C

AP Exam 2 - Diagrams

1. Direct Phosphorylation (Creatine Phosphate System)

Type: Anaerobic (no oxygen required)

Speed: Very fast

Duration: ~10–15 seconds

ATP yield: 1 ATP per creatine phosphate

🧩 Steps:

  1. Muscles store creatine phosphate (CP) — a high-energy molecule.

  2. When activity starts, creatine kinase enzyme transfers a phosphate from CP → ADP.
    Creatine phosphate + ADP → ATP + Creatine\text{Creatine phosphate + ADP → ATP + Creatine}Creatine phosphate + ADP → ATP + Creatine

  3. The new ATP provides immediate energy for muscle contraction.

  4. Once CP stores are used up, muscles switch to other systems.

✅ Used for: short, intense bursts (sprinting, jumping, heavy lifting).

 

🔥 2. Anaerobic Phosphorylation (Glycolysis / Lactic Acid System)

Type: Anaerobic (no oxygen)

Speed: Fast

Duration: ~30–60 seconds

ATP yield: 2 ATP per glucose

🧩 Steps:

  1. Glucose (from blood or glycogen) enters the cytoplasm.

  2. Glycolysis breaks glucose into 2 pyruvate molecules, producing 2 ATP and 2 NADH.

  1. Because there’s no oxygen, pyruvate can’t enter mitochondria.

  2. Instead, it’s converted into lactic acid (lactate) to regenerate NAD⁺, allowing glycolysis to continue.

  3. Lactic acid can later be transported to the liver and converted back to glucose when oxygen becomes available (Cori cycle).

✅ Used for: high-intensity, short-term activity (400m sprint, fast cycling).

⚠ Downside: Lactic acid buildup → muscle fatigue and burn.

 

🌬 3. Aerobic Phosphorylation (Oxidative Phosphorylation)

Type: Aerobic (requires oxygen)

Speed: Slow

Duration: Minutes to hours

ATP yield: ~34 ATP per glucose

🧩 Steps:

  1. Glucose (or fatty acids, amino acids) enters the cell.

  2. Glycolysis occurs → produces 2 pyruvate + 2 ATP + 2 NADH.

  1. Pyruvate enters mitochondria and is converted to Acetyl-CoA.

  1. Krebs Cycle (Citric Acid Cycle):

    • Acetyl-CoA enters → produces NADH, FADH₂, and CO₂.

  2. Oxidative Phosphorylation (Electron Transport Chain + ATP Synthase):

    • NADH and FADH₂ donate electrons to the ETC in the inner mitochondrial membrane.

    • Electrons move through carriers → energy pumps H⁺ across membrane.

    • H⁺ gradient powers ATP synthase → makes ~34 ATP.

    • Oxygen is the final electron acceptor → forms water (H₂O).

✅ Used for: endurance or steady activity (jogging, swimming, cycling).

💚 Benefit: Produces the most ATP and no lactic acid buildup.

 

🧠 Summary Chart

System

Oxygen Needed

Location

Main Fuel

ATP Produced

Duration

Example Activity

Direct Phosphorylation

❌ No

Cytoplasm

Creatine Phosphate

1 ATP

10–15 sec

Sprint, jump

Anaerobic Phosphorylation

❌ No

Cytoplasm

Glucose/Glycogen

2 ATP

30–60 sec

400m run

Aerobic Phosphorylation (Oxidative)

✅ Yes

Mitochondria

Glucose/Fats

~34 ATP

Minutes–hours

Jogging, biking

 

 

 

 

 

 

 

 

⚡ Steps of Neuromuscular Junction Transmission

1. Nerve impulse reaches the axon terminal

  • A motor neuron carries an action potential (electrical signal) down its axon.

  • When the impulse reaches the axon terminal at the NMJ, it triggers the next step.

 

2. Calcium channels open

  • The axon terminal membrane has voltage-gated calcium (Ca²⁺) channels.

  • The arrival of the action potential causes these channels to open.

  • Ca²⁺ ions from the extracellular fluid rush into the axon terminal.

 

3. Vesicles release acetylcholine (ACh)

  • The influx of calcium causes synaptic vesicles (tiny sacs) to fuse with the neuron’s membrane.

  • These vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic cleft (the gap between neuron and muscle fiber).

 

4. Acetylcholine binds to receptors on the muscle fiber

  • ACh diffuses across the cleft and binds to ACh receptors on the sarcolemma (muscle fiber’s cell membrane).

  • These receptors are part of ligand-gated sodium (Na⁺) channels.

 

5. Sodium channels open — muscle fiber depolarizes

  • Once ACh binds, the Na⁺ channels open, allowing sodium ions to enter the muscle fiber.

  • This causes depolarization (inside of the muscle cell becomes more positive).

  • This change in voltage generates a muscle action potential.

 

6. Action potential spreads across the sarcolemma

  • The muscle action potential travels along the sarcolemma and down the T-tubules (tunnel-like extensions into the muscle cell).

  • This triggers the sarcoplasmic reticulum (SR) to release Ca²⁺ inside the muscle fiber.

 

7. Calcium triggers muscle contraction

  • The released Ca²⁺ binds to troponin on the thin (actin) filaments.

  • This shifts tropomyosin, exposing binding sites for myosin.

  • Cross-bridges form between actin and myosin, leading to muscle contraction via the sliding filament mechanism.

 

8. Acetylcholine is broken down

  • To stop continuous stimulation, the enzyme acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft.

  • The breakdown products (choline and acetate) are reabsorbed by the neuron to make new ACh.

 

9. Muscle relaxes

  • When ACh is gone and Ca²⁺ is pumped back into the SR, the muscle fiber repolarizes and relaxes until the next signal.

 

Crossbridge Cycle

  1. Myosin binds actin (crossbridge forms).

  2. Power stroke (ADP + Pi released → myosin pulls actin).

  3. New ATP binds → myosin detaches.

  4. ATP hydrolyzed → myosin re-cocks.