2.1 Levels of organization
Summary: Do the trees outside look like a euglena? No, not really As well as being much larger, trees are much more complex. Large, complex organisms consist of many cells working together. A tree is an… Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- define a tissue, an organ, and an organ system
- describe the levels of organization in a multicellular organism.
Think back
- What is a unicellular organism?
- What does the nucleus of a cell do?
- Name some specialized animal Cells.
Key idea Multicellular organisms consist of many cells working together.
Key words multicellular organism, tissue, organ, organ system, skeletal system, muscular system, gas exchange system, digestive system, circulatory system
Do the trees outside look like a euglena? No, not really! As well as being much larger, trees are much more complex. Large, complex organisms consist of many cells working together. A tree is an example of a multicellular organism. Multicellular organisms are made up of many cells. They have five levels of organization. You can see the different levels in Figure 1.
Figure 1The levels of organization in a multicellular organism. Full text alternative to the content
What is a cell?
Cells are the building blocks of life. They make up the first level of organization. Nerve, muscle, and red blood cells are examples of animal cells. Root hair and leaf cells are examples of plant cells.
A Identify the multicellular organism from the list: bacteria, amoeba, fish.
What is a tissue?
The second level of organization is a tissue. A tissue is a group of similar cells that work together to perform a certain function.
One example of an animal tissue is nervous tissue. Nerve cells work together to carry messages around the body. Another example of an animal tissue is muscle tissue. Figure 2 shows muscle tissue. An example of a plant tissue is the xylem. These are tubes that carry water around the plant.
Figure 2Muscle tissue is a type of animal tissue. Muscle cells contract (pull together) to make the body move.
What is an organ?
The third level of organization is an organ. An organ is made up of a group of different tissues that work together to perform a certain function. Figure 3 shows examples of organs.
Figure 3The main organs in humans and plants. Full text alternative to the content
B Name two organs that help digest (break down) food.
What is an organ system?
The fourth level of organization is an organ system. An organ system is a group of different organs that work together to perform a certain function. Some examples of animal organ systems include:
- skeletal system (bones) – gives body shape, allows movement, protects organs
- muscular system (muscles) – responsible for movement
- gas exchange system (lungs) – takes in oxygen, removes carbon dioxide
- digestive system (stomach, intestines) – breaks down and takes in food Molecules
- circulatory system (heart, veins, arteries) – transports substances around the body in the blood
Plants contain fewer organ systems. The shoot organ system is made from leaves, stems, fruits, and flowers. The root organ system is made from all the parts that are underground.
C Name five organ systems in humans.
What is a multicellular organism?
The fifth level of organization is a multicellular organism. A multicellular organism is made up of several organ systems. They work together to perform all the processes that are needed to keep the organism alive.
Stretch zone
- Match the level of organization to its function. cell group of organs working together tissue group of tissues working together organ group of similar cells working together organ system group of organ systems working together organism building blocks of life
- Organize the terms into levels. Start with the bottom level. nervous tissuechimpanzeebrainnervous systemnerve cell
Stretch zone
- The circulatory system is an example of an organ system. Describe the function of the circulatory system.Name an organ that forms part of this system.Describe the function of this organ.