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online reading 2 - Biology 112 Cell Growth - 1 Unit 1...

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Biology 112 Cell Growth - 1 Unit 1 Structure and Function of Cells Topic 2 – Cell Growth Topic 2 Learning Objectives After reading this supplementary material and the related material in the textbook you will be able to: s Describe what is meant by growth in a unicellular organism. s Calculate the approximate number of cells in a growing population based upon the initial number of cells. s Identify the following phases on a growth curve graph: lag phase, exponential phase, stationary phase, death phase. s Contrast lag phase, exponential phase, stationary phase, and death phase in terms of division rate, cell composition, and cell viability. s Predict the response in cell growth to environmental conditions.
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Biology 112 Cell Growth - 2 2.1 Cell Growth versus Population Growth_______________ Most bacterial cells and many other kinds of cells reproduce by binary fission: a parental cell divides into two daughter cells of equal size. In cases where the daughter cell is smaller than the other the process is call "budding", but the effect is the same: two cells where there once was one cell. The cell cycle refers to the period between cell divisions. During the cell cycle the cell mass increases, the DNA is duplicated and then some trigger begins the process of creating two cells. This requires that the two chromosomes are separated and between them the cell membrane and wall encloses the two parts of the cell separately. The diagram below shows the process of a bacterial cell that contains a single double stranded DNA molecule as its chromosome. As the cell doubles in size all of its components, the ribosomes, tRNAs, energy generating proteins double as well. The chromosome is also replicated. After replication, a septum is formed in by inward growth of the cytoplasmic membrane and then afterwards the cell wall is completed. The details of the molecular events that control and coordinate the cell division are an active area of research. The end result of the cell cycle is to produce two cells that will be the same size as the parental cell. So while the cell increases in size during the cell cycle, at the end, the number of cells has increased, not the size of each cell. The same thing happens for most of the cells in a multicellular organism. They increase in number not size. (Some cells in multicellular organisms are exceptions to this statement, such as cells in the nervous systems).
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Biology 112 Cell Growth - 3 Because each cell is small, what biologists and microbiologists in particular, are interested in the increase in the number of cells, not the increase in size of a particular cell. So usually biologists talk about population increase, which means the increase in the number of cells in the population. This is sometimes called population growth or just growth.
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online reading 2 - Biology 112 Cell Growth - 1 Unit 1...

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