 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 video, 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 cells 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 13 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.