CH.2: ENERGY SYSTEMS

CH.2: ENERGY SYSTEMS (How we make the ATP we need for the SFM!)

ATP (ADENOSINE TRI-PHOSPHATE)

• ATP is used by muscles during the Sliding Filament Mechanism.

ATP HYDROLYSIS

• Hydrolysis (breakdown) of ATP provides energy for muscle contraction.

• When the 3rd phosphate group is removed, energy is released.

• 50 new X-bridges form per myosin head every second, requiring a lot of ATP.

ATP STORED IN MUSCLE CELLS

• A small amount of ATP is stored in muscle fibers to fuel a "flight or fight" response.

• This supply lasts only 1-2 seconds.

• We don’t store more ATP because it’s highly reactive with water and wouldn’t remain stable in cells.

ATP RE-SYNTHESIS

• The re-synthesis of ATP requires energy.

• The source of this energy comes from the food we eat.

• To re-synthesize ATP, a phosphate group (PO4 -3) is added back onto the ADP.

THE 3 ENERGY SYSTEMS

• Three different methods our body uses to put ATP back together:

  • ATP-PC System (Anaerobic Alactic)

  • Glycolytic System (Anaerobic Lactic)

  • Aerobic System

• All 3 systems make ATP, but 1 is usually more dominant depending on activity.

ATP-PC SYSTEM (Anaerobic Alactic)

• ATP re-synthesis from phosphocreatine (PC) breakdown.

When Do We Use It?

• During all-out maximum efforts

Starting Fuel?

• Phosphocreatine (PC)

# of Rxns Involved?

• 1 (which explains why it’s used for sprinting)

Duration?

• 10s

ATP Produced?

• 1/PC

Reason it Stops?

• Run out of PC (quickly!)

• We don’t store much PC in our muscles because our body prioritizes storing another fuel source (Glucose).

• There’s energy in those bonds!

GLYCOLYTIC SYSTEM (Anaerobic Lactic)

• ATP re-synthesis from glucose (C6H{12}O_6) breakdown.

STEPS OF GLYCOLYSIS:

• The original C6H{12}O_6 molecule of glucose starts to get broken down.

• As those bonds break, enough energy is released to re-generate 2 ATP’s.

• By the end of the 10 steps, all the carbons & oxygens are still intact, but some of the hydrogens have been removed (and get picked up by NAD).

When Do We Use It?

• During high-intensity work

Starting Fuel?

• Glucose

# of Rxns Involved?

• 10 (still pretty simple)

Duration?

• Dominates during high-intensity work lasting ~1 min

ATP Produced?

• 2/molecule of glucose

Reason it Stops?

• During high-intensity work, the hydrogens that are removed from glucose build up = pain!

• Otherwise, this system is always operating.

WHAT’S NAD?

THE FATE OF PYRUVATE..

• When enough oxygen is available to the muscle cell (since they’re not working very hard), the 2 pyruvate molecules will remain as pyruvate.

• They will move to the mitochondria, which is where the Aerobic Energy System occurs.

• When enough oxygen is NOT available to meet the muscle cells’ demand for it (since they’re working so hard you physically can’t breathe in enough O_2), pyruvate has to do NAD’s job and becomes a temporary “Taxi” for H’s

• When pyruvate takes the H’s, it becomes Lactic Acid/Lactate.

IS THE BUILD-UP OF LACTIC ACID “BAD”?

• No!

• But the build up of Hydrogen ions (H^+) is ‘bad’

• It’s the H^+ build up that causes the discomfort you feel at very high intensities

• Pyruvate accepting the H^+ buys you a little time (by temporarily ‘freeing up’ some NAD) before the H^+ ions overwhelm the muscles

IN FACT… LACTIC ACID IS YOUR HERO!

• The conversion of pyruvate into Lactic acid temporarily prevents H^+ build-up (buying you a little more time)

• A build up of Lactate (C3H5O_3) in the blood actually triggers your HR and respiratory rate to increase = more oxygen comes into muscles

• Plus, that Lactate can be converted back into pyruvate (once the H^+ has been removed) and used as fuel elsewhere in the body (brain, heart & other muscles)

SO…WHY ARE H+’ s “BAD”???

• They change the pH of the muscle tissue, making the muscles more acidic.

  • Therefore…. Glycolytic enzymes stop working

• They compete with Ca^{2+} for the binding sites on the troponin.

  • Therefore… Muscles don’t contract as efficiently

AEROBIC SYSTEM

• ATP re-synthesis from complete glucose (C6H{12}O_6) breakdown.

When Do We Use It?

• During low-intensity (or “sub-maximal” work)

Starting Fuel?

• Glucose (or fatty acids…or even amino acids if necessary!)

# of Rxns Involved?

• A lot!

Duration?

• Always the dominant system unless work rate gets too high

ATP Produced?

• 36/molecule of glucose

Reason it Stops?

• During high-intensity work, the body can’t take in enough oxygen to allow this system to work.

• Otherwise, this system is always operating.

THE 3 STAGES

• STAGE 1 – THE CONVERSION OF PYRUVATE

• Each 3-carbon pyruvate enters the mitochondria, where it is further broken down into a 2-carbon acetyl-CoA

  • The 3rd carbon is released as CO_2 (along with the oxygen attached to it)

  • The hydrogens that are removed are picked up by…NAD

• STAGE 2 – THE KREB’S CYCLE

  • Each 2-carbon acetyl-CoA is broken down completely

  • Both remaining carbons (and the oxygens attached to them) are released as CO_2

  • The remaining hydrogens are picked up by NAD (3) and FAD (1)

  • Plus, with all those bonds breaking, we release enough energy to resynthesize 1 ATP!

• STAGE 3 – THE ELECTRON TRANSPORT CHAIN

  1. NADH and FADH are oxidized (lose their hydrogens). Hydrogen’s proton & electron are split-up, and the proton is actively transported across the inner membrane.

  2. Those hydrogen protons start to build up, creating a very high concentration of hydrogen protons on one side of the mitochondria’s inner membrane, creating a gradient

  3. The protons want to diffuse to an area of lower concentration, but can ONLY cross the inner membrane through a special protein…ATP Synthase (which is where ADP and Pi are located)

  4. The movement of all the H^+’s through ATP synthase causes it to spin, literally ‘pushing’ ADP and Pi back together again!

  5. Oxygen picks up the H^+’s (and the electrons) once they’ve gone through ATP synthase (otherwise, H^+’s wouldn’t want to continue to diffuse!)

     • Picks up hydrogens as they come through ATP synthase, becoming H_2O

     • Produced as we remove the carbons (and the oxygen attached to them) off glucose molecules

OXYGEN’S IMPACT ON GLYCOLYSIS

• Why does “Club Mitochondria” close?

• When enough oxygen is NOT available to meet the muscle cells’ demand for it (since they’re working so hard you physically can’t breathe in enough O_2)…

• Pyruvate has to do NAD’s job and becomes a temporary “Taxi” for H’s

• When pyruvate takes the H’s, it becomes Lactic Acid/Lactate

• Although the Glycolytic System is Anaerobic (and can produce ATP in the absence of oxygen), a shortage of O_2 does explain why NADH can’t “drop off the H’s”

• Consider this… As H^+’s build-up in the “matrix”, the H’s attached to NADH wouldn’t want to “get out of the Taxi”, so NADH can’t drop the H’s it’s already carrying off!