You are a living organism, but so is every other animal and plant on Earth. An organism is a living system that can respond to stimuli, grow, reproduce, and maintain a consistent state (homeostasis). Or at least that's our current definition of a living organism; there is definitely room for debate. But we're pretty sure we know a living organism when we see it in most cases. Living organisms include animals, plants, fungi, and microorganisms.
But how are living organisms structured? What makes them up? Living organisms have lots of parts and those parts combine together to form a whole that works seemingly as one. It's an amazing system, especially in the case of plants and animals. Plants and animals are structured into cells, tissues, organs, and organ systems. Those organ systems together form the whole organism.
Cells are the basic unit of life - they are the smallest functional units of an organism and are microscopic objects which contain cytoplasm and a nucleus surrounded by a cell membrane. Microscopic organisms are often just a single cell: in that case, that's the whole organism. But humans have trillions of cells.
Tissues are groups of cells of the same type or from the same place that accomplish a particular task. For example, humans have muscle tissue, connective tissue, nervous tissue and others.
Organs are part of an organism that's usually self-contained and has a specific purpose or function. For example, the human heart is an organ with the function of pumping blood around the body.
An organ system is a group of organs that collectively do a particular job or function. For example, the digestive system contains the stomach, esophagus, small intestine, large intestine, pancreas, liver, gallbladder, rectum, and anus. An organ can be part of multiple organ systems.
So that's how organisms are organized. But how those individual parts structured? And how are the functions of organisms achieved?
Structure and Function
