Characteristics of Living Things

Making up a list of the characteristic of living things is not as easy as it might sound. In fact, scientists have argued for centuries over the basic characteristic that separate life and non life becomes blurred when we consider whether or not viruses are living things.

Despite these arguments, there do seem to be some generally accepted characteristics common to all living things. We can state with some confidence that all living thing.

• Are made up of one or more units called cells
•  Reproduce
•  Grow and develop 
• Obtain and use energy 
• Respond to their environment 

It will help in our understanding of living things to consider each of these characteristic in detail.

1. Living Things Are Made Up of Cells

Living things are made up of small self-contained units called cell. Each cell is collection of living matter enclosed by a barrier that separates the cell from its surroundings. Most cells can perform all the functions we associate with life. Cells are remarkably diverse. A single cell by itself can from an entire living organism. Organism consisting of only asingle cell are called unicellular . Most of the organism you are familiar with, such as dogs and trees, are multicellular. (The Latin prefix multi- means many, so multicellular means many-celled). Multicellular organism contain hundreds, thousands, even trillions of cells or more. Cells are never formed by nonliving things. They are found in nonliving matter only if that matter was once alive. Wood, for example, is made up largely of the walls that separated individual cells in the living tree.

2.  Living Things Reproduce

   Living things can reproduce, or produce new organisms of the same type. Because all individual organisms eventually die, reproduction is necessary if a group of similar organisms (what we will later call a species) is to survive. There are two basic kinds of reproduction:

a.  Sexual reproduction

       Sexual reproduction requires that two cells from different individual unite to produce the first cell of a new organism. Most familiar organism from maple trees to birds and bees reproduction sexually

b.  Asexual reproduction

       A single organism can reproduce without the aid of another. (The prefix a- means without, so asexual means without sex) Asexual reproduction can be very simple: Some single-celled organism merely divided in two to form two organisms.

3.  Living Things Grow and Develop

All living things, at one stage or another in their lives, are capable of growth. An acorn, when it sprouts, produces roots, stems, a trunk, and leaves that continue to grow for years. As it grows, the plant takes in substance from the air and soil and transforms these substances into living tissue. And long after the tree stops getting larger, it continues to add new material to replace existing parts that wear out.

A snowball, on he other hands, may seem to “grow” if you roll it over fresh snow. But a snowball grows bigger only if someone adds new snow onto it’s surface. A snowball won’t grow bigger by just sitting there. And it certainly cannot change liquid water or solid ice into new snow from which it can grow larger.

During growth, most living things go through a cycle of change called development. The single cell that starts an organism’s life divides and changes again and again to form the many and varied cells of an adult organism. You are probably well aware of growth and development since you are now in the midst of one of the most intense spurts of growth and development that you will ever encounter in your life.

As development continues, organism experience a process call againg. During againg, an organism become less efficient at the process of life. The ability to reproduce comes to an end. For virtually all organisms, death is the inevitable and of the life span of every individual. Death, too, is process of change that separates living and non living things.

4. Living Things Obtain and Use Energy

Living things obtain energy from their environment, or their surroundings, and use that energy to grow, develop, and reproduce. All organisms require energy to build the substance that make up their cells. Any process in a living thing that involves putting together, or synthesizing, complex substance from simpler substances is called anabolism.

Plants obtain their energy from sunlight in a process called photosynthesis. Photo- refers to light, and the suffix –synthesis means put together. Thus photosynthesis means put together with light. Animals cannot perform photosynthesis. Animals take in energy in the form of food. Food is broken down during a process called digestion. The final breakdown of complex substances into simpler ones, usually resulting in the release of energy, is called catabolism.

Living things must practice both anabolism and catabolism at the same time, just as a business or a household must take some money in as income and pay some money out as expenses. The total sum of all chemical reactions in the body-the balance of anabolism and catabolism is called metabolism.

5.  Living Things Respond to Their Environment

Living things respond to their environment. Such responses can be rapid, usually through changes in behavior, or slow, usually through change in metabolic process or through growth. Anything in the environment that causes a organism to react is called a stimulus. Organisms react to many stimuli, including light, temperature, odor, sound, gravity, heat, water, and pressure. (Miller, 1993: 8-9)

The ability of living things to react to stimuli is known as irritability. Both plants and animals exhibit irritability and can react to a variety of stimuli. Plants, however, usually respond to stimuli more slowly than animals. Plant leaves and stems, for example, grow toward light and away from the pull of gravity. Plants roots, on the other hand, respond to gravity by growing down into the soil.

In general, living things respond to stimuli in ways that improve their chances for survival. The process by which organisms respond to stimuli in ways that keep conditions in their body suitable for life is called homeostasis. (The prefix homeo- means similar or same. The suffix –stasis means standing or stopping). Homeostasis refers to an organism’s ability to maintain constant or stable conditions that are necessary for life. Just as a thermostat in your home turns on the heat when it gets down to certain temperature, your body has a thermostat that maintains a constant internal temperature. If you get hot too, you sweat and cool off. And if you to replace the water your body has lost.

You might point out that nonliving things also respond to the environment. However, the responses of nonliving things are purely mechanical (like a spring that jumps when compressed and released) and are not related to survival.

The most living thing, growth probably means an increase in size. As most organisms grow from the zygote, they increase not only in volume but in weight, cell number, amount of protoplasm, and complexity. In many studies, we must measure growth. In theory, we could measure any one of the growth features just cloned, but the two principal method of measurement determine increases in either volume or weight.The process by which cells become specialized is called differentiation, and the process of growth and differentiation of individual cells into tissues, organs, and organisms is often call development.