ATP is stored on the myosin cross bridges for immediate energy. After 5 to 6 seconds, this supply is exhausted and the muscle must rely on ............ to quickly produce more ATP.

Options

A. phosphocreatine
B. lactic acid
C. acetyl choline
D. pyruvic acid

Explanation

To solve the question, we need to understand the process of energy production in muscles during contraction. Here’s a step-by-step breakdown:

### Step 1: Understand the Role of ATP in Muscle Contraction
- ATP (adenosine triphosphate) is the primary energy currency in cells, including muscle cells. It is stored on myosin cross bridges and is essential for muscle contraction.

### Step 2: Recognize the Duration of ATP Supply
- The stored ATP on myosin cross bridges provides energy for muscle contraction for a very short duration, typically around 5 to 6 seconds.

### Step 3: Identify What Happens After ATP is Exhausted
- After this initial supply of ATP is exhausted, the muscle needs to find another way to produce ATP quickly to continue functioning.

### Step 4: Consider the Options Provided
- The options given are:
1. Phosphocreatine
2. Lactic acid
3. Acetylcholine
4. Pyruvate acid

### Step 5: Analyze Each Option
- **Phosphocreatine**: This compound can donate a phosphate group to ADP to quickly regenerate ATP, but it primarily supports energy needs for a few seconds and is not the main source after the initial ATP supply.
- **Lactic Acid**: This is produced during anaerobic respiration when oxygen levels are low. Muscles can rely on lactic acid fermentation to produce ATP quickly when the immediate ATP supply is depleted.
- **Acetylcholine**: This is a neurotransmitter involved in muscle contraction but does not directly produce ATP.
- **Pyruvate Acid**: This is a product of glycolysis and can enter aerobic respiration or be converted to lactic acid under anaerobic conditions, but it is not the immediate source of ATP.

### Step 6: Conclusion
- Based on the analysis, after the initial ATP supply is exhausted, the muscle must rely on **lactic acid** to quickly produce more ATP.

### Final Answer
The muscle must rely on **lactic acid** to quickly produce more ATP after the initial ATP supply is exhausted.

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