STEM Readiness: Physics Readiness - Energy/Work: Conservative vs., Non-conservative Forces, Path Independence, Potential Energy

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.
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes—supporting the endosymbiotic theory.
• Most prokaryotes have a cell wall made of peptidoglycan; archaea lack peptidoglycan and have different cell wall chemistry.
• Plant cells have a cell wall made of cellulose; fungal cells have a cell wall made of chitin; animal cells lack a cell wall.
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin that target peptidoglycan.
• Red blood cells in mammals lack a nucleus and most organelles, maximizing space for hemoglobin.
• Both prokaryotes and eukaryotes have a plasma membrane composed of a phospholipid bilayer with embedded proteins.
• Eukaryotes have internal membranes forming organelles (e.g., nuclear envelope, endoplasmic reticulum), while prokaryotes do not.
• The nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
• Chromosomal DNA in prokaryotes is typically a single circular molecule; eukaryotes have multiple linear chromosomes.
• Plasmids are small, circular extrachromosomal DNA molecules commonly found in prokaryotes, but rare in eukaryotes.
• Eukaryotic cells undergo mitosis for nuclear division; prokaryotes divide by binary fission.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and bend.
• Cilia are present in some eukaryotes (e.g., tracheal epithelium), absent in prokaryotes.
• Lysosomes (containing hydrolytic enzymes) are found only in animal cells, not in prokaryotes or plant cells.
• Plant cells contain chloroplasts for photosynthesis; animal cells lack chloroplasts.
• Vacuoles in plant cells are large and central; animal cells have small, temporary vacuoles.
• Peroxisomes are present in eukaryotes and contain enzymes for breaking down fatty acids and detoxifying hydrogen peroxide.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own DNA, 70S ribosomes, and double membranes.
• Archaea are prokaryotes but share some molecular features with eukaryotes, such as RNA polymerase and ribosome sensitivity to certain antibiotics.
• Gram-positive bacteria have a thick peptidoglycan layer; Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.

Difficulty Level

Intermediate – routinely tested in first-year biology courses with emphasis on comparison and organelle function.

Common Traps (3–5 factual traps)

Trap: All cells with a cell wall contain peptidoglycan – Fact: Only bacteria have peptidoglycan; plant cell walls are made of cellulose, fungal walls of chitin.

Trap: Ribosome size is the same across all cells – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes in the cytoplasm (but 70S in mitochondria and chloroplasts).

Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.

Trap: Prokaryotes have no DNA packaging – Fact: Prokaryotic DNA is supercoiled and associated with proteins, though not organized into histones like eukaryotes.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in eukaryotic cells but NOT in prokaryotic cells?
A) Plasma membrane
B) 70S ribosomes
C) Mitochondria
D) Circular DNA
Answer: C
Explanation: Mitochondria are membrane-bound organelles present only in eukaryotes.
Why the top distractor is wrong: Circular DNA is found in prokaryotes and also in mitochondria and chloroplasts, so it is not exclusive to prokaryotes.

Question: Which structure is present in plant cells but absent in animal cells?
A) Lysosome
B) Nucleus
C) Central vacuole
D) Ribosome
Answer: C
Explanation: The large central vacuole is a defining feature of mature plant cells.
Why the top distractor is wrong: Lysosomes are primarily found in animal cells, though some plant vacuoles have similar functions.

Question: A cell is observed to have a nucleus, mitochondria, and a cell wall made of cellulose. This cell is most likely from a:
A) Fungus
B) Bacterium
C) Animal
D) Plant
Answer: D
Explanation: Cellulose cell walls and chloroplasts (implied by context) are characteristic of plant cells.
Why the top distractor is wrong: Fungi have cell walls, but they are made of chitin, not cellulose.

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
D) Involvement in cellular respiration
Answer: A
Explanation: The double membrane and presence of 70S ribosomes and circular DNA support endosymbiotic theory.
Why the top distractor is wrong: Mitochondria use 70S ribosomes, not 80S; 80S ribosomes are cytoplasmic in eukaryotes.

Question: Which organism lacks a cell wall and is classified as a prokaryote?
A) Mycoplasma
B) Escherichia coli
C) Saccharomyces cerevisiae
D) Streptococcus
Answer: A
Explanation: Mycoplasma is a bacterium without a cell wall.
Why the top distractor is wrong: E. coli is a bacterium with a peptidoglycan cell wall.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have no nucleus; DNA in nucleoid.
• Eukaryotes have membrane-bound nucleus with nuclear pores.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal cells = no cell wall.
• Mycoplasma – bacterium without cell wall.
• Mature mammalian red blood cells lack a nucleus and organelles.
• Both cell types have phospholipid bilayer plasma membrane.
• Only eukaryotes have internal membrane systems (ER, Golgi, etc.).
• Nuclear envelope is a double membrane with nuclear pores.
• Prokaryotes have single circular chromosome; eukaryotes have multiple linear chromosomes.
• Plasmids are common in bacteria, rare in eukaryotes.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• Prokaryotic flagella = flagellin, rotate; eukaryotic = microtubules (9+2), bend.
• Cilia are eukaryotic only.
• Lysosomes are in animal cells, not in plant cells.
• Chloroplasts are in plant cells, not in animal cells.
• Plant vacuole = large and central; animal vacuoles = small and temporary.
• Peroxisomes contain catalase for H?O? breakdown.
• Mitochondria have double membrane, own DNA, 70S ribosomes – supports endosymbiotic theory.
• Archaea are prokaryotes but share transcription machinery with eukaryotes.
• Gram-positive bacteria = thick peptidoglycan; Gram-negative = thin peptidoglycan + outer membrane.
• Verify from standard textbook: exact protein composition of archaeal cell walls.