1b Cells, Tissue, Organs, and Organ Systems ------> living thing Cells, tissues, organs, organ systems, and eventually the organism are the layers of organization in the human body. The cell is the smallest organizational unit. Tissue is the next biggest unit, followed by organs and finally the organ system. The heart is an example of an organ; the immune system is a collection of organs that protect the body from infection; the brain is made up of organs such as the brain and spinal cord.
2a Atoms, Molecules, Elements, and Compounds ------> matter Atomism was the belief that everything that exists is made up of indivisible particles called atoms. Atomists believed that all forms of matter (solid, liquid, gas) were composed of identical particles called atoms. In ancient Greece, atomism was popular among philosophers, scientists, and laypeople who wanted a simple explanation for all phenomena. For example, Aristotle's theory of phyiscs was based on his understanding of the world as made up of small, indestructible parts called atoms. Atomism later found supporters among scholars in Europe studying mathematics and science before Newton came along. They argued that since gravity, optics, and chemistry could be explained without referring to any form of life, life must be made up of atoms too.
As a result, it is possible to conclude that the lowest level of biological structure is a cell. Cells are the building blocks of organisms. Cells are the fundamental units of life that can exist on their own. They are the smallest independent organisms.
The biosphere, which covers all other levels of organization for living things, is the greatest degree of organization for living things. Organelles, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystems, and the biosphere are the biological stages of structure of living things, organized from the simplest to the most complex. An organism's environment can play a role in determining its level of organization. For example, if an organism is living in a habitat that does not contain nutrients necessary for survival, then it will be reduced to the lowest possible level of organization, which in this case is the cellular level.
Biology classes often focus on the evolution of structures at these different levels of organization. For example, biologists study the anatomy of organisms from the molecular level down to individuals. They also study how organisms interact with each other at the population and community levels. Biologists sometimes refer to these studies as "organismal biology."
Finally, they study the ecology of organisms by looking at populations of organisms over time and across space. Biologists often refer to studies like this one that look at organisms as parts of an ecosystem over time and across multiple scales as "evo-devo" studies.
Ecosystems provide the physical conditions required for life to exist. They consist of plants, animals, minerals, and water, for example. Ecosystems are usually very large compared with organisms; thus, they include many more organisms than any single species.
It is easier to think of the body's architecture in terms of fundamental levels of organization that rise in complexity, such as molecules, cells, tissues, organs, organ systems, and organisms (from smallest to biggest). Each level has its unique set of properties and functions. Cells contain the genetic material necessary to produce new cells when needed. Tissues consist of many similar cells working together. Organs are composed of multiple tissues that perform specific tasks. Organ systems include all the organs involved in a particular function: the nervous system, the immune system, and the circulatory system are all part of the biological defense system. Animals are organized into groups called "classes" based on physical similarities between members of the group. There are five major classes of animals: protostomes, which include mollusks and insects; deuterostomes, which include fish, reptiles, and birds; cnidarians, which include jellyfish; mesozoans, which include mice, lizards, and snakes; and priapulids, which include porcupines. Within these broad categories, there are many different types of animals.
Animals are also divided up by size. Small animals tend to be less evolved biologically and have simpler lives than large ones. However, there are no true limits to an animal's size because bacteria can grow larger than elephants and viruses can infect humans, so the concept is not useful after all this time.