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:
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:
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
Problem 2: Having enough glucose
Problem 3: Getting ATP fast enough, since even with oxygen, ATP production isn't all that fast
Problem 4: When activity is high, lactic acid builds up
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.
© 2011 STCC Foundation Press