STEM Readiness: Biology Readiness - Physiology Preview: Endocrine System - Hormone Types, Steroid vs. Peptide, Target Cells, Feedback Loops

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, a region without 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).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan and have different wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport.
• Eukaryotes have internal membranes forming organelles (e.g., nuclear envelope, endoplasmic reticulum); prokaryotes do not.
• Prokaryotic DNA is typically a single circular chromosome; eukaryotic DNA is linear and organized into multiple chromosomes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes (e.g., 2-micron plasmid in yeast).
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide by mitosis and meiosis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin.
• Human red blood cells lack a nucleus and mitochondria in maturity, maximizing hemoglobin capacity.
• Plant cells contain chloroplasts for photosynthesis; animal cells do not; both are eukaryotic.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; rare in plant cells (vacuoles perform similar function).
• The endosymbiotic theory proposes that mitochondria and chloroplasts originated from engulfed prokaryotes; evidence includes their own DNA, 70S ribosomes, and double membranes.
• Mitochondria have circular DNA similar to bacterial chromosomes; nuclear DNA in eukaryotes is linear.
• Prokaryotes can have capsules (e.g., Streptococcus pneumoniae) for protection; some eukaryotes have extracellular matrices (e.g., animal connective tissue).
• Cilia and flagella in eukaryotes are anchored by basal bodies; prokaryotic flagella lack basal bodies.
• Gram-positive bacteria have thick peptidoglycan layers and retain crystal violet stain; Gram-negative have thin peptidoglycan and an outer membrane.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes; bacteria and eukaryotes have ester-linked phospholipids.
• Nuclear pores regulate transport between nucleus and cytoplasm in eukaryotes; no such structure exists in prokaryotes.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and recognizing exceptions.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (pseudopeptidoglycan or other) do not.

Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells, due to prokaryotic ancestry.

Trap: Prokaryotes have no DNA packaging – Fact: Prokaryotic DNA is supercoiled and associated with nucleoid-associated proteins, though not with histones like eukaryotes (except some archaea).

Trap: Eukaryotes are always multicellular – Fact: Many eukaryotes are unicellular, such as yeast (fungus) and Amoeba (protist).

Trap: The nucleus evolved after organelles – Fact: The nucleus is a defining feature of eukaryotes; endosymbiotic organelles (mitochondria, chloroplasts) were acquired later via endosymbiosis.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid region
D) Phospholipid bilayer
Answer: C
Explanation: The nucleoid is a distinct, non-membrane-bound region in prokaryotes where DNA is located.
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 cell type lacks a nucleus at maturity?
A) Escherichia coli
B) Onion root cell
C) Human red blood cell
D) Saccharomyces cerevisiae
Answer: C
Explanation: Mature human red blood cells lose their nucleus to accommodate more hemoglobin.
Why the top distractor is wrong: E. coli (A) is prokaryotic and never has a nucleus, but the question refers to cells that lose it during maturation.

Question: Which structure is present in plant cells but absent in animal cells?
A) Mitochondria
B) Plasma membrane
C) Central vacuole
D) Ribosomes
Answer: C
Explanation: The central vacuole is a large, membrane-bound organelle in plant cells that stores water and maintains turgor pressure.
Why the top distractor is wrong: Mitochondria (A) are present in both plant and animal cells.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to replicate independently
C) Contain 70S ribosomes and circular DNA
D) Use oxygen in metabolism
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic traits retained in mitochondria.
Why the top distractor is wrong: Double membranes (A) are suggestive but not exclusive evidence; some organelles have double membranes without endosymbiotic origin.

Question: A cell is observed to have a cell wall, no nucleus, and 70S ribosomes. It is most likely:
A) A fungal cell
B) A plant cell
C) A bacterial cell
D) An animal cell
Answer: C
Explanation: Bacterial cells are prokaryotic, with cell walls (peptidoglycan), no nucleus, and 70S ribosomes.
Why the top distractor is wrong: Fungal cells (A) are eukaryotic and have 80S ribosomes and a nucleus.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 80S
B) Ribosomes in mitochondria are 70S
C) Ribosomes are enclosed in a membrane
D) Ribosomes are only found in the cytoplasm
Answer: B
Explanation: Mitochondrial ribosomes are 70S, reflecting their prokaryotic ancestry.
Why the top distractor is wrong: Not all ribosomes in eukaryotes are 80S—mitochondria and chloroplasts have 70S.

Question: Which organism lacks a cell wall entirely?
A) Mycoplasma genitalium
B) Bacillus subtilis
C) Rhizopus stolonifer
D) Zea mays
Answer: A
Explanation: Mycoplasma is a bacterium without a cell wall, making it pleomorphic and resistant to beta-lactam antibiotics.
Why the top distractor is wrong: Bacillus subtilis (B) is a Gram-positive bacterium with a thick peptidoglycan cell wall.

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

• Prokaryotic cells: 0.1–5.0 ?m; eukaryotic cells: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have membrane-bound nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Peptidoglycan is found only in bacterial cell walls.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Mycoplasma – smallest known cells, no cell wall.
• Human red blood cells lack a nucleus and mitochondria at maturity.
• Both prokaryotes and eukaryotes have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Prokaryotic DNA = single circular chromosome; eukaryotic = multiple linear chromosomes.
• Plasmids are common in bacteria, rare in eukaryotes.
• Binary fission = prokaryotic division; mitosis = eukaryotic nuclear division.
• Prokaryotic flagellum = flagellin, rotates; eukaryotic = microtubules (9+2), bends.
• Lysosomes are primarily in animal cells; plant vacuoles perform degradation.
• Chloroplasts present in plants and algae, not in animals.
• Archaea lack peptidoglycan and have ether-linked membrane lipids.
• Gram-positive bacteria = thick peptidoglycan; Gram-negative = thin peptidoglycan + outer membrane.
• Nuclear pores regulate RNA and protein transport in eukaryotes.
• Endosymbiotic theory: mitochondria evolved from alpha-proteobacteria, chloroplasts from cyanobacteria.
• Cilia and flagella in eukaryotes anchored by basal bodies (derived from centrioles).
• All ribosomes in prokaryotes are 70S – no exceptions.
• 80S ribosomes are only in the cytoplasm and on rough ER of eukaryotes.
• Yeast (Saccharomyces) is a unicellular eukaryote (fungus).
• Amoeba and Paramecium are unicellular eukaryotes (protists).
• Verify from standard textbook: exact composition of archaeal cell walls varies widely.