Chapter 11.1 Cell Growth, Division & Reproduction Notes

Chapter 11.1 Cell Growth, Division & Reproduction Learning Objectives Single-celled organisms are so small, mostly invisible to the naked eye. Larger organisms like plants and animals are made up of trillions of little cells rather than a much smaller number of larger cells. Some organisms reprod...

Chapter 11.1 Cell Growth, Division & Reproduction Learning Objectives Single-celled organisms are so small, mostly invisible to the naked eye. Larger organisms like plants and animals are made up of trillions of little cells rather than a much smaller number of larger cells. Some organisms reproduce asexually and some reproduce sexually. Cells throughout a human body reproduce asexually all the time, even though humans as whole organisms reproduce sexually. Surface Area to Volume Ratio One of the main problems of a cell’s size is a matter of surface-area-to-volume ratio. Think of a cell as a cube and consider what happens as the cube, or cell, gets larger and larger. To find the surface area of a cube calculate the area of one side and then multiply by 6 to account for the 6 sides of the cube. Find the area of each side by multiplying length times width. To find the volume of a cube, multiply length times width times height. Surface Area to Volume in Growing Cells What happens when the cube, or cell, grows? Do both surface area and volume grow at the same rate when the cell gets bigger? No, both surface area and volume get bigger with a bigger cell, but volume grows relatively more than surface area does. Ratio of Surface Area to Volume in Cells Where is the ratio of surface area to volume greatest? In a small cube (cell) Where is the ratio of surface area to volume smallest? In a large cube (cell) Cell Growth Limitations Considering the problem of surface-area-to volume ratio, how could cell growth create a problem that is similar to the traffic jam in the growing town? As a cell grows, it needs more materials to cross its membrane. Traffic is comparable to the movement of materials such as nutrients, water, oxygen, and wastes across a cell’s membrane. As a cell grows larger (increasing volume), the number of traffic lanes (amount of surface area to cross) does not keep up, and materials cannot enter or leave the cell as quickly as necessary. Remember, if a cell grew too large, it would not have enough relative surface area to get sufficient amounts of oxygen and nutrients into the cell and waste products out. Cell Division To solve the surface-area-to-volume ratio faced by a growing cell, an individual cell divides. The division of a cell produces two daughter cells. Before a cell can divide to produce daughter cells, there has to be enough DNA for two cells. Before cell division, the cell must replicate, or make a copy of its DNA. This avoids the problem of information overload. Cell division solves the problem of increasing cell size by decreasing cell volume. Making more cells instead of larger cells keeps the surface-area-to-volume ratio high. Cell division slaves the problem of informtion overload by making sure each daughter cell has its own copy of the DNA Cells go through a period of growth after they divide. Asexual Reproduction Reproduction is one of the most important characteristics of living things. For a single-celled organism like a bacterium, simple cell division makes the process of reproduction pretty straightforward, and it can allow a population to grow very quickly. This kind of cell division is a form of asexual reproduction, called fission, which is the production of genetically identical offspring from a single parent. Since Bacteria can reproduce asexually, they can reproduce quickly in the right environment, such as a crowded school. Multicellular organisms can reproduce asexually. For example, tiny plantlets in the kalanchoe can break off and grow into full new plants. The bud on the hydra will eventually pinch off to become a separate individual. Humans can replace lost skin cells. The offspring share the same genetic material as their parent. Sexual Reproduction Humans, like other animals, grow and repair their bodies through cell division. But to reproduce themselves, humans use sexual reproduction. Asexual reproduction involves separation of a cell. Sexual reproduction involves fusion, or joining, of cells. Offspring from sexual reporduction are not genetically identical to either parent. They inherit some genetic information from each parent. Comparing Sexual & Asexual Reproduction Characteristics that make an organism well adapted to its environment are critical for survival. When resources such as food and space are unlimited it is helpful to be able to create a very large number of offspring in a short period of time. When environmental conditions are stable it is useful to have offspring that are genetically identical to the parent. When finding a suitable mate might be difficult or impossible it be helpful to need only one parent to reproduce When environmental conditions are changing and new combinations of traits may promote survival, it is useful to have offspring that are not identical to either parent. Asexual reproduction is fast, but does not allow for variety. Sexual reproduction is slower, but does allow for variety.