When a cell grows, its cellular activities
increase – it has more organelles that need more resources and produce more
waste. For instance, the mitochondria, which carry out cellular respiration,
require O2 and glucose to produce ATP, the body’s primary energy
molecule and CO2, which has to be removed from the cell because it’s
toxic. Not only that, they also need amino acids that are formed into the
enzymes that carry out the respiration process. The mitochondrial membranes are
composed of phosphorus and lipids. All of these parts required for cellular
respiration have to either enter or exit the cell in a timely fashion, or else
the mitochondria won’t be able to produce ATP at a sufficient rate to power the
rest of the cell’s components – and when that happens, cell health suffers.
The same goes for each of the organelles in the cell – each has its own vital
function that is required for keeping the cell alive and well. If the
organelles don’t have the resources to carry out their functions, the cell’s
health will deteriorate and it may die.
There are a variety of ways through which cellular resources
and wastes move through the membrane. These include:
- Diffusion (movement of molecules from an area of high to low concentration directly through the membrane, as happens when O2 moves from the lungs to the bloodstream)
- Facilitated diffusion (movement of ions from an area of high to low concentration via specialised channels in the membrane)
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- Osmosis (the movement of water from high to low concentration directly through the membrane)
- Ion pumps (the active (requiring energy) movement of ions through specialised proteins that act as channels in the membrane)
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The speed with which these membrane transport systems work
is entirely determined by the surface area of the membrane. As there is limited
surface area of the membrane, transport can only happen when there is
sufficient membrane space available. The more space there is available for
diffusion to occur, the faster diffusion will happen.
When a cell grows, as seen above, the Surface Area:Volume
ratio becomes smaller. This means that while the surface area is only a bit
larger, the volume is much larger. So there are more resources and wastes that
need to be moved through the membrane, but there is not enough surface area
through which they can move. If the rate of transport slows so dramatically
that the organelles don’t have access to the resources they need to carry out
their function, or cannot get rid of waste products quickly enough, the cell’s
health will suffer and the cell may die.
This is why cells reproduce. When a cell grows larger and
its SA:V ratio falls, it will reach a limit where it can only just maintain
fast enough cellular transport to maintain health. It will then undergo
mitosis, splitting into two daughter cells, and the SA:V ratio for each cell
will rise, allowing efficient cellular transport for the cells to thrive. This
process of growth and mitosis of cells continues, each daughter cell growing
and undergoing mitosis to maintain SA:V ratios that allow efficient cellular
resource and waste transport across the phospholipid bilayer membrane.
