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
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
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