EXERCISE INTENSITY & DURATION: SUBSTRATE UTILIZATION (CHO)

Question 1

Outline the dynamics of carbohydrate and fat
metabolism during physical activity: Low intensity

Answer 1

Low-Intensity Exercise (≤ 40% VO₂ max, Long Duration)
- Primary fuel source: Fat (plasma free fatty acids)
- Carbohydrate contribution: Minimal
- Metabolic pathway: Beta-oxidation of fatty acids → - - – Krebs cycle → Electron Transport Chain (ETC)
- Key takeaway: Fat oxidation is slow but sustainable for long durations.

Question 2

Outline the dynamics of carbohydrate and fat
metabolism during physical activity - Moderate intensity

Answer 2

Moderate-Intensity Exercise (40–65% VO₂ max, Moderate Duration)
- Primary fuel source: Mixture of carbohydrates and fat
- Fat oxidation increases but carbohydrate becomes more dominant as intensity rises.
- Muscle glycogen is broken down via glycolysis.
- Key takeaway: Fat contributes significantly but carbohydrate availability becomes more critical as intensity rises.

Question 3

Outline the dynamics of carbohydrate and fat
metabolism during physical activity - High intensity

Answer 3

High-Intensity Exercise (≥ 75% VO₂ max, Short Duration)
- Primary fuel source: Carbohydrates (muscle glycogen and blood glucose)
- Fat oxidation decreases due to limited time for beta-oxidation.
- Anaerobic glycolysis becomes dominant, producing lactate.
- Key takeaway: Carbohydrates provide rapid energy but deplete quickly.

Question 4

Outline the dynamics of carbohydrate and fat
metabolism during physical activity - Prolonged duration

Answer 4

Prolonged Exercise (≥ 90 min, Submaximal Intensity)
- Gradual shift from carbohydrate to fat metabolism as glycogen stores decline.
- Liver gluconeogenesis maintains blood glucose using lactate, glycerol, and amino acids.
- Fatty acids (from adipose tissue) become the primary fuel source as glycogen runs out.
- Key takeaway: Fat oxidation is critical for endurance but requires carbohydrate metabolism to continue.

Question 5

Outline the 3 major energy producing systems in cells: ATP-PCr System

Answer 5

ATP-PCr System (Phosphagen System) – Immediate Energy
*Cytoplasm!

quick energy, (very short, very intense)

• Duration: 0–10 seconds
• Fuel Source: Phosphocreatine (PCr)
• Speed: Fastest
• Oxygen Needed? No (Anaerobic)
• Used For: Explosive movements (sprinting, weightlifting)

Question 6

Outline the 3 major energy producing systems in cells: Anaerobic Glycolysis (Lactic Acid System)

Answer 6

Anaerobic Glycolysis (Lactic Acid System) – Short-Term Energy
*Cytoplasm!

breakdown of glucose to pyruvate can end aerobically (pyruvate ->acetyl CoA->TCA->ETC – moderate
durations/intensities) or can end anaerobically (Pyruvate->lactate + H+ ions; short durations, higher intensity) – replenished relatively quickly (high to moderate intensity and moderate to short durations

Breakdown of glucose into pyruvate; either pyruvate to Acetyl-COA (TCR cycle) and using oxygen as the final electron acceptor through electron transport cycle
or
fast build up of pyruvate generates lactate as a byproduct (happens without using oxygen to generate ATP, leads to greater fatigue)

Question 7

Outline the 3 major energy producing systems in cells: Aerobic System (Oxidative Phosphorylation)

Answer 7

Aerobic System (Oxidative Phosphorylation) – Long-Term Energy
*Mitochondria!

can derive acetyl-CoA from either CHO or LIPID sources to generate reducing equivalents - NADH and FADH2 to drive the electron transport chain (ETC) and generate lots of ATP – pretty much endless supply for lower intensity (mainly at rest and low intensity/long duration)

• Duration: 2 min – several hours
• Fuel Source: Carbohydrates (first) and fats (later), some protein in extreme cases
• Speed: Slowest
• Oxygen Needed? Yes (Aerobic)
• Used For: Endurance exercise (marathon running, cycling)

Question 8

What determines how fast you need to produce ATP?

Answer 8

Exercise intensity

Question 9

Fed state

Answer 9

Insulin promotes the building up of glycogen (storage molecule of glucose) AKA helps clear blood glucose and store it in the liver as glycogen

Glycogen is what maintains blood glucose during the day and during exercise

Question 10

What does epinephrine promote?

Answer 10

glycogen breakdown (glycogenolysis) and blood glucose maintenance

in fasted AND fed state

Question 11

Fasted state (exercise)

Answer 11

Glucagon levels rising promotes gluconeogenesis (building of glucose molecules) which promotes glycogen breakdown (glycogenolysis) and blood glucose maintenance

Question 12

Outline the role of the liver in maintaining blood glucose: Glycogenolysis

Answer 12

Releases glucose into the bloodstream during fasting or exercise.

Important during early stages of exercise when muscle glycogen is still available.

Question 13

Outline the role of the liver in maintaining blood glucose: Gluconeogenesis

Answer 13

Produces glucose from lactate, glycerol, and amino acids during prolonged exercise or fasting.

Essential when glycogen stores are depleted.

Occurs primarily in the liver

precursors must first convert to **Oxaloacetate

Glycolysis and gluconeogenesis are reciprocally regulated (opposite processes)

Question 14

Outline the role of the liver in maintaining blood glucose: Cori Cycle

Answer 14

AKA Lactate Recycling

Converts lactate (from anaerobic metabolism) into glucose in the liver.

Helps maintain blood glucose during high-intensity exercise.

Question 15

Outline the role of the liver in maintaining blood glucose: Glucose-Alanine Cycle

Answer 15

Converts muscle-derived alanine (from protein breakdown) into glucose.

Used in prolonged exercise when carbohydrate stores are low.

Question 16

Contrast the speed of energy transfer from carbohydrate and fat combustion: Carbohydrate Combustion

Answer 16

Fast ATP production (supports high power output).

Requires less oxygen per ATP (more efficient in oxygen-limited conditions).

Yields ~36 ATP per glucose molecule.

Limited storage capacity (~500g glycogen in liver and muscles).

Primary fuel for high-intensity, short-duration exercise.
Depletes quickly but provides rapid energy.

Question 17

Contrast the speed of energy transfer from carbohydrate and fat combustion: Fat Combustion

Answer 17

Slow ATP production (supports low power output).
Requires more oxygen per ATP (less efficient in oxygen-limited conditions).

Yields ~100+ ATP per fatty acid.

Nearly unlimited storage capacity (body fat stores).

Primary fuel for low-intensity, long-duration exercise.

More sustainable but takes longer to mobilize.

Key Takeaway:
Carbohydrates are ideal for quick energy during high-intensity exercise, while fats are better suited for endurance and prolonged activity.

Question 18

What are the carbohydrate fuel sources?

Answer 18

• Blood glucose from liver: Maintains energy supply, especially when muscle glycogen is running low
• Muscle glycogen: Rapidly broken down for ATP production during moderate to high-intensity exercise

Question 19

What are the fat fuel sources?

Answer 19

• Plasma fatty acids (from adipose tissue): Used by muscles for ATP during low-intensity, long-duration exercise
• Intramuscular triglycerides (IMTG): Broken down into fatty acids for ATP during low to moderate-intensity exercise or fasting states

Question 20

What is the major fuel source used by muscle cells during high intensity exercise?

Answer 20

Glucose/glycogen

Question 21

Which substrates are used concurrently in cells at all times?

Answer 21

Carbs and lipids

Question 22

What is the molecular endpoint of energy producing processes?

Answer 22

ATP

Question 23

What impacts substrate utilization?

Answer 23

Exercise intensity - higher intensity = CHO dominates

Exercise duration - longer duration = lipids dominate

Question 24

What changes the relative proportion of CHO to lipid?

Answer 24

Exercise intensity

Question 25

Glycolysis

Answer 25

breakdown of glucose to pyruvate *Cytoplasm!! Glucose is USED hormones: impact of exercise:

Question 26

Glycogenesis

Answer 26

glycogen synthesis (glucose to glycogen – anabolic) Glucose is STORED Cytoplasm!! hormones: impact of exercise:

Question 27

Gluconeogenesis

Answer 27

synthesis of glucose from non-CHO substrates (protein -> glucose) *Liver!! hormones: impact of exercise:

Question 28

Glycogenolysis

Answer 28

breakdown of glycogen to glucose (catabolic) *Cytoplasm!! Liver & skeletal muscle tissue!! hormones: impact of exercise:

Question 29

Major regulatory enzymes during glycolysis: Hexokinase

Answer 29

Trap glucose positive regulators (increase hexokinase activity) = glucose, insulin, Ca2+, epinephrine negative regulators = glucagon, Fructose-6-p This enzyme grabs glucose when it enters the cell and adds a phosphate to it. This keeps the glucose inside the cell so it can’t escape. What helps (activates) it? Glucose, insulin (which signals energy storage), calcium, and epinephrine (which signals energy release). What slows it down? Glucagon (which signals the need for glucose elsewhere) and high levels of fructose-6-phosphate (F6P), a later step in glycolysis.

Question 30

Major regulatory enzymes during glycolysis: Glycogen phosphorylase

Answer 30

positive = ADP, Pi, and AMP (from breaking down ATP!!), Ca2+ (calcium released when muscles contract), and epinephrine (for breaking down fuels) negative = high ATP/ADP ratio, citrate This enzyme breaks down glycogen (stored glucose) into smaller glucose units when the body needs energy. What activates it? ADP, Pi, AMP (signals that energy is low), calcium, and epinephrine (signals energy demand). What inhibits it? ATP (indicates high energy levels), citrate (a byproduct of energy metabolism).

Question 31

Major regulatory enzymes during glycolysis: Phosphofructo kinase (PFK)

Answer 31

positive = ADP, Pi, AMP (ATP breakdown) negative = high ATP/ADP ratio, citrate This is the most important control point in glycolysis. It decides whether the glucose will fully go through glycolysis. What activates it? ADP, Pi, AMP (signals that energy is low and glycolysis should continue). What slows it down? ATP (signals enough energy is available) and citrate (which means the cell is already making enough energy).

Question 32

Major regulatory enzymes during glycolysis: Pyruvate kinase (PK)

Answer 32

end of glycolysis positive = Fructose-1, 6bp, insulin negative = ATP, ADP, acetyl CoA, glucagon This enzyme helps convert the last molecule in glycolysis into pyruvate, making ATP in the process. What activates it? Fructose-1,6-bisphosphate (an earlier glycolysis product) and insulin (signals energy use). What slows it down? ATP (signals enough energy is available), acetyl-CoA (a product used in fat metabolism), and glucagon (signals energy conservation).

Question 33

What is the ATP/ADP ratio?

Answer 33

high ratio - sufficient amount of energy, signals that energy producing processes can slow down low ratio - more energy needed

Question 34

Muscle glycogen

Answer 34

1 g of muscle glycogen is Stored with 3 g of water What happens to water when glycogen stores are depleted -- weight loss What might happen to body weight if you go from a low carb to high carb diet initially? -- gain weight due to water weight

Question 35

Why do we store glucose in the liver?

Answer 35

blood glucose regulation and quick energy supply

Question 36

Can we normally access glycogen in the muscle?

Answer 36

yes from local muscle cells during exercise (not in the bloodstream)

Question 37

When would we store/build glycogen?

Answer 37

High carb diet, or post exercise the body builds more glycogen

Question 38

What happens to the excess glucose in the blood when muscle and liver glycogen are full/at capacity?

Answer 38

Excess carbs are converted and stored in adipose tissue; glucose converted into fat

Question 39

What determines level of glycogen depletion during exercise?

Answer 39

Intensity and duration; slope of line represents the rate of depletion; longer and harder is steeper line (on graph of muscle glycogen over exercise time)

Question 40

How does the body establish a new homeostatic set point at a low energy state (i.e. after muscle contraction)?

Answer 40

(within muscle cell) Sensing levels of phosphate, AMP, and Ca2+ WHICH ACTIVATE glycogenolytic enzymes to produce ATP (increase ATP production)

Question 41

How do hormones regulate the breakdown of glycogen? (muscle glycogenolysis process)

Answer 41

1. Epinephrine (!!!) binds to androgenic receptor on plasma membrane 2. Generates cyclic AMP 3. Cyclic AMP provides phosphate group to activate protein kinase 4. Protein kinase activates phosphorylase kinase 5. Phosphorylase kinase activates phosphorylase B into phosphorylase A (active enzyme!) 6. Phosphorylase A adds a phosphate group to a glycogen molecule (glucose molecule) 3. Results in glucose molecule moving to product G-1-P that can enter glycolysis (ATP generation)

Question 42

Where does the body get glucose if the liver is not full of glycogen?

Answer 42

From proteins/amino acids that store glucose; but has negative effect on performance

Question 43

What are the metabolic byproducts or substrates used in gluconeogenesis?

Answer 43

Amino acids (from protein breakdown) lactate (from exercise) glycerol (from fatty acid/triglyceride breakdown)

Question 44

What hormone best regulates glycogenolysis on the liver, and what is the direct response of the liver?

Answer 44

glucagon response is to maintain blood glucose fun fact it comes from pancreas

Question 45

How does glucose get into the muscle in order to ultimately be used for energy?

Answer 45

1. Insulin dependent (resting conditions) 2. Insulin independent (muscle contraction dependent)

Question 46

Most important: Insulin independent pathway (muscle contraction dependent)

Answer 46

1. Muscle contraction (releases calcium) 2. Increased intramuscular calcium levels activates signaling cascade and breakdown of ATP to AMPK 3. Energetic stress sensed by AMPK 4. GLUT4 vesicles moving to the muscle fiber membrane 5. Glucose removed from the plasma and into the muscle fiber 6. glucose “trapped” in muscle fiber by hexokinase converting glucose to glucose-6-phosphate Key players: calcium, energetic stress, GLUT4 vesicles, glucose moved to muscle fiber, trapped by hexokinase