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Prokaryotic Growth
Metabolism & Adaptation

Master how bacteria grow exponentially, harvest energy through diverse metabolic pathways, and adapt to environmental pressures: essential concepts for MCAT Foundational Concept 2B.

MCAT Biology Foundational Concept 2B

What is a Prokaryote?

Prokaryotes are single-celled organisms that lack a membrane-bound nucleus. They include two domains: Bacteria and Archaea. Their simplicity belies remarkable metabolic diversity and ecological dominance.

No Nucleus
DNA resides in a nucleoid region, not enclosed by a membrane
Rapid Reproduction
Binary fission allows doubling in as little as 20 minutes
Metabolic Diversity
Aerobes, anaerobes, phototrophs, chemotrophs, and more

The Bacterial Growth Curve

Time Log(Cell #) Lag Log (Exp) Stationary Death

Exponential Growth: The Math

During the log phase, bacteria divide by binary fission at a constant rate. The population at time \( t \) is given by:

Binary Fission: How Prokaryotes Divide

DNA replication 1. Replicate 2. Septum forms 3. Two daughters

Prokaryotic Metabolism Overview

Prokaryotes exhibit the widest metabolic diversity of any life forms. Classification is based on energy source and carbon source:

Phototrophs
Use light as energy source (e.g., cyanobacteria)
Chemotrophs
Use chemical compounds as energy (e.g., E. coli)
Autotrophs
Fix \( \text{CO}_2 \) as carbon source
Heterotrophs
Require organic carbon compounds

Aerobic vs Anaerobic Respiration

Aerobic Anaerobic
Final electron acceptor \( \text{O}_2 \) \( \text{NO}_3^-, \text{SO}_4^{2-}, \text{CO}_2 \)
ATP yield per glucose ~30-32 ATP ~2-4 ATP
ETC present? Yes (in plasma membrane) Yes (modified chain)
Example organisms Mycobacterium Desulfovibrio

Fermentation: No ETC Required

When no terminal electron acceptor is available, prokaryotes use fermentation to regenerate \( \text{NAD}^+ \) from NADH, allowing glycolysis to continue.

Lactic Acid
Pyruvate \( \to \) Lactate. Used by Lactobacillus. Net: 2 ATP per glucose.
Ethanol
Pyruvate \( \to \) Acetaldehyde \( \to \) Ethanol + \( \text{CO}_2 \). Used by Zymomonas.
Mixed Acid
Produces multiple organic acids. Used by E. coli under anaerobic conditions.

Oxygen Relationships

high O₂ low O₂ Obligate Aerobe Obligate Anaerobe Facultative Anaerobe Aerotolerant Micro- aerophile

Genetic Variation in Prokaryotes

Since prokaryotes reproduce asexually, genetic diversity arises through horizontal gene transfer and mutation:

1
Transformation: Uptake of free DNA from the environment through the cell membrane.
2
Transduction: DNA transfer via bacteriophage (virus) infection. Can be generalized or specialized.
3
Conjugation: Direct cell-to-cell transfer of plasmid DNA through a pilus (requires F-plasmid).
4
Mutation: Spontaneous errors during DNA replication; drives antibiotic resistance under selection.

Prokaryotic Adaptation

Prokaryotes adapt to environmental stress through both genetic and phenotypic mechanisms, often at remarkable speed:

Endospore Formation
Durable, dormant structures resisting heat, desiccation, and chemicals (Bacillus, Clostridium)
Biofilm Formation
Cooperative communities encased in extracellular matrix: increased antibiotic resistance up to 1000x
Quorum Sensing
Cell-density-dependent gene regulation via autoinducer molecules (e.g., AHL signals)

lac vs trp Operon

lac Operon trp Operon
Type Inducible Repressible
Default state OFF (repressor bound) ON (repressor inactive)
Inducer / Corepressor Allolactose Tryptophan
Function Catabolism (break down lactose) Anabolism (synthesize tryptophan)
CAP-cAMP role Positive regulator (low glucose) Not involved

Solving Growth Problems: Step by Step

1
Identify the phase: Is the question about lag, log, stationary, or death phase? Only log phase uses the exponential equation.
2
Extract knowns: Look for \( N_0 \), \( t \), and \( g \) (generation time). Convert units to match.
3
Calculate generations: \( n = \frac{t}{g} \). For example, 3 hours with a 30-min doubling time: \( n = \frac{180}{30} = 6 \).
4
Apply the formula: \( N_t = N_0 \times 2^n \). With \( N_0 = 100 \) and \( n = 6 \): \( N_t = 100 \times 64 = 6{,}400 \).
5
Check reasonableness: Each generation doubles the count. Verify your answer is \( 2^n \) times the start.

Example 1: Population Count

1
Convert time: \( t = 2 \text{ hr} = 120 \text{ min} \)
2
Calculate generations: \( n = \frac{120}{20} = 6 \)
3
Apply formula: \( N_t = 500 \times 2^6 = 500 \times 64 = \mathbf{32{,}000} \) cells

Example 2: Finding Generation Time

1
Find the multiplication factor: \( \frac{64{,}000}{1{,}000} = 64 = 2^6 \), so \( n = 6 \) generations
2
Solve for generation time: \( g = \frac{t}{n} = \frac{180 \text{ min}}{6} = \mathbf{30 \text{ min}} \)

Common Mistakes

✕ Confusing Phases
Applying the exponential equation during lag or stationary phase. The formula \( N_t = N_0 \times 2^n \) only applies during log phase.
✕ Mixing Up Operons
Saying the lac operon is repressible or the trp operon is inducible. Remember: lac = inducible, trp = repressible.
✕ ETC Location
Placing the prokaryotic ETC in mitochondria. Prokaryotes lack organelles: ETC is in the plasma membrane.

Question 1 of 5

A bacterial culture starts with 200 cells and has a generation time of 25 minutes. How many cells are present after 75 minutes?

Question 2 of 5

Which type of organism can grow with or without oxygen, but prefers aerobic conditions?

Question 3 of 5

In the lac operon, what happens when lactose is present and glucose is absent?

Question 4 of 5

The primary purpose of fermentation in prokaryotes is to:

Question 5 of 5

Which mechanism of horizontal gene transfer requires a bacteriophage as an intermediary?