FUNCTIONS OF A CELL. reflect

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1 reflect The image at the right shows a model of a spaceship from a popular movie. You may not be able to tell by looking at it, but it was constructed completely out of plastic building blocks. Imagine taking apart this model. You might begin by removing large sections. But, suppose you keep breaking down the model until the pieces can no longer be broken down further. What do you have? Can you picture the smallest building blocks that make up this model? Now imagine taking apart a leaf from a plant. Suppose that you do this with the help of a microscope. What is the smallest unit you can reduce the leaf to and still identify it as part of a plant? Do you need the microscope to be able to see this unit? The Cell Theory When broken down to its tiniest building block, a leaf is reduced to individual cells. In fact, any living thing can be reduced to cells. Once scientists discovered that cells are the basic unit of living things, they developed the cell theory. This theory covers three main ideas. All organisms are made up of one or more cells. The cell is the smallest unit that makes up an organism. Some organisms are composed of only one cell. These are called unicellular, or single-celled organisms. The prefi x uni- means one, like a unicycle with one wheel. Other organisms are composed of many cells. These are known as multicellular organisms. The prefi x multi- means more than one. For example, multiple items means more than one item. Cells are the basic units of structure and function in organisms. The cell is the smallest living unit of an organism. Cells are made of smaller structures called atoms, but atoms are nonliving units. The cell represents the basic building block of all organisms. Plastic building blocks were used to make this model of a spaceship. A bean plant is multicellular, as shown by a close-up of the bean plant s root cells. Cells come only from other living cells. A single cell can divide to produce two cells. This process of cell division provides a way for cells to increase in number. Cell division is the process by which living cells produce new cells. New cells can only come from existing cells. 1

2 what do you think? A long time ago, people were confused when they observed fl ies emerging from rotting meat. They assumed that the fl ies arose spontaneously from the meat. This idea was called spontaneous generation. In 1668, an Italian scientist named Francesco Redi did an experiment to test the idea of spontaneous generation. He prepared three identical jars with raw meat. He left one jar open. He covered the second jar with cloth and the third jar with heavy paper. Flies were able to enter the fi rst jar but not the other two jars. After a few days, Redi observed that fl ies emerged from meat in the fi rst jar but not in the others. Which of the three main ideas of the cell theory does this observation support? Explain your reasoning. Discover Science: Cell Discovery and Technology How were scientists able to develop the cell theory? The invention of the microscope occurred before cell theory was developed. Once microscopes were made with sufficient magnifi cation, scientists could observe cells. These observations led to the cell theory. Robert Hooke was the first scientist to observe cells. Hooke built a simple microscope and used it to observe a variety of organisms. He described his findings in a book titled, Micrographia that was published in In his book, Hooke provides an illustration of the structure of cork, which is made from plant stems. He describes the repeating small structures making up the cork as cells. The structures reminded him of the cells or rooms in a monastery. Since Hooke s time, more powerful microscopes have been developed. Today, scientists use electron microscopes to observe bacterial cells, which are A scanning electron microscope allows scientists to study the structure of bacteria. tiny, simple cells without a nucleus. Bacterial cells are 10 to 100 times smaller than more complex cells that have a nucleus. Electron microscopes can also be used to view the structures of tiny organelles within eukaryotic cells. 2

3 Cellular Energy Earlier, you learned that one of the main ideas of the cell theory was that the cell is the smallest living unit in organisms. In order to live, organisms must take in energy from their environment. Their cells convert this energy into forms that they can use to grow and reproduce. Animals also use energy to move about their environment. Some animals use energy to sense their environment and to process information. For example, an animal may use energy to sense seasonal changes in temperature and react by seeking cooler or warmer places to live. These are all forms of biological energy. So, how do cells convert the energy obtained from food into useful energy? Metabolism is the overall process that cells use to make these conversions. Metabolism involves many chemical reactions. These reactions are linked so that the products (what is made in a reaction) of one reaction are the reactants (what goes into a reaction) of another. Some products of metabolism are not useful and are expelled as waste. Metabolic Reactions: Cellular Respiration The term metabolism refers to all of the chemical reactions that take place inside a cell. One important subset of these reactions is cellular respiration. In eukaryotes, cellular respiration occurs in the mitochondria. In prokaryotes, cellular respiration occurs in the cytoplasm near the cell membrane. Cells use metabolic processes to convert the energy in food into useful forms of energy. Waste products from these processes are expelled. In cellular respiration, carbohydrates, fats, and proteins are broken down. Energy is released and used by cells. The gas carbon dioxide, which is a compound with the formula CO 2, is also released during cellular respiration. The mitochondria carry out cellular respiration in eukaryotes. They are often called the power factories of cells. 3

4 look out! Photosynthetic organisms carry out cellular respiration, too. It is important to remember that both plant and animal cells need the energy released during cellular respiration. Animals obtain the substances broken down during respiration by eating plants or other animals. In contrast, plants obtain these substances through photosynthesis. They use energy from sunlight to convert carbon dioxide and water into food substances. These food substances are broken down during cellular respiration. Important Cell Structures A single cell is made up of many different parts that work together. Each individual part of a cell performs a specific function or set of functions. This is similar to the way in which an organism is made up of many structures that perform specifi c functions. For example, some structures of a frog include legs that help it jump, eyes that allow it to see, and skin that protects its body and helps it breathe. You learned that mitochondria are involved in the transformation of energy from food into a form cells can use. Let s learn about some other important cell structures found in eukaryotic cells. Cell membrane: A cell is surrounded by a cell membrane. This structure regulates which substances can enter and exit the cell. All things needed by a cell, such as nutrients and water, enter through the cell membrane. Waste products leave a cell through the cell membrane. Skin acts in a similar way to a cell membrane. Skin allows some substances to enter the body and prevents some substances from entering the body. Skin also lets some substances, like sweat, out of the body. Cell wall: Plant cells have a stiff cell wall that surrounds the cell membrane. The cell wall provides protection and support for the cell. Its function is similar to the function of a skeleton in an organism. Animal cells do not have a cell wall. Nucleus: The nucleus is the control center of a eukaryotic cell. It controls most of the cell s activities, including cell division and growth. The nucleus is like the brain of a cell. 4

5 Cytoplasm: The cytoplasm is a gel-like substance that includes all the living material in a cell except the nucleus. Many of the necessary cell reactions take place in the cytoplasm. Lysosomes: The function of the lysosome is to break down large food molecules into smaller food molecules. Lysosomes also break down waste products and old cell parts. They can be thought of as garbage collectors for the cell. Vacuoles: Plant and animal cells contain vacuoles. However, most plant cells have a larger central vacuole. Animal cells have smaller vacuoles. These structures are the storage houses of a cell. They store water and food materials. Endoplasmic reticulum: This structure works as a kind of factory for making important substances for the cell like proteins and lipids. The endoplasmic reticulum also acts as a highway for the cell; it transports these substances throughout the cell. Golgi apparatus: The Golgi apparatus is structure that modifi es and packages proteins. It can be thought of as the shipping center of a cell. Chloroplasts: Plant cells have green chloroplasts. These structures help trap energy from sunlight. This energy is used during photosynthesis. Animal cells do not have chloroplasts. What do you know? Now that you have explored the functions of cells, use your knowledge to compare a whole organism to a cell. Complete the chart by writing the correct terms from the word box below in the chart on the next page. Note that there are more words than you will need to complete the chart and some words may be used twice. Word Box Endoplasmic reticulum Lungs Small intestine Mitochondria Golgi apparatus Lysosome Mouth Urinary system Cell membrane 5

6 Take in food Digest food Respiration Process Process waste for disposal Human organ or structure Cell organelle or structure Now take some time to compare animal and plant cells. Read each description in the top row of the chart. Decide whether the description applies to each type of cell. Circle Y for yes and N for no. Then explain your answer in the space provided in each block of the table. Cannot be broken down further into a living unit Animal and Plant Cells Obtains energy by eating other organisms Carries out cellular respiration Animal cell Y N Y N Y N Y N Distributes materials in the cell Plant cell Y N Y N Y N Y N 6

7 connecting with your child Exploring Cellular Respiration and Mitochondria in Different Cell Types You can observe cellular respiration with Here are some questions to discuss with your child by following this procedure. students before they begin their research: 1. Place one-quarter cup of warm tap water in a sealable plastic sandwich bag. 2. Add one teaspoon of table sugar and one tablespoon of active dry yeast to the water in the bag. Carefully press to remove most of the air in the bag and then seal the bag. 3. Gently mix the contents. Then, place the bag in a warm spot for about 30 minutes. 4. Check the bag and record your observations. What change suggests the yeasts have been carrying out cellular respiration? To expand students understanding of the role of mitochondria in human cells, have them conduct research to locate information about the numbers of mitochondria in various human cells. Students can research the average numbers of mitochondria found in muscle cells, blood cells, skin cells, heart cells, and liver cells. Encourage students to think about the reasons for any differences and to research what scientists understand to be reasons for the differences. Why might different cells in the human body have different numbers of mitochondria? Which cell types do you think have large numbers of mitochondria compared to others? Here are some questions to discuss with students after they conduct their research: Were your predictions correct? What surprised you about the information you found? 7