Cells to organ systems
Describe organisation in humans and other complex multicellular organisms: cells, tissues, organs, organ systems and the whole organism.
What a learner can do afterwards
- Places cells, tissues, organs, organ systems and the organism in the correct order of organisation
- Gives a specific example of each level, such as cardiac muscle cell, muscle tissue, heart, circulatory system and human
- Explains why specialised cells are needed in a multicellular organism
1 · Read
Some organisms consist of one cell. Humans are multicellular: they have many cells. A cell is the smallest living unit. Cells working together form tissues, such as muscle cells forming muscle tissue. Two or more tissue types work together in an organ. Several organs work together in an organ system. The organism is the whole living thing. This five-level plan describes humans and many other complex organisms.
Cardiac muscle cells form muscle tissue in your heart. This tissue contracts to help pump blood. Other tissue types also help the heart work. The heart and blood vessels are parts of the circulatory system. Skin is another organ, with several tissue types. The brain is an organ in the nervous system, which also includes nerves. Your organ systems work together in you, the organism.
Specialised cells have structures suited to their jobs. Muscle cells contract, nerve cells carry signals, and red blood cells carry oxygen. Different tasks need cells and tissues working together. A tissue can contain more than one cell type: nervous tissue includes nerve cells and support cells.
Cells group into tissues, tissues into organs, organs into organ systems, and organ systems into you.
Three readings and six flashcards written for this dataset using organisation facts studied from OpenStax and Khan Academy. The linked video is Making sense of how life fits together by Bobbi Seleski, from TED-Ed.
More to read
Follow a heart cell
A cardiac muscle cell is a cell specialised for contraction. Many cardiac muscle cells work together as muscle tissue in the heart. Their contractions help pump blood.
The heart also contains other tissue types. An organ combines two or more tissue types that work together. Calling the whole heart muscle tissue would leave out those other tissues.
The heart belongs to the circulatory system. Blood vessels are also parts of that system. Blood moves through them and carries substances such as oxygen and dissolved nutrients. Blood is a tissue, not another organ.
The circulatory system cooperates with other systems in a human. Digestion supplies nutrients that circulation can transport.
The membership path is cardiac muscle cell, muscle tissue, heart, circulatory system, human. Each name refers to a different level.
Cells working together
A tissue contains cells working together on a shared function. The cells need not all be the same type. Nervous tissue includes nerve cells and support cells. Their different roles contribute to the tissue's work.
Several tissue types combine in an organ. Skin is an organ even though it spreads over a large area. Its level depends on how it is organised, not just its size.
Some tissues do not sit inside one solid organ. Blood is a tissue that moves through the circulatory system. This is why the five-level plan should not be read as a rule that every cell must stay inside one organ.
Humans and many other complex multicellular organisms have tissues, organs and systems. A single-celled organism has none of those levels inside it: its one cell is the whole organism.
Which level is it?
To classify a body structure, ask what it contains and what it belongs to. A cell is a living unit. A tissue contains cooperating cells. An organ contains different tissue types. An organ system contains cooperating organs. The organism is the whole living thing.
The brain is an organ. It works with nerves as part of the nervous system. Brain and nervous system are therefore not two names for the same level.
An organelle is different from an organ. A nucleus or mitochondrion is a structure inside a cell. It is not an organ built from tissues.
Try checking a path in reverse. A human contains organ systems. The circulatory system includes the heart. The heart contains muscle tissue, and that tissue contains cardiac muscle cells. Reversing the path changes the reading direction, not the relationships.
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2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.