Example of Living Organisms, Plant Cells Energy and Microscopic Bacteria on Earth

How to Define Life

Life on Earth takes on a staggering variety of forms, often functioning and behaving in ways strange to humans. For example, gastric-brooding frogs swallow their embryos and give birth to them later by throwing them up! Some species of puffballs, a type of fungus, are capable of producing trillions of spores when they reproduce.

Fetal sand sharks kill and eat their siblings while still inside their mother. Some Ophrys orchids look so much like female bees that male bees try to mate with them. Octopuses and squid have remarkable problem-solving abilities despite a small brain. Some bacteria live their entire life in 15 minutes, while bristlecone pine trees outlive ten generations of humans. Simply put, from the deepest oceanic trenches to the upper reaches of the atmosphere, life is plentiful and diverse.

Figure 1 illustrates the major groups of living things, also called organisms. From left to right, bacteria are widely distributed, tiny, microscopic organism with a very simple structure. A Paramecium is an example of a microscopic protist. Protists are larger in size and more complex than bacteria. The other organism in Figure 1 are easily seen with the naked eye. They can be distinguished by how they get their food. A morel is a fungus that digests its food externally. A sunflower is a photosynthetic plant that makes its own food, and a snow goose is an animal that ingests its food.

FIGURE 1 Diversity of life. Biology is the scientific study of life. Many diverse forms of life are found on planet Earth.
FIGURE 1 Diversity of life. Biology is the scientific study of life. Many diverse forms of life are found on planet Earth.


Because life is so diverse, it seems reasonable that it cannot be defined in a straightforward manner. Instead, life is best defined by several basic characteristics shared by all organism. Like nonliving things, organism are composed of chemical elements. Also, organism obey the same laws of chemistry and physics that govern everything within the universe. The characteristics of life, however, will provide great insight into the unique nature of organisms and will help us distinguish living things from nonliving things.

Living Things Are Organized

The levels of organization depicted in Figure 2 begin with atoms, which are the basic units of matter. Atoms combine with other atoms of the same or different elements to form molecules. The cell, which is composed of a variety of molecules working together, is the basic unit of structure and function of all living things. Some cells, such as unicellular paramecia, live independently. Other cells, for example, the colonial alga Volvox, cluster together in microscopic colonies.

 Many living things are multicellular, meaning they contain more than one cell. In multicellular organisms, similar cells combine to form a tissue; nerve tissue is a common tissue in animals. Tissues make up organs, as when various tissues combine to form the brain. Organs work together in systems; for example, the brain works with the spinal cord and a network of nerves to form the nervous system. Organ systems are joined together to form a complete living thing, or organism, such as an elephant. The levels of biological organization extend beyond the individual organism. All the members of one species in a particular area belong to a population. 

A nearby forest may have a population of gray squirrels and a population of white oaks, for example. The populations of various animals and plants in the forest make up a community. The community of populations interacts with the physical environment and forms an ecosystem. Finally, all the Earth’s ecosystems make up the biosphere.

Emergent Properties

Each level of biological organization builds upon the previous level, and is more complex. Moving up the hierarchy, each level acquires new emergent properties that are determined by the interactions between the individual parts. When cells are broken down into bits of membrane and liquids, these parts themselves cannot carry out the business of living. For example, you can take apart a lump of coal, rearrange the pieces in any order, and still have a lump of coal with the same function as the original one. But, if you slice apart a living plant and rearrange the pieces, the plant is no longer functional as a complete plant, because it depends on the exact order of those pieces.

In the living world, the whole is indeed more than the sum of its parts. The emergent properties created by the interactions between levels of biological organization are new, unique characteristics. These properties are governed by the laws of chemistry and physics.

FIGURE 2 Levels of biological organization.
FIGURE 2 Levels of biological organization.


Living Things Acquire Materials and Energy

Living things cannot maintain their organization or carry on life’s activities without an outside source of nutrients and energy (Fig. 3). Food provides nutrients energy, which are used as building blocks or for energy. Energy is the capacity to do work, and it takes work to maintain the organization of the cell and the organism. When cells use nutrient molecules to make their parts and products, they carry out a sequence of chemical reactions. The term metabolism [Gk. meta, change] encompasses all the chemical reactions that occur in a cell.  

The ultimate source of energy for nearly all life on Earth is the sun. Plants and certain other organisms are able to capture solar energy and carry on photosynthesis, a process that transforms solar energy into the chemical energy of organic nutrient molecules. All life on Earth acquires energy by metabolizing nutrient molecules made by photosynthesizers. This applies even to plants.

Remaining Homeostatic

To survive, it is imperative that an organism maintain a state of biological balance or homeostasis [Gk. homoios, like, and stasis, the same]. For life to continue, temperature, moisture level, acidity, and other physiological factors must remain within the tolerance range of the organism. Homeostasis is maintained by systems that monitor internal conditions and make routine and necessary adjustments.

Organisms have intricate feedback and control mechanisms that do not require any conscious activity. These mechanisms may be controlled by one or more tissues themselves, or by the nervous system. When a student is so engrossed in her textbook that she forgets to eat lunch, her liver releases stored sugar to keep blood sugar levels within normal limits. Many organisms depend on behavior to regulate their internal environment. These behaviors are controlled by the nervous system, and are usually not consciously controlled. The same student may realize that she is hungry and decide to visit the local diner. A lizard may raise its internal temperature by basking in the sun or cool down by moving into the shade.

FIGURE 3 Acquiring nutrients and energy. a. An eagle ingesting fish. b. A human eating an apple. c. A cypress tree capturing sunlight. d. An amoeba engulfing food. e. A fungus feeding on a tree. f. A bison eating grass.
FIGURE 3 Acquiring nutrients and energy. a. An eagle ingesting fish. b. A human eating an apple. c. A cypress tree capturing sunlight. d. An amoeba engulfing food. e. A fungus feeding on a tree. f. A bison eating grass.


Living Things Respond

Living things interact with the environment as well as with other living things. Even unicellular organisms can respond to their environment. In some, the beating of microscopic hairs or, in others, the snapping of whiplike tails moves them toward or away from light or chemicals. Multicellular organisms can manage more complex responses. A vulture can detect a carcass a kilometer away and soar toward  dinner. A monarch butterfly can sense the approach of fall and begin its flight south where resources are still abundant.

 The ability to respond often results in movement: the leaves of a land plant turn toward the sun, and animals dart toward safety. Appropriate responses help ensure survival of the organism and allow it to carry on its daily activities. All together, these activities are termed the behavior of the organism. Organisms display a variety of behaviors as they maintain homeostasis and search and compete for energy, nutrients, shelter, and mates. Many organisms display complex communication, hunting, and defense behaviors.

Living Things Reproduce and Develop

Life comes only from life. Every type of living thing can reproduce, or make another organism like itself (Fig. 4). Bacteria, protists, and other unicellular organisms simply split in two. In most multicellular organisms, the reproductive process begins with the pairing of a sperm from one partner and an egg from the other partner.The union of sperm and egg, followed by many cell divisions, results in an immature stage, which grows and develops through various stages to become the adult.

An embryo develops into a humpback whale or a purple iris because of a blueprint inherited from its parents. The instructions, or blueprint, for an organism’s metabolism and organization are encoded in genes. The genes, which  contain specific information for how the organism is to be  ordered, are made of long molecules of DNA (deoxyribonucleic acid). DNA has a shape resembling a spiral staircase with millions of steps. Housed within this spiral staircase is the genetic code that is shared by all living things.

When living things reproduce, their genes are passed on to the next generation. Random combinations of sperm and egg, each of which contains a unique collection of genes, ensure that the new individual has new and different characteristics. The DNA of organisms, over time, also undergoes mutations (changes) that may be passed on to the next generation. These events help to create a staggering diversity of life, even within a group of otherwise identical organisms. Sometimes, organisms inherit characteristics that allow them to be more suited to their way of life.

FIGURE 4 Rockhopper penguins with their offspring. Rockhopper penguins, which are named for their skill in leaping from rock to rock, produce one or two offspring at a time. Both male and female have a brood patch, a feather-free area of skin containing many blood vessels, which keeps the egg(s) warm when either parent sits on the nest.
FIGURE 4 Rockhopper penguins with their offspring. Rockhopper penguins, which are named for their skill in leaping from rock to rock, produce one or two offspring at a time. Both male and female have a brood patch, a feather-free area of skin containing many blood vessels, which keeps the egg(s) warm when either parent sits on the nest.

Living Things Have Adaptations

Adaptations [L. ad, toward, and aptus, suitable] are modifications that make organisms better able to function in a particular environment. For example, penguins are adapted to an aquatic existence in the Antarctic. An extra layer of downy feathers is covered by short, thick feathers that form a waterproof coat. Layers of blubber also keep the birds warm in cold water. Most birds have forelimbs proportioned for flying, but penguins have stubby, flattened wings suitable for swimming. Their feet and tails serve as rudders in the water, but the flat feet also allow them to walk on land. Rockhopper penguins have a bill adapted to eating small shellfish.

 Penguins also have many behavioral adaptations to living in the Antarctic. Penguins often slide on their bellies across the snow in order to conserve energy when moving quickly. Their eggs—one or at most two—are carried on the feet, where they are protected by a pouch of skin. This also allows the birds to huddle together for warmth while standing erect and incubating eggs.

From penguins to fire ants, life on Earth is very diverse because over long periods of time, organisms respond to ever-changing environments by developing new adaptations. Evolution [L. evolutio, an unrolling] includes the way in which populations of organisms change over the course of many generations to become more suited to their environments. Evolution constantly reshapes the species, providing a way for organisms to persist, despite a changing environment.

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