1. Ch. 10: Cell Growth and Division
10-1: Cell growth, Division, and Reproduction
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When living things grow and get larger,
Do all the cells just increase in size?
Do the cells increase in number?
A is hair cell of a 4 day old owl.
B is hair cell of an adult owl.
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On average,
The cells of an adult are no larger than the cells of a young individual.
This tells us that, in general, living things grow by producing more cells!
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Limits to Cell Growth
What problems does growth cause for cells?
That is, why do cells divide (increase in number) rather than continue to grow indefinitely?
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Limits to Cell Growth
There are 2 reasons that there are limits to cell growth.
The larger a cell becomes, the more demands the cell places on its DNA.
In addition, the cell has more trouble moving enough nutrients and wastes across the cell membrane to keep the cell alive.
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DNA Overload
DNA Overload
Recall that DNA stores information that controls a cell’s function.
If a cell is small, the amount of DNA present can control the cell’s function.
If a cell grows, the amount of DNA does not change.
Therefore, when a cell grows, there will not be enough “information” for the entire cell to function.
The amount of DNA present cannot serve the needs of the growing cell.
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Exchanging Materials
Exchanging Materials
Recall that nutrients and wastes enter and leave the cell through the cell membrane.
The speed of the exchange of these substances depends on how much surface area is present for this exchange to take place. Rate depends on the total area of the cell membrane.
Therefore, the rate at which food, oxygen, water, and wastes are moved in and out of the cell is dependent on the surface area of the cell.
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Exchanging Materials
However, the rate at which food, oxygen and water are used and waste is produced depends on the cell’s volume.
Specifically, we need to consider the ratio of surface area to volume.
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Ratio of SA to V
Ratio of Surface Area to Volume = SA
V  
Simply put, as a cell grows, its volume increases faster than the surface area.
The SA/V ratio, therefore, decreases.
The decrease in the cell’s ratio of surface area to volume means the cell cannot move needed materials in and waste products out quickly enough for the cell to survive.
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10. Notice that as cell size increases, the ratio of SA to V decreases.
Limits to Cell Growth
As the length of a cell increases, its volume increases faster than its surface area. The resulting decrease in the cell’s ratio of surface area to volume makes it more difficult for the cell to move needed materials in and waste products out.
Notice that as cell size increases, the ratio of SA to V decreases.
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Division of the Cell
Division of the Cell
Before it becomes too large, a growing cell divides forming two “daughter” cells.
The process by which a cell divides into two new daughter cells is called cell division.
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Division of the Cell
Before cell division occurs, the cell replicates (copies) all of its DNA.
The problem of “DNA overload” is solved because each daughter cell gets a copy of all the DNA.
The problem of increasing cell volume is also solved. Each daughter cell has an increased SA to V ratio.
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13. Asexual Reproduction
The production of genetically identical offspring from a single parent is known as asexual reproduction.
As the definition implies, it involves a single parent producing an offspring that are genetically identical to the single parent cell that produced them.

14. In both prokaryotic and eukaryotic single-celled organisms, cell division is a form of reproduction. It is a simple, efficient, and effective way for an organism to produce a large number of offspring.
Bacteria reproduce by binary fission.

15. In multicellular organisms, cell division leads to growth
In multicellular organisms, cell division leads to growth. It also enables an organism to repair and maintain its body.
Many multicellular organisms also reproduce asexually.
Kalanchoe plants form plantlets. Hydras reproduce by budding.

16. Sexual Reproduction
Sexual Reproduction
In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.
The offspring produced inherit some genetic information from both parents.
Most animals and plants, and many single-celled organisms, reproduce sexually.

17. Comparing Sexual and Asexual Reproduction