What Is a Plant?

Biologists like to put things into categories. In another reading we deal with principles of how botanists put different types of plants into categories, i.e., how botanists name and classify plants. For this present reading, in order to answer the question, "What is a plant?", we must look at the major groups of living organisms that most biologists recognize.

Introduction to the Five Kingdoms

The major groups of living organisms are called kingdoms. Biologists used to recognize two kingdoms: 1) The plant kingdom (Kingdom Plantae) and 2) the animal kingdom (Kingdom Animalia). As more organisms were observed, it became obvious that not all organisms neatly fit into either the plant or animal kingdom. In some cases zoologists considered an organism an animal, but botanists considered the same organism a plant!

A three-kingdom system for some time was recognized by many biologists, then a four-kingdom system became popular, eventually a five-kingdom system was recommended, and many biologists currently recognize six kingdoms. As biologists study different organisms, new information becomes available for making decisions about how organisms may be grouped, and there probably never will be a system of classification that will satisfy all biologists.

In this set of on-line readings we use the five-kingdom system because it is convenient and is used in a variety of introductory textbooks. The five kingdoms are : 1) Monera, 2) Fungi, 3) Protista, 4) Plantae, and 5) Animalia. The basic criteria used to place organisms in the five kingdoms include cell structure, modes of nutrition, and level of complexity of the organism.

Although associated with living organisms, particularly in diseases, the viruses are a group that many biologists would not consider as living, and, therefore, do not fit into the classification system used in this set of readings about botany. Viruses do not consist of cells: Instead, they generally are several complex molecules that contain information for replicating themselves if they are in a suitable environment such as a plant or animal cell. The virus itself does not have the ability to replicate: It requires a living cell to provide the materials and machinery to perform the replication of new viruses.

In order to categorize various organisms into the five kingdoms, we use criteria that are based on properties of living things. The criteria that have been found to be the most useful are based on the cells that make up the organisms and how organisms obtain their nutrition.

Cell Structure

The cell theory states that all living things (with very few exceptions) consist of microscopic units called cells or of products of cells. Most cells can be seen only if observed with a compound light microscope, an instrument that allows the biologist to study material magnified approximately 2,000 times. The cell theory and the study of cells have helped biologists to understand many basic processes that occur in all types of organisms. By studying cell structure, biologists became better at distinguishing between and among different groups of organisms.

In the 1950s the transmission electron microscope (TEM) became available to biologists. The TEM allows biological specimens to be viewed at magnifications ranging from 2,000 to 500,000 times and even greater. Two basic cell types became evident from the early studies with TEM. Prokaryotic cells are smaller and do not contain the elaborate internal structures as do eukaryotic cells.

Organisms that consist of prokaryotic cells are either single-celled (unicellular organisms) or simple colonies of similar cells. Organisms consisting of prokaryotic cells are called the prokaryotes.

Organisms that are made up of eukaryotic cells are the eukaryotes. Eukaryotes can be either unicellular or multicellular (made up of many cells). Multicellular eukaryotes may have cells that become specialized in their function within the organism. Cells that become specialized are said to be differentiated; cells that are not specialized are undifferentiated. Animals and plants are multicellular, eukaryotic organisms that have cellular differentiation. Another way of stating this is that plants and animals are complex multicellular organisms. Other eukaryotes are unicellular while others are multicellular, but with undifferentiated cells.

Nutrition for Living Things

Three basic properties of living organisms are growth, reproduction, and movement. In order for a living organism to carry out its functions, it needs energy and raw materials. Organisms have different ways of obtaining raw materials and energy.

Food is what organisms consume that perpetuate the life processes. Foods that in part contribute to your life processes include, among others, eggs, milk, potatoes, tomatoes, hamburgers, bagels, cheese, and salad. All of these things we recognized as foods come either from animals or plants. The foods humans consume are complex structures that either were other organisms or products produced by other organisms. The term used for the mode of nutrition for organisms like humans is heterotrophic nutrition. Organisms that rely on heterotrophic nutrition are referred to heterotrophs.

Not all organisms require complex foods like heterotrophs. Plants survive with soil, air, water, and light. These do not fit into what we generally use for food for ourselves. Plants need only a few inorganic minerals such as nitrates, phosphates, and iron; water; carbon dioxide; and light to maintain their life processes. Plants use photosynthesis to convert simple molecules (carbon dioxide and water) into more complex molecules that are used in plant growth and reproduction. The type of nutrition in which the organism makes its own complex molecules from relatively simple molecules is autotrophic nutrition. Plants are autotrophs. The process of making food by using light energy is photosynthesis. Plants specifically, then, are photosynthetic autotrophs.

The Five Kingdoms

Now that you have been introduced to some basic concepts of modes of nutrition and cell structure, you are ready to answer the basic question of "What is a plant?" by looking at each of the five kingdoms with cell structure and nutrition in mind.