Cells are really picky. They're always either too hot, too cold, too salty, or too watery.
In order to survive, cells need to maintain a specific internal environment.
They're constantly adjusting the amounts of glucose, water, oxygen, carbon dioxide,
sodium, potassium, calcium, and many other things.
But how do cells do this? How do they make sure that they maintain a perfect
internal environment? Stick with us, we're going to find out.
The process of constantly adjusting certain variables is known as "dynamic
homeostasis". Usually it involves feedback mechanisms called negative feedback
loops. Cells use both passive transport and active transport to move materials
in and out. However, the size and polarity of the molecules will determine how they
can get across the membrane. Remember that cells are surrounded by a
double membrane made of phospholipids and cholesterol. This barrier is
excellent at stopping most molecules from passing through, while still
allowing certain particles to cross. As we go through the different types of
particle transport, we'll be keeping our information organized in a grid that
looks like this. We'll be starting off with passive transport and a process
called diffusion. In diffusion, particles flow from an area of higher
concentration to an area of lower concentration. The particles spread out
until they're equally spaced apart. Diffusion becomes much more complex when
we add a membrane. The particles still want to flow from high concentration to
low concentration, or as we call it "down the concentration gradient", but the
membrane gets in the way. Luckily, small and non-polar molecules can slip right
through the membrane. These include both oxygen and carbon dioxide. To summarize,
in diffusion, the particles only flow from high to low concentration,
do not use protein channels, and also do not need to spend any ATP to do it. Next
up is a slightly different type of passive transport called facilitated
diffusion. Remember that large or polar molecules can't fit through the cell
membrane. Water isn't a very large molecule, but its polar structure makes
it unable to slip through the membrane on its own. In facilitated diffusion, the
cell uses channel proteins to allow large and/or polar molecules to pass
through. Water diffuses through the cell membrane using special channel proteins
called Aquaporins. Whenever water diffuses across a membrane, we call it
"osmosis". To continue with our chart, in facilitated diffusion particles still
flow from high to low concentration. They do use protein channels, but they still
don't use ATP. Next we're going to move into the realm of active transport.
Active transport requires the input of energy, usually in
form of ATP. Remember that ATP or Adenosine Triphosphate is a rechargeable,
renewable source of cellular energy, almost like a battery. It stores energy
in its bonds, and that energy is released when the bonds are broken. The ATP is
reduced to lower energy ADP, then is recharged into ATP via cellular
respiration. Active Transport often uses channel proteins similar to those used
in facilitated diffusion. These channel proteins charge a toll to admit a
molecule. On return for a small amount of ATP, these carrier proteins allow certain
molecules to pass through the cell membrane. These carrier proteins are
especially useful because they can pump items AGAINST their concentration
gradients. Passive transport can only allow particles to flow from high
concentration to low concentration, but active transport can pump from low to
high, thereby making the concentration more extreme rather than only moving
towards equilibrium. A great example of this is the sodium potassium pump, which
pumps sodium out of cells and potassium into cells allowing neurons to work.
Three sodium ions enter the carrier protein, which is then charged by a
molecule of ATP in a process called Phosphorylation. The protein then
undergoes a conformational change, delivering the sodium to the outside of
the membrane. The process is then reversed; this time delivering potassium
to the inside and returning the protein to its original shape. In active
transport, it's possible to pump particles from low to high concentration,
as well as from high to low. The process uses protein channels, and the cell has
to spend some ATP. Sometimes, however, it's not efficient enough to transport
molecules through the membrane just a few at a time. So, cells use a process
called bulk transport. There are two major types of bulk transport, and
they're related. They're called endocytosis and
exocytosis. Neurotransmitters are a great example of bulk transport. These
molecules allow neurons to communicate. Just like other cells, neurons are
surrounded by a phospholipid bilayer. Neurotransmitters are encased in
vesicles that are made of the same material. These vesicles are brought to
the edge of the presynaptic neuron, the vesicles dock against the
membrane, and then release their contents into the synaptic cleft between the two
neurons. The vesicle is then incorporated into the neuron's cell membrane. The
presynaptic neuron has just performed exocytosis. When the particles of
neurotransmitter lock into specific ligands on the postsynaptic neuron,
endocytosis can occur. The particles are again wrapped up in a bubble of cell
membrane, that becomes a new vesicle. Endocytosis and exocytosis are capable of moving
particles both with and against the concentration gradient. Instead of using
protein channels, they use vesicles to transport particles. The vesicles can
hold more molecules, but opening and closing them requires energy, so the cell
does need to spend a little bit of ATP. So, there we have it; our grid that
compares and contrasts passive and active transport. If you'd like to learn
more about homeostasis, click here. And if you'd like to learn more about the cell
membrane, click here. YouTube thinks that you'll like this video, and I hope you'll
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