Despite the fact that these two characteristics are combined in one post they are absolutely different from each other and are considered separately below.
Growth
This is defined as the irreversible increase in the dry mass of an organism. It is brought about by an increase in cell size or number.
Growth results in organisms getting larger over time. It occurs all over the body until adulthood is reached. Animal regeneration is limited and can occur only in a few tissues, for example the skin and liver.
Growth is regulated by hormones. Hormones are chemical messengers, in animals, produced in the endocrine gland and released into the bloodstream to be taken to their site of action. The pituitary gland in humans is responsible for the production of many hormones. Specific to growth the necessary hormones are somatotrophin and thyroid stimulating hormone.
Irritability
This term is sometimes used interchangeably with sensitivity. This is the ability of a cell to respond to stimuli. All living things respond to stimuli. A stimulus is a change in the cell’s environment that can make it change its activity. The change in activity is the response to the stimulus.
The cell responds both internally and external to changes. The external stimuli are attributed to environmental factors, whereas an internal stimuli is change within the cell itself.
The next post in the Characteristics of Living Organisms series will be: Movement and Reproduction.