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The smallest unit of life capable of independent reproduction. Generally contains nucleic acid, cytoplasm, a cell membrane, and many other proteins and structures.
Using the circulatory system as an example, a cell in this system is a red blood cell, the heart's cardiac muscle is a tissue, an organ is the heart itself, and the organ system is the circulatory system.
An organism is made up of four levels of organization: cells, tissues, organs, and organ systems. These levels reduce complex anatomical structures into groups; this organization makes the components easier to understand.
Level 1: Cells
The first and most basic level of organization is the cellular level. A cell is the basic unit of life and the smallest unit capable of reproduction. While cells vary greatly in their structure and function based on the type of organism, all cells have a few things in common. Cells are made up of organic molecules, contain nucleic acids (such as DNA and RNA), are filled with fluid called cytoplasm, and have a membrane made out of lipids. Cells also contain many structures within the cytoplasm called organelles, which perform various cellular functions.
Cells may be prokaryotic (without a nucleus) in bacteria and archaea (single-celled organisms), or eukaryotic (with nucleus-enclosing DNA) in plants, animals, protists, and fungi. In humans, most cells combine to form tissues, but some cells are found independent of solid tissues and have their own functions. A red blood cell found circulating in the bloodstream carrying oxygen throughout the human body is an example of an independent cell.
Level 2: Tissues
Tissues are a group of similar cells of the same origin that carry out a specific function together. Humans have four different types of basic tissues. Connective tissues such as bone tissue are made up of fibrous cells and give shape and structure to organs. Muscle tissue is made up of cells that can contract together and allow animals to move. Epithelial tissues make up the outer layers of organs, such as the skin or the outer layer of the stomach. Nervous tissue is made of specialized cells that transmit information through electrochemical impulses, such as the tissue of nerves, the spinal cord, and the brain.
Level 3: Organs
An organ is a structure made up of different tissues that perform specific bodily functions. Most organs contain tissues such as parenchyma (used to perform the organ functions), stroma (connective tissue specific to organs) and epithelial. Organs may be solid or hollow, and vary considerably in size and complexity. The heart, lungs, and brain are all examples of organs.
Level 4: Organ Systems
An organ system is a collection of organs that that work together to perform a similar function. There are eleven different organ systems in the human body, each with its own specific functions. One example is digestive system, which is made up of many organs that work together to digest and absorb nutrients from food. While most organ systems control a few specific physiological processes, some processes are more complex and require multiple organ systems to work together. For example, blood pressure is controlled by a combination of the renal system (kidneys), the circulatory system, and the nervous system.