Cells Our world contains both living and non-living things. All living things are made of at least one cell. Cells have important needs and important jobs. They are the building blocks of life.

All Organisms are Made of CellsAll living things, such as plants and animals, are made up of at least one cell. Cells perform the functions that help keep an organism alive. For example, cells help keep conditions inside the organism constant. This can include maintenance, such as keeping the body temperature the same and keeping the amount of water in the body the same. Cells also help organisms make the materials and energy they need to grow, repair themselves, reproduce, and stay alive.

Most cells are alike in some ways. They can perform many of the same basic functions, such as making energy from food. However, different kinds of cells can also have different structures, appearances, and special functions. For example, plant cells and animal cells are not exactly the same.

Animal cell

Plant cell

Small organisms, such as bacteria and protists, are made up of one cell each. This single cell must be able to perform all the life processes the organism needs to survive.

Large, complex organisms, such as plants and animals, are made up of many cells. Within these complex organisms, there are many different kinds of cells that have different jobs. These cells are different sizes and shapes.

Single-Celled & Multi-Celled Organisms All living things contain at least one cell. Some organisms contain many cells.

Cells are necessary for life. Cells have many different parts, and each part has a different set of functions. Some cells act as complete organisms. Other cells are part of multi-celled organisms.

Single-Celled OrganismsSingle-celled, or unicellular, organisms perform all life processes within a single cell. They may live alone or as part of a colony or group. Bacteria and protozoa are two types of tiny single-celled organisms. The image below shows a single protozoan, magnified many times.

Image courtesy Dr. Stan Erlandsen and Dr. Dennis Feely, CDC

There are more single-celled organisms than multi-celled organisms in the world. They are just so small that they cannot be seen without a microscope.

Each single-celled organism must find its own resources, such as food, water, and air. To help with this, many single-celled organisms can move themselves through their environments with special structures. Single-celled organisms can also often reproduce by themselves, although some do reproduce through interaction with other organisms like themselves.

Multi-celled OrganismsMulti-celled, or multicellular, organisms may have many different kinds of cells. Just like different people in a factory have different jobs, these cells are specialized. This means that each type of cell has its own purpose. Cells are different sizes and shapes, depending on the job that kind of cell does. Larger organisms are usually made up of many cells, instead of just one cell. This is because it is more efficient to have specialized cells.

Cells from multi-celled organisms cannot survive on their own for very long. The cells must work together in order to get food, air, and other resources, and to help the organism as a whole to move, grow, and reproduce. For example, in multi-celled organisms, only the surface cells that are in contact with the external environment are able to exchange substances with it. Cells within the organism are too far away from the environment for direct exchange. This is the reason multicellular organisms have developed transport systems made up of many similar cells working together. These transport systems include the circulatory and digestive systems in humans.

Multi-celled organisms may be very small, made up of only a few cells. Or they may be very large, like the human body, which has over 1 trillion cells. Examples of multi-celled organisms include both the butterfly and the much larger person shown below.

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Cell Specialization In multi-celled organisms (living things that contain more than one cell), there are many different types of cells. These different cell types carry out different functions that are necessary in order for the organism to survive. This is known as cell specialization.

Specialized Cells CooperateIn single-celled organisms, all of the tasks needed for the organism to survive can be carried out by the one cell. But in complex, multi-celled organisms like humans, different cells are needed to perform different, specialized jobs for the organism.

Having many different kinds of cells that each specialize in a different process helps multi-celled organisms perform the different functions efficiently and at the same time. All the different kinds of cells working together are cooperating to keep the organism alive. This cooperation is a little like students acting out different parts in the school play. Students in the play are participating in the same overall activity, but each one has a different role.

Examples of Specialized CellsDifferent types of cells may look very different. Their size and shape help them to perform their special roles. The four diagrams below show different types of human cells—nerve cells, a red blood cell, muscle cells, and a fat cell.

Nerve cells—Nerve cells have parts that are very long, which allows them to send messages a long way. The structure of a nerve cell allows the cell to more easily send and receive signals to

and from all parts of the body.

Red blood cells—Red blood cells have simple structures. These cells carry oxygen to all parts of the body as they move in the blood stream. The smooth, round shape of red blood cells allows

the cells to flow smoothly through blood vessels.

Muscle cells—Muscle cells help the body move. These cells have a long, thin shape and structures that allow the cells to shorten or lengthen as they contract or relax.

Fat cells—Fat cells store food until it is needed. These cells have large storage spaces that can

expand to hold large amounts of fat.

A FAT CELL

Other Specialized Cell Types

There are many other types of specialized cells in the human body as well. These include:

bone cells that give the body support white blood cells that fight disease in the body skin cells that protect and cover the body reproductive cells that allow humans to pass inherited traits on to their children

All of the different specialized cells benefit from cooperating to keep the body alive.

Overview of Organ Systems Living systems are made up of small parts that work together to make larger structures. Organ systems are groups of organs that work together to perform a specific task.

In the bodies of most animals, cells, tissues, and organs group together to form organ systems. These systems perform different functions that help meet the basic needs of the animals.

Major Organ SystemsSome of the major organ systems and their functions in humans and other animals include:

The Circulatory System—The circulatory system carries food, gases and wastes throughout the body in the blood.

The Respiratory System—The respiratory system allows the body to take in oxygen and excrete, or get rid of, waste gases.

The Skeletal System—The skeletal system protects the body's organs, provides support for the body, and helps the muscular system move the body.

The Muscular System—The muscular system is made up of stretchy tissue called muscles. Muscles connect to the skeletal system and help the body move.

The Digestive System—The digestive system is responsible for getting food into the body, getting solid wastes out of the body, and breaking food down into usable parts.

The Nervous System—The nervous system helps the body respond to signals and information from both inside and outside the body.

The Circulatory System The circulatory system is made up of the heart, blood, and blood vessels. Types of blood vessels include arteries, capillaries, and veins. The circulatory system is in charge of carrying water, gases, nutrients, and wastes throughout the body.

One job of the circulatory system is to make sure all the body's cells have enough oxygen. To do this, blood that is low in oxygen is collected from the body and carried toward the lungs and heart through blood vessels called veins. At the same time, blood that is rich in oxygen travels from the lungs and heart to the rest of the body through a different kind of blood vessel, known as arteries.

Circulatory System Organs & their Functions Veins—Veins are the larger blood vessels that carry blood back to the heart after its oxygen has

been carried to body tissues. Veins are normally drawn blue in diagrams.

Arteries—Arteries move blood that is rich in oxygen away from the heart. The pressure from the heart's pumping action keeps this blood moving in the right direction. Arteries are normally drawn red in diagrams.

Heart—The heart is made up of four sections, or chambers. Two of them, the right atrium and the right ventricle, push blood returning from the body to the lungs. The other two, the left atrium and the left ventricle, get oxygen-rich blood from the lungs and send it back out to the body. The pulse you can feel in your wrist is the blood hitting the walls of the artery there with each pump of the heart.

Capillaries—Capillaries are tiny blood vessels that carry oxygen to tissues. They can only be seen under a microscope because they are about one-tenth the width of a human hair. Capillaries have thin walls that allow oxygen and nutrients from the blood to move through them to the

tissues. Carbon dioxide and other wastes leave the body cells, cross through the capillary walls, and enter the blood for the trip back to the heart and lungs.

The Respiratory System Air taken into the body travels through the organs of the respiratory system in the order:

nose → trachea → bronchial tubes → lungs → air sacs

The respiratory system provides oxygen for all of the body's cells and removes the carbon dioxide waste from the cells.

Air flows into the body through the nose. From there, it moves throughout the body along the path shown below. While air does not travel through the muscle called the diaphragm, the diaphragm is an important part of the respiratory system.

Respiratory System Organs & their Functions Nose—The nose is lined with mucus and tiny hairs. The hairs trap particles, such as dirt and

bacteria. The mucus adds moisture to the air and also helps trap particles.

Trachea—The trachea, or windpipe, is a tube that allows air to pass to the lungs. It is lined by a mucous membrane that traps particles that were able to travel through the nose. Close to the lungs, the trachea divides into two branches called the bronchial tubes.

Bronchial tubes—Each of the bronchial tubes carries air into one of the lungs. Once inside the lung, the bronchial tubes divide into smaller and smaller tubes that eventually lead to small air sacs.

Air Sacs—The air sacs are lined with a thin layer of skin cells. Gases like oxygen and carbon dioxide move through the walls of the air sacs and into capillaries that surround them. Air sacs are too small to be seen without a microscope. There are about 480 million air sacs in each human lung.

Diaphragm—The diaphragm is a large muscle that sits just below the lungs. When it flexes, it causes the lungs to expand and take in more air. When the diaphragm relaxes, air moves out of the lungs.

The Skeletal System The skeleton is a human body system that is made up of over 200 bones. Every time you walk to school or stand up from your desk, you are using your skeleton.

The bones of the skeleton are important for:

providing support for muscles and other body tissues protecting organs, such as the brain, heart, and lungs allowing movement, such as walking

The video below talks about how the human skeleton works—how it moves, supports the body, provides protection, and how different bones connect together.

Clip provided by Education Clip Library with permission from ITN Source

The Muscular System Muscles work with bones and joints to allow us to move.

The human body has more than 650 tough, elastic pieces of tissue called muscles. Muscles attach to the bones of the skeletal system and allow the body to move.

The muscular system connects to the skeletal system and moves the body.

Muscle MovementThe pull of a muscle occurs when the muscle contracts and become shorter and tighter. Often muscles work in pairs so that one relaxes while the other tightens.

For example, when the muscle in the front of the upper arm tightens or contracts, the muscle in the back of the upper arm relaxes so that the arm can bend at the elbow.

To straighten the arm back out, the muscle at the back of the upper arm tightens or contracts and the muscle at the front of the arm relaxes.

The Digestive System The digestive system is responsible for getting food into and out of the body.

The organs in the digestive system include the mouth, esophagus, stomach, small intestine, and large intestine.

Digestive System Organs & their Functions Mouth & Teeth—In the digestive system, the mouth and teeth change food into a soft, moist

mass through the processes of chewing and adding saliva. This is the first step in the process of food digestion.

Esophagus—The esophagus is a tube-shaped structure that has muscular rings around it. The esophagus transports food from the mouth to the stomach.

Stomach—The stomach is a muscular sac. It releases acid, which breaks down food into smaller parts. The stomach also mixes the food, which helps to break it down.

Small intestine—The small intestine is a long tube-like organ that is lined with muscle and special cells. These cells absorb the nutrients from digested food into the blood. Muscles in the intestine wall help food to move through to the large intestine.

Large intestine—Like the small intestine, the large intestine is tube-shaped and lined with muscle. The cells in the wall of the large intestine absorb water from the partially digested food. They absorb about a gallon and a half of water daily. Also, bacteria in the large intestine help break down some of the remaining undigested materials. The material that cannot be digested moves out of the body as waste.

The Nervous System The nervous system is made up of the brain, the spinal cord, and nerve cells.

When humans use their senses (sight, smell, touch, taste, hearing), the sense organs (eyes, nose, skin, mouth, and ears) send information to the brain using the nervous system's pathways. The brain processes the information and creates a response.

The following video describes how the nervous system works. Click on the video to watch.

Clip provided by Education Clip Library with permission from ITN Source

Nervous System Components & their Functions Brain—The brain is the control center of the body. It controls body functions and helps humans

to respond to conditions and events in the environment.

Spinal cord—The spinal cord carries information to and from the brain. Nerves reach from the spinal cord to the rest of the body. The spinal cord is protected by the bones of the spine.

Nerve cells—Nerve cells have long fibers that reach to different parts of the body. These nerve cells collect information from the environment and send it through the spinal cord to the brain. The brain then processes the information and comes up with the proper response.