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Introduction

This resource provides you with an introduction to cells.

Everyone's body is made of the same basic stuff. All living things, large or small, plant or animal, are made up of cells. Most living things are made up of one cell and they are called unicellular organisms. Many other living things are made up of a large number of cells that form a larger plant or animal. These living things are known as multicellular organisms. Water makes up about two thirds of the weight of cells.

Cells are very small; most cells can only be seen through a microscope. Cells are the smallest living units that are capable of reproducing themselves. Each cell in your body was made from an already existing cell. All plants and animals are made up of cells. In this article, we will talk about the cells that make up You.

All the parts of your body are made up of cells. There is no such thing as a typical cell. Your body has many different kinds of cells. Though they might look different under a microscope, most cells have chemical and structural features in common. In humans, there are about 200 different types of cells, and within these cells there are about 20 different types of structures or organelles.

All cells have a membrane. Cell membranes are the outer layers that hold the cell together. They let nutrients pass into the cell and waste products pass out. Not everything can pass through a cell membrane. What gets through and what doesn't depends on both the size of the particle trying to get in and the size of the opening in the membrane.

Cells also have a nucleus. This is the cell's control center. Cells continually divide to make more cells for growth and repair in your body. The nucleus contains the information that allows cells to reproduce, or make more cells. Another important part of a cell is the mitochondrion. This is the part of the cell where food and oxygen combine to make energy.

You know that you need air to breathe. It is the oxygen in air that your body really needs. Every cell in your body needs oxygen to help it metabolize (burn) the nutrients released from food for energy. You also know that you need food. Food gives you energy, but oxygen is needed to break down the food into pieces that are small enough for your cells to use This is known as cellular respiration and it is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The energy released is chemically trapped for use by all the energy-consuming activities of the cell. Your cells are the energy converters for your body.

Different cells have different jobs to do. Each cell has a size and shape that is suited to its job. Cells that do the same job combine together to form body tissue, such as muscle, skin, or bone tissue. Groups of different types of cells make up the organs in your body, such as your heart, liver, or lungs. Each organ has its own job to do, but all organs work together to maintain your body. A group of different organs working together to do a job makes up a system. All the systems in your body are like members of a team whose job it is to keep you alive and healthy.

The different types of cells in your body have different, specialized jobs to do. The specialization of cells depends almost always on the exaggeration of properties common to cells. For example, cells that line the intestine have extended cell membranes. This increases the amount of surface area that is available to absorb food. Nerve cells can be very long, which makes them efficient in sending signals from the brain to the rest of your body. Cells in heart muscle process a lot of energy, so they have a large number of mitochondrion, the part of the cells where energy is made.

Like all living things, cells die. The number of cells that an adult male loses per minute is roughly 96 million. Fortunately, in that same minute, about 96 million cells divided, replacing those that died. Just as you shed dead skin cells, dead cells from internal organs pass through and out of the body with waste products. The length of a cell's life can vary. For example, white blood cells live for about thirteen days, cells in the top layer of your skin live about 30 days, red blood cells live for about 120 days, and liver cells live about 18 months.

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This resource provides you with an introduction to cells. Everyone's body is made of the same basic stuff. All living things, large or small, plant or animal, are made up of cells. Most living things are made up of one cell and they are called unicellular organisms. Many other living things are made up of a large number of cells that form a larger plant or animal.

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Oxygen, Carbon Dioxide, and Energy | CK-12 Foundation

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When we breathe, oxygen gets into our bodies. Every day about 2,000 gallons of circulating blood travel through an estimated 60,000 miles of blood vessels which link all the cells and organs of our body.

Oxygen is traveling in that circulating blood. How does it move from our lungs to other places, like the brain?

This video clip provides an animated overview of the process.

All cells need oxygen to live. Freely available in the air around us, oxygen is the essential fuel to help cells stay alive and carry out their jobs.

How does oxygen reach our body's cells? It follows a systematic transportation route which depends on another substance called hemoglobin.

Getting oxygen to the body's cells requires three major events:

Uptaking oxygen from the air to the lungs;

Transporting that oxygen in the blood; and

Delivering the oxygen to cells throughout the body.

How does a person's body uptake oxygen? Blood flows through our bodies via fine capillaries in the walls of our lungs' air sacs. Those air sacs are called alveoli.

The oxygen molecules undergo a change once they are inside the body. They change from gas molecules, which circulate in the air, to dissolving into a solution within the blood's plasma located within the capillaries of the alveoli.

Once those dissolved oxygen molecules are in the solution of the blood, 98% of the dissolved oxygen is taken-up by red blood cells which are passing by. The other 2% of the dissolved oxygen remains in the physical solution.

Red cells are great vehicles for transporting the dissolved oxygen. That's because red blood cells contain a special oxygen-binding protein known as " hemoglobin."

Each hemoglobin molecule contains something else which is important for transporting oxygen: Four molecules of " heme," which is an iron-containing pigment capable of binding oxygen loosely and reversibly. The four heme groups are displayed in this graphic (from Washington University's Department of Chemistry).

When hemoglobin is fully saturated with oxygen, its color is bright red, and it's called "oxyhemoglobin." When hemoglobin is not fully saturated with oxygen, its color is purplish-blue, and it's called "deoxyhemoglobin."

It is the "heme" in hemoglobin which makes it possible for red blood cells, which are passing by, to pick-up oxygen dissolved in the blood and then transport it. Combined with hemoglobin, the dissolved oxygen is then released back into the blood as oxygen in solution.

Now it is ready for delivery to various cells throughout the body.

Hemoglobin gives-up its oxygen as red blood cells travel through capillaries in tissues where there is a low content or partial pressure of oxygen. The partial pressure of oxygen represents the level of dissolved oxygen in plasma.

Oxygen is released in such a way that it can:

Travel to the body's tissues; and

Reach the cells, which need oxygen to survive and thrive.

Hemoglobin, thus, plays a really important role in transporting oxygen from its entry point in the human body to the cells which put the oxygen to work.

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Breathing - How Oxygen Travels in the Body

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When we breathe, oxygen gets into our bodies. Every day about 2,000 gallons of circulating blood travel through an estimated 60,000 miles of blood vessels which link all the cells and organs of our body. Oxygen is traveling in that circulating blood. How does it move from our lungs to other places, like the brain?

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Oxygen is used by many cells to process energy resources, and oxygen-based metabolism is the most efficient, but not all cells require it. A few that live in low-oxygen environments do not use it at all, and it is often toxic to these organisms. Fermentation is an example of cells using energy sources without oxygen, but it is far less efficient than cellular respiration.

Oxygen is actually a dangerous chemical in general, and while the cells that use it benefit from it greatly, they must also have defenses against its toxic effects. Oxygen is useful because it reacts strongly with the carbon and hydrogen in glucose, but these are major components of cellular DNA and protein structures as well, all of which can be attacked and destroyed by oxygen. This is particularly true in the case of free radicals, highly reactive singular oxygen atoms that are constantly produced by cellular respiration.
Learn more about Cells

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Oxygen is used by many cells to process energy resources, and oxygen-based metabolism is the most efficient, but not all cells require it. A few that live in low-oxygen environments do not use it at all, and it is often toxic to these organisms. Fermentation is an example of cells using energy sources without oxygen, but it is far less efficient than cellular respiration.

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(Nice to Know)

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Kids learn about cell mitochondria in the science of biology. These tiny organelles inside cells produce energy for the rest of the cell through respiration.