Biological concepts for intermediate level

Abstract

To be considered as living, an organism must be able to perform all the following functions: Reproduction - all living organisms can produce offspring thereby ensuring survival of the species and continuity of life. Nutrition - all living organisms need food which provides energy and the building blocks for their bodies. Respiration - food must be broken down in a series of chemical reactions in the body to release energy which can be used to perform other life processes. Sensitivity - this is the ability to respond to changes within the body and in the surrounding environment. Movement - animals can move about in search of food, mates, space and conditions which ensure survival while escaping from danger. Plants cannot move about from place to place but can perform slow, often unnoticeable movements. Growth - organisms increase in size using materials obtained from food which they incorporate into their bodies. Excretion - any substances produced during chemical reactions which are harmful or useless are removed from the organism. Finally, all organisms are made up of one or more cells. A cell is like a sack of chemicals which can survive and replicate itself. The smallest living organisms such as bacteria, some protoctists and some fungi are made up of just one cell and hence are described as being unicellular. Larger organisms are composed of more than one cell (multicellular) which can become modified and specialised to carry out different functions. All cells arise from pre-existing cells. These three principles make up the modern cell theory.