From the towering sequoia to the tiny moss growing at its base, plant life is so diverse that it may be challenging to see beyond the differences among the myriad species belonging to the plant kingdom (Plantae) to reveal what they have in common. Indeed, even botanists can't agree all of the time, and there is ongoing debate about whether species like lichen and algae belong with in the plant kingdom or elsewhere. Regardless of where you classify organisms that fall into the gray area between kingdoms, all plants share several common traits that define their kingdom.
Are plants even alive? Sitting still and silent, they sometimes seem to share more in common with nonliving objects in the landscape than with animal life. Seven processes are common to all life--nutrition, growth, reproduction, excretion, sensitivity, movement and respiration--and plants do carry out all of those processes, though often in very different ways than we do.
Photosynthesis is one of the key characteristics that distinguish plants from animal life. Most organisms have to eat in order to acquire nutrients and energy. While plants do draw water and minerals from the soil, they use a metabolic process called photosynthesis to produce their energy from sunlight. For this reason, plants are also called autotrophs or primary producers because they do not rely on any other organisms to provide their food. Photosynthesis also defines the position of plants in the web of life on Earth. Because they alone can convert sunlight to chemical energy, they are at the center of any food web, and all other life on Earth relies on them for survival.
Plant cells differ in many ways from animal cells. Among the most notable distinctions, plants have a more rigid structure. While all cells are enclosed by a membrane that regulates the passage of materials in and out of the cell, plant cells also have a tough cell wall that provides extra structure to the cell. Plasmodesmata, another feature of plant cells, connect cells to each other, while a central vacuole provides storage space and structural support. Of course, plant cells are also distinct because they have chloroplasts, tiny structures that carry out photosynthesis.
Plants demonstrate alternating generations as part of their life cycle. In most familiar plant species, the sporophyte generation dominates, and plant cells contain a full array of chromosomes. The gametophyte generation, in contrast, functions to produce reproductive cells for the plant. Its cells contain only half of the expected chromosomes. Primitive mosses, called bryophytes, are the only plants where the gametophyte generation dominates and is necessary for the survival of the sporophyte generation.
Like animals, plants exchange gases with their environment, collecting carbon dioxide necessary for photosynthesis and releasing oxygen as a byproduct. The anatomy of plants reflects this essential function, and plant leaves contains pores called stomata that regulate the flow of gas in and out of the plant. Leaves are also coated with a waxy cuticle because, during gas exchange, water vapor often escapes as well, dehydrating the plant. The waxy cuticle minimizes water loss.