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Cell Specialization and Differentiation What is Cell Differentiation? Living organisms can be made of a single cell, such as bacteria and protists, or they can be multicellular like plants, animals, and fungi. Unicellular organisms, like bacteria, are able to perform all life functions within one single cell. They can transport molecules, metabolize nutrients, and reproduce within this one cell. Organisms made of more than one cell need many different types of cells to carry out the same life processes. Each of these special types of cells has a different structure that helps it perform a specific function. Humans have many different types of cells with different jobs, such as blood cells that carry oxygen and nerve cells that transmit signals to all parts of the body. Cell differentiation is the process by which cells become specialized in order to perform different functions. Even multicellular organisms begin as just a single cell. Getting from one single cell to billions of specialized cells that perform different functions is a process that happens with the regulation of DNA and RNA. It can also be influenced by factors in the environment. Roles of DNA and RNA in Cell Differentiation Dexoyribonucleic Acid, or DNA, is the molecule that controls the way cells function. It also holds the key to determine what type of specialized cells will be made. Stem cells are cells that have the ability to become any type of specialized cell in the body. After an egg cell and sperm cell unite to begin forming a new organism, all of the DNA in each cell of that organism will be virtually identical. If every part of the DNA in each cell is the same, then how does it end up with different types of cells? Let’s look more closely at the DNA to find out. DNA is wound tightly into chromosomes. There are different regions of the chromosome code for every different function and cell type. Not all sections of a chromosome are turned on, or expressed, at the same time. Only the regions that are needed to perform a specific function are expressed in each cell. These regions are often depicted as bands or stripes on a drawing of a chromosome. These bands are called genes, and whether or not a gene is expressed determines what type of cell it will become. For example, genes that are expressed, or turned on, in a nerve cell are different from the genes that are expressed in a muscle cell. Both cells have the same DNA but expressing specific genes determines what type of cell it will become. This is known as gene expression. Ribonucleic Acid, or RNA, is involved in making proteins that the cell needs to function. Since RNA translates and transcribes the DNA code, it can also influence cell differentiation. Specialized Plant Cells Now that you know how living organisms get from a single cell to many differentiated cells, let’s discuss some of the specialized cells that are made as a result of this process. In plants, cells are specialized for the role they play in the plant’s survival. Three of the many specialized cell types in plants include the cells of the roots, stems, and leaves. Root Cells Root cells are specialized plant cells that absorb dissolved minerals and water from the ground. Root cells grow in long lengths called root hairs, in order to increase the surface area of the root system. This also helps anchor the plant. Since root cells grow underground, you would not expect to find chloroplasts in these cells since the purpose of chloroplasts is to absorb energy from the sun. The tip of the root also has specialized cells and is called the root cap. These cells discharge a slippery substance that allows them to burrow down into the soil more easily. Cells of Plant Stems Not to be confused with the type of embryonic stem cells that can differentiate, the stem cells of plants are specialized cells that transport water, nutrients, and the products of photosynthesis throughout the plant. They connect the photosynthesis factories in the leaves with the storage site in the roots. Xylem cells transport water from the roots in the upward direction. Phloem cells transport nutrients in both directions to reach all parts of the plant. Some plants have specialized stem cells that form the wood to support the plant. Leaf Cells The cells found in the leaves of plants are highly specialized and there are many different types. Palisade cells are column-shaped and are found near the surface of the leaf. They are full of chloroplasts ready to capture radiant energy from the sun and turn it into chemical energy. Along the bottom of the leaf, guard cells open and close to allow the transfer of gases with the atmosphere as well as the transpiration of water. These cells can help protect the plant from drying out. Specialized Animal Cells As you learned in the previous section about gene expression, animals require many different specialized cells in order to function. Some cells must be able to move, like sperm cells, while other cells need to contract, like muscle cells. Look at the three types of specialized cells below found in many animals, including humans. Red Blood Cells Red blood cells are specialized cells found in animals that transport oxygen throughout the body. Since transport is their primary function, they do not need a nucleus. The lack of a nucleus leaves more space in the cell for hemoglobin, the protein that binds to oxygen. This also gives red blood cells their unique shape. Unlike most cells, red blood cells do not reproduce through mitosis; they are created in the marrow found in the bones. Muscle Cells Muscle cells are long, fibrous cells that have the ability to contract. They allow animals to move and allow their organs to function in circulation as well as digestion. Muscle cells are made up of two special types of proteins called actin and myosin. Because of the actin and myosin, muscle strands are able to slide past each other. This sliding action allows the muscle cell to contract. Since a lot of energy is needed in your muscle cells, you will find more mitochondria than is found in most cells. Epithelial Cells Epithelial cells are the cells that line the outside, as well as the inside, of your body. They are the cells that make up your skin. Skin epithelium is actually located just below the surface of your skin. They are responsible for making new cells. They also produce the pigment melanin, which protects us from the sun’s harmful rays. Epithelium also lines the inside of your digestive tract as well as your respiratory system. They come in many shapes and sizes depending on their function.