There are nine characteristics of living things.
- The need for energy
- Cellular structure and organization
- Growth and development
- Maintenance and repair
- Response to stimuli
- Variation and adaptation
All living things require a continuous supply of energy to support their more obvious characteristics such as movement, growth, and reproduction.
Figure: All living things have common characteristics. These characteristics are functions that require energy.
Almost all the energy used by living things comes originally from the sun (Figure). Green plants, through photosynthesis, store some of the sun’s energy in compounds such as glucose. These plants, through respiration, then ‘burn’ or breakdown the glucose, releasing the energy needed to support their life processes. Animals get their supply of energy by eating the plants or by eating other animals that have eaten plants. By doing this, they obtain glucose and other compounds which they too, break down through respiration to release energy to support their life processes.
One of the most obvious characteristics of living things is movement. Most animals show obvious signs of movement when they are alive.
Although movement in plants is not as obvious, it does occur. This movement can be very slow, such as (1) the opening of buds on a tree or (2) the turning of leaves of a plant toward the sun. In contrast, (3) the tiny sundew of northern bogs and (4) the Venus flytrap of Carolinian bogs show much more rapid motion. One of the most interesting examples of motion in plants is shown by (5) the Mimosa pudica, commonly called the Sensitive Plant. If this plant is touched, its leaves quickly fold up.
Many animals, plants, and microscopic organisms show few or no outward signs of movement. Yet under the microscope, you can see that the cell contents of these organisms are in continuous motion. This proves that in one way or another, all living things show movement.
Some organisms show a special type of movement called locomotion. Locomotion is the movement of an organism from one place to another. Most animals can carry out locomotion but very few plants can. Remember that both movement and locomotion, in a biological sense, must be initiated or caused by the organism itself. Locomotion does not occur when the wind blows a plant from one place to another, nor does movement occur when the wind moves the branches of a tree.
3. Cellular Structure and Organization
All living things are made up of cells. Some have only one cell; others have millions of cells. Some cells are very simple and others are very complex. However, from bacteria and amoebas to trees and humans, the cell is the basic unit in which substance are organized to produce a living thing.
Living cells contain a complex mixture of substances that is called protoplasm. This mixture is found only in living cells. The protoplasm itself, however, is not alive. None of the materials of which protoplasm is made ¾ carbohydrates, fats, proteins, waters, and other compounds ¾ are alive. Yet, living cells have the ability to organize all these materials into what biologists call a living condition. Protoplasm differs from one kind of organism to another and even from one individual to another of the same kind. It even differs from one part of an individual to another part of the same individual. In fact, the composition, or makeup, of any particular sample of protoplasm is always changing.
Living things have the ability to organize materials into protoplasm and to organize protoplasm and other substances into cells. Living things are therefore called organisms because of this ability to organize substances.
4. Growth and Development
All living things grow at some time during their lives. The total growth may be very small, as in the case of a bacterium or an amoeba. Total growth can be quite extensive, as in the case of a whale or a large tree. Yet, whether great or small, growth is a characteristic of all living things.
However, many nonliving things can also grow. For example, crystals of sugar, salt, and bluestone can be made to grow larger. You probably have seen an icicle grow. How, then, can we say that growth is a characteristic of living thing? What kind of growth are we referring to?
The crystals and the icicle grow larger by adding more material of the same kind to their surfaces. The growth of living things is quite different from this. A dog does not grow by the collection of more dogs on its surface; nor does a mango plant grow by the collection of more mango plants on its surface. Yet, neither of these organisms grows simply by taking in food. They must organize the food, along with water, minerals, and other chemicals, into the complex materials that make up protoplasm and the other parts of living cells.
Living things grow, not by adding more of their own material to their surfaces, but by organizing materials that they take in to form their own special kind of protoplasm.
If you plant a bean seed, it will become a bean plant. It never becomes a potato plant or a tree. It becomes a unique living thing with specialized parts that make it different from other living things. The series of changes that take place as an organism grows toward its final form is called development. All living things undergo development.
5. Maintenance and Repair
Most living things live long after growth appears to have stopped. Yet, in one sense, they continue to grow as long as they are alive. They may not grow any larger but they must continually maintain and repair the materials of which they are made.
1) Skin cells on your body wear away and must be replaced by new ones. A cut on your finger heals because new tissues are produced to cover the cut.
2) Some organisms, such as the salamander, house lizard and crayfish, can even grow new limbs or tail to replace lost ones by recognizing old, and adding new, material.
Living things use great deal of energy in the maintenance and repair of worn-out and damaged parts. This is a characteristic of all living things.
Only living things can produce offspring similar to themselves. Shrimps lay eggs that hatch and develop into shrimps; bluebirds lay eggs that eventually produce bluebirds; horse give birth to horses; apple seeds grow into apple plants; and mango seeds become mango trees.
It is a basic law of biology that only life can produce life and like produces like. Reproduction is the process whereby all living organisms produce offspring.
Organisms must be able to reproduce themselves because they have a limited life span. After most organisms are formed, they go through a period of rapid growth. They eventually reach a stage called maturity at which growth in size usually ceases but maintenance and repair may continue. They then enter a period of decline in which maintenance and repair of worn-out and damage materials are no longer fast enough to keep the organisms in a stable state. Finally death occurs.
Life spans vary considerably from one type of organism to another. Some insects live only a few weeks. A person in Western world can expect to live, on the average, about seventy years. A horse lives about thirty years. Some trees live for a few decades and others for hundreds of years. Some redwood trees in California have lived for several thousand years. Some simple organisms such as bacteria and amoebas appear to have an indefinite life span. In a sense, they live forever, because they reproduce by splitting in two. The offspring repeat this process. Clearly such organisms never die of old age!
Organisms use a great deal of energy in the reproduction of offspring. This also is a characteristic of all living things.
7. Response to Stimuli
All living things are able to respond to certain stimuli or change in their environment (surroundings). A dog comes when you whistle. A fly moves when you try to swat it. A Mimosa (Sensitive) plant folds its leaves in response to darkness, touch, and heat. A plant in a window turns its leaves toward the light. Earthworms seek out moist soil containing decaying vegetation. In all these examples a stimulus¾sound, touch, heat, light, moisture¾causes a response by a living thing.
A living thing’s response to a stimulus is called irritability.
Irritability is valuable to animals in many ways. It helps them obtain food and avoid predators. It is most highly developed in those animals that have nervous systems and keen sense organs such as eyes, nose, and ears.
Plants usually respond slowly to stimuli because they lack sense organs, muscles, and other parts needed for a quick response. However, they usually respond to light by turning their leaves towards it. They also respond to gravity by sending roots downward into the soil. Many homeowners have discovered, to their dismay that Poplar and Willow trees often respond to the presence of water around a home by clogging the water trains with roots.
Even single-celled organisms such as amoebas show irritability. Such organisms respond to touch, light, heat, and other environmental stimuli.
Response to stimuli must be coordinated if they are to be effective. Even simple organisms have many parts and each part must do the right thing at the right time if the proper response is to be carried out. For example, when you call a dog to supper, stimuli will be received by one or more of the eyes, ears, and nose. The responses to these stimuli must be coordinated within the dog before it can respond properly. Some muscles must contract; others must relax; digestive juices must be secreted. A system of nerves and a system of chemical regulators called hormones coordinates these responses in a dog and many other animals. In plants, only hormones are involved in the coordination of responses.
Organisms respond to stimuli by changing their relationship to it. For example, a dog usually comes when you whistle. It changes its location in response to the stimulus. Such responses, which often occur in definite pattern, are called behaviour. Remember that behaviour must begin with the organism. A ball rolling down a hill is not showing behaviour. It is simply being pulled along by the force of gravity. However, a dog that responds to a whistle creates a change in its relationship to its environment. Your whistle does not pull the dog to you. Organisms use a considerable amount of energy as they respond to stimuli within their environments.
8. Variation and Adaptation
Change occurs as a result of a characteristic called variation.
Offspring always differ in some ways from one another and from their parents. These differences are called variations.
- Most variations do not affect an organism’s chances of survival. For example, the fact that your hair is a different colour from your parents will not likely affect your chances of survival.
- However, now and then a variation occurs that does give an organism a better chance of surviving in a changing environment. Suppose that the climate of an area is changing and deeper snow is produced each winter. Clearly a variation that produced longer legs in a deer would increase that deer’s chances of surviving in that area. If this variation is passed on to the offspring of that deer, they, also, would have an increased chance of survival. Gradually the only deer of that type to be found in the deep snow area may be the long-legged types. The others would have moved away or died.
The process by which a certain type of organisms becomes better suited to survive in its environment is called adaptation.
Keep in mind that organisms do not change in order to survive in a changing environment. The deer in our example did not grow long legs because they needed them to survive in the deep snow. Organisms do not change to survive; they survive because they change.
Metabolism is the exchange of matter and energy between an organism and its environment, and the changes that occur in this matter and energy when they are within the organism.
In effect, metabolism is the sum of all the processes occurring in an organism.
- It includes taking in food, or ingestion, as well as taking in water and air.
- It also includes all the changes in food materials that occur within organisms during digestion.
- It includes all changes that occur as the products of digestion are assimilated, or put together, during growth, maintenance, and repair.
- Finally, metabolism includes the release of energy through respiration.
- Finally, it includes the elimination of by-products through excretion.
Metabolism has two distinct phases, anabolism and catabolism.
- Anabolism is a constructive or building-up phase; it includes assimilation, or building of protoplasm from simple compounds and elements that were obtained as a result of ingestion and digestion. It also includes the process of photosynthesis.
- Catabolism is a destructive or breaking-down phase; it involves the release of energy by the breakdown of food materials through respiration.