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Name: _________________________________________ Block: _______ Date: _______________ Chemical Activity in the Cell Introduction to the Cell Membrane The inside of a single-celled organism is very much alive. However, the physical environment outside the cell is the opposite—a nonliving place where many changes occur. What stands between a cell and the potentially hostile environment that surrounds it? An ultrathin, extremely important layer separates the living world inside a cell from the nonliving world outside. This is the cell membrane, or plasma membrane. The cell membrane performs two primary, yet very different, functions: it separates the cell from its environment and it enables communication and movement of materials between the cell and its environment. Without a cell membrane, there could be no cell. Protein molecules, which float within lipids in the membrane, enable much of the movement of materials across the cell membrane. (See Figure 6-1.) These protein molecules often extend from one side of the membrane to the other. In multicellular organisms, some cell membrane proteins function as receptor molecules to which chemical signals may attach as one cell communicates with another cell. Transport Across the Cell Membrane For a cell to remain alive, it must have a very special collection of chemicals inside it. These chemicals may be quite different from the chemicals located in the outside environment. Some substances that are abundant outside the cell are not found inside the cell. Other substances that are scarce outside the cell are present in larger quantities inside the cell. The cell membrane creates and maintains this special environment inside the cell. How does it do this? The cell membrane allows some substances—that is, molecules—to pass through but keeps other substances out. This ability to determine which molecules can pass through is called selective permeability. The cell membrane is selectively permeable—it determines which molecules move through it and whether the molecules go into or out of the cell. It also makes possible the rapid transport of some molecules across it, while other molecules pass through slowly. Passive Transport Typically, there is an overall or net movement of molecules from an area of high concentration — a place where molecules are crowded together — to an area of low concentration. This kind of movement is called diffusion. Molecules are constantly in motion and they naturally move from where they are more concentrated to where they are less concentrated. This movement happens automatically with a cell if its membrane is permeable to the molecules and if there is a difference in concentration of the molecules on either side of the membrane. This is called passive transport, because no energy is used by the cell and no work is done. For example, one of the basic needs of most cells is oxygen. There are few oxygen molecules inside a cell, but there is usually an abundance of oxygen molecules Adapted from AMSCO Chem–Cells Revised Edition Page 1 in the water or other liquid that surrounds the cell. Thus, oxygen molecules diffuse across the cell membrane into the cell by passive transport. (See Figure 6-2.) The diffusion of water molecules across a cell membrane—so important for living cells—is given a special name, osmosis. When plant cells are put in a strong salt solution, the abundant fresh water inside the plant cells automatically moves out of the cells, to where there is more salt and relatively fewer water molecules. Plant cell membranes can be seen pulling away from the cell walls as the cells lose water. The reverse happens when limp celery stems are put in fresh water. The celery stems are limp because their cells have too little water in them. When the celery is put in the water, osmosis occurs and water molecules move into the cells. The cells expand, the cell membranes push against the cell walls, and the cells—and thus the celery stems—become firm again Active Transport The movement of a substance against the concentration gradient is known as active transport. When substances are moved from an area of low concentration to an area of high concentration, energy is used and work is done. This kind of transport of materials across the cell membrane is one of the most important activities of cells. Other than using energy from your food to keep you warm, the most important use of energy in your body is to help pump substances across the membranes of your cells by active transport—a process that goes on all the time. Cells get the energy for active transport from ATP molecules. A Tour of the Cell: Organelles Some living things are made up of a single cell, but most have many cells that work together on behalf of the organism. Yet, almost everything an organism does to stay alive is accomplished by each individual cell: getting food, using food for energy, transporting substances, growing, reproducing, and eliminating wastes. Each of these activities involves a large number of chemical reactions. Organization is needed for all of these reactions to take place under the precise control of so many enzymes. In many cells, these reactions take place in special internal structures called organelles. Organelles are dispersed throughout the cytoplasm, which fills the cell and transports materials within it. Cytoplasm is a thin gel, made up mostly of water, with many other chemicals dissolved in it. The cell membrane encloses the cytoplasm. In a typical eukaryotic cell (that is, a cell with a nucleus), there are many ribosomes, the organelles at which proteins are built. The lysosomes, which are scattered throughout the cell, are the structures involved in the breaking down of food. Complicated-looking Adapted from AMSCO Chem–Cells Revised Edition Page 2 mitochondria, shaped like tiny kidney beans, are the organelles in which the cell’s energy is released. (See Figure 6-7.) By far the largest structure in a cell is the nucleus, which is responsible for information storage. The nucleus often fills the entire central portion of the cell. In addition, other specialized structures may be present in a cell to handle other functions, such as: the Golgi complex, which packages many materials; vacuoles, which store materials such as wastes; and, in plants, chloroplasts, the organelles in which plants convert energy from the sun into food. (See Figure 6-8.) Review Questions 1. Which letter indicates the cell structure that directly controls the movement of molecules into and out of the cell? A. B. C. D. A B C D 2. Which statement about the functioning of the cell membrane of all organisms is not correct? A. The cell membrane forms a boundary that separates the cell’s contents from the outside environment. B. The cell membrane forms a barrier that keeps all substances that might harm the cell from entering it. C. The cell membrane is capable of receiving and recognizing chemical signals. D. The cell membrane controls the movement of molecules into and out of the cell. Adapted from AMSCO Chem–Cells Revised Edition Page 3 3. What happens during diffusion? A. Molecules move automatically from an area of higher concentration to an area of lower concentration. B. Molecules are pumped from an area of lower concentration to an area of higher concentration. C. An enzyme joins with a particular molecule. D. A catalyst speeds up the rate of a chemical reaction. Base your answer to question 4 on the following diagram, which represents a cell in water. Formulas of molecules that can move freely across the cell membrane are shown. Some molecules are located inside the cell and others are in the water outside the cell. 4. Based on the distribution of these molecules, what would most likely happen to them after a period of time has passed? A. The concentration of O2 will increase inside the cell. B. The concentration of O2 will remain the same outside the cell. C. The concentration of CO2 will remain the same inside the cell. D. The concentration of CO2 will decrease outside the cell. 5. A plant cell shrinks when placed in salt water due to the osmosis of A. water molecules out of the cell B. water molecules into the cell C. salt into the cell D. salt out of the cell 6. Placing limp celery in water will make the celery stalk firm again due to A. diffusion B. osmosis C. active transport D. a catalyst 7. A high concentration gradient means that the concentration of a substance is A. low on both sides of the cell membrane B. high on both sides of the cell membrane C. about the same on both sides of the cell membrane D. high on one side of the cell membrane and low on the other side Adapted from AMSCO Chem–Cells Revised Edition Page 4 Base your answer to question 8 on the diagram below, in which the dark dots represent small molecules. These molecules are moving out of the cells, as indicated by the arrows. The number of dots represents the relative concentrations of the molecules inside and outside of the two cells. 8. ATP is being used to move the molecules out of A. cell A only B. cell B only C. both cell A and cell B D. neither cell A nor cell B Refer to the set of diagrams below, which shows the movement of a large molecule across a cell membrane, to answer question 9. 9. Which process is best represented by this set of diagrams? A. active transport B. protein building C. diffusion D. gene transfer 10. While viewing a slide of rapidly moving sperm cells, a student concludes that these cells require a large amount of energy to maintain their activity. The organelles that most directly provide this energy are known as A. vacuoles B. chloroplasts C. ribosomes D. mitochondria 11. The organelle that stores wastes for the cell is the A. vacuole B. chloroplast C. ribosome D. Golgi complex Adapted from AMSCO Chem–Cells Revised Edition Page 5 12. Chloroplasts are important because they A. are necessary to release stored energy B. store wastes in both plants and animals C. use energy from the sun to make food D. are in the nuclei of both plant and animal cells 13. Discuss the meaning of “selective permeability” for a cell membrane by explaining the following: why the cell membrane is said to be selectively permeable - _________________________ _______________________________________________________________________ _______________________________________________________________________ why this characteristic is important to the health of a cell - __________________________ _______________________________________________________________________ _______________________________________________________________________ Base your answers to questions 14 through 16 on the diagram below, which represents a unicellular organism in a watery environment. The small triangles represent molecules of a specific substance. 14. What kind of transport is represented by arrow A? ___________________________________ 15. State two ways in which active transport is different from diffusion (passive transport). (1) _____________________________________________________________________ (2) _____________________________________________________________________ 16. In the cells of multicellular organisms, structure B often contains special proteins known as receptor molecules. What specific function do these protein molecules carry out for the cell? ________________________________________________________________________ ________________________________________________________________________ 17. What, specifically, would happen to a cell if its mitochondria were removed? Explain why. ________________________________________________________________________ ________________________________________________________________________ Adapted from AMSCO Chem–Cells Revised Edition Page 6 Refer to the diagrams below to answer questions 18 and 19. 18. The diagrams represent the movement of molecules from an area of A. low concentration to an area of high concentration B. high concentration to an area of low concentration C. low concentration to an area of equal concentration D. high concentration to an area of equal concentration 19. The diagrams could be used to illustrate all of the following types of transport except A. diffusion B. osmosis C. active D. passive 20. Just like complex organisms, cells are able to survive by coordinating various activities. Complex organisms have a variety of systems, and cells have a variety of organelles that work together for survival. Describe the roles of two organelles. In your answer be sure to include: the names of two organelles and the function of each (1) _______________________________________________________________________ (2) _______________________________________________________________________ an explanation of how these two organelles work together _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Adapted from AMSCO Chem–Cells Revised Edition Page 7