12.3 Survey of Major Groups of Organisms

Life on Earth spans an enormous range of forms, from particles of genetic material that hijack cells to complex multicellular organisms with specialized organ systems. Understanding how these major groups differ, and what unites them, is central to taxonomy and classification.

This survey covers microorganisms (viruses, bacteria, archaea, protists), the eukaryotic kingdoms (fungi, plants, animals), and a closer look at animal diversity and biodiversity.

Microorganisms

Viruses and Bacteria

Viruses are non-living infectious agents that cannot reproduce on their own. They require a host cell's machinery to replicate. Each virus particle consists of genetic material (either DNA or RNA, never both) surrounded by a protein coat called a capsid. Some viruses also have a lipid envelope. Viruses can infect every type of organism, including bacteria (those viruses are called bacteriophages), plants, and animals.

Because viruses lack cells, ribosomes, and the ability to carry out metabolism independently, most biologists do not classify them as living organisms. That's a distinction worth remembering for exams.

Bacteria are prokaryotic, single-celled organisms that lack a membrane-bound nucleus and membrane-bound organelles. They're classified by shape:

• Cocci (spherical)
• Bacilli (rod-shaped)
• Spirilla (spiral)

Bacteria occupy virtually every habitat on Earth, from deep soil to the human gut. Some are beneficial: gut bacteria aid digestion, and Streptomyces bacteria produce many of the antibiotics used in medicine. Others are pathogenic, causing diseases like strep throat (Streptococcus) and tuberculosis (Mycobacterium tuberculosis).

Archaea and Protists

Archaea are also prokaryotic and single-celled, so they superficially resemble bacteria. However, archaea differ in key biochemical ways, including distinct cell membrane lipids and unique ribosomal RNA sequences. These molecular differences are why archaea are placed in their own domain, separate from Bacteria.

Archaea are famous for thriving in extreme environments:

• Thermophiles in hot springs and hydrothermal vents
• Halophiles in salt lakes
• Methanogens in oxygen-free environments like swamps and animal guts, where they produce methane gas

They also play important roles in global nutrient cycling, particularly in nitrogen and carbon cycles.

Protists are a diverse, catch-all group of eukaryotic organisms that don't fit into the plant, animal, or fungus kingdoms. They include:

• Algae (e.g., kelp, diatoms) that carry out photosynthesis and produce a significant portion of Earth's oxygen
• Protozoa (e.g., amoebae, paramecia) that are heterotrophic and often motile
• Slime molds that can behave like both single-celled and multicellular organisms at different life stages

Protists can be unicellular or multicellular, and their ecological roles are just as varied: photosynthesizers, predators, parasites, and decomposers.

Eukaryotic Kingdoms

Fungi

Fungi are eukaryotic organisms that obtain nutrients through absorption. Rather than ingesting food like animals, fungi secrete digestive enzymes into their surroundings and absorb the broken-down molecules. This makes them critical decomposers, recycling nutrients from dead organic matter back into ecosystems.

The group includes mushrooms, yeasts, and molds. Key traits to know:

• Cell walls made of chitin (not cellulose like plants)
• Reproduction primarily through spores, which can be produced sexually or asexually
• Body structure often consists of thread-like hyphae that form a network called a mycelium

Fungi also form important symbiotic relationships. Mycorrhizae are associations between fungi and plant roots that help plants absorb water and minerals. Lichens are partnerships between fungi and photosynthetic organisms (algae or cyanobacteria). Humans use fungi extensively too: yeast for bread and beer, molds for cheese, and Penicillium for the antibiotic penicillin.

Plants

Plants are multicellular eukaryotes that produce their own food through photosynthesis. Their cells contain chloroplasts, organelles that convert light energy into chemical energy (glucose), and have cell walls made of cellulose.

Plant diversity is classified into two broad categories based on internal transport structures:

• Non-vascular plants (bryophytes) like mosses and liverworts lack true vascular tissue. They tend to be small and grow in moist environments because they need water for reproduction and nutrient transport.
• Vascular plants have xylem and phloem for transporting water and sugars. This group includes:
  • Ferns (seedless vascular plants)
  • Gymnosperms (seed plants without flowers, like conifers)
  • Angiosperms (flowering plants, the most diverse group of plants on Earth)

Plants provide oxygen as a byproduct of photosynthesis, serve as the base of most terrestrial food webs, and create habitat for countless other organisms.

Animals

Animals are multicellular eukaryotic heterotrophs, meaning they obtain energy by consuming other organisms. Unlike plants and fungi, animal cells lack cell walls. Most animals are motile during at least some stage of their life cycle.

Animals display an enormous range of body plans, from structurally simple sponges (which lack true tissues) to vertebrates with complex organ systems. They're broadly divided into two groups:

• Invertebrates: animals without a backbone (the vast majority of animal species)
• Vertebrates: animals with a backbone and internal skeleton

Animals function in ecosystems as consumers at various trophic levels, as predators, prey, pollinators, seed dispersers, and decomposers.

Animal Diversity

Vertebrates

Vertebrates possess a vertebral column (backbone) and an internal endoskeleton. They belong to phylum Chordata and include five major classes:

• Fish (aquatic, gills, fins)
• Amphibians (moist skin, aquatic larvae, terrestrial adults; e.g., frogs, salamanders)
• Reptiles (dry scaly skin, amniotic eggs; e.g., lizards, snakes, turtles)
• Birds (feathers, lightweight bones, endothermic)
• Mammals (hair/fur, mammary glands, endothermic)

Vertebrates share several advanced features: a well-developed nervous system with a brain, a closed circulatory system, and specialized respiratory and excretory organs.

A key distinction within vertebrates is thermoregulation:

• Ectotherms (fish, amphibians, most reptiles) rely on external heat sources to regulate body temperature.
• Endotherms (birds and mammals) generate internal body heat through metabolism, allowing them to maintain a stable body temperature regardless of the environment.

Invertebrates

Invertebrates lack a backbone and make up roughly 95% of all animal species. The major invertebrate phyla include:

Arthropods alone account for more species than all other animal phyla combined, largely because of the incredible diversity of insects.

Many invertebrates undergo metamorphosis, passing through distinct life stages (e.g., larva → pupa → adult in insects). Others, like cnidarians, alternate between polyp and medusa body forms.

Biodiversity

Biodiversity refers to the variety of life on Earth, measured at three levels:

• Species diversity: the number and variety of species in a given area
• Genetic diversity: the variation in genes within a species (critical for adaptation)
• Ecosystem diversity: the range of different habitats, communities, and ecological processes

Biodiversity matters because it underpins ecosystem stability and resilience. Diverse ecosystems are better at providing ecosystem services like pollination, water purification, nutrient cycling, and climate regulation. When biodiversity declines, these services weaken.

Human activities are the primary drivers of biodiversity loss:

• Habitat destruction (deforestation, urbanization)
• Overexploitation (overfishing, poaching)
• Pollution (pesticides, plastics, nutrient runoff)
• Climate change (shifting habitats faster than species can adapt)
• Invasive species (outcompeting or preying on native organisms)

The current rate of species extinction is estimated to be 100 to 1,000 times higher than the natural background rate. Conservation efforts include establishing protected areas, implementing species recovery plans, and international agreements like the Convention on Biological Diversity.