SBI4U – Active Transport
ATP and ADP
When you decide to go on a vacation you might load up the car with your personal
belongings and then go to the gas station to fill up with gasoline. The gasoline in the tank
of your car is stored energy that your car will use to get you to your destination. As the
fuel is consumed it will be turned into something new, specifically the combustion of
gasoline will produce carbon dioxide, water vapour, and large amounts of energy to move
you, your car, and your stuff to your destination.
Photo by Leo Koskiluoma and
used under the Creative
Commons Attribution 2.5
Generic license.
Cells may not go on vacation, but they do need energy to do things. Instead of using
gasoline they use adenosine triphosphate (ATP) as an energy source. As the ATP is
consumed, it turns into adenosine diphosphate (ADP), an inorganic phosphate (P i), and
releases usable energy. This energy can be used by the cells in your body to do many of
the things you need to survive. For example, it allows muscles to contract or to move
compounds through the cell membrane.
ATP is consumed in huge quantities in the cells of your body. Lucky for us, we don’t have to go to a gas station to refuel;
instead we can enjoy a good meal. The food we consume is the fuel we use to regenerate ATP from ADP. We recycle the ADP
back to ATP so that the cell can have the energy in a form that can be used.
ATP
(Image in public domain)
ADP
(Image in public domain)
The formation of ADP and an inorganic phosphate from ATP releases energy that cells can use as an
energy source.
(Image courtesy of C. Fusco)
Active Transport
When we eat a meal, our digestive system breaks down the food into its basic nutrients that can be absorbed and used by the
body. However, if we relied only on passive transport, we would only be able to extract half the nutrients that we consume.
With passive transport, the nutrients move by diffusion into our body until it reaches an equilibrium. That would mean only
half the nutrients would move into our body before equilibrium would be reached and diffusion would stop. Lucky for us, we
also have an active transport system that will move the rest of these nutrients into our body against the concentration gradient.
This type of transport needs energy to work and therefore must use ATP.
The Sodium-Potassium Pump
One of the most studied mechanisms for active transport is the sodium-potassium pump. It is termed a pump because it creates
and maintains a concentration gradient across the cell membrane by actively pumping sodium ions out of the cell and
potassium ions into the cell. The concentration gradient produced between these two ions allows electrical signals to move
along our nerves, which results in the contraction of muscles.
If the pump stopped working, passive transport would allow these ions to flow back through the cell membrane until the
concentration is the same on both sides; in other words, until equilibrium is reached. You might think about it as being in a
leaky boat. To avoid sinking you will pump the water out of the boat to keep it floating. If you stopped pumping, the water
would flow into the boat until it was full and the boat would sink.
There are many examples of these types of pumps in cells. They not only keep a concentration gradient between sodium and
potassium ions to allow electrical signals to pass through nerve cells, but these pumps are also used to move wastes out of our
bodies and bring nutrients in.
Sodium-Potassium Pump
(Image in public domain)
Endocytosis
What happens when the cell wants to consume something larger than an ion? Cells use bulk transport to bring things into the
cytoplasm. This process is called endocytosis and there are three types we will be looking at. The first is phagocytosis that is
very much like a cell eating. The others are pinocytosis and receptor-mediated endocytosis that are more like a cell drinking.
Types of Endocytosis
(Image in public domain)
Phagocytosis
If you have a dog you are probably already aware of how phagocytosis works. Dog owners in many communities have to
follow “stoop and scoop” laws. They often do this by taking plastic bags with them when they take their dogs out for a walk.
When their dog does his business, they stoop and use the plastic bag to scoop up the dog’s waste. Many people will do this
without touching the waste to get it into the bag. The technique they use is to put their hand into the bag, then pick up the
waste, and turn the bag inside out to remove their hand and to pull the waste into the bag.
Phagocytosis works in a very similar manner. When the cell wants to consume a large particle it will extend pseudopods to
wrap around the particle. It will then pull the particle into the cell and in the process turn the membrane inside out. A
phagosome (food vacuole) will be formed within the cytoplasm.
One type of cell that is very good at this is white blood cells. They will attack bacteria that can make us ill and they will ingest
them using phagocytosis. The bacteria are brought into the cell by phagocytosis and a food vacuole (also known as a
phagosome or phagocytotic vesicle) is formed. The vacuole will fuse with lysosomes that contain enzymes that will digest the
bacteria.
Pinocytosis
Where phagocytosis can be described as a cell eating, pinocytosis is a cell drinking. Instead of engulfing a large particle to
bring into the cell, the membrane forms a pinocytotic vesicle that contains a small amount of the extracellular fluid and the
materials dissolved in it.
Receptor-Mediated Endocytosis
A special type of pinocytosis is called receptor-mediated endocytosis. In this case, there are small receptors on the surface of
the cell membrane that specific molecules can attach to. The cell then creates a pinocytotic vesicle that contains these
molecules along with a small amount of the extracellular fluid. This is the mechanism by which cholesterol can be brought into
the cell. Processes in the cell can modify the cholesterol to produce hormones.
Exocytosis
There are two mechanisms that the cell can
move things from inside the cell to the
extracellular fluid. The first is constitutive
secretion and the other is regulated secretion.
Both of these types of secretion are tied to the
endoplasmic reticulum and Golgi apparatus.
Materials formed by the endoplasmic
reticulum are prepared for secretion by the
Golgi apparatus that forms the secretory
vesicle.
Types of Exocytosis
(Image in public domain)
In constitutive secretion the Golgi apparatus
continually forms secretory vesicles to be
discharged into the extracellular fluid. In
regulated secretion, the secretory vesicles are
only released into the extracellular fluid when
in the presence of a particular molecule
(called a signal molecule). When this
molecule is present, it triggers the need for the
products located inside the secretory vesicles.
Assignment
1.
2.
What is the difference between passive transport and active transport?
List the types of active transport and explain the mechanism they use to move material.