ATP-PC and glycosis

ATP-PC pathway (write the reaction)

Phosphocreatine (PC) + ADP → ATP + Creatine (Cr), catalyzed by creatine kinase.

Creatine kinase

The enzyme that catalyzes the ATP-PC reaction; activated by rising ADP, inhibited by high ATP. Also the rate-limiting enzyme of the ATP-PC system.

How does creatine (PC) help resynthesize ATP?

PC acts as a high-energy phosphate donor — it transfers its phosphate group directly to ADP to rapidly reform ATP at the very start of exercise, before glycolysis/aerobic metabolism ramp up.

ATP-PC (phosphagen) system — how long does it dominate?

Provides energy for the first ~5-15 seconds of high-intensity exercise; a simple one-enzyme reaction, so it's the fastest ATP source available.

Lactic acid vs. lactate — what's the difference?

Lactic acid is the acid form, carrying an extra H+. Lactate is the ionized conjugate-base form left once that H+ dissociates. At physiological pH, lactate (not lactic acid) is essentially what's actually formed and present in the body.

Glycolysis — starting substrate, location, and net ATP (from glucose)?

Breaks down glucose (or glycogen) in the cytosol/sarcoplasm; net gain of 2 ATP starting from glucose.

Glycolysis ATP investment/generation — starting with GLUCOSE

2 ATP invested (energy investment phase) → 4 ATP generated (energy generation phase) → NET = 2 ATP.

Glycolysis ATP investment/generation — starting with GLYCOGEN

1 ATP invested → 4 ATP generated → NET = 3 ATP. (Only 1 ATP needed because glycogen is phosphorylated using intracellular Pi instead of a full ATP.)

Glucose

The primary 6-carbon blood sugar that enters glycolysis.

Glycogen

The stored, branched polysaccharide form of glucose, kept in muscle and liver cells; broken down via glycogenolysis.

Glucose 6-phosphate (G-6-P)

Formed by phosphorylating glucose; traps glucose inside the cell since the charged molecule can't cross back out through the membrane.

Fructose 6-phosphate (F-6-P)

A structural isomer of G-6-P; the substrate immediately before glycolysis's main regulatory (rate-limiting) step.

Fructose 1,6-bisphosphate (F-1,6-BP)

Formed by PFK during the second ATP-investment step; this 6-carbon molecule splits into two 3-carbon molecules, beginning the energy generation phase.

Phosphofructokinase (PFK)

The rate-limiting enzyme of glycolysis; catalyzes F-6-P → F-1,6-BP. Stimulated by ADP/Pi, inhibited by ATP, H+, and citrate.

Pyruvate

The 3-carbon product of glycolysis when NADH does NOT donate its hydrogen to it; aerobically it goes on to form acetyl-CoA.

Lactate

The dominant 3-carbon end-product of glycolysis, formed when pyruvate accepts a hydrogen atom from NADH (via lactate dehydrogenase) — this regenerates NAD+ so glycolysis can continue.

Why is lactate formed even when oxygen is available?

Because lactate dehydrogenase activity is high and glycolysis produces large quantities of both pyruvate and NADH, which drives the reaction toward lactate — lactate is the primary end-product of glycolysis at rest AND during exercise, not just under low-oxygen conditions.