You know that you need to have ATP in order for your muscles to contract. And you know that every time you use an ATP molecule up for energy, you end up with the products: ADP and inorganic phosphate. Those products are not waste products... instead, our cells recycle and reuse them; that should make sense to you, because it would be a huge waste and very energetically-costly to have to reacquire these materials to re-make ATP again. Now comes time for the question-- how do I recycle my ADP and inorganic phosphate back into ATP?

Answering that question is the focus of this page. It helps if you already know how most cells make ATP before we talk about the specific additional mechanisms that muscle cells have to help them recycle their ADP into ATP. So we will start with the basics, which are roughly described on pages 106 - 108 in your textbook.

How most cells make ATP

You may remember the term "cellular respiration," the term used for how our cells make ATP. It implies that cells are breathing, or taking in air, which they are doing by taking up oxygen from the blood. Every cell in our body uses that oxygen gas in its mitochondria to make ATP. However, our cells also need glucose to make ATP. If a cell has both glucose and oxygen, the following formula represents how it makes ATP:

cellrespformula.jpg (8926 bytes)

Notice that the process of breaking down one molecule of glucose leads to the production of 38 ATPs! Basically, as the bonds of glucose are broken, they release energy which gets transferred to the production of ATP. Again, exactly how that works is beyond the scope of this class.

In order for the chemical reaction drawn above to occur, your cells need to have both glucose and oxygen gas. However, when you are really moving a lot and contracting muscles, they run out of oxygen gas quicker than they can replenish them. Not all of the chemical steps to break down glucose and make ATP require oxygen... but most do. Here is a figure that summarizes how much ATP a cell can make with oxygen versus without oxygen:

cellresp.jpg (19453 bytes)

I hope you can tell from this figure that the majority (36) of the ATP are made when oxygen is present.

Therefore, our muscle fibers, which use up ATP and oxygen gas so quickly, have to have specializations for making ATP. The rest of this web page describes those specializations.

Muscle specializations for making ATP

Let's discuss these specializations problem-by-problem...

Problem 1: Muscle fibers need to have more access to oxygen gas

Solution: Muscle fibers contain a protein called myoglobin that is similar to hemoglobin. Myoglobin is an oxygen carrier. When blood goes to supply most cells, oxygen diffuses from the blood to the cell for immediate use. But when blood goes to supply muscle fibers, oxygen diffuses from the blood to myoglobin for oxygen storage within the muscle fiber as well as from the blood to the muscle fiber for immediate use.

Please note that having myoglobin doesn't totally solve the problem of getting enough oxygen for long periods. Instead, it just helps muscle fibers have more oxygen available to them than other cells.

Problem 2: Having enough glucose

Solution: Muscle fibers store glucose (a monosaccharide) as glycogen (a polysaccharide). In this manner, muscle fibers always have a reserve of glucose in them.

Problem 3: Getting ATP fast enough, since even with oxygen, ATP production isn't all that fast

Solution: When the muscle cell has a chance, it makes ATP. Then it converts its ATP into another form that won't get used until needed. The chemical form for storing the previously-made energy is a molecule called creatine phosphate. You see, muscles take their newly-made ATP and hydrolyze it-- but the energy from ATP hydrolysis isn't released but instead is stored again in a new high-energy bond in creatine phosphate. The inorganic phosphate yanked off the ATP is stuck onto a creatine molecule to make creatine phosphate. Whenever energy is needed, creatine phosphate is broken down to restore the ATP molecule so it can be used by the myosin heads for the powerstroke.

Problem 4: When activity is high, lactic acid builds up

    This problem doesn't really have a solution. Basically, lactic acid begins to form whenever your muscles are active for more than a minute or two, and they just cannot get enough oxygen. As lactic acid builds up, it diffuses out of your muscle fibers and into the blood... the blood takes it to the liver, which will eventually (with the expenditure of ATP) turn it back into glucose.

    Meanwhile, back in your muscle, lactic acid levels will be rising. Eventually the level of lactic acid in your muscle will rise high enough to cause your muscle to fatigue. When your muscle is fatigued, it will not be capable of all the work it can normally do.

    This whole situation with lactic acid building up occurs when your muscles are said to be in oxygen debt, not being able to get enough even with myoglobin available.

The reading material for this unit ends on page 293 in the section on muscle fatigue. We will cover fast and slow muscles in our next unit.