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Lesson Video: Tissues and Organs Biology

In this video, we will learn how to describe the organisation of multicellular organisms in terms of cells, tissues, organs, and organ systems.

16:22

Video Transcript

In this video, we will explore the levels of biological organization from cell to organism. We’ll investigate some examples. Then we’ll answer some practice questions together. And finally, we’ll summarize what we’ve learned. So, let’s get started.

One of the core concepts of biology is this idea of hierarchical organization. The word hierarchical means based on rank. And in biology, we tend to rank things based on size and complexity. For example, you may already know that a cell is made of several organelles. The functional parts of those organelles are composed of macromolecules. Macromolecules are made from more simple molecules. And molecules are composed of atoms. Even atoms consist of smaller parts that you might’ve learned about in chemistry. For the purposes of this lesson, we’ll be focusing on the levels of organization from cell to organism.

Let’s begin with the smallest or least complex of these, the cell. You may already know the cell as the basic unit of life. This means that a cell is the smallest thing that can be considered independently alive. The human body contains trillions of individual cells that could be categorized into about 200 different types. Different cells have different shapes and different parts because they have different jobs or perform different functions. Skin cells and other cells whose job it is to line the surfaces of our bodies tend to be thin and flat, while fat cells will be big and round with lots of space for storage. Muscle cells are long and full of special proteins that allow them to shorten or contract.

Nerve cells are also very long, and they’re specialized to transmit signals rapidly from place to place within the body. Mature red blood cells are so simple they don’t even have a nucleus. They have one job which is to transport oxygen from the lungs to the tissues of the body that need it. Each cell has a special shape or form that allows it to do its job or function. But our bodies are made of trillions of cells. How do they organize themselves to do all of the jobs it takes to keep us alive? Let’s return to our definition of a cell as the basic unit of life.

A unit is not only a thing that can be considered to be complete on its own, but also a building block that can come together to form a larger whole. Cells are the units that make up tissues. Tissues are the units that make up organs. Organs are the units that make up organ systems. And organ systems can be considered the units that come together to make up one multicellular organism. So next, let’s take a closer look at tissues. What exactly is a tissue? A tissue is a group of cells that work together to perform a specific function. There are four types of tissue in the human body: nervous tissue, connective tissue, muscle tissue, and epithelial tissue.

Within the body, nervous tissue handles information processing, communication between body parts, and control of bodily functions. The function of connective tissue is to support, anchor, and connect the different parts of the body. Muscle tissue is responsible for all movement, while epithelial tissue functions as the lining for most parts of the body. Epithelial tissue is also responsible for secretion and absorption. Let’s look at an example. Let’s recall our definition. A tissue is a group of cells that work together to perform a specific function. A tissue you may already be familiar with is blood. Blood is a type of connective tissue.

Blood is made up of red blood cells that carry oxygen, white blood cells that protect the body from infection, and platelets that help your body to form clots to prevent bleeding. All of these are suspended in a liquid known as blood plasma. The main function of blood is transportation. It moves things around the body like oxygen, nutrients, waste, hormones, and more. Blood also serves the purpose of protecting the body from invasion and infection. It also has some regulatory functions such as helping to maintain body temperature and fluid levels.

If you think of the different cells in the body as an alphabet, the tissues are like words. The different types of cells within a tissue work together to perform a specific function the same way that different letters come together to give each word a specific meaning. Next, let’s take a closer look at organs.

An organ is a group of tissues that work together to perform a specific function. Your body contains many, many different organs such as your tongue, both of your eyes, each of your individual teeth, every separate hair on your body, the bones that make up your skeleton, each of your various glands, and so many more. Out of all of the organs in your body, five of them are considered vital or organs that you simply could not live without. These five organs are your brain, your liver, your kidneys, your heart, and your lungs. Let’s look at an example and more closely examine the heart.

Let’s recall our definition that an organ is a group of tissues that work together to perform a specific function. Your heart is one of your five vital organs. Your heart contains all four types of tissues. It’s primarily made of a special type of muscle tissue called cardiac muscle. Cardiac muscle is only found in the heart. It possesses nervous tissue which helps to control the speed of your heartbeat. There are several types of connective tissue found in your heart, the most obvious of which is blood. And your entire heart is lined inside and out with epithelial tissue. All of these tissues work together to perform the heart’s primary function, which is to pump blood to and from the lungs and the tissues of the body.

Returning to our analogy, if cells are like an alphabet and tissues are like words, then organs are like sentences. Tissues work together so that an organ can perform a specific function the same way that words work together to give each sentence one specific meaning. Next, let’s take a look at organ systems. Well, you may have already guessed that an organ system is a group of organs that work together to perform a specific function. The human body has about 11 organ systems. And they work together in closely integrated ways to carry out your life functions, functions like protection, support and movement, control and integration, transport of materials, and reproduction. Let’s take a closer look at your circulatory system.

Let’s recall our definition. An organ system is a group of organs that work together to perform a specific function. This is a highly simplified diagram of your circulatory system. The circulatory system contains one of our vital organs, the heart, which we discussed in more detail a little bit earlier. The circulatory system also contains veins, which carry blood towards the heart, as well as arteries, which carry blood away from the heart, and also the capillaries, which are tiny blood vessels. These are the sites of gas and nutrient exchange between the blood and the tissues of the body.

The circulatory system has many functions. Its primary function is to transport materials throughout the body, materials like gases, hormones, nutrients, and wastes. The circulatory system also helps to fight off infection and to maintain body temperature. Returning to our analogy, if the cells of our body are like letters in an alphabet and the tissues are like words formed from those letters and the organs are like sentences built from those words, then our organ systems are like paragraphs. Organs work together within an organ system to perform a specific function the way that written sentences come together into a complete paragraph to convey one idea.

Finally, let’s take a look at this term, “organism.” An organism is what we call one living thing. Some organisms are made of just one cell, organisms like bacteria, yeast, and most species of protists. In these organisms, just one cell carries out all the functions necessary for life. We refer to these organisms as unicellular, uni- meaning one. Organisms like you, me, and other animals, all plants, and most species of fungi have many cells which are arranged into issues which are arranged into organs which are arranged into organ systems. These are known as multicellular organisms, multi- meaning many.

Returning one last time to our alphabet analogy, if cells are like letters and tissues are like words, organs are like sentences and organ systems are like paragraphs, then a multicellular organism is like an entire story. Organ systems come together to carry out all the essential functions of life within an organism the way that paragraphs come together to convey the meaning within a story. Now that we’re familiar with the levels of organization and we’ve looked at examples of cells, tissues, organs, organ systems, and organisms, we’re ready to try some practice questions.

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.

This question is asking us to place these four key terms, tissues, cells, organ systems, and organs, in order from smallest to largest. The four terms in question represent four of the levels of biological organization. We’re being asked to rank them from smallest to largest or from least complex to most complex. In order to rank these terms, we’ll use this chart and this definition. A blank is a group of blank that work together to perform a specific function.

The only term that doesn’t fit this definition is the cell, which we know is the basic unit of life. And since that means it’s the smallest thing that can be considered to be independently alive, we know that the term cell goes in the smallest spot on our chart. Now, we have three more terms to place. Next, let’s try to place the term tissues. A tissue is a group of cells that work together to perform a specific function. So, we know where to place tissue in our levels of biological organization chart, directly above cells.

Next, let’s try organ systems. An organ system is a group of organs that work together to perform a specific function. Since we didn’t say it’s a group of tissues, we’ll go ahead and skip a space and put it at the top of our levels of biological organization chart. Let’s go ahead and define our last term organs. An organ is a group of tissues that work together to perform a specific function. Since we know that tissues make organs and organs make organ systems, we know exactly where to place the term organ in our levels of biological organization chart.

Using the definition of each term and this handy organizational chart, we’re ready to answer our question. The order of these structures from smallest to largest is cells, tissues, organs, organ systems.

Let’s try another question.

What organ system are the organs in the picture key components of?

The question is telling us that this diagram is of organs. And it’s asking us which organ system these organs belong to. So our first step is to identify the organs in the picture. The organs pictured are the lungs. The human body has about 11 organ systems, and we need to figure out which of these 11 your lungs belong to. Well, we know that our lungs allow us to breathe in oxygen and to breathe out carbon dioxide. This process is commonly known as breathing or, more technically, as respiration. And here’s our most important clue. Since our lungs play a key role in respiration, we can conclude that they’re key components of the respiratory organ system.

So, returning to our question, “What organ system are the organs in the picture key components of?”, the lungs in the picture are key components of the respiratory system.

Let’s take a moment and review what we’ve learned in this lesson. In this video, we’ve learned that cells make up tissues, tissues make up organs, and organs make up organ systems. We can call this hierarchical ranking from smallest to largest or from most simple to most complex the levels of biological organization. We also learned that there are four types of tissues in the human body. We reviewed the five vital organs. And we learned that there are about 11 different organ systems that carry out our life processes.

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