yeast) are fungi and have a nucleus; all are eukaryotic except the exception in C.

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

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Nucleolus
B) Rough endoplasmic reticulum
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies, sorts, and packages proteins into vesicles for export.
Why the top distractor is wrong: B synthesizes proteins but does not package them for secretion.

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m
• Eukaryotic cell size: 10–100 ?m
• Prokaryotic ribosome = 70S (30S + 50S)
• Eukaryotic ribosome = 80S (40S + 60S)
• Mitochondria and chloroplasts have 70S ribosomes
• Bacterial cell walls contain peptidoglycan
• Plant cell walls = cellulose
• Fungal cell walls = chitin
• Animal cells have no cell wall
• Mycoplasma: no cell wall
• DNA in prokaryotes: nucleoid (no membrane)
• DNA in eukaryotes: membrane-bound nucleus
• Mature mammalian RBCs: no nucleus
• Nuclear envelope: double membrane with pores
• Prokaryotes: circular DNA, usually one chromosome
• Eukaryotes: linear chromosomes, multiple
• Plasmids common in bacteria, rare in eukaryotes
• Endosymbiotic theory: mitochondria and chloroplasts from engulfed prokaryotes
• Evidence: organelles have circular DNA, 70S ribosomes, binary fission
• Mitochondria have double membrane
• Lysosomes: in animal cells, not common in plants
• Flagellin = prokaryotic flagella protein; eukaryotic = microtubules (9+2)
• Pili: prokaryotic only
• Nucleolus: site of rRNA synthesis and ribosome assembly
• SER: lipid synthesis, detoxification
• RER: protein synthesis for export
• Golgi: modifies, sorts, packages proteins
• verify from standard textbook: archaeal cell wall composition varies (pseudopeptidoglycan, polysaccharides, or proteins)