Characteristics of life

Characteristics of life  

Biology is the study of plants and animals. It is the study of living things as opposed to nonliving things. A summary of the characteristics that distinguish living things from nonliving things will be examined in this chapter.  Detailed treatment will be found in appropriate sections.

Metabolism

 Living things use food to provide energy in order to carry out body activities such as walking, breathing, playing and so forth. Food is also required for growth and repair of worn out cells. The body also needs mineral substances in order to remain healthy. Autotrophic organisms such as bacteria are chemosynthetic deriving their energy from inorganic compounds. Chemosynthesis is a process certain organisms use to produce energy from the oxidization (burning) of chemicals which seep up from the Earth's crust. The organisms that use chemosynthesis, mainly bacteria, manufacture carbohydrates and other organic molecules from the oxidization of sulfates or ammonia.  These organisms  are found around hydrothermal vents on the ocean floor.  

Other autotrophs include green algae and plants. These are photosynthetic , using solar energy to convert organic compounds to energy. The heterotrophs which comprise most of the bacteria, protozoa, fungi and multicellular animals feed on ready-made food substances from plant or animal sources

The major role of the food taken by living things is to provide energy. Once inside the cells, the food substances are broken down with the aid of oxygen from the atmosphere to release energy, carbon dioxide and water. The energy is in form of heat and is used by the body to do work.

Carbon dioxide is a waste product that is removed from the body through the lungs. Water is a major solvent in the body in addition to being a transport medium for food substances, hormones and enzymes.
Some organisms, including the kangaroo rat, depend on metabolic water for their existence because they live in very arid deserts which hardly receive rainfall. 
 Growth
Growth is an increase in the living matter in an organism, resulting from mitotic cell division.  The cell is the most basic structural and functional unit of life. Cells can be broken down into smaller parts like atoms but these are not alive and therefore do not contain characteristics of life.

Some organisms, such as amoeba and bacteria, consist of only one cell. However, the majority of the organisms are larger and more complex. In these multicellular organisms, large numbers of similar cells are organized into different tissues that perform specific roles. Several types of tissues form organs that together perform various tasks as systems.

 Reproduction
Living things give rise to other living things either by sexual or asexual means. Asexual reproduction usually involves the division of an organism into two by what is known as binary fission, resulting into two genetically identical daughter cells. Sexual reproduction, on the other hand, involves the fusion of female and male gametes to produce a zygote that has genetic characteristics of both parents. 

 Reproduction is necessary for the continuation of life. Without reproduction, life would cease to exist.
Response to stimuli

Living things response to the environment around them in several ways. One-celled organisms like the amoebae respond to a stimulus by either moving away from or moving towards the direction of the stimulus. More advanced animals have sensory organs that respond to stimuli. Vertebrates or animals with backbones have a nervous system that acts as the information centre for the animal.

Plants respond to stimuli in a variety of ways.  Some respond by growing towards the direction of the light. This type of response is called phototropism.  In a forest ecosystem, plants compete for light by growing tall and forming extensive canopies so as to trap as much light as possible. One well known plant response is that of mimosa. This plant responds to touch by folding its leaves.

Homeostasis
 An aquatic invertebrate organism maintains the same osmotic environment as that of the surrounding medium. On the other hand, aquatic vertebrate have various ways of regulating the amount of water and salt in their bodies. Mammals and birds maintain a constant body temperature that is independent of the external temperature.

The process by which animals maintain a stable internal environment with only minor fluctuations is called homeostasis. Homeostasis enables animals maintain their metabolic activities at optimum level at all times. Homeostasis is achieved by three important processes – thermoregulation, excretion and osmoregulation. The liver, pancreas, skin, kidneys and lungs are the major organs of homeostasis.

Movement

Movement of the body from one place to another is called locomotion. Some animals show no locomotion but move their bodies by swaying while fixed in one place. These animals are sessile or sedentary like the sponges living in the water. Plants are generally stationary. 

Movement allows the animal to search for food, find mates or territory. It also allows the animal to escape from its enemies. The skeleton provides support for the body, gives the animal its shape and protects the internal organs. It also provides attachment for the muscles, necessary for movement.

          Death

Eventually, living things must die. Death occurs when all the cell functions cease to work. Death may be by accident, through disease or through old age. Death is necessary to check overpopulation so that life is in keeping with the available resources.