Energy MiraCosta Biology


Uploaded by MeerdesIrrtums on 05.01.2012

Transcript:
Think about the last time you put gasoline in your car. Let’s assume for now that you
drive a standard gasoline-powered vehicle. If you use a bicycle, or an electric car or
a bio-diesel or some other alternative fuel vehicle, just play along with me for right
now.
Now think abou this question very carefully and answer. As far as your car is concerned,
what is gasoline?
Okay, if your answer was “energy,” well, sorry but you’re wrong. Energy is not a
material substance that you can touch or carry or pump into your gas tank. But that doesn’t
mean that it’s not a real thing.
Energy is real but it’s fundamentally different from matter, which is the stuff of material
substances—the words “matter” and “material” share a common root.
If your answer to my question was “fuel” give yourself a gold star. A fuel like gasoline
is a material substance—not energy. But it would be accurate to say that gasoline
is a substance that represents a great deal of chemical energy potential. When we use
substances like gasoline for fuel, we’re taking advantage of the way that energy gets
released when high energy potential substances like gasoline are converted into low energy
potential substances, and we usually call these “waste molecules.” Carbon dioxide
and water are the waste molecules left over after we combust gasoline. This conversion
results in the release of energy.
The energy being released will typically be manifest in some combination of heat as well
as work, which in the case of the energy released from gasoline in your car’s engine means
the work of physically moving you and your car from place to place. Point is, you can’t
achieve the work without the input of energy, and in a physics class you would learn that
work is equal to force times distance. Think of the force that must be applied by your
car’s motor in order to move you and your car—times the distance that you travel in
your car, and that’s the scientific definition of work.
Note that—in a different situation now—if you were coasting from the top of a hill to
the bottom of the hill, this would not be considered work because you don’t need to
exert any force over that particular distance. No energy is required, and in fact you could
achieve that amount of downhill distance traveled without any consumption of fuel or any output
of energy.
Okay, now back to the biology class. In the cellular context, the term “work” applies
to any cellular activity that requires the input of energy. One form is mechanical work
which is the same as the physical sciences use of the word. With nearly every movement
you make, you’re exerting a force over a distance and this requires energy input—in
order to power whatever muscles are used to make that movement happen. An exception to
this would be the movement of falling—say out of your chair because you’re dozing
off as you’re listening to this. That part of your movement in going from chair height
to flat on the floor would not require any energy at all. However, if you’re going
to do anything to break your fall—like extending an arm out and applying pressure to the floor
so you don’t land too forcefully, this would require the input of energy and it would be
work.
Another kind of work that we’ve talked about at length in a previous module is transport
work. That’s the pumping of ions or some other solute from areas where they are in
lower concentration into an area where they are in higher concentration. I’m guessing
(and hoping) that you don’t want or need me to go into the differences between active
transport and facilitated diffusion again, but if you need to go back and review that
section please go right ahead.
Yet another kind of cellular work that is very important is chemical work. Earlier we
addressed this point when we talked about the anabolism of polymers from monomers. Remember
that macromolecules are often constructed out of organic building blocks, and the process
of dehydration synthesis was said to require the input of energy. This would be an example
of chemical work.
The opposite reaction—the catabolic breakdown of polymers, making monomers via the process
of hydrolysis—is not energy-requiring and would thus not be an example of chemical work.
This would be more like the car rolling down the hill with no exertion of force.
Now let’s go through a specific example involving glucose and starch. You should know
already that starch is a polysaccharide comprised of many molecules of glucose that are covalently
bonded together in long, branching chains. Now in a plant, where the glucose is being
produced by photosynthesis in all the green parts of the plant above the ground a lot
of the glucose gets stored in the form of starch molecules. Think of a potato plant
photosynthesizing in its leaves up here…
…and sending all the glucose down to the tubers where they are polymerized into the
starch of a potato. The cells making the starch out of the glucose must provide an outside
energy source in order to drive this polymerization reaction.
Now say you dig up that potato, clean it, and bake it for your dinner. When you eat
the potato, the enzymes in your digestive tract carry out the exact opposite reaction
of polymerization. Hydrolytic enzymes like the amylase in your saliva break down the
starch molecules into glucose. But this doesn’t require an outside energy source in order
to drive this catabolic reaction.
What the plant is doing when it makes the starch is cellular work, while what you are
doing when you digest the starch is not work because no added energy is required.
Now the real reason why dehydration synthesis reactions tend to be energy-requiring work,
while hydrolysis reactions are usually energy-independent non-work has entirely to do with the covalent
bonds that are changing as
the result of the chemical reactions.
Well one of those nucleoside triphosphate packets of potential energy—the one with
ribose for sugar and adenine for the nitrogenous base—is the molecule known as adenosine
triphosphate, or ATP. And this is used not only for RNA synthesis—it has actually become
the gold standard for storing nearly all the energy that we get from food, and it provides
this energy to the cells to drive basically all forms of cellular work. ATP is that “outside
energy source” we were talking about in the first part of this video.
You could say that we are like our cars in the sense that we need fuel in order to meet
our energy requirements, and we use the food that we eat as this fuel. Now there are many
kinds of food molecules that we use for fuel and they have varying amounts of potential
energy. But the energy potential of food molecules is not directly useable by enzymes or transport
proteins or motor proteins—the molecules that carry out our cells’ work. Instead,
these protein workhorses of our cells require energy input in the form of ATP, a standard
packet of fuel that interacts well with proteins and doles out an appropriate-sized amount
of energy, where it’s needed and when it’s needed.
We’re going to be focusing a lot on ATP in this module. So get ready.