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Structure and Function of Molecules and Cells1 The structure of a molecule is related to its function. These figures illustrate how the overall structure of Collagen is a long protein with three polypeptides twisted some proteins matches the function together like a rope or cable. Collagen gives strength to the of the protein. tendons and ligaments which connect bones and muscles. A porin protein The motor provides a protein, channel for a kinesin, walks small molecule along a to diffuse microtubule across a cell carrying the membrane. vesicle with its contents. Most protein enzymes have a similar overall globular shape, but each enzyme has an active site with a unique structure that matches the enzyme’s function. 1. The diagrams on the right show how the shape of the active site of an enzyme matches the shape of its substrate molecule, maltose. This enzyme breaks down maltose to two glucose molecules. Explain why this enzyme cannot break down proteins into amino acids. 2. Many proteins have a structure that allows them to change shape in order to accomplish their function. How could the motor protein kinesin twist, bend or otherwise change shape to accomplish its function? 1
By Dr. Ingrid Waldron, Dept Biology, Univ. Pennsylvania, © 2014. This Student Handout, Teacher Notes with instructional suggestions, background information and links to recommended videos are available at http://serendip.brynmawr.edu/exchange/bioactivities/SFMolecCell. All living organisms are made up of one or more cells. A cell is the smallest unit that can carry out the activities of life. These figures show some of the organelles and other parts of animal and plant cells. (Figure from Krogh, Biology – A Guide to the Natural World) 3. Complete the following table. Explain how one or more of the parts of an animal cell help the cell to carry out each Activity of Life of the listed activities of life. Homeostasis (maintaining a stable internal environment) Metabolism (using energy) Growth (e.g. making proteins and making new cells) 4a. Explain why biologists say that a cell is alive, but none of the individual molecules inside the cell are alive. 4b. If you ground up a cell and put all the molecules from the cell in a test tube, would this mixture of molecules be alive? Explain why or why not. 2
5. Plant cells have several structures that are not found in animal cells – chloroplasts, cell walls and central vacuoles. Choose two of these structures and explain why each of these is useful for plant cells, but not needed or even a disadvantage for animal cells. cell wall chloroplast central vacuole Many cells in our bodies do not look like the "typical" animal cell shown on the previous page. Differences in cell structure are related to differences in the function of different types of cells, as illustrated by the following examples. 6. Sperm cells have a very specialized structure, including a flagellum and very little cytoplasm. Explain how the structure of a sperm cell contributes to its function. 7. A capillary is a tiny blood vessel where: • Oxygen and nutrients diffuse from the blood to the cells near the capillary. • Carbon dioxide and other waste molecules produced by the cells near the capillary diffuse into the blood. Explain why it is useful for the walls of capillaries to consist of a single layer of thin, flattened cells. 3
8. For some types of cell, a flexible structure that allows the cell to change shape is crucial for its function. The white blood cell shown in this figure defends our body against infection by engulfing, killing and digesting bacteria (phagocytosis). Why do white blood cells need to be able to change shape in order to accomplish their function? White blood cell Another aspect of cell structure that is related to the cell’s function is the amount of each type of organelle in the cell. An extreme example is the mammalian red blood cell which has no nucleus, ribosomes or mitochondria. Instead, red blood cells are filled with hemoglobin, the protein that carries oxygen. The large amount of hemoglobin allows the red blood cells to carry lots of oxygen which the cells need for the high rate of metabolism in mammals. However, the specialized structure of red blood cells has a disadvantage – reduced capacity for repair, so each red blood cell only survives about four months. 9. Most cells are constantly replacing damaged molecules and organelles. Explain why a red blood cell is unable to replace damaged proteins. 4
10a. Explain why it is useful for the pancreas cells that secrete digestive enzymes to have lots of rough endoplasmic reticulum and Golgi apparatus. A protein that will be secreted from the cell is • made by a ribosome on the rough endoplasmic reticulum • processed in the rough endoplasmic reticulum • transported in a vesicle to the Golgi complex • processed some more in the Golgi complex • transported in a vesicle to the plasma membrane = cell membrane 10b. How is the vesicle containing the protein moved from the rough endoplasmic reticulum to the Golgi complex and then to the plasma membrane? (Hint: Review the top of page 1.) 5
11. Based on the examples in this activity, one student argues for the claim that "The structure of a molecule or cell is related to its function." A second student disagrees and argues for the claim that "There is no relationship between the structure of a molecule or cell and its function." His argument includes the following observations: • Many enzymes have a similar overall globular shape, but different enzymes have different functions. • Both sperm and phagocytic white blood cells move in order to accomplish their functions; sperm use a flagellum to move, but phagocytic cells do not have a flagellum. A third student argues for the claim that "The structure of a molecule or cell is related to its function, but different structures can accomplish a similar function." Which of these claims do you think is best supported by the evidence? Write an argument with the evidence and reasoning that supports this claim. Be sure to include your response to the arguments of the second student. Briefly describe any additional evidence you would like to have to evaluate these claims. 12. In a typical diagram of a cell, it looks as though nothing much is happening. In reality, cells are highly dynamic, with lots of activity at the molecular, organelle and cellular levels. Give examples to illustrate the kinds of activity observed in cells. 6