[Nicos]

Glycogen phosphorylase, concerned with the breakdown of glycogen, is activated by phosphorylation and inactivated by dephosphorylation. By using common kinases and phosphatases, glycogen synthase can be activated when glycogen phosphorylase is inactivated, and vice versa.
There is an excellent linear correlation between the activity of glycogen synthase a (active form) and the rate of glycogen synthesis in liver. This positive correlation is also observed in fast-twitch skeletal muscle, where glycolysis from glycogen is a vital source of ATP for contraction.
(Medical Sciences - Jeannette Naish, Patricia Revest & Denise Syndercombe Court, p. 102)

I'm not sure if I understand the last sentence. How can glycolysis be a source for ATP if glycolysis is a process by which, indeed, ATP is produced, but it is not a substance. Glucose is a source which can be obtained from glycogen by glycogenolysis, which subsequently can be used to produce ATP.

Can anyone elaborate on this quote from one of my textbooks? I don't really 'get' it, especially the underlined part. My understanding of biochemistry is fairly limited.

Thank you in advance.

[Arkcon]

Well, you're kinda all over the place with this question, but to try to answer some of the many questions:

How can glycolysis be a source for ATP if glycolysis is a process by which, indeed, ATP is produced, but

Glycolysis, the breakdown of glucose into pyruvic acid, does produce ATP, and it does require ATP ... it simply produces more than it consumes.

it is not a substance

.

Don't know what you mean here, maybe you meant to say substrate, and not substance. 

Glucose is a source which can be obtained from glycogen by glycogenolysis, which subsequently can be used to produce ATP.

Seems like a correct statement, what's the exact problem?

Can anyone elaborate on this quote from one of my textbooks? I don't really 'get' it, especially the underlined part. My understanding of biochemistry is fairly limited.

Glycogen phosphorylase, concerned with the breakdown of glycogen, is activated by phosphorylation and inactivated by dephosphorylation. By using common kinases and phosphatases, glycogen synthase can be activated when glycogen phosphorylase is inactivated, and vice versa.
There is an excellent linear correlation between the activity of glycogen synthase a (active form) and the rate of glycogen synthesis in liver. This positive correlation is also observed in fast-twitch skeletal muscle, where glycolysis from glycogen is a vital source of ATP for contraction.
(Medical Sciences - Jeannette Naish, Patricia Revest & Denise Syndercombe Court, p. 102)
Thank you in advance.

[/quote]

Maybe you can rewrite this, to make it clearer for yourself?  You'll be able to use the explanation you write on an exam, so you can show you understand the process.  Try rewriting it as bullet points, filling in other steps as you go.

[Babcock_Hall]

I can make some educated guesses.  In fast-twitch muscle, anaerobic catabolism of glycogen is a major pathway by which ATP is synthesized from ADP and inorganic phosphate.  It probably is the major pathway for short durations (such as a 100 meter dash).  In other types of tissue, aerobic catabolism of glycogen or other substances the most important route, and anaerobic catabolism is unimportant.  Anaerobic catabolism means that the pyruvate will probably be converted into lactate and exported.  Aerobic catabolism means that pyruvate is oxidized ultimately into carbon dioxide.

Stored glycogen in muscle is broken down into glucose 1-phosphate, then glucose 6-phosphate.  This compound is run through glycolysis, to produce 2 pyruvate molecules per every glucose 6-phosphate (free glucose is not an intermediate in this process).  Free glucose can also be taken up from the bloodstream, and then it is converted into glucose 6-phosphate and run through glycolysis.  Glycolysis is a process in which the favorable free energy from certain chemical reactions is coupled to the unfavorable free energy of synthesizing ATP from phosphate and ADP.

[Nicos]

Thank you for your prompt response, Arkcon and Babcock_Hall and apologies for my not so promopt one.

Yes, I meant 'substrate'. It's crystal clear now, I don't know why it didn't make sense in the first place.