A CLOSER LOOK AT LIVING THINGS nitrogen oxygen and hydrogen were concen trated in warm pools bathed in the ultraviolet rays of the Sun Out of this mix chemical elements combined in reactions that grew increasingly complex forming organic mol ecules such as proteins and nucleic acids As they combined and recombined these mole cules eventually formed a highly primitive cell capable of reproducing itself Over millions of years the process of natural selection then aided the evolution of single and multicelled organisms from an ancient common ancestor There are seven key functions or pro cesses necessary for life To be categorized as a living thing an organism must be able to do all of these MoVEMENT Living things have the ability to move in some way without outside help The move ment may consist of the flow of material within the organism or external movement of the organism or parts of the organism SENSITIVITY Living things respond to conditions around them For example green plants grow toward 12
THE CHARACTERISTICS OF LIVING THINGS sunshine certain microorganisms shrink into tiny balls when something touches them and human beings blink when light shines into their eyes RESPIRATION All living organisms must be capable of releasing energy stored in food molecules through achemical process known as cellular respiration In aerobic respiration oxygen is taken up and carbon dioxide is given off In single celled organisms the exchange of these gases with the environment occurs across the organism s cellular membrane In multicellular organisms the exchange of the gases with the environment is slightly more complex and usually involves some type of organ specially adapted for this pur pose Large multicellular animals such as birds and mammals must breathe in oxygen IST which travels to the lungs and is transferred to the blood flow of the body s arteries The arterial system carries this fresh oxygen to all the tissues and cells of the body where it is exchanged for carbon dioxide a cell lular waste product that must be carried back to the lungs so that the organism can exhale it 13
A CLOSER LOOK AT LIVING THINGS Different ways of breathing Spider Chicken book lung lung Sponge outflow Fish SC inflow Caterpillar gills OO spiracles Insects breathe through holes in the sides of their bodies Fish and young amphibians have organs called gills to take in oxygen from water Mammals birds reptiles and adult amphibians breathe through lungs Encyclopaedia Britannica Inc 14
THE CHARACTERISTICS OF LIVING THINGS Plants respire too but they do it through openings called stomata which are found on the underside of their leaves Certain types of bacteria and archaea use a type of cellular respiration called anaerobic respi ration in which the role of oxygen is carried out by other substances Anaerobic respi ration may make use of carbon dioxide or nitrate nitrite or sulfate ions and it allows the organism to live in an environment with out oxygen NUTRITION Living things require energy to survive The energy is derived from nutrients or food Green plants algae and certain archaea and bacteria can make food from water and carbon dioxide via photosynthesis Plants FAS called legumes can make proteins by taking up nitrogen provided by bacteria that live in nodules in the plant s roots Animals fungi protozoa and many archaea and bacteria need to get food from an outside source They do this in different ways all of which depend on what physical adaptations the organism has Some animals such as mam mals bite into their food with teeth while certain insects suck up nectar from flowers 15
