Of the six kingdoms into which life is classified, four fall under the eukaryote domain, which encompasses organisms whose cells contain a well-defined nucleus surrounded by a membrane. Inside are chromosomes that contain the organism's genetic material. Eukaryote cells also have specialized organelles such as mitochondria, which generate energy. Eukaryotes are generally more complex and collectively more diverse that prokaryotes, which lack internal membranes and therefore also lack nuclei and organelles. Most prokaryotes are bacteria.
Plants, the basis of most food chains, are multicellular, autotrophic organisms. The latter term refers to plants' ability to synthesize abiotic elements to produce the organic nutrients they need to survive. Chlorophyll---the green pigment that gives plants their coloring---allows leaves to trap sunlight and convert it into sugars in the presence of water and carbon dioxide. This process is known as photosynthesis.
The majority of plants are Earth are made up of roots and shoots. The former refers to the parts of the plant that extend below the soil, absorbing water and inorganic nutrients and serving as an anchor; the latter encompasses the stem and the other structures that grow above the ground. About 250,000 known species of plants make it the second-largest kingdom.
Humans are among the most complex organisms that make up the animalia kingdom, which, with more than 1 million known species, is the largest of the six. All animals are heterotrophs, or consumers, which means they must consume organic matter to obtain the energy needed for life processes.
Predators are animals that eat other organisms or parts thereof. Carnivores are predators that eat other animals. Herbivores, known also as primary consumers, are predators that feed on plant matter. Detrivores feed on dead organisms. Parasitic animals like tapeworms feed off other animals, known as hosts, but without completely destroying them.
Most animal species have male and female individuals. During mating, the former's sperm fertilizes the latter's egg. The result is a zygote that develops into a new organism. Hermaphrodites are animals that posses both male and female reproductive organs.
Fungi, considered a kingdom in their own right, were once mistaken for plants because they too are sedentary and do not have senses. Fungi, however, do not have the means to produce their own organic compounds. Therefore, like animals, they are heterotrophs. Many fungi absorb nutrients from the dead or decayed organic matter on which they grow. Others, like ringworm, are parasitic. Some fungi, such as mushrooms and yeasts, are used in cooking and baking. Mold and mildew are also types of fungi.
The kingdom protista comprises a hodgepodge of eukaryotic organisms grouped together because they do not fit in any of the other kingdoms. The vast majority of protists consist of only one cell and are therefore among the simplest eukaryotes. Some protists are photoautotrophs, which means that, like plants, they can generate their own foodstuff. Examples of photoautotrophic protists include algae, which play important roles as producers in aquatic food chains. Protozoans such as amoebas, on the other hand, are heterotrophic protists.