6 Cell Structure and Function Biologists study life at many levels, from the whole biosphere to the molecules

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1 6 Cell Structure and Function Biologists study life at many levels, from the whole biosphere to the molecules that make up the cells of organisms, as shown in the diagram below. Cell biology is the study of the compositions and functions of cells; cell structures and organelles; and cell molecules. Challenge 00What are the fundamental structures and functions of cells? Materials For each student sticky notes biosphere The levels of organization of life studied by biologists ecosystem organism population community organ system organ stomach tissue cell organelle molecule 186

2 Procedure 1. Follow the Read, Think, and Take Note strategy as you complete the Reading. Reading The Cell Principle Englishman Robert Hooke was an influential scientist in the 17th century, who, while looking at thin slices of cork under a microscope, became the first scientist to observe and record cells. What he saw in the cork looked like a series of boxes that reminded him of the rooms, or cells, of a monastery. His observations led him to call these structures cells. It was not until 1839, however, that German botanist Matthias Schleiden, from studies with a more powerful microscope, declared that entire plant organisms were made up of cells. This was a momentous discovery at the time, but within a year Schleiden s colleague, Theodor Schwann, a professor of animal studies, proposed that animals were also made up of cells. In 1855, physician Rudolf Virchow theorized that new cells could only come from existing cells. The ideas of Schleiden, Schwann, and Virchow led to the development of the cell principle, which has been confirmed by many scientists. The cell principle states that all living organisms are made of cells. cells are the basic units of structure and function in living organisms. new cells are made from existing cells. cell structure and function Activity 6 Robert Hooke s drawing of cork cells,

3 Science & Global Issues/Biology cell biology These prokaryotic cells (a) lack a nucleus. The single chromosome is located in the cytoplasm. In the enlarged human liver cell (b), the chromo somes are inside the nucleus. a b All Cells Contain Genetic Information All living cells contain hereditary information in the form of a molecule called deoxyribonucleic acid (DNA). From the information that is stored in DNA the cell makes the proteins and other molecules it needs to carry out all of its chemical processes and, for multicellular organisms, function within the larger organism. Based on the location of the DNA in the cell, scientists classify living cells into two major categories eukaryotes and prokaryotes. Eukaryotes are organisms with cells that contain a nucleus, a large organelle surrounded by a double membrane, where the DNA is stored. Of the organisms you have observed, animals, plants, and protists are eukaryotes. In most of the cells of multicellular eukaryotes, the nucleus contains the genetic instructions for the entire organism. Protists are s ingle-celled microbes that have a nucleus. Examples of protists are the paramecium and amoeba, most algae, and some fungi. Prokaryotes are organisms with cells that do not have a nucleus, and their DNA is located in the cytoplasm. The bacteria you observed in Activity 3, What Is a Cell? were single-celled prokaryotes. Prokaryotes also lack many other cell structures found in eukaryotes. All Cells Have a Cell Membrane and Cytoplasm Every living cell has a cell membrane that serves as a barrier between the cell and its environment. The cell membrane lets some substances into and out of the cell, while preventing the movement of other substances. Within the boundary of the cell membrane of all cells, but outside the nucleus in eukaryotes, is a semi-fluid substance called the cytoplasm. All of the cell s internal structures and organelles are suspended in the cytoplasm. As you will see 188

4 cell structure and function Activity 6 through the rest of this unit, many of the cell functions and reactions of metabolism occur in the nucleus and cytoplasm. Metabolism is the term for all of the chemical processes that maintain life and that occur within living cells. All Cells Must Maintain Internal Balance The external conditions in the environment surrounding cells can vary. To survive, however, cells must maintain constant internal conditions, such as water content and temperature just as a whole organism, such as a human, must do. The process by which cells maintain constant internal conditions is called homeostasis, and it depends greatly on the membrane s regulation of what substances go into and out of the cell. The Cytoskeleton Inside the cytoplasm is a system called the cytoskeleton. As you might guess from its name, the cytoskeleton provides structure and organization to the cytoplasm and maintains the shape of the cell. But the cytoskeleton has other important functions. It plays a role in the transport of materials within cells, in the division of cells, and in the movements of cells when they crawl, swim, or contract. The cytoskeleton is made of three types of long, thin structures built of proteins. These are the actin microfilaments, intermediate filaments, and microtubules. These tubules and filaments are both able to assemble and disassemble in the cell when and where they are needed. Motor proteins cause microfilaments and microtubules to slide past each other as well, producing shortening or lengthening movements. These cytoskeletons radiate from near the nuclei. Cells in Multicellular Organisms Specialize Multicellular organisms have anywhere from a few to many types of cells. In humans there are more than 220 types of cells, each with a specialized function and its own arrangement of cellular structures needed for the body to survive and reproduce. Muscle cells, for example, are specialized to contract, whether as part of the skeletal muscles that interact with and move bones or the muscles surrounding digestive organs that contract to move food through your digestive system. These cells contain a highly developed system of microfilaments, which function to shorten the muscle cells when the muscle contracts. Muscle cells also contain many mitochondria, which supply the large amounts of energy needed for contraction. Red blood cells, too, are specialized cells, but, unlike other human cells, they do not have nuclei, mitochondria, or other cell structures. Instead, red blood cells are packed with hemoglobin, a protein that carries oxygen. 189

5 Science & Global Issues/Biology cell biology A third example of specialized cells is the intestinal epithelial cells of the small intestine, which have microscopic structures called microvilli that function to increase the surface area of the cell. The increased surface area increases the rate of nutrient absorption. Analysis 1. What are the similarities between all cells? 2. What is the major characteristic that classifies a cell as a prokaryote or eukaryote? 3. How did technology, namely the microscope, advance scientists understanding of cells and microbes? 4. Some substances are damaging to cell membranes. What effects on the function of the membrane might the cell experience if the cell membrane is damaged? Key vocabulary bacteria cell cell biology cell membrane, membrane cell principle cytoplasm cytoskeleton deoxyribonucleic acid, DNA eukaryote, eukaryotic homeostasis metabolism nucleus prokaryote, prokaryotic protein protist 190