In this explainer, we will learn how to describe the organization of multicellular organisms in terms of cells, tissues, organs, and organ systems.
There are trillions of cells in the human body and they fall into about 200 different varieties. How are these cells organized? How do they work together to accomplish a task like absorbing nutrients from our food? One of the key concepts in biology is the concept of hierarchical organization. Hierarchical organization is when the components of something large and complex can be divided into groups based on their common traits, for instance, all being part of an organ system like the digestive system. Components of these groups can be divided into even smaller groups based on more specific traits, and this continues to make many very specialized groups.
Figure 1 demonstrates how this hierarchical organization can be used in organisms to group biological components from a large group (in this case, the whole organism) into smaller, more specific groups (in this case, cells).
The levels of biological organization shown in Figure 1, from largest to smallest, are organism, organ system, organ, tissue, and cell. This means that many cells make up a tissue, several tissues make up an organ, different organs work together in an organ system, and an organism contains more than one organ system.
Example 1: Identifying the Correct Order of Levels of Organization of the Human Body
Put the following structures in the order in which they are organized in the human body from smallest to largest: tissues, cells, organ systems, organs.
Answer
The human body is made of trillions of cells. Cells are the basic unit of life. There are many different types of cells in the human body that have varying structures and serve different, specific functions.
These cells are organized into tissues. A tissue is a group of cells that work together to serve a specific function. A tissue’s function is determined by the types of cells it is made of.
Several types of tissue work together as an organ. Different organs in the body carry out different functions. For example, the heart pumps blood throughout the body, the stomach breaks down food, and the brain controls and coordinates the activities of the body.
A group of organs will work together in organ systems that perform our life functions. For example, the stomach and small intestine work together within the digestive system to break down food and then absorb the nutrients, releasing them into the bloodstream. The heart and blood vessels work together in the circulatory system to move the blood and the nutrients it carries around the body.
Therefore, we can conclude that the order of these structures, from smallest to largest, is cells tissues organs organ systems.
An organism is one living thing. An organism may be made of one cell, in which case they are referred to as a unicellular organism. Some examples of unicellular organisms are bacteria, paramecia, and yeast. Unicellular organisms have all of their essential life processes carried out by a single cell.
Other organisms may be made of many cells, in which case they are referred to as multicellular organisms. In multicellular organisms, different cells that perform different functions will be organized into larger structures. This is in contrast to unicellular organisms, where one cell does it all. Humans are made up of trillions of cells, so we are a good example of multicellular organisms!
Definition: Organism
An organism is one living thing that may be unicellular or multicellular.
A cell is the basic unit of life. Cells contain smaller structures called organelles, which carry out their life functions. However, cells, rather than organelles, are the smallest structures that are considered living as some types of cells are able to live and reproduce independently. Everything we consider to be alive is made of at least one cell.
Definition: Cell
A cell is the smallest biological unit of an organism; all living things are made of one or more cells.
Different cells have different shapes and different parts because they perform different functions. A nerve cell has branches to communicate with other cells. A muscle cell is full of special proteins that allow it to shorten, or contract. Mature red blood cells have cytoplasm that is rich in a special protein called hemoglobin, which gives them their red color and allows them to carry oxygen efficiently. The basic structure of these cells is shown in Figure 2. As you can see, each cell has a particular shape, and this along with special components help it to do its job. The process by which cells change and adapt for a specific role is called specialization.
Definition: Specialization
Specialization is the development of cells and structures they contain that results in the cells being adapted to perform a specific function.
We have mentioned that our bodies have trillions of cells and that those cells are organized into other structures. A group of cells that work together to perform a specific function is called a tissue. In animals, some types of tissues can be considered simple if they are made up of only one layer of cells or can be considered compound if they are composed of more than one layer.
Definition: Tissue
A tissue is a group of specialized cells that work together to carry out specific functions.
There are 4 main types of tissues in the human body: nervous tissue, muscular tissue, epithelial tissue, and connective tissue. These types of tissue and their basic functions are outlined in Table 1.
Table 1: A table describing the four major tissue types in the human body.
| Tissue Type | Description |
|---|---|
| Epithelial tissue | Lining tissue made up of single or multiple layers of cells that cover the external surface of the body and line the internal surfaces (e.g., the digestive tract) |
| Nervous tissue | Communicative tissue composed of cells (neurons) that send and receive electrical impulses and cells that support them |
| Muscular tissue | Motor tissue made up of cells that cause movement of body parts by shortening, or contracting, and relaxing |
| Connective tissue | Connective tissue composed of cells suspended in a matrix that supports, protects, binds, or otherwise connects other structures |
Just like how cells have special parts that allow them to do their job, the job a tissue can do is determined by the types of cells it is made of. Blood is an example of a type of connective tissue. Blood consists of red blood cells, white blood cells, platelets, and plasma. These components and their relative proportions are given in Figure 3.
Red blood cells carry oxygen, white blood cells fight infection, platelets prevent bleeding, and plasma is a medium that transports these cells and other substances like nutrients throughout the body. These all work together to carry out the primary function of your blood, which is to carry nutrients and materials throughout the body, from place to place.
Example 2: Identifying the Type of Cells that Line the Stomach and Small Intestine
What type of specialized cells line the stomach and small intestine?
- Nervous cells
- Epithelial cells
- Connective cells
- Alveoli
- Stem cells
Answer
Different types of cells have different structures that make them adapted to different functions. A group of cells that work together to carry out a particular function are called a tissue. Tissues that line the inside and outside of the structures of our bodies are called “epithelial tissues.” The prefix “epi-” means “upon” or “around.” Epithelial tissue is made of specialized lining cells called “epithelial cells.” These cells form different types of barriers depending on their shapes and structures. They are also usually easily replaceable since, in their role as coverings, they tend to become damaged and worn out quickly.
So, the cells that line the stomach and small intestine are B: epithelial cells.
A group of tissues that work together to perform a specific function is called an organ. There are many, many different organs in the human body. Some are familiar, such as your heart, brain, liver, lungs, and kidneys—as seen in Figure 4. Others are less well known. Every hair follicle and sweat gland in your skin can be considered an organ, and each tooth is an organ. Also, each blood vessel, bone, and nerve in our bodies are considered individual organs!
The organs of our bodies are usually made of more than one tissue type. The specific tissues, their quantities, and their arrangements are what gives each organ the tools it needs to carry out a specific function. Both our stomachs and our hearts contain muscular tissue, but the muscle of the heart is adapted to beat constantly and automatically, and the muscle of the stomach is adapted to mix and churn the food we eat. Each hair follicle also has a tiny muscle that makes your hair stand up when you are startled or cold!
Definition: Organ
An organ is a group of specialized tissues that work together to perform specific functions.
A group of organs that work together to perform a specific function is called an organ system. Your body contains approximately 11 distinct organ systems. Each organ system performs an important life function. Some examples of human organ systems, their component organs, and their functions are summarized in Table 2.
Table 2: A table of some examples of human organ systems with descriptions of their functions and major organs.
| Organ System | Primary Function | Organs Included |
|---|---|---|
| Digestive system | To absorb nutrients from food | Mouth, stomach, and small intestine |
| Circulatory system | To transport materials like blood cells, nutrients, and hormones throughout the body | Heart and blood vessels |
| Respiratory system | To take in oxygen and expel carbon dioxide from the body | Lungs, diaphragm, and trachea |
| Skeletal system | To support, protect, and help move the body | Bones and joints |
| Muscular system | To move the body | Skeletal muscles |
| Nervous system | To allow rapid communication between the brain and the rest of the body | Brain, spinal cord, nerves, and sensory organs (e.g., eyes, ears, skin) |
| Immune system | To protect the body from infection | Lymph nodes, lymph vessels, and the thymus gland |
The functions of the different organs in an organ system determine the function of the organ system itself. Your skeletal system consists of all of your bones and joints. Bones support the body, and joints allow movement. So, the function of the skeletal system is to act as the framework of our body, providing the support and structure that allows movement.
Definition: Organ System
An organ system is a group of organs that work together to carry out a specific function.
Example 3: Defining the term Organ System
Which of the following best defines an organ system?
- Multiple, similar tissues that work together to carry out a particular function
- A mass of cells that work together to perform multiple, different functions
- A group of the same organs that work together to perform different functions
- A group of different organs that work together to perform a particular function
- A group of different tissues that work together to perform a particular function
Answer
The biological levels of organization in a complex, multicellular organism, from the smallest unit to the largest unit, are cells tissues organs organ systems organism. This means that organisms are composed of several organ systems, which are composed of organs, which are made of tissues, which are made of cells. Each of these structures, from cells to organ systems, are able to carry out a particular function because they possess structures that are adapted to that function. A good example is the digestive system, which allows us to absorb nutrients from our food. The digestive system possesses digestive organs, such as the stomach, that all participate in the process of breaking down food to absorb nutrients and remove indigestible wastes. These organs are made of tissues, like muscular tissue and epithelial tissue, that contribute to the function of the organ.
This tells us that an organ system is a group of different organs that work together to perform a particular function.
Now that we have familiarized ourselves with these levels of organization, let’s take a look at some examples of organ systems within the human body.
Key Term: Digestive System
The digestive system is the organ system concerned with breaking down food to absorb nutrients and removing unnecessary remains as waste.
The main function of the digestive system is to break down the food we eat, absorb the useful nutrients, and get rid of anything our bodies cannot use as waste. The major organs of the digestive system include the mouth, esophagus, stomach, small intestine, and large intestine. These organs and their approximate location in the human body are shown in Figure 5.
Example 4: Identifying the Organ System to Which the Stomach Belongs
What organ system is the stomach a key component of?
Answer
An organ system is a group of organs that work together to perform a particular function. The major organ systems in the human body are the respiratory system, the circulatory system, the digestive system, the reproductive system, the immune system, and the nervous system.
Each of the organs in the organ system contributes to the function of the system it is a part of. For example, the function of the respiratory system is to facilitate gas exchange. The major organs of the respiratory system are the lungs, which are adapted to take in oxygen into the bloodstream and remove carbon dioxide from it. Similarly, the circulatory system functions to move materials throughout the body, so the organs of the circulatory system, such as the heart and blood vessels, all play a role in that function.
The function of the stomach is to break down the food that we eat so that nutrients can be absorbed in the small intestine. The stomach is well adapted to do this, as it has a very acidic pH that helps break down food and a muscular wall that helps churn the food. So, the stomach plays a key role in digestion.
Using this information, we can say that the stomach is a key component of the digestive system.
The main function of the circulatory system is to transport materials from place to place in the body. The major organs of the circulatory system include the heart, veins, arteries, and capillaries. The major blood vessels of the body and the heart are shown in Figure 6.
Key Term: Circulatory System
The circulatory system is the organ system concerned with transporting materials throughout the body within the vast network of blood vessels.
The function of the respiratory system is to take in oxygen that our cells need and to remove carbon dioxide from the body that our cells produce as waste. The major organs of the respiratory system include the mouth and nose, the trachea, and the lungs, which are demonstrated in Figure 7.
Key Term: Respiratory System
The respiratory system is the organ system concerned with taking oxygen into the body from the air and releasing carbon dioxide that our cells produce as waste.
Complex, multicellular organisms, like humans, are made of many cells, which are organized into tissues, which make up organs, which organize into organ systems to perform our life functions. The function of each of these structures relies on the properties of the components it is made of.
Let’s summarize what we have learned in this explainer.
Key Points
- Multicellular organisms are made of cells, which make up tissues, which make up organs, which make up organ systems.
- Some examples of the body’s major organ systems are the circulatory system, the respiratory system, and the digestive system.
- The function of an organ system is determined by the functions of the organs it is composed of in the same way that an organ’s function is determined by the tissues it contains.
