Topic 2 — Diversity of Living Organisms

§Introduction

Chapter 1 makes one argument twice. First: every living cell on Earth — from a stomach-dwelling bacterium to a neuron in your cortex — runs on the same molecular operating system. Second: on top of that shared system, life has diversified so wildly that microbes alone occupy boiling vents, frozen glaciers, acid springs, and the inside of your gut, and collectively outweigh and out-diversify every plant and animal combined.

These two facts are not in tension — they are the same fact seen from two ends. The shared operating system (double-strand DNA, transcription, translation, a near-universal genetic code, a phospholipid membrane, ATP energetics) is compelling evidence that all life descends from a single last universal common ancestor (LUCA) that existed roughly 3.5–3.8 billion years ago. Everything after that is variation on an inherited theme, generated by mutation and natural selection acting on a genome that changes slowly enough to still be readable across the entire tree of life.

The chapter then resolves the living world into three domains — Bacteria, Archaea, and Eukaryotes — a structure invisible to the microscope and revealed only by sequencing the one gene conserved in everything: ribosomal RNA. It closes on the deepest structural event in the history of complex life: the endosymbiotic origin of mitochondria and chloroplasts, which is why you carry bacterial DNA in every cell you own. Where the lecture audio and the textbook disagree, the textbook wins, and the disagreement is flagged in amber.

◎Learning Objectives

1Unity of Life — The Universal Features

Start with the paradox the professor opened on: before you can talk about what makes organisms different, you have to see how staggeringly similar they are. A human and a gut bacterium share almost nothing in appearance and yet run the identical molecular machinery. That shared machinery is the definition of cellular life. The textbook frames it as a short list of features present in every cell.

The cell is the minimal unit of life

All organisms are made of cells — either unicellular (bacteria, yeast) or multicellular (a human body is >10¹³ cells, all descended by division from a single fertilized egg). The single cell is therefore the minimal self-reproducing unit: it holds the complete hereditary information and all the machinery needed to build a new copy of itself from raw materials. Heredity — parents specifying offspring in molecular detail — is what separates life from a candle flame or a growing crystal, both of which also consume free energy but never transmit a blueprint.

The features every cell shares

1. Double-strand DNA stores hereditary information as a linear sequence of four nucleotides (A, T, C, G).
2. Templated polymerization copies that information: strands separate, each templates a complementary partner (A–T, C–G).
3. Transcription reads DNA into RNA (ribose replaces deoxyribose; uracil replaces thymine).
4. Translation reads mRNA into protein via the genetic code — codons of 3 nucleotides, read by tRNAs on the ribosome.
5. Proteins as catalysts: 20 amino acids fold into enzymes that run essentially all of the cell's chemistry.
6. A plasma membrane of amphiphilic phospholipids encloses the cell and selectively controls what crosses it.
7. Continual free-energy input, carried by ATP, drives the whole far-from-equilibrium system.

Because these are shared with only minor exceptions across all of life, the machinery of one cell can read the information of another: insert human DNA into a bacterium and it is transcribed and translated correctly. That interoperability is the single strongest piece of evidence that all cells inherited one ancestral system — LUCA, ~3.5–3.8 billion years ago.

Templated polymerization: one mechanism, used three times

The same trick — build a new polymer by reading an existing template one monomer at a time — underlies replication, transcription, and the selection of tRNAs in translation. Base-pairing (A–T/U, C–G) is what makes it faithful: the template physically selects which monomer can be added next. Covalent sugar–phosphate bonds hold each strand together strongly; the hydrogen bonds between the strands are weak, which is exactly why the double helix can be pulled apart for copying without breaking the backbone.

The membrane and the energy budget

The plasma membrane self-assembles from amphiphilic phospholipids — one hydrophilic head, two hydrophobic tails. In water, the tails hide from water by clustering together, spontaneously forming a bilayer that closes into a vesicle. No template, no enzyme: the molecules' own chemistry drives the assembly. Embedded transport proteins then decide what crosses. Meanwhile the whole cell is a chemical system held far from equilibrium; let the free-energy supply (ATP, ultimately from food or sunlight) fail, and the cell decays toward equilibrium and dies.

2Cell Size, Scale & the Microscopic World

The lecture walked down a magnification ladder — finger, pin, dust mite, human cells, bacteria, viruses — to make one point: life spans several orders of magnitude in size, and most of it is invisible. Fixing the scale bar in your head matters because size differences drive real biology (a eukaryote can eat a bacterium; a virus can't be trapped by a bacteria-catching filter).

The ruler

• Millimeter (mm) = 10⁻³ m — a dust mite, barely visible.
• Micrometer (µm) = 10⁻⁶ m — the world of cells. Bacteria are typically a few µm (E. coli ~1–2 µm rod); a human red blood cell ~7–8 µm; yeast ~5–10 µm.
• Nanometer (nm) = 10⁻⁹ m — most viruses. Rhinovirus ~20–30 nm; a filovirus like Ebola can be ~1000 nm long (a thin thread).

Eukaryotic cells are typically 10–30× larger in linear dimension and 1000–10,000× larger in volume than a typical prokaryote. Bacterial shape vocabulary you should recognize: cocci (spheres, e.g. Streptococcus), bacilli (rods, e.g. E. coli, Salmonella), and spirals (e.g. Treponema pallidum). Size is not destiny, though: the cigar-shaped bacterium Epulopiscium fishelsoni reaches ~600 µm — bigger than many eukaryotic cells.

3Prokaryotes vs. Eukaryotes

The oldest structural division in cell biology is nucleus or no nucleus. "Eukaryote" is Greek for truly nucleated (eu = true, karyon = kernel/nucleus). Under an early-20th-century light microscope, that one visible feature split the living world in two. We now know the split is real but the labels are uneven: "eukaryote" is one domain, while "prokaryote" is a descriptive grouping of two domains (Bacteria + Archaea) united only by the absence of a nucleus.

What the eukaryotic cell has that the prokaryote lacks

• A nucleus: a double-membrane envelope, perforated by nuclear pores, enclosing the DNA (packaged with protein into chromosomes).
• Membrane-bound organelles: ER, Golgi, lysosomes, peroxisomes, mitochondria (and chloroplasts in plants/algae), plus the membraneless nucleolus and biomolecular condensates.
• A cytoskeleton: actin filaments, microtubules, and intermediate filaments — mechanical support, transport tracks, and the machinery to pull chromosomes apart at division.
• Much bigger, gene-richer genomes: humans and corals have >20,000 protein-coding genes vs. ~4,000–6,000 for a typical bacterium or archaeon.

What the nucleus actually buys

The professor's key point: putting DNA behind a membrane separates transcription (in the nucleus) from translation (in the cytosol). That separation creates a window in which the primary RNA transcript can be processed — spliced, edited, capped — before it is exported and translated. Prokaryotes, with no such barrier, translate mRNA while it is still being transcribed, leaving little room for this kind of processing. Compartmentalization is a big part of why one eukaryotic gene can yield multiple protein variants.

Cell walls and their consequences

Bacteria (and plant and fungal cells) have a tough external cell wall; typical animal cells do not. That absence is what lets animal cells change shape rapidly and perform phagocytosis — engulfing other cells and particles. Alberts classifies bacteria as Gram-negative (an outer membrane plus the inner plasma membrane) or Gram-positive (no outer membrane). Archaea differ from bacteria in cell-wall chemistry and membrane lipids.

4The Three Domains & the Tree of Life

Classification once relied on appearance — a fish has a backbone, a rosebush resembles an apple tree. That works for large organisms with visible traits and fails completely for microbes, where one rod or sphere looks like any other, and where most species can't even be cultured in a lab. The fix, pioneered by Carl Woese in the late 1970s, was to stop looking and start sequencing.

Ribosomal RNA as a molecular ruler

To compare all organisms, you need a gene present in every one of them. Woese chose ribosomal RNA (rRNA) — a core component of the ribosome, which every cell has because every cell translates. rRNA is so essential that its sequence changes very slowly; the differences that do accumulate act as a clock. The more similar two organisms' rRNA, the more recently they diverged. This single measurement shattered the old two-kingdom picture of "prokaryotes" and revealed three domains:

• Bacteria — by far the most evolutionarily diverse group, reflecting their much earlier appearance and their colonization of essentially every niche.
• Archaea — superficially bacterial in appearance, but genomically closer to eukaryotes; differ in membrane lipids and wall chemistry.
• Eukaryotes — larger cells with a nucleus and organelles; the least diverse of the three as a group, having appeared latest.

Every domain traces back to LUCA, the last universal common ancestor, ~3.5–3.8 billion years ago. More than 200 gene families (specifically ~264) are conserved across all three domains — the shared inheritance from that ancestral cell. The largest categories are genes for translation and for amino-acid metabolism and transport.

How new genes arise, and the vocabulary of relatedness

Genomes change over time by four mechanisms — worth memorizing as a set:

1. Intragenic mutation — an existing gene is altered by replication/repair errors.
2. Gene duplication — a gene is copied; the two copies can diverge to new functions.
3. DNA segment shuffling — pieces of two genes recombine into a hybrid gene.
4. Horizontal (intercellular) gene transfer (HGT) — DNA moves between cells, even between species — contrasted with the usual vertical (parent→offspring) transfer. HGT is a major force in Bacteria and Archaea.

Two related genes are homologs (a general umbrella term). The subtypes are the classic exam trap:

• Orthologs — homologs in different species that descend from the same gene in their common ancestor (arose by speciation). They usually keep corresponding functions. Example: the human and chimp versions of a given gene.
• Paralogs — homologs within one genome that arose by gene duplication. They have often diverged to different functions. Example: the members of the human hemoglobin gene family.

5Extremophiles & Environmental Diversity

The lecture spent its longest stretch on a single theme: for almost any physical extreme — scalding, freezing, acidic, salty, oxygen-free — there is some microbe that not only tolerates it but requires it. Life is bounded by chemistry, not by comfort. Two organizing frameworks make this manageable: classification by niche (what conditions an organism lives in) and classification by energy/carbon source (how it makes a living).

Niche vocabulary

• Temperature: psychrophiles (cold-loving; spoilage bacteria like Listeria grow even in a refrigerator), mesophiles (moderate; most human pathogens, ~35–40 °C), thermophiles (heat-loving, >~45 °C), hyperthermophiles (>100 °C). An autoclave uses 121 °C to kill vegetative cells — but resistant endospores can survive.
• pH: acidophiles (e.g. Helicobacter pylori in the stomach at pH 1–2), neutrophiles (most organisms, ~pH 7), alkaliphiles (high pH).
• Salt: halophiles thrive at very high salt (the Dead Sea, salt-evaporation ponds — the red/orange pigment is halophilic microbes).
• Water: desiccation-resistant bacteria form endospores — dormant, metabolically inert, extraordinarily stable in dry conditions, germinating back to vegetative cells when water returns.
• Oxygen: aerobes require O₂; anaerobes may be poisoned by it. A full gradient of oxygen tolerance exists between the two.

Energy and carbon: the trophic categories

The textbook's precise vocabulary (worth using on the exam) sorts primary metabolism by where the energy and carbon come from:

• Organotrophic — feed on organic molecules made by other organisms (animals, fungi, us, much of our gut flora).
• Phototrophic — capture energy from sunlight (plants, algae, many bacteria). Their biosynthesis put the O₂ in our atmosphere.
• Lithotrophic — literally "rock-feeding": capture energy from energy-rich inorganic chemicals (H₂, Fe²⁺, H₂S, elemental sulfur, etc.), far from equilibrium. Mostly microscopic, in habitats humans avoid.

Organotrophs like us depend utterly on the primary energy converters (photo- and lithotrophs). At hydrothermal vents, entire ecosystems run on geochemical energy instead of light: anaerobic lithotrophic bacteria and archaea harvest energy from vent chemicals, and animals (giant Riftia tube worms with no mouth or gut) live in symbiosis with sulfur-oxidizing bacteria inside them. The vent temperature gradient runs from ~350 °C at the core down to 2–3 °C in the surrounding ocean, with different organisms at each temperature band.

Textbook note. The lecture cited hydrothermal-vent water at ~380 °C. Alberts (Fig. 1–15) gives ~350 °C near the vent core, cooling to 2–3 °C in the surrounding water. Use the textbook figure; the exact number matters less than the concept of a steep temperature gradient hosting distinct organisms at each band. (Ch. 1, Fig. 1–15) Flag this for the exam.

Nitrogen and carbon fixation: who makes the building blocks

Cells are built from six main elements (H, C, N, O, S, P), all abundant in the environment — but atmospheric N₂ and CO₂ are extremely unreactive, and it takes large free-energy input to "fix" them into usable organic form. Not every cell can do this; many depend on those that can.

• Cyanobacteria are the star example: they perform oxygen-generating photosynthesis and fix nitrogen. In Anabaena, the labor is divided among cell types — most cells photosynthesize, specialized heterocysts fix N₂, and others form spores.
• Plants can fix CO₂ (photosynthesis) but cannot fix N₂; they depend on nitrogen-fixing bacteria — e.g. the symbiotic bacteria in the root nodules of legumes.
• We animals depend on plants (directly or indirectly) for fixed carbon and nitrogen.

Archaea as extremophiles

Archaea were first found in extreme habitats — acid hot springs, salt lakes, cattle stomachs — and famous archaeal lifestyles include methanogens (produce methane; found in wetlands, swamps, and the rumen of cows) and extreme halophiles. But the modern picture is broader: archaea also live in soil, seawater, and on skin, and they are now thought to be a predominant life-form in soil and ocean, playing major roles in recycling nitrogen and carbon.

6Origin of Eukaryotes: Endosymbiosis

This is the deepest structural event in the history of complex life, and the section the exam is most likely to probe for precise detail. The headline: you carry bacterial DNA in every cell you own, because two of your organelles were once free-living bacteria that got permanently swallowed. Getting the order, the host, the guest, and the membrane counts right is what separates a correct answer from a plausible-sounding wrong one.

The timeline

• ~4.6 billion years ago (Gya): Earth forms.
• ~3.5–3.8 Gya: first cells (LUCA); a prokaryote-only world begins.
• For >1.5 billion years: only prokaryotes exist.
• ~2 Gya: the first eukaryotic cell arises when an archaeal cell and a bacterial cell merge.
• ~1.5 Gya: mitochondria are already present before the plant, animal, and fungal lineages diverge (which is why their mitochondria are essentially the same).

Mitochondria first: an aerobic bacterium captured by an anaerobic archaeon

All eukaryotic cells contain (or once contained) mitochondria. The evidence they were once bacteria is overwhelming: mitochondria have their own DNA with bacterial-like genes, their own ribosomes and translation factors resembling bacterial ones, they are the size of small bacteria, and they reproduce by dividing. The best-supported model: an anaerobic archaeal host captured an aerobic bacterium (~2 Gya), and the two evolved an endosymbiotic partnership. The host was likely from the Asgard archaeal lineage, whose genomes carry many eukaryote-like genes.

Chloroplasts second: a cyanobacterium captured by a eukaryote that already had mitochondria

Chloroplasts perform photosynthesis in plants and algae. Like mitochondria, they have their own circular genome and divide. But the order is critical: a chloroplast arose when an already-mitochondriate eukaryotic cell engulfed a photosynthetic (cyano)bacterium, probably by phagocytosis. So mitochondria are universal in eukaryotes; chloroplasts are a later, additional acquisition found only in the plant/algal lineage.

Textbook note. The lecture described chloroplasts as having two membranes. That counts only the envelope (outer + inner membranes). A chloroplast has a third, distinct internal membrane system — the thylakoids — where the chlorophyll sits and the light reactions occur (visible in textbook Fig. 1–28 as the "chlorophyll-containing internal membranes"). So the accurate count is three membrane systems: outer envelope, inner envelope, and thylakoid. Contrast with mitochondria, whose two membranes (smooth outer, convoluted inner cristae) are the whole story. (Ch. 1, Fig. 1–28) Flag this for the exam.

Textbook note. The endosymbiotic theory is most associated with Lynn Margulis, who revived and championed it in her landmark 1967 paper (published as Lynn Sagan, "On the Origin of Mitosing Cells") — not 1981. If the lecture cited 1981, that's an error; 1981 is closer to the date of a later book, but the original influential proposal is 1967. (General history; Ch. 1 covers the theory, not the date) Flag this for the exam.

Textbook note. If the lecture implied ATP generation (including glycolysis) happens "in the mitochondria," correct it: glycolysis occurs in the cytosol, not in mitochondria. Mitochondria run the citric acid cycle and oxidative phosphorylation (the O₂-dependent, high-yield ATP steps). Alberts is explicit that the cytosol "is also where most of the cell's other metabolic reactions take place," while mitochondria harness energy from oxidation of food molecules. Keep the compartments straight: glucose is split in the cytosol; its products are finished off in the mitochondrion. (Ch. 1, cytosol vs. mitochondria; detailed in Ch. 2) Flag this for the exam.

7Eukaryotic Microbes: Fungi & Protozoa

Not all microbes are prokaryotes. A large, ecologically vital slice of the microbial world is eukaryotic — single-celled (or simply constructed) organisms with a nucleus and organelles. The lecture highlighted two groups: fungi and protozoa. Both are eukaryotes, so both have mitochondria; the exam-relevant distinctions are about walls, chloroplasts (absent in both), lifestyle, and — critically — not misclassifying a parasite.

Fungi

Fungi are eukaryotes with mitochondria but no chloroplasts (they cannot photosynthesize) and a tough cell wall (of chitin, not the cellulose of plants). Ecologically they are scavengers/decomposers: they secrete digestive enzymes onto organic matter and absorb the breakdown products. Two body plans:

• Yeasts — unicellular fungi. Saccharomyces cerevisiae (budding yeast) is ~5–10 µm, reproduces by budding, and is a workhorse model organism. Its metabolism is a useful contrast: growing aerobically it makes CO₂ (leavening bread); growing anaerobically (fermentation) it makes ethanol (brewing).
• Molds — filamentous fungi that grow as thread-like hyphae, often coenocytic (many nuclei sharing cytoplasm without complete cell divisions). Penicillin was discovered by Fleming from a Penicillium mold that killed surrounding bacteria.

Medically, many fungi are opportunistic: normally harmless, they cause serious disease in immunocompromised people (e.g. Pneumocystis pneumonia and Cryptococcus in AIDS patients).

Protozoa (protists)

Protozoa are free-living, mostly single-celled, motile eukaryotes — the "animal-like" protists. They are often larger and more internally elaborate than bacteria, some with intricate surface coverings, feeding structures, and swimming appendages (cilia/flagella). Many are harmless or ecologically important; some are major pathogens. The parasite Plasmodium, which causes malaria, is a protozoan — and this is a classic exam trap.

Textbook note. The lecture referred to malaria's cause, Plasmodium, as a "virus." This is wrong. Plasmodium is a protozoan (a single-celled eukaryote), transmitted by Anopheles mosquitoes, with a complex life cycle in both mosquito and human hosts. It is not a virus, not a bacterium — it is a eukaryotic parasite. The distinction matters mechanistically (eukaryotic cell biology, drug targets, life cycle) and is exactly the kind of detail an MCQ will test. (Ch. 1, eukaryotic microbes / protozoa; Fig. 1–33) Flag this for the exam.

Model organisms and their genomes

A recurring lecture theme: biologists concentrate on a handful of model organisms because what's learned in one illuminates all, given shared descent. Approximate protein-coding gene counts worth recognizing (not memorizing to the digit):

Textbook note. If the lecture gave the adult C. elegans somatic-cell count as 956, the standard figure is 959 somatic cells in the adult hermaphrodite (the complete cell lineage of all 959 was mapped — a landmark achievement). Minor, but the kind of exact number an MCQ can hinge on. (Ch. 1, C. elegans as a model organism) Flag this for the exam.

8Viruses

Viruses sit at the edge of the definition of life, and the exam will exploit exactly that ambiguity. The disciplined position: a virus is not a cell and not "strictly alive" — it has no metabolism, no ribosomes, generates no ATP, and cannot reproduce on its own. It is a mobile packet of genetic information that hijacks a host cell's machinery to copy itself. Yet by numbers, viruses are the most abundant biological entities on Earth — there are more virus particles than all cellular organisms combined.

Structure: minimal by design

• A nucleic-acid genome — and this is the key teaching point: it can be DNA or RNA, and single- or double-stranded. All four combinations exist (dsDNA, ssDNA, dsRNA, ssRNA). This is the concrete counterexample to "all genomes are double-stranded DNA."
• A protein coat (capsid) enclosing and protecting the genome; capsids self-assemble from repeated protein subunits (often into helical or icosahedral shells).
• Sometimes a lipid envelope — an outer membrane stolen from the host cell, studded with viral proteins (e.g. HIV, influenza, coronaviruses). "Enveloped" vs. "non-enveloped" has practical consequences: enveloped viruses are easier to destroy with soap/alcohol because disrupting the lipid coat inactivates them.

Reproduction: an obligate intracellular parasite

A virus must enter a host cell and redirect that cell's transcription/translation to manufacture new viral genomes and capsids, which assemble and exit (often lysing the cell). This absolute dependence is why viruses are obligate intracellular parasites with strict host ranges. Bacteriophages ("phages") are viruses that infect bacteria — the injection mechanism the lecture showed — and they are major regulators of bacterial populations in every ecosystem (the oceans turn over an enormous fraction of bacteria via phage lysis daily).

Retroviruses reverse the "usual" flow

Most information flow is DNA → RNA → protein. Retroviruses (like HIV) carry an RNA genome and an enzyme, reverse transcriptase, that copies RNA back into DNA — RNA → DNA — which then integrates into the host genome. This does not overturn the central dogma so much as reveal its full generality: information can move between DNA and RNA in both directions; what has never been observed is protein → nucleic acid. HIV also illustrates the immunocompromise theme from §7: by destroying helper T cells, it opens the door to the opportunistic fungal and protozoal infections that define AIDS.

Viruses and cancer

Several viruses cause cancer by disrupting host growth control. The cleanest example: human papillomavirus (HPV) is responsible for essentially all cervical cancer — which is why the HPV vaccine is, in effect, a cancer-prevention vaccine. This parallels the H. pylori→gastric-cancer link from §5: a subset of cancers have infectious causes and are therefore preventable.

Size and history

Viruses span a wide size range but are generally far smaller than cells: a rhinovirus is ~20–30 nm; a large filovirus like Ebola can be ~1000 nm long. Historically, viruses were defined operationally as infectious agents that passed through filters fine enough to trap bacteria and could not be seen by light microscopy — they became visible only with the electron microscope. That "filterable agent" definition is a nice reminder that the size gap between viruses and cells is itself diagnostic.

9Astrobiology & "Life 2.0"

The lecture closed on a genuinely open question: is the specific molecular system all Earth life shares — four nucleotides, one near-universal genetic code, the central dogma — the only way to build life, or just the one that happened to win here? Chapter 1's "frozen accident" framing leans toward the latter: many features of our system are contingent, not chemically forced.

Life as we don't know it

• Expanded genetic alphabets: synthetic biologists have engineered cells using unnatural base pairs beyond A-T/G-C, producing semi-synthetic organisms with a six-letter (and in vitro, eight-letter) genetic alphabet. The four-nucleotide system is not the only chemistry that can store and replicate information.
• Expanded codes: engineered orthogonal tRNA/aminoacyl-tRNA synthetase pairs let cells incorporate non-canonical amino acids beyond the standard 20 — reprogramming the "universal" code in the lab.
• Implication: if the code and alphabet are reprogrammable by us in a few decades, then the universality of Earth's system is best read as common descent (all life inherited one accident), not chemical necessity (the only possible system).

Where to look

Astrobiology targets liquid water: subsurface oceans on Europa and Enceladus, and past/present water on Mars. A key methodological caveat the professor flagged: contamination. Detecting "life" on a probe that carried Earth microbes proves nothing — which is why planetary protection and sterilization protocols matter, and why any biosignature must be scrutinized for terrestrial origin. This connects to the chapter's Europa end-of-chapter problem: what would count as evidence of independent life?

✦Core Concepts — Rapid Review

Everything the exam is likely to test, compressed. If a line here is unfamiliar, reread the linked section before the final.

✎End-of-Topic Problems

Every Alberts end-of-chapter problem for this topic, one multiple-choice item each. Click any option to check it instantly — the correct choice is marked Answer as lectured. No submit step; feedback is immediate.