STEM Readiness: Physics Readiness - Electricity Foundations: Electric Potential - V = kq/r, Potential Energy, U = qV, Work Done by Electric Force

Must?Know

• 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, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, supporting endosymbiotic origin.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cell walls are made of cellulose; fungal cell walls are composed of chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have an endomembrane system (nuclear envelope, ER, Golgi, vesicles, lysosomes); prokaryotes do not.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Eukaryotic nuclei are surrounded by a double membrane (nuclear envelope) with nuclear pores for RNA export.
• Nucleolus is present in eukaryotic nuclei and is the site of ribosomal RNA synthesis; absent in prokaryotes.
• Mycoplasma species are bacteria without a cell wall, making them resistant to penicillin.
• Human red blood cells lack a nucleus and mitochondria, maximizing hemoglobin capacity.
• Chloroplasts are found in plant cells and some protists (e.g., Chlamydomonas); absent in animal and fungal cells.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; rare in plant cells (vacuoles perform similar function).
• Peroxisomes contain enzymes for fatty acid oxidation and detoxify hydrogen peroxide; present in both plant and animal eukaryotic cells.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 array) and undulate.
• Cytoskeleton elements (microtubules, actin filaments, intermediate filaments) are present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ, MreB) but no true cytoskeleton.
• 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; some live in extreme environments (e.g., Methanogens in anaerobic conditions).
• Plant vacuoles are large, central, and maintain turgor pressure; animal cells may have small, temporary vacuoles.

Difficulty Level

Intermediate – requires precise differentiation of structures and exceptions commonly tested in first-year biology exams.

Common Traps

• Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose) and fungi (chitin) have different wall compositions.
• Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S (except in mitochondria and chloroplasts, which have 70S).
• Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
• Trap: Eukaryotes are always multicellular – Fact: Many eukaryotes are unicellular (e.g., Amoeba, Paramecium, yeast).
• Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein compartments (e.g., carboxysomes), but no membrane-bound organelles.

Practice MCQs

Question: Which of the following is a feature found in eukaryotic cells but NOT in prokaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nuclear envelope
D) Plasma membrane
Answer: C
Explanation: The nuclear envelope is a double membrane surrounding the nucleus, present only in eukaryotes.
Why the top distractor is wrong: 70S ribosomes are found in prokaryotes and also in mitochondria/chloroplasts, so they are not exclusive to prokaryotes.

Question: A cell is observed to have a cell wall, chloroplasts, and a large central vacuole. It is most likely:
A) Fungal cell
B) Animal cell
C) Bacterial cell
D) Plant cell
Answer: D
Explanation: Plant cells have cellulose cell walls, chloroplasts for photosynthesis, and large central vacuoles.
Why the top distractor is wrong: Fungal cells have chitin cell walls and lack chloroplasts and large central vacuoles.

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Presence of a nucleolus
B) 80S ribosomes in the cytoplasm
C) Circular DNA in mitochondria
D) Linear chromosomes in the nucleus
Answer: C
Explanation: Mitochondria have circular DNA similar to bacteria, supporting their origin as free-living prokaryotes.
Why the top distractor is wrong: The nucleolus is involved in ribosome assembly but does not support endosymbiosis.

Question: Which organism lacks a cell wall and is classified as a prokaryote?
A) Saccharomyces cerevisiae
B) Mycoplasma pneumoniae
C) Escherichia coli
D) Streptococcus pyogenes
Answer: B
Explanation: Mycoplasma is a bacterium without a cell wall, making it naturally resistant to beta-lactam antibiotics.
Why the top distractor is wrong: Saccharomyces cerevisiae is a fungus (eukaryote) with a chitin cell wall.

Question: Where are 70S ribosomes found in eukaryotic cells?
A) In the nucleus
B) In the Golgi apparatus
C) In mitochondria
D) In the cytoplasm
Answer: C
Explanation: Mitochondria contain 70S ribosomes, similar to those in bacteria, due to their endosymbiotic origin.
Why the top distractor is wrong: Cytoplasmic ribosomes in eukaryotes are 80S, not 70S.

Question: Which of the following is true about the DNA of prokaryotes?
A) It is enclosed within a nuclear membrane.
B) It is typically linear and associated with histones.
C) It is located in the nucleoid region.
D) It is found in multiple chromosomes.
Answer: C
Explanation: Prokaryotic DNA is located in the nucleoid, an unbound region of the cytoplasm.
Why the top distractor is wrong: Prokaryotic DNA is usually a single circular chromosome and lacks histones (except in some archaea).

Question: Which cellular component is present in both prokaryotes and eukaryotes?
A) Mitochondria
B) Endoplasmic reticulum
C) Plasma membrane
D) Lysosome
Answer: C
Explanation: Both cell types have a phospholipid bilayer plasma membrane.
Why the top distractor is wrong: Mitochondria are membrane-bound organelles found only in eukaryotes.

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: DNA in nucleoid; eukaryotes: DNA in nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have membrane-bound organelles; prokaryotes do not.
• Mycoplasma has no cell wall – smallest known prokaryote.
• Human red blood cells lack a nucleus and mitochondria.
• Nuclear envelope has nuclear pores for RNA transport.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Chloroplasts found in plants and some protists.
• Lysosomes are common in animal cells, not in plant cells.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Flagellin = prokaryotic flagella protein; 9+2 microtubule array = eukaryotic flagella.
• Cytoskeleton (microtubules, actin) = eukaryotic feature; prokaryotes have FtsZ/MreB analogs.
• Gram-positive = thick peptidoglycan; Gram-negative = thin peptidoglycan + outer membrane.
• Archaea have ether-linked lipids and no peptidoglycan.
• Endosymbiotic theory supported by double membranes, circular DNA, and 70S ribosomes in mitochondria/chloroplasts.
• Plasmids = small circular DNA, common in bacteria.
• Binary fission = prokaryotic division; mitosis/meiosis = eukaryotic.
• All mitochondria are derived from endosymbiotic alpha-proteobacteria – verify from standard textbook.
• Eukaryotic organelles with double membranes: nucleus, mitochondria, chloroplasts.
• FtsZ protein in prokaryotes is homologous to eukaryotic tubulin.