Cell Theory &
Prokaryotic Structure
Master the three tenets of cell theory and the architecture of prokaryotic cells: essential foundations for MCAT Biological Sciences.
MCAT Bio/Biochem
What Is Cell Theory?
Cell theory is one of the unifying principles of biology. Developed in the 1800s by Schleiden, Schwann, and Virchow, it provides the framework for understanding all living organisms at their most fundamental level.
The Three Tenets of Cell Theory
1
All living organisms are composed of one or more cells. From single-celled bacteria to multicellular humans, the cell is the universal building block.
2
The cell is the basic unit of structure and function. Cellular processes (metabolism, signaling, replication) are the basis for all life functions.
3
All cells arise from pre-existing cells. Spontaneous generation does not occur: cells divide via binary fission or mitosis/meiosis.
Why This Matters for the MCAT
The MCAT tests your understanding of cell theory both directly and indirectly. Passages on microbiology, genetics, and evolution all assume you know these principles. Questions may ask you to:
Identify
Which tenet of cell theory a passage or experiment supports or challenges
Distinguish
Prokaryotic from eukaryotic structures and explain functional consequences
Apply
Cell theory to novel experimental scenarios involving viruses or prions
Anatomy of a Prokaryotic Cell
Key Prokaryotic Structures
Nucleoid
Region containing a single circular chromosome: no nuclear envelope
Ribosomes
\(70\text{S}\) ribosomes (\(30\text{S} + 50\text{S}\) subunits): target of many antibiotics
Plasmids
Small circular extrachromosomal DNA: often carry antibiotic resistance genes
Cell Wall
Peptidoglycan layer providing structural rigidity and shape
Flagella
Rotary motor-driven appendages for motility and chemotaxis
Pili
Short projections for adhesion and conjugation (DNA transfer)
Prokaryote vs Eukaryote
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
| DNA Shape | Circular chromosome | Linear chromosomes |
| Ribosomes | \(70\text{S}\) (\(30\text{S} + 50\text{S}\)) | \(80\text{S}\) (\(40\text{S} + 60\text{S}\)) |
| Organelles | None membrane-bound | Mitochondria, ER, Golgi, etc. |
| Cell Wall | Peptidoglycan (bacteria) | Cellulose (plants) or absent |
| Reproduction | Binary fission | Mitosis / Meiosis |
| Size | \( 1\text{--}10\ \mu\text{m} \) | \( 10\text{--}100\ \mu\text{m} \) |
Gram-Positive vs Gram-Negative
Reproduction: Binary Fission
Genetic Variation in Prokaryotes
Although prokaryotes reproduce asexually, they achieve genetic diversity through three horizontal gene transfer mechanisms. These are high-yield MCAT topics.
Transformation
Uptake of free DNA from the environment through the cell membrane
Transduction
Bacteriophage (virus) transfers DNA from one bacterium to another
Conjugation
Direct transfer via pilus: F-plasmid passed from F+ to F- cell
Edge Cases: Viruses and Prions
The MCAT loves to test your understanding of entities that challenge cell theory. Know why these are exceptions.
Viruses
Not cells: no ribosomes, no metabolism, require a host cell to replicate. Contain DNA or RNA, but are not considered "alive" under cell theory.
Prions
Misfolded proteins that propagate without nucleic acid. No cells, no DNA/RNA: the ultimate challenge to classical cell theory.
The Plasma Membrane
How to Approach MCAT Questions
1
Identify the organism type. Is the passage describing a prokaryote, eukaryote, virus, or prion? This determines which structures apply.
2
Check for membrane-bound organelles. If the question mentions a nucleus, mitochondria, or ER, it is eukaryotic. If none, think prokaryotic.
3
Recall ribosome sizes. Antibiotics targeting \(70\text{S}\) ribosomes affect prokaryotes selectively. This is a common MCAT trap.
4
Connect to cell theory. If asked about cell origins, remember: all cells from pre-existing cells. Viruses require host cells to replicate.
5
Eliminate wrong answers by structure. Prokaryotes lack: nucleus, mitochondria, ER, Golgi, lysosomes, cytoskeleton (mostly). Use this to eliminate.
Example: Antibiotic Targeting
1
Identify ribosome type. The \(30\text{S}\) subunit is part of the \(70\text{S}\) prokaryotic ribosome (\(30\text{S} + 50\text{S}\)).
2
Recall eukaryotic ribosomes. Eukaryotes have \(80\text{S}\) ribosomes (\(40\text{S} + 60\text{S}\)), so they would be unaffected.
3
Conclusion: The antibiotic selectively targets bacteria without harming human cells. This is the basis for selective toxicity.
Example: Gram Staining Results
1
Purple = Gram-positive. Crystal violet is retained by thick peptidoglycan layers.
2
No outer membrane. Gram-positive bacteria lack the LPS-containing outer membrane found in Gram-negative species.
3
Clinical implication: Gram-positive bacteria are generally more susceptible to penicillin, which targets peptidoglycan synthesis.
Common Mistakes
✕ Confusing \(70\text{S}\) and \(80\text{S}\)
Prokaryotes have \(70\text{S}\) ribosomes. Eukaryotes have \(80\text{S}\). The "S" values do not add up arithmetically (\(30\text{S} + 50\text{S} = 70\text{S}\), not \(80\text{S}\)).
✕ Calling Viruses "Cells"
Viruses are not cells and do not satisfy cell theory. They lack ribosomes, metabolism, and independent reproduction.
✕ Forgetting Plasmids
Prokaryotic DNA is not only the chromosome. Plasmids carry accessory genes (e.g., antibiotic resistance) and are transferred during conjugation.
Question 1 of 5
Which of the following is NOT a tenet of cell theory?
Cell theory states that all organisms are made of cells, cells are the basic unit of life, and all cells come from pre-existing cells. Prokaryotes lack a membrane-bound nucleus, so option C is not a tenet: it is a characteristic only of eukaryotes.
Question 2 of 5
A bacterium is found to have an outer membrane containing LPS and a thin peptidoglycan layer. This bacterium is most likely:
Gram-negative bacteria have two defining features: an outer membrane with lipopolysaccharide (LPS) and a thin peptidoglycan layer. Gram-positive bacteria have thick peptidoglycan but no outer membrane. Viruses lack cell walls entirely.
Question 3 of 5
During conjugation, genetic material is transferred between bacteria via:
Conjugation involves direct cell-to-cell contact via a pilus (sex pilus), through which the F-plasmid is transferred. Option A describes transduction, and option B describes transformation. Endocytosis is a eukaryotic process.
Question 4 of 5
An antibiotic selectively inhibits \(50\text{S}\) ribosomal subunits. Which of the following would also be affected?
Mitochondria contain \(70\text{S}\) ribosomes (with \(50\text{S}\) and \(30\text{S}\) subunits) due to their endosymbiotic origin from ancestral prokaryotes. An antibiotic targeting the \(50\text{S}\) subunit would also affect mitochondrial protein synthesis, which is why some antibiotics cause mitochondrial side effects.
Question 5 of 5
Which statement best explains why viruses are not considered living organisms under cell theory?
Cell theory requires living things to be composed of cells. Viruses lack cells, ribosomes, and independent metabolism: they require a host cell's machinery to replicate. Option A is irrelevant to cell theory, option C is wrong (many viruses have DNA), and option D is wrong (viruses infect both prokaryotes and eukaryotes).