DAT Review · Biology
Diversity of Life
On this page 7 sections
In 30 seconds
- Viruses are not cells — no ribosomes, no metabolism, not alive. Know lytic vs lysogenic cycles and the structure (capsid + nucleic acid, sometimes an envelope).
- Gram-positive vs Gram-negative bacteria — peptidoglycan thickness (thick vs thin) and the Gram stain result. This is a classic DAT question.
- Archaea ≠ Bacteria. Distinct membrane lipids, no peptidoglycan, extremophiles, more closely related to eukaryotes.
- Protists are a paraphyletic grouping, not a true clade. Know the three functional categories: protozoa (animal-like), algae (plant-like), slime molds (fungus-like).
- Fungi: chitin cell walls, heterotrophic by absorption, hyphae → mycelium. Decomposers.
- Plants: cellulose cell walls, autotrophic, alternation of generations. Four major groups (bryophytes, seedless vascular, gymnosperms, angiosperms) in order of evolutionary appearance.
- Animals: major phyla — Porifera through Chordata. Chordate hallmarks: notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail. Arthropods = most diverse phylum.
The college version
Core Review
Viruses
Viruses occupy a gray zone between living and non-living. They are obligate intracellular parasites — they cannot reproduce or carry out metabolism outside a host cell. Structure: a nucleic acid genome (DNA or RNA, single-stranded or double-stranded) enclosed in a protein coat called a capsid. Some viruses also have a lipid envelope derived from the host cell membrane. Viruses lack ribosomes, so they must hijack host machinery to translate their genes.
Lytic cycle: The virus infects a host cell, replicates its genome, assembles new virions, and lyses (bursts) the cell, releasing progeny. This is virulent, destructive replication. Example: T4 bacteriophage.
Lysogenic cycle: The viral genome integrates into the host chromosome as a prophage (or provirus in eukaryotes) and replicates passively with host cell division for many generations. An environmental trigger (stress, UV light) can induce the prophage to excise and enter the lytic cycle. Example: lambda phage (λ) in E. coli.
Retroviruses (e.g., HIV) use reverse transcriptase to convert their RNA genome into DNA, which integrates into the host genome (lysogenic-like). The integrated DNA is transcribed to produce new viral RNA genomes and proteins.
Three Domains of Life
| Feature | Bacteria | Archaea | Eukarya |
|---|---|---|---|
| Cell type | Prokaryotic | Prokaryotic | Eukaryotic |
| Nucleus | None | None | True nucleus |
| Peptidoglycan in cell wall | Yes (most) | No | No |
| Membrane lipids | Ester-linked, unbranched | Ether-linked, branched (more stable) | Ester-linked, unbranched |
| First amino acid in translation | Formyl-methionine | Methionine | Methionine |
| Histones | No (usually) | Yes (some, histonelike) | Yes |
| Introns | Rare | Present in some genes | Common |
| Habitat | Diverse | Often extreme (extremophiles) | Diverse |
Bacteria: Most have a cell wall of peptidoglycan. Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain (purple). Gram-negative bacteria have a thin peptidoglycan layer sandwiched between inner and outer membranes; they stain pink (safranin counterstain). The outer membrane of Gram-negatives contains lipopolysaccharide (LPS), an endotoxin. Reproduction is primarily asexual via binary fission. Bacteria exchange genetic material through conjugation (direct transfer via pilus), transformation (uptake of free DNA from environment), and transduction (viral-mediated transfer).
Archaea: Prokaryotes that are more closely related to eukaryotes than to bacteria. Their cell membranes use ether-linked isoprenoid lipids, which are more stable at high temperatures. Cell walls lack peptidoglycan. Many archaea are extremophiles: thermophiles (hot springs), halophiles (salt lakes), methanogens (anaerobic environments, produce methane). Archaea are not pathogenic — no archaeal diseases are known.
Protists
Protista is a traditional, paraphyletic grouping of mostly unicellular eukaryotes that are not fungi, plants, or animals. In modern phylogenetics, protists are distributed across multiple eukaryotic supergroups. Three functional categories:
- Animal-like (protozoa): Heterotrophic, motile. Amoebas (pseudopodia), paramecia (ciliates), Plasmodium (apicomplexan, causes malaria).
- Plant-like (algae): Autotrophic, photosynthetic. Diatoms (silica shells), dinoflagellates (cause red tides), euglenoids (flagella + chloroplasts), green algae (ancestral to land plants), brown algae (kelp), red algae.
- Fungus-like: Heterotrophic, decomposers. Slime molds (plasmodial and cellular), water molds (oomycetes — phylogenetically closer to brown algae than fungi).
Fungi
Fungi are eukaryotic heterotrophs that absorb nutrients after external digestion (they secrete enzymes into the environment and absorb the breakdown products). Cell walls are made of chitin (not cellulose like plants). The body is a network of threadlike hyphae, collectively called a mycelium. Some fungi have septa (cross-walls) between cells; others are coenocytic (no septa). Reproduction: asexual (fragmentation, budding, spores) and sexual (fusion of hyphae → heterokaryotic stage → karyogamy → meiosis → spores). Fungi are critical decomposers in ecosystems, along with bacteria. Mycorrhizae are mutualistic associations between fungi and plant roots (fungus provides minerals; plant provides sugars). Lichens are mutualistic associations between fungi and algae or cyanobacteria.
Plants
Plants are multicellular, autotrophic eukaryotes with cell walls of cellulose. The defining life cycle feature is alternation of generations: a multicellular diploid sporophyte (produces spores by meiosis) alternates with a multicellular haploid gametophyte (produces gametes by mitosis). The relative dominance of these phases varies across groups:
| Group | Key Features | Sporophyte/Gametophyte | Examples |
|---|---|---|---|
| Bryophytes | Nonvascular (no xylem/phloem); anchored by rhizoids; require water for fertilization | Gametophyte dominant | Mosses, liverworts, hornworts |
| Seedless vascular | Vascular tissue (xylem + phloem); true roots, stems, leaves; still require water for sperm to swim | Sporophyte dominant | Ferns, horsetails, club mosses |
| Gymnosperms | Vascular; seeds "naked" (not enclosed in fruit); pollen (no water needed for fertilization); cones | Sporophyte dominant; gametophyte microscopic | Conifers (pines, spruces), cycads, ginkgo |
| Angiosperms | Vascular; flowers; seeds enclosed in fruit; double fertilization | Sporophyte dominant; gametophyte microscopic | Flowering plants (grasses, roses, oaks, orchids) |
Angiosperm double fertilization: One sperm fertilizes the egg → diploid zygote (2n). The second sperm fuses with two polar nuclei → triploid endosperm (3n), which nourishes the developing embryo.
Animals
Animals are multicellular, heterotrophic eukaryotes with no cell walls. Most have specialized tissues (except Porifera).
Major phyla (survey level):
| Phylum | Key Features | Examples |
|---|---|---|
| Porifera | No true tissues; asymmetrical; filter feeders via choanocytes; spongocoel, osculum | Sponges |
| Cnidaria | Radial symmetry; diploblastic (ectoderm + endoderm); cnidocytes (stinging cells); gastrovascular cavity; polyp + medusa forms | Jellyfish, corals, sea anemones, Hydra |
| Platyhelminthes | Bilateral symmetry; triploblastic; acoelomate (no body cavity); flat body (diffusion); gastrovascular cavity or absent | Planaria, tapeworms, flukes |
| Nematoda | Bilateral; triploblastic; pseudocoelomate; complete digestive tract (mouth→anus); tough cuticle | Roundworms, C. elegans, hookworms |
| Annelida | Bilateral; triploblastic; true coelomate; segmented body; closed circulatory system | Earthworms, leeches, polychaetes |
| Mollusca | Bilateral; triploblastic; coelomate; muscular foot, visceral mass, mantle; radula (most) | Snails, clams, octopus, squid |
| Arthropoda | Bilateral; triploblastic; coelomate; segmented body; jointed appendages; chitinous exoskeleton; most diverse phylum (~80% of known animal species) | Insects, crustaceans, spiders, millipedes |
| Echinodermata | Secondary radial symmetry (adults); deuterostome development; water vascular system; tube feet | Starfish, sea urchins, sea cucumbers |
| Chordata | Notochord; dorsal hollow nerve cord; pharyngeal slits; post-anal tail (at some life stage) | Vertebrates (fish, amphibians, reptiles, birds, mammals), tunicates, lancelets |
Deuterostome vs Protostome: Chordates and echinoderms are deuterostomes (anus forms from blastopore first). Most other bilaterian phyla are protostomes (mouth forms from blastopore first).
Common Traps
- Viruses are NOT cells — they lack ribosomes. Questions that imply viruses are alive are wrong.
- Archaea and Bacteria are both prokaryotes, but archaea are more closely related to eukaryotes.
- Protists are not a monophyletic group. The DAT may ask whether Protista is a valid clade (it is not).
- Bryophytes don't have seeds or vascular tissue. Their gametophyte is dominant. This is the opposite of all other plants.
- Fungi are NOT plants. Chitin vs cellulose; heterotrophic vs autotrophic.
- Porifera lack true tissues — they are the only phylum without tissue-level organization.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Life on Earth comes in three big flavors: Bacteria, Archaea, and Eukaryotes (that's us!). Bacteria are everywhere — on your skin, in your gut, in the soil. Some are good, some cause disease. The Gram stain is like a bacteria sorting hat: thick-walled bacteria stay purple (Gram-positive); thin-walled ones turn pink (Gram-negative). Archaea look like bacteria but are weirdos living in boiling hot springs or super-salty lakes. Viruses are the oddballs — they're not alive, more like a USB stick of instructions that hijacks a cell to make copies of itself. Among the big complex eukaryotes, plants use sunlight to make food (thanks to chloroplasts and cellulose walls); fungi are the cleanup crew (chitin walls, they digest stuff outside their bodies and slurp it up); and animals are the eaters (no cell walls, they swallow or absorb other organisms). Animals start simple — sponges are just a tube of cells — and get fancier: jellyfish, worms, snails, bugs, starfish, and finally us chordates with our backbones (and that weird tail you had as an embryo).
Key takeaways
- Lytic = burst and destroy; lysogenic = integrate and lie dormant. A prophage can switch to lytic.
- Gram stain: thick peptidoglycan = Gram+ (purple); thin + outer membrane = Gram− (pink).
- Archaea: ether-linked lipids, no peptidoglycan, extremophiles, no known pathogens.
- Plant evolution order: bryophytes → seedless vascular → gymnosperms → angiosperms. Know which have vascular tissue (all but bryophytes), seeds (gymnosperms + angiosperms), fruits/flowers (only angiosperms).
- Double fertilization = 1 sperm + egg (2n zygote) + 1 sperm + 2 polar nuclei (3n endosperm).
- Fungi: chitin cell walls, heterotrophic absorption, hyphae → mycelium.
- Arthropods = most diverse phylum. Chordate hallmarks: four features.
- Deuterostomes: echinoderms + chordates. Protostomes: everything else bilaterian.
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
A bacteriophage infects an E. coli cell. The phage DNA integrates into the bacterial chromosome and is replicated along with the host genome for many generations. After UV exposure, the phage DNA excises and begins producing new virions that lyse the cell. Which life cycles are represented in this scenario?
Show answer
The phage initially entered the lysogenic cycle (integration as a prophage, passive replication with the host). UV exposure triggered induction — the switch to the lytic cycle (excision, viral replication, cell lysis). Many temperate phages can alternate between these two cycles.
A botanist discovers a plant that produces seeds but has no flowers or fruits. It has well-developed xylem and phloem and needle-shaped leaves. To which plant group does it belong?
Show answer
Gymnosperms. Seeds ("naked seeds" — not enclosed in fruit), vascular tissue (xylem + phloem), and cone-bearing/needle-shaped leaves are diagnostic of gymnosperms. Angiosperms have flowers and fruits; seedless vascular plants have vascular tissue but lack seeds; bryophytes lack both vascular tissue and seeds.
An organism is multicellular, heterotrophic, has cell walls containing chitin, and grows as a network of threadlike filaments that secrete digestive enzymes. What kingdom does it belong to, and what is the function of the filament network?
Show answer
This organism belongs to Kingdom Fungi. The threadlike filaments are hyphae, which collectively form a mycelium. The mycelium secretes hydrolytic enzymes into the environment (external digestion), breaking down complex organic matter, then absorbs the resulting nutrients — a strategy called absorptive heterotrophy. The chitin cell walls distinguish fungi from plants (cellulose) and animals (no cell walls).
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Compare and contrast viral lytic and lysogenic life cycles.
- Distinguish the three domains of life (Bacteria, Archaea, Eukarya) by key cellular features.
- Describe the structural and reproductive characteristics of the four major plant groups.
- List the defining features of the major animal phyla and recognize representative organisms.
- Identify the four key chordate characteristics.
Sources & references
- OpenStax Biology 2e, Chapters 21–28: Viruses, Prokaryotes, Protists, Fungi, Plants, and Animals
- NCBI Bookshelf search: "diversity of life" (the cited book has been removed from Bookshelf)
- NIH National Library of Medicine: "Bacteria, Viruses, Fungi, Parasites"
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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