Unity in Diversity: Different Plants and Animals, Same Metabolism


By Robert HazenGeorge Mason University

Every organism has to adapt to its surroundings and grow in accordance with its environment. All organisms, be it microbes, plants, or animals, have to develop their survival strategy to compete for resources, and derive energy to perform the basic physiological functions.

picture of a caterpillar on a plant
Even though their way of obtaining energy is different, all organisms share a common metabolic process. (Image: Mehmetbahadrunal/Shutterstock)

Algae: A Plant or an Organism?

The classification of plants is very difficult and not at all a matter of universal consensus. Algae are problematic because they can thrive as single-celled organisms; but then they often form large colonies with plant-like structures, such as kelp and seaweed.

Is algae a plant? Is it a single-celled organism? It’s sort of an arbitrary definition. Nature does what nature does, and our definitions, and human classifications, do not necessarily restrict nature in what it’s going to do. Many scientists classify some kinds of algae as plants; some kinds as monera, or single-celled organisms.

Multicellular Plants

The main divisions of multicellular plants, most of which live on land, are based on the way they reproduce, and also how they circulate water throughout their vascular systems.

The simplest plants are bryophytes, including mosses, and they absorb their moisture directly through above-ground structures. Most other plants are vascular plants. They have root systems, stems, leaves; they have an internal plumbing system that moves water by capillary action.

There are three big groups of vascular plants that do this: ferns, which have spores; gymnosperms, such as evergreens that have seeds but no flowers; and finally the angiosperms, with seeds and flowers.

All plants reproduce sexually, but they can also be cloned. That is, one can get genetically identical plants, just by growing them from a single cell, or small pieces of an older plant.

diagram explaining photosynthesis in plants
Green plants get their energy by the process of photosynthesis. (Image: BlueRingMedia/Shutterstock)

Photosynthesis of Plants

The great kingdom of plants and algae obtain their energy from the light of the Sun, converting photons to chemical energy by the process of photosynthesis. All plants and algae use this process, by which solar energy is converted into energy-rich molecules such as the sugar glucose.

The raw materials of photosynthesis are nothing more than water and carbon dioxide, fed by the Sun’s radiant energy. The net reaction to form glucose is simply six CO2 molecules, plus six water molecules, plus photons, light energy from the Sun, produce glucose. That’s C6H1206; the sugar molecule and three oxygen molecules then go off into the atmosphere.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Diversity in Animal Kingdom

Animals include multicellular organisms that obtain their energy and raw materials from the biomolecules of other organisms; they eat other organisms. There are more than 1.3 million living species of animals that are known, and then there are numerous extinct ones from the fossil record as well.

Although some aspects of animal classification are under debate, there are about 30 to 35 distinct phyla, each of which represents a basic design, a different structural design for organisms.

Vertebrates include all the animals with a bony skeleton, and a backbone that protects a spinal cord. There are about 40,000 known living species of vertebrates. These vertebrates are divided into seven different classeslike fish, which include the familiar ray-finned fish and the earliest vertebrate fossils, which are also fish, with bony armored plates.

Amphibians are the first land vertebrates; these include frogs and salamanders. They have a moist, permeable skin, so they have to remain near a wet environment.

Reptiles, Birds, and Mammals

Modern reptiles include lizards, snakes, turtles, alligators, and crocodiles. They have thick, scaly skin, and this skin resists dehydration, so reptiles have been able to adapt to many different land environments.

There are about 9,000 species of modern birds, and they’re distinguished by their feathers. Feathers give the birds the ability to fly; they also help the birds to regulate their internal body temperature, which is typically about 106 degrees Fahrenheit.

Mammals are represented by about 4,500 known species. Mammals have two distinctive characteristics: they have hair, and they have the ability to suckle their young.


The vast majority of animal species are invertebrates, which include all the animals without backbone. The simplest invertebrates are in the phylum of sponges, or poriphera, which are aggregates of individual cells.

There’s another phylum that contains jellyfish, corals, sea anemones, and other marine creatures that are radially symmetric. All of these in this phylum have a three-layer body structure, with a wall surrounding a stomach-like chamber. Here, as in all phyla, it’s the structure, not the size or shape, that establishes the common lineage. A three-layer skin structure and a central stomach are sufficient to define this group.


There are several phyla that contain different kinds of worms. There are flatworms and roundworms and segmented worms. All these have similar external shapes, but different internal structures, and so they form different phyla.

The mollusk phylum has 100,000 different species; it is the second largest of all the phyla. It includes gastropods, like snails; bivalves, like clams; cephalopods, including squids and octopi.

They have hard shells, and they have worldwide abundance. They evolve very, very rapidly, and so mollusks provide wonderful markers for geological history. If you find a stratum of sediment, and it has fossils of mollusks in it, the chances are you can date that layer of sediment very accurately, as the mollusks have evolved so rapidly.


Arthropods number almost a million known species, and certainly the vast majority of arthropod species are unknown at this point. This huge phylum includes all the invertebrates with jointed legs.

There are four large groups of arthropods:

  • the crustaceans, like lobsters and crabs
  • the insects, which have three body segments and six legs
  • the chelicerates, like spiders, scorpions, horseshoe crabs, with two main body segments
  • the myriapoda, like millipedes and centipedes, with lots of segments and lots of legs

Insects account for 70% of all the arthropods; indeed, most of the known species are insects, and beetles represent the vast majority of those.

Unity in Diversity

Irrespective of their classification, it must be remembered that all organisms use the same basic metabolism for distributing energy throughout the cells. The strategy is to synthesize large numbers of small, energy-rich molecules that act as tiny batteries.

Plants obtain energy through photosynthesis of sunlight, and animals obtain energy by eating other organisms.

Common Questions about Different Plants and Animals, Same Metabolism

Q: Is algae a plant or an organism?

Many scientists classify some kinds of algae as plants; some kinds as monera, or single-celled organisms.

Q: Give an example of crustaceans.

Lobsters and crabs are examples of the crustaceans.

Q: How do plants and animals obtain energy?

Plants obtain energy through photosynthesis of sunlight, and animals obtain energy by eating other organisms.

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