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Cell Structure

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In this lesson we will begin examining cells, which are the most basic units of life. Specifically, we will answer the following questions: what are cells, what are some roles that cells play in anatomy, and what is the general structure of a cell? The root cyto- means cell, and the suffix –ology means the study of, so cytology is the study of cells. We study cells in anatomy and physiology because we, like all living organisms, are made of cells, trillions of them, in fact. Cells are the most basic structural and functional units of life. Put another way, life begins at the cell. Some organisms are merely a single cell. These organisms are called unicellular organisms, with uni- meaning one. All others, like us, are multicellular, meaning they are made of multiple cells. In our bodies cells join to form our tissues, such as muscle tissue, bone, and cartilage, and then the tissues, made of cells, organize to form organs, such as our muscles, heart, and kidneys. Our organs, in turn, organize into organ systems, such as the respiratory system, cardiovascular system, and reproductive system. You likely know what these systems do but you may not realize that all the work done within the body is actually being done by the cells and all the trillions of cells that make up our bodies arise from a single cell, the zygote, which forms by the union of an egg and a sperm. All of our cells are what are called eukaryotic cells, which means that they have a nucleus. Eukaryotic cells have three major components: a cell membrane, a nucleus, and the cytoplasm. The cell membrane, also known as the plasma membrane, separates the cell's inside, the intracellular compartment, from the world around it, known as the extracellular compartment. This barrier allows the cell to maintain the proper internal environment to stay alive and do its work. But the cell needs nutrients and it produces wastes, so the cell membrane must allow some substances to pass through. The cell membrane is semipermeable. It is permeable to some substances, allowing them to pass through. But it is not permeable to everything. Here is another view of a cell with more detail. Again, we see the cell membrane, the nucleus, and the cytoplasm. We'll use this representation to explore the cell in more depth. The cell's nucleus, shown in two views here, houses the DNA, organized into diffuse strands called chromatin or condensed rod-like structures called chromosomes. The DNA contains genes. Each gene is basically the instructions for making a specific polypeptide, and polypeptides become functional proteins or unite with others to do so. Thus, your genes determine what proteins your cells make, which determines what your cells, and thus you, can do. The nucleus is enclosed by a double-membraned nuclear envelope and contains a liquid called nucleoplasm. The nucleus also contains a structure called the nucleolus that makes ribosomes, which we'll get to shortly. The space between the cell membrane and the nuclear envelope is filled with the cytoplasm. The cytoplasm is composed of a liquid called cytosol and the cell's functional parts, collectively referred to as organelles. Notice that organelle sounds a bit like a small organ. That's a simple way to think of organelles, they are like mini-organs inside cells, each with a specific job. Let's review some of those. We already mentioned one organelle, the ribosome. Ribosomes are where proteins are built. They may be free in the cytoplasm or attached to the endoplasmic reticulum. The endoplasmic reticulum is a network of membranous channels. Let's take a closer look. Endoplasmic reticulum that is studded with ribosomes is called rough endoplasmic reticulum, or rough ER, and it makes proteins. That's always what ribosomes do. Endoplasmic reticulum that lacks ribosomes is called smooth endoplasmic reticulum, or smooth ER, and it instead makes lipids and carbohydrates. The Golgi apparatus, or Golgi complex, processes items made by the endoplasmic reticulum, packages them in membrane, and releases them out into the cytoplasm of the cell or prepares them to be shipped out of the cell. Mitochondria are the cell's powerhouses. They produce most of the ATP in our cells, and our cells use ATP for the energy they need to do their work. You can think of the mitochondria as being like little energy generators fueling the cell. Lysosomes are the cell's digestive system. These sacs contain digestive enzymes that break down damaged organelles or solid materials that enter the cell. The cytoskeleton is a network of delicate filaments and tubes that form a scaffold inside the cell that helps to maintain its shape and provides attachments for the organelles and ways for them to move around within the cell. The centrisome is made of two cylindrical structures called centrioles, which direct the movement of chromosomes when a cell divides. Some cells have cilia, which are hair-like projections on the surface that move in a wave-like manner, sweeping things over the cell membrane. For example, in your respiratory tract, the cilia sweep a layer of mucus across the cells, sweeping away any debris that might otherwise plug up the air sacs in your lungs. And sperm contain a flagellum, which is a whip- like tail that allows the sperm to swim. Cells in the human body are quite diverse. Sperm are tiny compared to the much larger eggs that they fertilize. Red blood cells are flattened discs that lack nuclei, so that they can carry more oxygen, while some white blood cells hunt for foreign invaders or debris to eat and destroy. Skeletal muscle cells can be quite long and contain proteins that slide across each other to produce body movement. Neurons have long processes that conduct electrical signals for communication. Each cell has its own job, and collectively all the jobs our cells do provide everything that we need to live and to function as human beings.