WARM UP #2: Cells and Organelles
With selected student responses

QUESTION 1: Cells are generally microscopic, ranging in size from about 50 microns for animal cells to as small as 1 micron for bacterial cells. Why do you think that cells - animal, plant, or bacteria - don't get any bigger than this? Why do you think we are made of 75 trillion very small cells, rather than just a bunch of very large cells?

From AnneM: Maybe having 75 trillion very small cells allows for a variety of tasks to be done by all the cells .

From HA: Being made up of many small cells helps to insure an oirganism's survival. If an organism was made up of only a few larger cells and one died, is injured, or the information is lost, it would make the organism more vulnerable. Whereas with many small cells it is easier to recover.

From BK: By being made with more cells we can become more complex. I think plants, animals, and bacteria cells don't get any bigger than they are because it fits the environment, The size balances with the ecosystem.

From Dr. Marrs: I would add that cells are sooo tiny due to limitations in their ability to take up oxygen and get rid of waste products. A small cell has a lot of "surface area" for oxygen to diffuse in, and has a small volume inside for waste products to diffuse out. However, as the size of a sphere increases, its volume grows much more than its surface area. After a certain size, the volume of the cell is too big to allow oxygen to get into the middle of the cell and for all the waste products to get out. Thus, the smaller the object, the greater the amount of surface area : volume. This limits the size of the cell.

QUESTION 2: How do you think cells become specialized for their function? For instance, we all started our life as one single fertilized egg cell, but within weeks or months of conception, we had liver cells, heart cell, brain cells, muscle cells. How does one cell give rise to many cells with many different functions??

From LE: Once we start to develop other cells develop too. Our bodies know how to form and therefore the cells we need to survuve are made. I think cells just know what to do to forn a human.

From TC: All cells come from preexisting cells, so I think that the fertilized egg has all the necessary information and it divides into many cells that have the information to perform their job.

From DS: Cells become specialized due to their DNA and protein makeup. One cell has all the information to create other cells, but each cell has a different makeup which enables them to perform their specific function.

From Dr. Marrs: Each cell does come from a preexisting cell - and had DNA identical to the other cells of the body. However, although each cell has the potential to make about 100,000 different protein from their DNA, no cell makes all the proteins! Different pieces of DNA (genes) get 'turned on' in different cells, allowing muscle cells to only make proteins needed in muscle cells, heart cells to only make the proteins that are needed in the heart... What turns the genes on? Developmental signals in the embryo from hormones, and other regulatory chemicals. More about this in later chapters.

QUESTION 3: In what cells of your body would you expect to find a lot of mitochondria? Why?

From SD: I would expect to find a lot of mitochindria in the respiratory part of the body. It gives energy and the respiratory part of the body is where this comes from.

From TT: I think that there would be a lot of mitochindria in the cells in the brain and muscle because they need the most energy.

From Dr. Marrs: Any cell that is very metabolically active (using a lot of glucose for energy) will have lots of mitochondria. Muscle cells, liver cells, brain cells, and sperm cells have thousands of mitochondria in each cell!!!

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