STEM Readiness: Chemistry Readiness - Phases/Solutions: Phase Diagrams - Triple Point, Critical Point, Reading Phase Transitions

Must?Know

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• Prokaryotes lack a membrane-bound nucleus; DNA is located in the nucleoid region.
• Eukaryotes have a membrane-bound nucleus containing linear chromosomes.
• Prokaryotic ribosomes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli).
• Archaea lack peptidoglycan; their cell walls are made of pseudopeptidoglycan or other polysaccharides.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin.
• Animal cells lack a cell wall; only a plasma membrane is present.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membrane systems: endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, nucleus.
• Prokaryotes lack membrane-bound organelles; metabolic functions occur in the cytoplasm or plasma membrane.
• Bacteria (e.g., Streptococcus) and archaea (e.g., Methanogens) are prokaryotes.
• Animals, plants, fungi, and protists are eukaryotes.
• Mycoplasma species are bacteria that lack a cell wall—an exception among prokaryotes.
• Mature mammalian red blood cells lack a nucleus and mitochondria—maximizing hemoglobin capacity.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport.
• Nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
• Endosymbiotic theory proposes mitochondria and chloroplasts originated from engulfed prokaryotes.
• Evidence for endosymbiosis: mitochondria and chloroplasts have circular DNA, 70S ribosomes, and double membranes.
• Chloroplasts are found in plants and some protists (e.g., Chlamydomonas); absent in animals and fungi.
• Lysosomes (in animal cells) contain hydrolytic enzymes for intracellular digestion; absent in most plant cells.
• Peroxisomes contain enzymes for fatty acid oxidation and detoxification of hydrogen peroxide.
• Cytoskeleton (microtubules, microfilaments, intermediate filaments) is present in eukaryotes; prokaryotes have analogous proteins (e.g., FtsZ) but no true cytoskeleton.
• Flagella differ structurally: prokaryotic flagella are rotary filaments of flagellin; eukaryotic flagella are 9+2 microtubule arrays.

Difficulty Level

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

Common Traps

Trap: All cells with cell walls have peptidoglycan.
Fact: Only bacterial cell walls contain peptidoglycan; plants (cellulose), fungi (chitin), and archaea (pseudopeptidoglycan or other) differ.

Trap: Ribosome size correlates with cell complexity; eukaryotes only have 80S ribosomes.
Fact: Eukaryotic cytoplasmic ribosomes are 80S, but mitochondria and chloroplasts have 70S ribosomes.

Trap: Prokaryotes have no internal membranes.
Fact: While lacking organelles, some prokaryotes (e.g., cyanobacteria) have thylakoid membranes for photosynthesis.

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

Trap: All eukaryotic cells have a cell wall.
Fact: Animal cells lack a cell wall; only plant, fungal, and some protist cells have one.

Practice MCQs

Question: Which of the following is a defining feature of prokaryotic cells?
A) Presence of mitochondria
B) 80S ribosomes in the cytoplasm
C) DNA enclosed within a nuclear envelope
D) DNA located in a nucleoid region
Answer: D
Explanation: Prokaryotes lack a nucleus; DNA resides in the nucleoid.
Why the top distractor is wrong: Distractor C describes eukaryotes, which have a membrane-bound nucleus.

Question: A cell is observed to have a cell wall, chloroplasts, and 80S ribosomes. It is most likely from which organism?
A) Fungus
B) Bacterium
C) Animal
D) Green plant
Answer: D
Explanation: Green plants have cellulose cell walls, chloroplasts, and eukaryotic (80S) ribosomes.
Why the top distractor is wrong: Fungi (A) have cell walls and 80S ribosomes but lack chloroplasts.

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

Question: Which of the following provides evidence for the endosymbiotic origin of mitochondria?
A) Presence of linear DNA
B) Use of 80S ribosomes
C) Double membrane and 70S ribosomes
D) Connection to the endoplasmic reticulum
Answer: C
Explanation: Mitochondria have double membranes and 70S ribosomes, resembling bacteria.
Why the top distractor is wrong: Mitochondria have circular DNA (not linear) and 70S (not 80S) ribosomes.

Question: Which cell type lacks a nucleus but contains DNA?
A) Human neuron
B) Escherichia coli
C) Mature human red blood cell
D) Yeast cell
Answer: B
Explanation: E. coli is a prokaryote with DNA in the nucleoid but no nucleus.
Why the top distractor is wrong: Mature human red blood cells (C) lack both nucleus and DNA.

Question: Which of the following organelles is found in animal cells but typically absent in plant cells?
A) Mitochondrion
B) Lysosome
C) Nucleus
D) Peroxisome
Answer: B
Explanation: Lysosomes are common in animal cells; plant vacuoles perform similar functions.
Why the top distractor is wrong: Mitochondria (A) are present in both animal and plant cells.

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

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m
• Eukaryotic cell size: 10–100 ?m
• Prokaryotic ribosome = 70S (50S + 30S)
• Eukaryotic cytoplasmic ribosome = 80S (60S + 40S)
• Mitochondria and chloroplasts have 70S ribosomes
• Mitochondria and chloroplasts have circular DNA
• Prokaryotes store DNA in the nucleoid (no membrane)
• Eukaryotes store DNA in the nucleus (double membrane)
• Nuclear pores regulate mRNA and protein transport
• Peptidoglycan is unique to bacterial cell walls
• Archaea lack peptidoglycan-use pseudopeptidoglycan or other polymers
• Plant cell wall = cellulose
• Fungal cell wall = chitin
• Animal cells = no cell wall
• Both cell types have phospholipid bilayer membranes
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes do not
• Mycoplasma-bacteria without cell walls
• Mature red blood cells lack nucleus and mitochondria
• Endosymbiotic theory supported by: double membrane, 70S ribosomes, circular DNA in mitochondria/chloroplasts
• Lysosomes: present in animal cells, rare in plants
• Flagellin-based flagella = prokaryotes; 9+2 microtubules = eukaryotes
• Thylakoid membranes in cyanobacteria (prokaryotes) perform photosynthesis
• Nucleolus = site of rRNA synthesis
• FtsZ protein in prokaryotes is homologous to eukaryotic tubulin
• Peroxisomes break down fatty acids and detoxify H?O?