Source: Unit 1 Review, Introduction to Biology I
Difficulty: Introductory | Prerequisites: None, though a basic familiarity with cell biology vocabulary helps.
This set of notes covers the fundamental distinction between prokaryotic and eukaryotic cells, plus the endosymbiotic theory that links them. It also walks through the four phases of bacterial growth, which tend to reappear on exams in graph-interpretation questions. If you are comfortable with the basic idea that cells are the unit of life and that organisms can be single-celled or multicellular, you have what you need to start here. This material underpins everything that follows in BIO 101, from macromolecules to membrane transport.
Bacteria are prokaryotes: no membrane-bound nucleus, circular chromosome stored in a nucleoid region. Eukaryotic cells have a dynamic membrane, linear chromosomes, and membrane-bound organelles, some of which (mitochondria, chloroplasts) likely originated from ancient prokaryotes swallowed by a host cell. Bacterial populations grow through four predictable phases: lag, log, stationary, and death.
Prokaryote
An organism whose cells lack a membrane-bound nucleus. DNA sits in a region called the nucleoid.
In simple terms, a prokaryote keeps its genetic material loose in the cell rather than locking it away in a compartment.
Eukaryote
An organism whose cells contain a true nucleus and other membrane-bound organelles.
Think of it as the upgraded model: compartments inside the cell let different jobs run at the same time.
Nucleoid
The region in a prokaryotic cell where the circular chromosome is concentrated. It is not enclosed by a membrane.
In simple terms, it is where the DNA lives in a bacterium, even though there is no wall around it.
Endosymbiotic theory
The hypothesis that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by an ancestral eukaryotic cell, eventually becoming permanent organelles.
Think of it as a partnership that started as one cell eating another, then both benefiting so much that neither left.
Lag phase
The initial period of bacterial growth in which cells adapt to a new environment, synthesise new proteins, and increase in size but do not yet divide.
In simple terms, the bacteria are getting settled in before they start multiplying.
Log (exponential) phase
The phase of bacterial growth characterised by rapid, exponential cell division under optimal conditions with excess nutrients.
This is the steep upward curve on a growth graph, where the population doubles at a constant rate.
Stationary phase
The phase in which the rate of cell division equals the rate of cell death, producing zero net growth, typically because nutrients are depleted and waste has accumulated.
The population plateaus: new cells are still being made, but old ones are dying off at the same rate.
Death phase
The phase in which the rate of cell death exceeds cell division, causing the population to decline.
In simple terms, the colony is running out of resources and the population is shrinking.
Bacteria are prokaryotes, meaning they have no membrane-bound nucleus.
Their genetic material is a single, circular chromosome housed in a region called the nucleoid.
Prokaryotic cells are generally smaller and structurally simpler than eukaryotic cells.
Despite that simplicity, bacteria are enormously diverse in metabolism and habitat.
Eukaryotic cells possess a dynamic membrane and membrane-bound organelles, including a true nucleus.
Their DNA is organised into multiple linear chromosomes.
Multiple chromosomes means many replication origins, which allows faster DNA replication overall.
Organelles compartmentalise cellular functions, letting different chemical processes run simultaneously without interfering with each other.
Certain eukaryotic organelles closely resemble prokaryotic cells, particularly mitochondria and chloroplasts.
Evidence supporting the theory:
Both organelles have a double membrane (the inner membrane is thought to be the original prokaryote's membrane).
Both contain their own circular DNA, separate from the nuclear genome.
Both have their own ribosomes, which are more similar in size to bacterial ribosomes than to eukaryotic cytoplasmic ribosomes.
Both grow and replicate semi-independently within the eukaryotic cell, dividing by binary fission rather than mitosis.
When bacteria are introduced to a new environment with fresh medium, their population passes through four distinct phases. Exam questions often present a growth curve and ask you to identify each phase or explain what is happening at the cellular level.
Lag phase: Cells adjust to the new environment. They are metabolically active, synthesising enzymes and proteins, and increasing in size, but they are not yet dividing. The population count stays roughly flat.
Log (exponential) phase: Conditions are optimal and nutrients are plentiful. Cells divide at a constant, maximum rate, so the population doubles at regular intervals. On a semi-log plot this appears as a straight line.
Stationary phase: Nutrients become depleted and waste products accumulate. The rate of new cell division equals the rate of cell death, producing zero net change in population size. The curve levels off.
Death phase: Resources are exhausted. Cell death outpaces division, and the population declines. On the growth curve, the line falls.
Understanding bacterial growth phases is central to microbiology, medicine, and food science. Antibiotics are most effective during the log phase, when bacteria are actively dividing, which is why timing and dosing matter.
Endosymbiotic theory explains why mitochondrial DNA is used in forensic science and ancestry testing: it is inherited maternally and evolves at its own rate, separate from nuclear DNA.
Students often think bacteria have no DNA organisation at all. They do: the nucleoid is a defined region, even though it lacks a membrane.
Students sometimes say mitochondria "are" bacteria. They are not. They descended from an ancient prokaryote, but after roughly two billion years of co-evolution they have lost most of their original genes to the host nucleus.
A common error is to assume the lag phase means nothing is happening. Cells are very active during lag; they are simply not dividing yet.
Students often confuse stationary phase with death phase. In stationary phase the population holds steady (births equal deaths). In death phase, deaths outpace births and the count drops.
⚠️ Be ready to identify each phase on a bacterial growth curve and explain what is happening at the cellular level.
⚠️ Know the three main lines of evidence for endosymbiotic theory (double membrane, own DNA, own ribosomes) and be able to explain why each one supports the hypothesis.
⚠️ Distinguish between prokaryotic and eukaryotic chromosome structure: circular vs. linear, single vs. multiple, nucleoid vs. nucleus.
⚠️ Understand why multiple replication origins on linear chromosomes allow faster replication than a single origin on a circular chromosome.
True or false: Bacteria store their DNA inside a membrane-bound nucleus.
Fill in the blank: Mitochondria and chloroplasts both have their own ______ and ______, which is key evidence for the endosymbiotic theory.
True or false: During the lag phase of bacterial growth, the population is rapidly increasing.
Fill in the blank: In the stationary phase, the rate of cell division ______ the rate of cell death.
True or false: Eukaryotic chromosomes are circular.
(Answers: 1. False, it is in the nucleoid with no membrane. 2. DNA, ribosomes. 3. False, cells are growing but not dividing. 4. Equals. 5. False, they are linear.)
Q: Name three pieces of evidence that support the endosymbiotic theory for the origin of mitochondria and chloroplasts.
A: (1) Both organelles have a double membrane, consistent with engulfment by the host cell. (2) Both carry their own circular DNA, separate from the nucleus. (3) Both have their own ribosomes, which resemble bacterial ribosomes in size.
Q: A bacterial culture is introduced to fresh medium. For the first two hours the cell count does not increase, yet the cells are metabolically active. Which growth phase is this, and what are the cells doing?
A: This is the lag phase. The cells are adjusting to the new environment, synthesising enzymes and other proteins needed for division, and increasing in size, but they have not yet begun to divide.
Q: Explain why eukaryotic DNA replication can proceed faster than prokaryotic replication, even though eukaryotic genomes are much larger.
A: Eukaryotic chromosomes are linear and contain many origins of replication. Replication starts simultaneously at multiple sites along each chromosome, so the genome is copied in parallel. Prokaryotes typically have a single circular chromosome with one origin of replication.
Q: On a bacterial growth curve, what distinguishes the stationary phase from the death phase?
A: In the stationary phase the rate of cell division equals the rate of cell death, so the total population holds steady. In the death phase, cell death exceeds division and the population declines.
This material connects directly to cell membrane structure and transport (covered in the next set of notes), because the dynamic membrane of eukaryotic cells is what makes compartmentalisation possible.
Endosymbiotic theory also ties into evolution and natural selection: it is one of the best-supported examples of how symbiotic relationships can drive major evolutionary transitions.
Bacterial growth phases reappear whenever the course covers population ecology or microbiology lab techniques, since growth curves are the standard way to measure bacterial fitness and antibiotic efficacy.
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