STEM Readiness: Biology Readiness - Physiology Preview: Homeostasis - Negative Feedback, Positive Feedback, Examples from Each Organ System

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 store DNA in a nucleoid, an unenclosed region lacking a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, supporting their bacterial origin.
• Most prokaryotes have a cell wall made of peptidoglycan; exceptions include Mycoplasma, which lacks a cell wall entirely.
• Eukaryotic cell walls vary: fungi have chitin, plants have cellulose, and animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; only eukaryotes have extensive internal membrane systems.
• Endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, and nucleus are organelles found exclusively in eukaryotes.
• The nuclear envelope is a double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
• Chromosomes in prokaryotes are typically a single, circular DNA molecule; eukaryotes have multiple linear chromosomes.
• Histone proteins associate with DNA in both archaea and eukaryotes, but most bacteria lack histones.
• Binary fission is the primary mode of reproduction in prokaryotes; eukaryotes divide via mitosis and meiosis.
• Flagella differ structurally: prokaryotic flagella are made of flagellin and rotate, while eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Plasmids are small, circular DNA molecules common in prokaryotes; rare in eukaryotes (e.g., 2-micron plasmid in yeast).
• Cytoskeleton components (actin, tubulin) exist in some prokaryotes, but complex forms (microtubules, intermediate filaments) are eukaryotic innovations.
• Lysosomes are membrane-bound organelles containing hydrolytic enzymes; absent in prokaryotes and plant cells (vacuoles perform similar function).
• Plant cells contain chloroplasts for photosynthesis; these organelles have double membranes, 70S ribosomes, and circular DNA.
• Mitochondria generate ATP via oxidative phosphorylation; they have cristae (inner membrane folds), matrix, 70S ribosomes, and circular DNA.
• Endosymbiotic theory proposes mitochondria originated from alpha-proteobacteria and chloroplasts from cyanobacteria.
• Evidence for endosymbiosis includes organelle independent replication, double membranes, 70S ribosomes, and circular DNA.
• Red blood cells in mammals lack a nucleus and most organelles, maximizing hemoglobin content; they are anucleate.
• Archaea are prokaryotes but differ from bacteria in membrane lipid composition (ether linkages, branched hydrocarbons) and ribosome structure.
• Peroxisomes are single-membrane eukaryotic organelles that break down fatty acids and detoxify hydrogen peroxide; absent in prokaryotes.
• Nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome subunit assembly.
• Smooth ER synthesizes lipids and detoxifies drugs; rough ER has bound ribosomes and synthesizes secretory and membrane proteins.

Difficulty Level

Intermediate – expected foundational knowledge in first-semester biology but requires precise differentiation of structures and exceptions.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; fungi (chitin) and plants (cellulose) have different compositions.
Trap: Ribosome size correlates with cell complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes have protein-bound compartments (e.g., carboxysomes), but lack membrane-bound organelles.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Eukaryotes evolved directly from bacteria – Fact: Phylogenetic evidence shows eukaryotes share a common ancestor with archaea, not bacteria.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) Mitochondria
C) Phospholipid bilayer plasma membrane
D) Endoplasmic reticulum
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (ER) is only found in eukaryotes; prokaryotes lack internal membrane systems.

Question: A cell is observed to have 70S ribosomes, a nucleoid, and no mitochondria. Which of the following is most likely true?
A) It is a plant cell
B) It is a fungal cell
C) It is a bacterial cell
D) It is an animal cell
Answer: C
Explanation: 70S ribosomes and a nucleoid are diagnostic of prokaryotes, such as bacteria.
Why the top distractor is wrong: A (plant cells) are eukaryotic and have 80S ribosomes and mitochondria.

Question: Which structure is present 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 exclusive to eukaryotes.
Why the top distractor is wrong: A (ribosomes) are present in both, though differing in size.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to synthesize proteins using 80S ribosomes
C) Linear chromosomes with telomeres
D) Replication via mitosis
Answer: A
Explanation: The double membrane and presence of 70S ribosomes and circular DNA support bacterial origin.
Why the top distractor is wrong: B is incorrect because mitochondria use 70S, not 80S, ribosomes.

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: A (yeast) is a eukaryotic fungus and has a nucleus.

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

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Nucleolus
B) Smooth ER
C) Rough ER
D) Golgi apparatus
Answer: D
Explanation: The Golgi apparatus modifies, sorts, and packages proteins from the ER for transport.
Why the top distractor is wrong: C (rough ER) synthesizes proteins but does not package them for secretion.

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

• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Prokaryotes: 0.1–5.0 ?m; eukaryotes: 10–100 ?m.
• DNA in prokaryotes is in the nucleoid; in eukaryotes, it’s enclosed in the nucleus.
• Bacterial cell walls contain peptidoglycan; archaeal walls do not.
• Mycoplasma lacks a cell wall – no peptidoglycan.
• Plant cell wall = cellulose; fungal cell wall = chitin; animal cells = no cell wall.
• Both cell types have phospholipid bilayer membranes.
• Only eukaryotes have membrane-bound organelles (e.g., ER, Golgi, lysosomes).
• Nuclear envelope has double membrane and nuclear pores.
• Prokaryotes have circular DNA; eukaryotes have linear chromosomes with histones.
• Histones are present in eukaryotes and archaea, but not in most bacteria.
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis/meiosis.
• Eukaryotic flagella: 9+2 microtubule arrangement; prokaryotic: flagellin-based, rotating.
• Plasmids are common in bacteria; rare in eukaryotes.
• Cytoskeleton complexity (microtubules, intermediate filaments) is eukaryotic-specific.
• Lysosomes are eukaryotic-only; absent in prokaryotes and plant cells.
• Chloroplasts have double membrane, 70S ribosomes, circular DNA.
• Mitochondria originated from alpha-proteobacteria; chloroplasts from cyanobacteria.
• Red blood cells are anucleate – no nucleus or organelles.
• Archaea have ether-linked, branched hydrocarbon membrane lipids.
• Peroxisomes break down fatty acids and detoxify H?O?; eukaryotic-only.
• Nucleolus = site of rRNA synthesis and ribosomal subunit assembly.
• Smooth ER = lipid synthesis, detoxification; rough ER = protein synthesis.
• Mitochondria and chloroplasts have double membranes – like the nucleus.
• Verify from standard textbook: presence of cytoskeletal homologs in some prokaryotes (e.g., FtsZ, MreB).