Cell and Tissues

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History of the cell


For a long time, no one knew that plants and animals were made of. Then, in the seventeenth century, the microscope was invented. Scientists used the microscope to study plants and animals. When sections of living things were magnified under the microscope, tiny structures could be seen for the first time-structures that human eye was never able to see. In 1665, a scientist named Robert Hooke looked at pieces of cork under his microscope. He saw that the cork was made of very tiny boxes which he called cells. Other scientists believed that if cork were made of tiny cells, other living things might be made of cells, too. By using their microscopes, they found that every living plant and animal they examined was made of cells. As microscopes were improved, scientists were able to see smaller and smaller organisms. They found that no matter how large or small the organism was, it was made of cells. This led to the cell theory. According to this theory, all living things are made of cells, and all life activities of each organism take place in these cells. Today, very powerful microscopes are used to learn even more about the cell. The cell has proven to be very complex.


What is a cell?


Your body is full of cells. Every part of every living individual is developed from cells. No two individuals are alike. Most cells are so tiny; they can be seen only under a microscope. Some cells are so small, you could put thousands of them on the period at the end of this sentence. Different kinds of cells have different shapes. Some cells are round. Some are oval. Some are shaped like spiral rods. And some are long and skinny. Certain kinds of cells group together to form a tissue. A tissue is a group of the same kind of cells that work together to do a certain job. Tissues group together to form an organ. An organ is made of groups of the same kind of tissues that work together to do a certain job. The different kinds of cells each do a special job. Some living things, such as bacteria, have only one cell. Larger organisms, like people, are made of billions of cells. You can think of a cell as the basic unit of life. All cells come from other cells. Life is passed on from one living thing to another by reproduction. If you cut an orange in half, the fibers that you see are cells. When the orange was cut, the cell walls were broken, and so the juice ran out. You probably didn’t even realize that you were dealing with cells!





What is a cell made of?


Your cells are covered by a thin membrane, or layer of tissue. This cell membrane holds all the cell material inside it. The membrane also lets food, oxygen, water, and wastes flow in and out of the cell. Inside of each cell is a jellylike substance called cytoplasm. Cytoplasm contains chemicals that break up food taken into the cell. It produces building materials and energy for the cell to carry on its life activities. In the middle of the cytoplasm lies a sphere, or ball-shaped structure, surrounded by another membrane. This sphere is called the cell’s nucleus. It controls how the cell gets rid of wastes. It controls how the cell reproduces. Without the nucleus, the cell would die. The nucleus also contains the materials of heredity called chromosomes. Chromosomes determine the sex of an individual. Chromosomes contain genes that determine the color of the eyes and hair, the physical appearance, and even some talents. Through heredity, chromosomes pass on to an individual some of the traits of parents.


What is protoplasm?


The cell membrane, the cytoplasm, and the nucleus make up the protoplasm of the cell. Protoplasm is a chemical substance that regulates all the cell’s activities. Protoplasm takes in food and oxygen. It changes food into substances that the cell can use. It gets rid of cells wastes and causes the cell to produce itself.


What is a tissue?


Cells work together. They are organized into groups. Each group consists of special kinds of cells that do a particular job. Such a group of cells is called a tissue.


What are the main kinds of tissue?


Your body contains five main kinds of tissues. For example, tissues that hold muscles together and keep organs in place are called connective tissue. Fat tissue, cartilage, and bone are examples of connective tissue. Fat makes up about 18 to 8 percent of your body weight. It pads your body, supplies it with energy, and helps you keep a normal body temperature. Another kind of tissue is epithelial tissue. The outer layer of skin and the inside lining of the nose, mouth, throat, stomach, and blood vessels are all made of epithelial tissue. A third kind of tissue is muscle tissue. It enables the body and parts inside the body to move. One type of muscle tissue is called skeletal muscle. It is attached to the bones by ligaments and tendons. Another type is called smooth tissue. It controls the movement of parts inside the body. Stomach walls are made of smooth muscle tissue. A third kind of muscle tissue is cardiac muscle tissue. It is found in the heart wall. The fourth kind of tissue is blood tissue. Blood tissue is 55 percent liquid and 45 percent solid. It supplies cells with food and oxygen and materials needed for repair. It removes wastes from the cells. It also contains cells that fight disease. The fifth kind of tissue is nerve tissue. Nerve tissue sends messages between the brain and the body to make all parts of the body work together smoothly.


What is an organ?


Different tissues are organized into groups that work together to do a particular job. These groups of tissues are known as organs. One example of an organ is the ear. The ear is an organ of hearing. The different parts of the ear are made of certain types of tissue. Each tissue does a particular job. All the tissues work together so that the ear can perform its job of hearing. Different examples of organs are the eyes, heart, brain, lungs, and even skin. All the organs have different kinds of tissues that work together so that the organs can work properly. With all the cells organized in this way, they can efficiently carry on the body’s activities.


What is a system?


Organs are organized into systems. Each system of your body does a particular job. Each organ in each system depends on other organs for the body to function properly. For example, to perform the job of digesting food, the digestive system needs the mouth, teeth, tongue, throat, stomach, intestines, and glands.


How do the systems work together?


Each system in the body depends on other systems to help it to do its work. For example, the circulatory system sends blood to all parts of the body. This system relies on the work of the blood, the heart, and the lungs. The circulatory system depends on the respiratory system to send oxygen into the blood and remove waste material such as carbon dioxide. The circulatory system also depends on the digestive system to break down food so that the blood can carry it to the body cells. These different systems work together to get food and oxygen to all the cells. The cells are then provided with energy. This energy enables your body’s system of muscles to do its job of moving body parts. It allows the nervous system to do its job of sending and receiving messages. Now you can see how the different systems depend on each other so that your body will do all the things it’s supposed to do!





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