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Chapter 1 • Lesson 1 Objectives: 1,1.1,1.1.2 Cell Structure Key Words • cell • eukaryote • organelle • prokaryote • cytoplasm • nucleus • nuclear membrane • chromosome • plasmid • plasma membrane • ribosome • mitochondria • cell wall • chloroplast • vacuole Getting the Idea While living organisms share a few common structures and functions, they exhibit an amazing diversity of sizes, shapes, and features. The same is true of cells, the basic units of structure and function in all organisms. Each cell is a complete system that carries out all the processes necessary for life. All cells, both single-celled organisms and cells that are part of larger organisms, have some features in common. However, cells also have a wide range of specialized structures that aid in their survival. Prokaryotic Cells and Eukaryotic Cells The two basic types of cells are prokaryotic cells and eukaryotic cells. Each kind of cell performs similar functions in the same ways. Both kinds of cells are enclosed by an outer structure called a plasma membrane. Both are filled with cytoplasm and contain structures called ribosomes, in which proteins are synthesized. How then do the two kinds of cells differ? You can see some of the differences in the diagram below. One obvious difference is that the eukaryotic cell is more complex than the prokaryotic cell. The two kinds of cells have very different internal structures. For example, the eukaryotic cell has a distinct nucleus and other cell structures, called organelles, that are enclosed within membranes. Organisms whose cells have a distinct nucleus and membrane-bound organelles are called eukaryotes. The cells of eukaryotes are larger than prokaryotic cells. A typical eukaryotic cell ranges in size from about 2 to 100 micrometers. By contrast, most prokaryote cells are only 0.5 to 2 micrometers in size. Organelles are cell structures that are specialized for different functions. Each type of organelle has a structure that is suited to its function. You will learn more about how organelle structure is related to function as you read about the different types of organelles. Many eukaryotic organisms consist of large numbers of cells that work together. Plants, animals, protists, and fungi are all eukaryotes. All plants and animals, and some protists and fungi, are multicellular, or made up of many cells. In eukaryotes, the organelles interact to carry out most of the cell's life processes. Notice that the prokaryotic cell does not have a nucleus or membrane-bound organelles. Organisms whose cells lack a nucleus and membrane-bound organelles are prokaryotes. In prokaryotes, most of the processes of life occur in the cytoplasm. The cytoplasm is the thick, semifluid material that is enclosed by the plasma membrane. In eukaryotes, the cytoplasm contains the nucleus and the organelles that carry out most of the cell's life processes. For example, eukaryotes have specialized organelles that release energy. In prokaryotes, the processes that release energy occur in the cytoplasm or the plasma membrane. All prokaryotes are one-celled organisms—either bacteria or archaea. The Nucleus The largest structure in most eukaryotic cells is the nucleus. The nucleus directs and controls most cellular activities. It is enclosed by a structure called the nuclear membrane, or nuclear envelope. This membrane controls the passage of materials between the nucleus and the cytoplasm. The nucleus contains deoxyribonucleic acid, or DNA. The DNA molecules control protein production and cell functions. DNA also stores the genetic information that is passed from parent to offspring during reproduction. In eukaryotes, DNA is bundled into structures called chromosomes, which are located in the nucleus. You will read more about DNA and chromosomes in Lessons 5 and 20. Prokaryotic cells lack nuclei, but they do contain DNA. Their DNA is found floating in the cytoplasm. In addition to a large, tangled piece of DNA, many prokaryotes have ring-shaped pieces of DNA called plasmids, which are also suspended in the cytoplasm. Structures Common to Most Eukaryotic Cells The organelles common to most eukaryotic cells are shown in the diagram of the animal cell below. Plasma Membrane The plasma membrane is a thin, flexible layer that surrounds the cytoplasm. This membrane supports and protects the cell and gives it shape. The plasma membrane is made up of two layers of lipids, in which protein molecules are embedded. Tiny openings in the plasma membrane enable it to control which materials enter and leave the cell, either from the external environment or from other cells of the same organism. The plasma membrane is also called the cell membrane. You will learn more about the role of the plasma membrane in the movement of materials into and out of cells in the next lesson. Ribosomes Structures called ribosomes are scattered throughout the cytoplasm of a cell. Ribosomes make a variety of proteins that are used throughout the cell. Unlike most organelles, ribosomes are not enclosed by membranes. Many ribosomes float freely within the cytoplasm of both prokaryotes and eukaryotes. In eukaryotes, ribosomes may also be attached to the endoplasmic reticulum (ER). The endoplasmic reticulum is a network of membranes and sacs that transports molecules from one part of the cell to another. Mitochondria Mitochondria (singular mitochondrion) are the organelles that carry out cellular respiration. Cellular respiration is the process by which living things obtain energy from food. Mitochondria have folded inner membranes that increase the surface area available for the release of energy. Cells that need a lot of energy, such as muscle cells, have many more mitochondria than cells with lower energy requirements. Other Organelles Two other organelles common to many eukaryotes are the Golgi apparatus (also called Golgi bodies) and lysosomes. The Golgi apparatus is a system of membranes that modifies proteins and lipids according to where they will be used. Lysosomes are small, spherical organelles that break down old organelles and carry out digestion in the cell. Lysosomes are common in the cells of animals and fungi, but they are rare in plant cells. Recall that individual cells carry out all of the functions required for life. These functions include energy production and use, the synthesis and transport of molecules, and the removal of wastes. Cell organelles must interact to carry out all of these functions. For example, the DMA in the nucleus directs the ribosomes to assemble proteins that are used as enzymes in other parts of the cell. Mitochondria use enzymes made by the ribosomes to generate the energy that is used by other organelles. See “Wacky History of Cell Theory” Structures Found in Some Eukaryotic and Prokaryotic Cells The organelles discussed so far are common to most eukaryotic cells. They are found in animals, plants, protists, and fungi. The cells of some eukaryotes, particularly plants, have structures that are not found in other types of eukaryotic cells. Many of these structures are shown in the diagram of the plant cell below. Cell Wall The plant cell shown is surrounded by a cell wall. A cell wall is a rigid structure that surrounds the plasma membrane of some cells, giving them additional protection and support. Cell walls are thicker than plasma membranes but also have openings that enable the cell to control the passage of materials into and out of the cell. The cells of all prokaryotes, fungi, and plants have cell walls. Some protists also have cell walls. The cell walls of different kinds of organisms are composed of different materials. Plant cell walls are made of cellulose, a substance made up of sugars. The cell walls of fungi contain chitin. Chitin is made from sugars modified by the addition of nitrogen. Bacteria have cells walls made up mostly of peptidoglycan. This molecule is made up of sugar and protein. The cell walls of archaea do not contain peptidoglycan. Instead, they are composed of a variety of other molecules. Chloroplasts Plant cells contain chloroplasts. Chloroplasts are organelles that capture the energy of sunlight and use it for photosynthesis. Photosynthesis is a process in which energy from sunlight is used to make sugar from water and carbon dioxide. A green pigment called chlorophyll captures the energy from sunlight. Some protists, including algae, also have chloroplasts. Some bacteria carry out photosynthesis, but they do not have chloroplasts. In these bacteria, chlorophyll is scattered through the cytoplasm. Animals and fungi do not have chloroplasts or carry out photosynthesis. Vacuoles Plant cells have a large central vacuole, an organelle that stores water and other important materials, including salts, proteins, and carbohydrates. In plants, pressure from the liquidfilled vacuole also helps support heavy structures such as leaves and flowers. Many animal cells have small vacuoles, which store substances and transport them within the cell. The cells of some protists also contain vacuoles that may store either useful materials or wastes. Discussion Question Which cell structures are common to prokaryotic cells, plant cells, and animal cells? Why do you think these features are found in all types of cells? Comparing Structures Found in Different Kinds of Cells The table below summarizes the functions of the main components of prokaryotic cells and two types of eukaryotic cells. Internet Search Find three photograph of eukaryotic cell under the microscope: Internet Search Find three photograph of eukaryotic cell under the microscope: Cell City Analogy In a far away city called Grant City, the main export and production product is the steel widget. Everyone in the town has something to do with steel widget making and the entire town is designed to build and export widgets. The town hall has the instructions for widget making, widgets come in all shapes and sizes and any citizen of Grant can get the instructions and begin making their own widgets. Widgets are generally produced in small shops around the city, these small shops can be built by the carpenter's union (whose headquarters are in town hall). After the widget is constructed, they are placed on special carts which can deliver the widget anywhere in the city. In order for a widget to be exported, the carts take the widget to the postal office, where the widgets are packaged and labeled for export. Sometimes widgets don't turn out right, and the "rejects" are sent to the scrap yard where they are broken down for parts or destroyed altogether. The town powers the widget shops and carts from a hydraulic dam that is in the city. The entire city is enclosed by a large wooden fence, only the postal trucks (and citizens with proper passports) are allowed outside the city. Match the parts of the city (underlined) with the parts of the cell. 1. Mitochondria ________________________________________ 2. Ribosomes ________________________________________ 3. Nucleus ________________________________________ 4. Endoplasmic Reticulum ________________________________________ 5. Golgi Apparatus ________________________________________ 6. Protein ________________________________________ 7. Cell Membrane ________________________________________ 8. Lysosomes ________________________________________ 9. Nucleolus ________________________________________ Using the “cell city analogy activity” as a model, come up with your own analogy that describe the function of a cell and its organelles: