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section 28.1 Levels of Organization Key Concept The human body has five levels of organization. Humans, like plants and other animals, are multicellular organisms. This means our bodies are made up of many cells. However, these cells are not all the same. Human bodies include many different types of cells. Each type of cell has a specialized function*. For example, red blood cells deliver oxygen to all the parts of the body. Muscle cells contract to make it possible for the body to move. Neurons transmit and receive messages from other neurons. The body needs all these types of cells to survive. Specialized cells develop from a single zygote. Before a human being develops, an egg and sperm unite to form a zygote. A zygote is a fertilized egg. It is made of just one cell. Then the zygote begins to divide, and the cells that it forms also divide. This process continues for a few weeks. The cells that form during this time are called embryonic stem cells. At this point these cells are all the same. Every embryonic stem cell contains all the information it needs to become any kind of specialized cell in the body. sperm cell (1n) egg cell (1n) Fertilization zygote (2n) Division Embryonic stem cells all look alike. Later they can become blood cells, muscle cells, neurons, skin cells, or any other kind of specialized cell. Determination A few weeks after the zygote forms, a process called determination begins. In this process, each stem cell commits* to becoming one type of cell, such as a muscle cell or a neuron. Each stem cell still contains all the genetic information it would need to become any type of specialized cell. However, the stem cells no longer use all that information. Now, they will become only one type of specialized cell. For example, a cell that is going to become a neuron cannot become a blood cell later. * Academic Vocabulary function action, role commit to follow a course of action 460 McDougal Littell Biology Differentiation Once a cell has committed to becoming a particular specialized cell, it must develop into that cell. This process is called differentiation, because it is the process in which cells develop their specialized shapes and functions. The diagram on the right shows some cells that have gone through differentiation. sperm cells bone cells Different types of cells need different structures. Sperm cells need whiplike tails to help them swim. Other cells, such as those in the stomach, skin, and bones, do not need to move. zygote epithelial cells connective cells What is the difference between determination and differentiation? Specialized cells function together in tissues, organs, organ systems, and the whole organism. If you think about your body, you cannot help but notice that there is more to you than individual cells. You also have tissues and organs. In fact, there are five levels of organization in a human body. 1.Cells Each type of specialized cell has its own job to do. Blood cells carry oxygen to all the parts of the body. Some lung cells make mucus that traps particles and protects the lungs from bacteria and viruses. 2.Tissues Groups of cells working together are called tissues. There are four types of tissues: epithelial (skin and lining of many organs); connective (ligaments and tendons); muscle; and nervous tissues. Your digestive system is an organ system. It includes your mouth, esophagus, stomach and intestines. mouth esophagus stomach intestines Interactive Reader 461 3.Organs Different types of tissue that work together form an organ. Your heart, lungs, liver, and stomach are only a few of the organs in your body. 4.Organ systems Two or more organs working together form an organ system. For example, one organ system allows you to breathe. This organ system includes your lungs, sinuses, nasal passages, pharynx, and larynx. 5.Organism Together, all of the organ systems make up an organism. You are an organism. Which of the following is a cell, a tissue, an organ, or an organ system? sperm, heart and blood vessels, liver, lining of the stomach 28.1 Vocabulary Check determination differentiation tissue organ organ system Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. Fill in the blanks with the correct term from the list above. 1. A(An) work together. is made up of different types of tissue that 2. The process through which embryonic stem cells commit to becoming only one type of specialized cell is called . 3. Groups of cells that work together to perform a specialized function . are called 4. is the process by which committed cells develop their specialized shapes and functions. 5. Together, your lungs, nasal passages, larynx, pharynx, and sinuses form a(an) . 28.1 The Big Picture 6. Put the following five terms in order from simplest to most complex: tissue, organ system, organ, cell, organism. 7. How do specialized cells differ from each other? 462 McDougal Littell Biology