Biological Classification · Lesson 8 of 10
Kingdom Plantae, Animalia and Alternation of Generations
“Compare plants and animals and understand how haploid and diploid phases alternate during plant life cycles.”
• Identify important cellular and nutritional traits of Plantae and Animalia. • Explain why some plants are parasitic or insectivorous. • Distinguish gametophyte from sporophyte and interpret chromosome states. • Trace the roles of meiosis and fertilisation in alternation of generations. • Avoid treating plant movement and animal mobility as absolute classifications.
Two familiar kingdoms viewed more carefully
Green plants and animals are easy to recognise in everyday life, but scientists distinguish them more reliably through cell organisation, nutrition, development and reproductive life cycles. Both kingdoms contain eukaryotic organisms with specialised cellular machinery. Most familiar land plants perform photosynthesis and have cell walls containing cellulose, whereas animals lack cell walls and obtain organic material by ingesting food.
Kingdom Plantae
Plants are generally multicellular eukaryotes with cellulose-containing cell walls. Photosynthetic cells contain chloroplasts, which use light energy to produce organic compounds from carbon dioxide and water. Photosynthesis supplies the energy-rich molecules used by the plant itself, but every plant cell also needs metabolism such as cellular respiration. Some plant tissues, for example many root cells, lack functioning chloroplasts and depend on organic substances made elsewhere in the organism.
The broad plant grouping described in the chapter includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. They differ in bodies, conducting tissues, seeds and reproductive structures. Their differences form the basis of the next chapter's deeper treatment; here the key objective is to recognise general plant organisation and understand its important exceptions.
Insectivorous and parasitic plants
The Venus flytrap and bladderwort are capable of photosynthesis, yet capture small animals. These organisms commonly live where important nutrients such as nitrogen are limited. Capturing prey helps supplement mineral nutrition; it does not mean the whole plant stops using sunlight to build sugars. A Venus flytrap leaf closes after appropriate stimulation; bladderworts use small traps to capture aquatic organisms.
Cuscuta is a parasitic flowering plant whose specialised connections penetrate host tissues and obtain water and organic substances. Many Cuscuta species have reduced chlorophyll and depend strongly on host plants. These are useful contrasts: the first group supplements nutrients with prey, while parasitic plants can directly withdraw resources from hosts.
Alternation of generations
The haploid (n) phase of a plant life cycle that produces gametes, usually by mitotic division.
The diploid (2n) phase of a plant life cycle that produces haploid spores by meiosis.
Most land-plant life cycles alternate between a haploid gametophyte and a diploid sporophyte. In the sporophyte, specialised cells undergo meiosis, halving chromosome number to form spores (n). A spore can divide mitotically and develop into a gametophyte, which produces gametes (n). Fusion of two gametes at fertilisation creates a diploid zygote (2n), which grows into a new sporophyte. The sequence is therefore a biological cycle rather than an alternation of two ordinary seasons.
Both gametophytes and sporophytes can be multicellular in land plants, but their relative sizes and independence differ by group. Mosses prominently display a gametophyte with a dependent sporophyte. Ferns have a large, recognisable sporophyte and a much smaller gametophyte. In flowering plants the sporophyte is the familiar plant, whereas gametophytes are highly reduced and occur within reproductive structures.
Notice that meiosis occurs in the sporophyte, before spores form, while fertilisation is the transition from gametes to a diploid zygote. Students sometimes assume gametes are always formed by meiosis, because that is common in animal life cycles. In land plants, gametes are produced by a haploid gametophyte through mitosis, so chromosome number remains n until fertilisation.
Kingdom Animalia
Animals are multicellular eukaryotes whose cells lack cell walls. Most obtain food by ingestion, followed by digestion and absorption of nutrients. This is often called holozoic nutrition. They may store energy-rich reserves as glycogen and fat. Animals develop specialised tissues and systems; in many groups, these include sensory structures and nervous and muscular tissues that coordinate responses to surroundings.
Many animals can move from place to place, yet movement is not a universal adult trait: corals and sponges may remain fixed to a substrate. Animal reproduction frequently involves fusion of gametes followed by embryonic development, but many groups also have asexual reproductive methods. The shared developmental and cellular history is more informative than a single stereotype such as 'all animals run'.
A heterotrophic feeding mode involving the ingestion of organic food, its digestion and the absorption and use of nutrients.
| Feature | Plants (general pattern) | Animals (general pattern) |
|---|---|---|
| Cell wall | Commonly cellulose-rich | Absent |
| Energy and nutrition | Usually photosynthetic production of organic matter | Usually ingestion of organic food |
| Food reserves | Often starch | Often glycogen and fat |
| Body organisation | Organs and tissues vary by group | Specialised tissues, organs and organ systems in many groups |
| Growth and movement | Growth often localised to meristems; non-locomotory responses occur | Growth patterns vary; many are motile at some stage |
| Reproductive pattern | Gametophyte–sporophyte alternation in land plants | Embryonic development typical; reproductive strategies vary |
Problem
A fern sporophyte is diploid. What ploidy will its spores, gametophyte, gametes and next zygote have?
- 1.The diploid sporophyte undergoes meiosis to produce haploid spores (n).
- 2.Haploid spores grow into haploid gametophytes, whose gametes remain n.
- 3.Fertilisation combines two haploid gametes to create a diploid zygote (2n), which develops into the next sporophyte.
Insectivorous plants often remain photosynthetic and mainly use prey to supplement nutrients. Plant gametophytes produce gametes by mitosis; do not automatically transfer animal gametogenesis rules to plants.
Quick Quiz
Quiz
Which cell-wall material is prominent in many plant cells?
Why does a Venus flytrap capture insects?
Which stage is diploid in a typical land-plant life cycle?
How are gametes normally formed in a haploid gametophyte?
Which feature is typical of animal cells?
Which process restores the diploid phase in land plants?
Practice Problems
- Explain why carnivorous plants do not necessarily stop being autotrophs.
- Describe the relationship between sporophyte, meiosis, spores, gametophyte, gametes and fertilisation.
- Contrast moss, fern and flowering plant dominance of gametophyte and sporophyte phases.
- Compare the main cellular features and nutritional methods of Plantae and Animalia.
- Explain why movement alone is not a reliable test for animal classification.
Many carnivorous plants photosynthesise to make sugars while trapped prey supplies minerals such as nitrogen and phosphorus. This supplemental nutrition does not replace all their autotrophic activity.
Key Takeaways
Plantae includes predominantly photosynthetic eukaryotes with cellulose-containing walls. Insectivorous plants supplement mineral nutrition; some plants such as Cuscuta are parasitic. A plant sporophyte is diploid (2n); a gametophyte is haploid (n). Meiosis produces haploid spores and fertilisation restores the diploid zygote. Animals are multicellular eukaryotes without cell walls, generally using ingested food. Single features such as movement or photosynthesis do not capture every biological exception.