Characteristics of Living Things

Living things are characterized by an increase in both size and complexity.
Development is defined as the specific process by which an organism becomes more complex as it grows.
Feeding is a prerequisite for growth in all organisms.
There are distinct differences in growth patterns across species:
- Plants generally continue growing throughout their entire lives.
- Animals and humans typically stop growing once they reach a certain age.

All living things engage in respiration to obtain the energy necessary for vital activities, including growth, movement, and the replacement of worn-out structures.
Respiration is the process through which energy is released by breaking down nutrients found in food.
There are two main types of respiration based on the presence of oxygen:
- Aerobic Respiration: Requires the presence of oxygen to release energy.
- Anaerobic Respiration: Can take place in the absence of oxygen.
The energy released during these processes is stored in the form of a molecule known as ATP (Adenosine Triphosphate).

All living things possess irritability, also known as sensitivity, which is the ability to detect and respond to changes in their environment.
A stimulus is defined as any change in the environment, whether occurring outside the body or internally.
A response is defined as the direct change in the behavior or position of an organism resulting from a specific stimulus.
Types of stimuli include:
- Chemicals
- Heat
- Light
- Sound
- Touch
Response mechanisms differ between plants and animals:
- Animals generally respond via movement, specifically locomotion.
- Plants generally respond through growth. For example, plant shoots grow toward light sources, while their roots grow toward moisture.

Movement is a fundamental characteristic of all living things and is defined as a change in the position of parts of an organism or the movement of the whole organism.
Locomotion is the specific term for the movement of an entire body from one location to another.
Animals generally utilize locomotion to seek out food or to avoid being eaten by predators.
In higher plants, movement is often restricted either to specific parts (such as the opening and closing of flower petals) or occurs via growth directed by external stimuli.
Continuous internal movement occurs within an organism even if the organism as a whole appears stationary.

Nutrition is the process through which living organisms either obtain or manufacture food to secure energy and facilitate growth.
There are two primary modes of nutrition:
- Heterotrophs (Heterotrophic): These organisms, primarily animals, must obtain "ready-made" food. They intake complex (organic) substances that must be broken down internally.
- Autotrophs (Autotrophic): These organisms, primarily plants, manufacture their own food. They take in simple (inorganic) substances and build them up into complex substances.

Metabolism, or metabolic processes, refers to the collective chemistry of the many processes taking place within an organism.
Metabolic processes are categorized into two types:
- Building up substances (e.g., processes involved in growth).
- Breaking down substances (e.g., respiration).
Excretion is the process by which all living things remove the by-products generated by these metabolic processes.