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TR 7-28 MHR • Unit 3 Cycling of Matter in Living Systems CHECK YOUR UNDERSTANDING ANSWERS 10. Thinking Critically 1 Student Book Page 284 1.Chloroplasts, a cell wall, and sometimes a central vacuole are present in plant, but not animal cells. 2. Ribosomes on the endoplasmic reticulum carry out protein synthesis. 3. The Golgi apparatus receives vesicles from the endoplasmic reticulum and packages proteins into vesicles for transport to the cell membrane (secretion) or to the lysosomes within the cell, much like a post office receives and then redistributes mail. 4. Having separate compartments allows various chemical reactions to be kept separate from one another, which prevents these reactions from interfering with each other or damaging the cell, and allows for greater efficiency of cell activities. Also, having separate compartments (i.e., organelles) permits the organelles to have specialized functions within a cell. 5. In plant cells, vacuoles give added internal support to the plant cell. As well, they may provide storage space for water and other substances, including toxins that make the cell (and thus the plant) taste bad to animals. 6. Mitochondria and chloroplasts have their own DNA, and probably evolved from bacteria that were living symbiotically within other cells. 7. Thinking Critically Lysosomal contents leaking into the cell would effectively digest the cell and destroy it from the inside out. 8. Thinking Critically Because red blood cells have no nuclei to direct their activities, the cells die when their cell structures become worn out. Thus, red blood cells would be expected to have a shorter life span compared to cells that can repair themselves. (Students may be interested to know that red blood cells have nuclei during the early stage of the cells’ development. The nuclei are lost before the cells enter the blood stream. Red blood cells live for about 120 days before they break down and are removed as waste.) 9. Thinking Critically The instructions for making proteins are carried on DNA molecules, found in the cell’s nucleus. The DNA is transcribed into a messenger molecule, which is read and used to make proteins in the ER. Since protein production is dependent on the instructions encoded in the genetic material, it is efficient to have the ER close to the nucleus. ER * * lysosome Golgi apparatus * energy 2 mitochondria * vesicles * * cell membrane CHAPTER AT A GLANCE Student Book Page 285 (a) Hooke was the first to see that a plant was made of numerous cells. Van Leeuwenhoek developed a single-lens microscope and was the first person to describe single-celled micro-organisms. Schleiden looked at hundreds of plant specimens and concluded that all plants are composed of cells. Schwann recognized that the nucleus was common to almost all cells and after viewing numerous specimens, he concluded that animals were also composed of cells. (b) The concept of spontaneous generation is that life forms can arise from non-living matter. The theory of biogenesis says that living organisms can come only from other living organisms. (c) Pasteur’s experiment supported the theory of biogenesis because when he prevented micro-organisms in the air from reaching the broth, growth did not occur in the flask. In broth exposed to particulate matter in the air, micro-organisms were able to enter and grow. Thus, Pasteur showed that these micro-organisms arose from other micro-organisms, not by spontaneous generation. (d) (i) Living cells: compound light microscope since no preparation is needed. (i) Shape of a molecule: STM since it allows visualization of structures as small as molecules. (ii) Shape of a large or thick specimen: compound light microscope would allow one to see the outline of the specimen; whereas an SEM would allow one to see the superficial three-dimensional view of the specimen; a CLSM would also produce a three-dimensional view of the specimen. (e) Computers can save the microscope image. Researchers can then retrieve, manipulate, and study the image long after they create it. Chapter 7 The Basis of Life• MHR Computers can also be used to operate microscopes and adjust specimens. Computers result in more precise and often, better-quality images. (f) Since most cells are transparent, stains are used to make cells visible under a microscope. Stains bind to and colour cellular structures, making cells more visible. (g) Cancer-causing genes are sequenced to determine how they differ from healthy genes. Determining the gene sequence helps researchers learn how the gene affects cell activity and, hopefully, how it changes healthy cells into cancer cells. Cell lines from cancerous cells are also developed to help researchers have a supply of cells. Cancer cells can grow indefinitely in the laboratory whereas normal cells cannot. (h) Stem cells are able to produce other specialized cells, unlike most cells, which can only divide to produce cells like themselves. Scientists hope to use this ability of stem cells to develop treatments and cures for a variety of diseases or even to grow new tissues or organs for transplants. (i) Organelles are the specific internal parts of cells, such as ribosomes or chloroplasts. Except for ribosomes, organelles are separated by membranes or boundaries that compartmentalize the organelles, enabling them to perform specific cell functions. The compartments allow specific chemical reactions to proceed unhindered by other cellular constituents. (j) Plant cells have rigid cells walls, chloroplasts, and a large central vacuole, while animal cells do not. The difference in their structures relates to the differing functions of plant and animal cells. (k) (i) nucleus: storage of genetic information; directs cell activities (ii) endoplasmic reticulum: synthesis and/or modification of proteins, lipids, and other substances, and transport of proteins via vesicles (iii) cell membrane: defines the cell boundary, and regulates the passage of molecules in and out of cell (iv) mitochondria: cellular respiration; provides energy for cell nucleus rough endoplasmic reticulum smooth endoplasmic reticulum cell membrane mitochondrion TR 7-29 Prepare Your Own Summary • Students’ timelines should include the events shown in the following example: Hooke observes cork cells under microscope (mid 1600s). Redi shows that maggots on rotting meat develop from flies’ eggs, not via spontaneous generation (1668). Van Leeuwenhoek observes single-celled organisms (1674). Scheiden states that all plants are composed of cells. Extending this idea, Schwann states that all organisms are composed of one or more cells (1839). Virchow suggests theory of biogenesis (1858). Pasteur’s experiments provide supporting evidence for biogenesis (1800s). • Student charts could resemble: Microscope Explanation Maximum magnification - light used to illuminate simple specimens viewed (light) microscope through a single lens - about 200× compound - light used to illuminate specimens viewed light microscope through two or more stacked lenses - 2000× transmission - beam of electrons is shone through thinly electron microscope sliced specimen to create image - over 1 million× (less than 100 000× usually more useful) - beam of electrons scanning reflected off surface of electron microscope specimen to create three-dimensional image - 300 000× (most specimens viewed more easily at 10 000× or less) - laser beam directed at confocal series of planes in a laser specimen; resulting scanning microscope “optical slices” are stacked to create three dimensional image - not given in student book (students may suggest about 300 000×, same as a scanning electron microscope) - electrons flow between scanning a probe and atoms on tunnelling microscope specimen’s surface; computer generates three-dimensional image from the information - not given in student book (students may suggest that this microscope is powerful enough to obtain images of atoms) TR 7-30 MHR • Unit 3 Cycling of Matter in Living Systems • Some possible answers: – obtaining energy: mitochondria (transforms energy in food molecules), chloroplasts (fixes carbon dioxide for food) – producing wastes: vacuoles, vesicles, lysosomes, endoplasmic reticulum (smooth) – responding and adapting to the environment: nucleus, cell membrane, vacuole – reproduction: nucleus, cell membrane – growing: nucleus, cell membrane, cell wall, endoplasmic reticulum (rough), Golgi apparatus CHAPTER 7 REVIEW ANSWERS Student Book Pages 286–287 Understanding Key Concepts 1. Redi was an Italian physician who used a controlled experiment to test the idea of spontaneous generation. He demonstrated that maggots did not spontaneously generate from rotting meat, thus furthering support for the concept of biogenesis (life from life). 2. Microscope improvements include: development of different-shaped lenses; use of multiple lenses; use of electrons instead of light; using laser beams and computer imaging and interpretation. Different types of microscopes give a variety of options to view cells in two dimensions or three dimensions as well as to view internal structures of live cells. Microscopes were revolutionary because they allowed scientists to view organisms and parts of organisms that were previously not visible to the naked eye. 3. Plants need chloroplasts because they are the structures in which the process of photosynthesis takes place. Animal cells do not photosynthesize. 4. The development of microscopes allowed researchers to finally observe individual microorganisms. Advances in microscopy, staining techniques, cell culture, and gene sequencing are all technologies that have helped develop and promote the understanding of cells. For example, gene replication techniques have allowed researchers to study cancer cells. Advances in microscopes have allowed researchers to obtain incredibly detailed images even of DNA itself. 5. Both Virchow and Pasteur believed that cells did not generate from non-living material, but rather that cells came from pre-existing cells. The experiments of both scientists supported the theory of biogenesis. Redi’s fly experiment and Pasteur’s swan-necked flask experiment both included controls against which to compare the effects of the manipulated variable. As well, both experiments involved preventing organisms from entering into a nutrient-rich medium, with the effect that these same organisms did not grow in the meat or broth. 6. Stem cells have the ability to differentiate into other cell types. As adults, people retain a limited amount of stem cells. Embryonic stem cells are thought to be able to differentiate into any cell type, while adult stem cells can differentiate into a limited number of cell types. 7. This is a plant cell because it has a rigid cell wall, chloroplasts, and a large central vacuole. 8. Vesicles transport materials around the cell. Vacuoles are membrane-bound spaces inside cells, which are used for temporary storage. They can store food, enzymes, and other materials, including waste products. Lysosomes are a type of vacuole created by the Golgi apparatus. They contain digestive enzymes and digest worn out organelles, food particles, viruses, or bacteria. Lysosomes can fuse with other vacuoles carry particles to be digested. 9. Many cancers may be caused by genes with mistakes or mutations; therefore, genes are the focus of a good deal of cancer research. 10. Gene sequencing, cell culture (the development of cell lines), and sequencing projects, such as the Human Genome Project, all contribute information on the links between genes and cancer. 11. The cell wall is found in plant cells, fungi, and most bacteria. It is a rigid structure outside of the cell, or plasma, membrane that provides support and protection. The cell membrane is semi-permeable and is the more flexible boundary of a cell. The cell wall is porous and allows molecules to pass through; however, it does not regulate the passage of materials across itself as does the cell membrane. 12. (a) ribosome (iii) makes proteins (b) mitochondrion (ii) converts energy in food molecules into a form that cells can use (c) vacuole (v) stores water in plants cells (d) vesicle (i) transports materials from ER and throughout the cell Developing Skills 13. (a) Answers may vary. One option would be: Collect water from a fish tank. Sterilize half the water using a filter and put the water in a clean fish tank. Put the rest of the water in a separate clean tank. Cover both tanks. Monitor the fish tanks for the growth of algae over a period of two weeks. Growth of algae in the unfiltered water with no growth of algae in the filtered water would support the theory of biogenesis. (b) If spontaneous generation were a better explanation for the development of algae in fish tanks, one would expect to see algae in both the filtered and unfiltered water. Chapter 7 The Basis of Life• MHR 14. H2O 15. (a) The organelle is 927.5 nm long. (b) The organelle is 4000 nm long. (c) 500 nm Problem Solving/Applying 16. One could begin viewing the mitochondria at a low power. After focussing on the specimen, one could turn to a higher microscope objective to view more detail. Methylene blue could be used to make the mitochondria more visible. Also, using a TEM, which is capable of much greater magnification than a light microscope, could be helpful. 17. The student could take a small sample from each tube and grow the cells in cell culture. She would have to monitor the cells from time to time under the microscope. The samples in which the cells began to change form or differentiate would probably be the stem cells. 18. Examine sections from the tissue under the microscope, and examine the cells. Look for a central vacuole, chloroplasts, and cell walls, which would indicate that the cells were plant cells. 19. Staining the cells with methylene blue would make the nuclei more visible. 20. One possibility would be to use stem cells to grow new pancreatic cells for diabetic patients. 21. If the ribosome could no longer function as a result of the genetic mutation, the cell may not be able to produce proteins. Or, the ribosome may be altered in such a way that it puts together proteins improperly. Critical Thinking 22. Red blood cells do not have readily available energy stores and as a result, would not be expected to live as long as other cells that have mitochondria. TR 7-31 23. Hooke only saw cells in one plant (cork) and these cells were dead. At the time, Hooke did not realize the significance of his observation and thought the cells were unique to cork. Thus, he did not realize that the cells were common to all organisms as the cell theory now states. Observations of cells in other organisms were needed before scientists could develop the cell theory. 24. Cell theory states that all cells come from pre-existing cells. Scientists studying diseases learn a lot about unhealthy, infected, or diseased cells by watching them replicate and comparing them with healthy cells. In addition, stem cells may one day be used in disease treatment. 25. Students may speculate that it would have taken longer for scientists to develop the cell theory had van Leeuwenhoek not published his results, because his work would not have sparked an interest in the microscopic world. By pooling information, scientists often come up with new ideas or see old data in a new light; an example would be the conversations of Schwann and Schleiden, which led them to propose the cell theory. 26. Answers will vary. It is the nature of science to continue to revise explanations for natural phenomena. It is important to separate facts from theories. In science, a theory is not a guess; it is a well-thought out argument based on a set of verifiable facts. For example, the theory of gravity may be “adjusted” in its details from time-to-time, but it is a fact that objects on Earth continue to fall to the ground. If students have difficulty answering any of the Focussing Questions, direct them back to relevant sections in the textbook.