Go through the following statements
During muscle contraction, the width of A-band remains constant while I-band shortens.
During muscle contraction, the `Ca^(2+)` released from the SR bind to the troponin component of thin filament
The process of muscle contraction is faster in smooth muscle as they have a well developed sarcoplasmic reticulum.
The red muscle fibres have abundant mitochondria, low glycogen and poorly formed sarcoplasmic reticulum.

Options

A. (i),(ii), and (iii)
B. (i),(ii) and (iv)
C. (i),(iii) and (iv)
D. All are correct

Answer

To analyze the given statements about muscle contraction, we will evaluate each statement step by step.

### Step 1: Evaluate the first statement
**Statement:** "During muscle contraction, the width of A-band remains constant while I-band shortens."

- **Explanation:** The A-band is the region of the sarcomere that contains thick filaments (myosin) and remains the same length during contraction. The I-band, which contains thin filaments (actin), shortens as the filaments slide past each other during contraction.
- **Conclusion:** This statement is **correct**.

### Step 2: Evaluate the second statement
**Statement:** "During muscle contraction, the Ca²⁺ released from the SR bind to the troponin component of the thin filament."

- **Explanation:** When muscle contraction is initiated, calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum (SR) and bind to troponin, a protein on the thin filament. This binding causes a conformational change that exposes the active sites on actin, allowing myosin heads to attach and initiate contraction.
- **Conclusion:** This statement is **correct**.

### Step 3: Evaluate the third statement
**Statement:** "The process of muscle contraction is faster in smooth muscle as they have a well-developed sarcoplasmic reticulum."

- **Explanation:** Smooth muscle contraction is generally slower compared to skeletal muscle contraction. Skeletal muscles have a well-developed sarcoplasmic reticulum and can contract more rapidly due to their structure and function. Therefore, this statement is misleading.
- **Conclusion:** This statement is **incorrect**.

### Step 4: Evaluate the fourth statement
**Statement:** "The red muscle fibers have abundant mitochondria, low glycogen, and poorly formed sarcoplasmic reticulum."

- **Explanation:** Red muscle fibers (slow-twitch fibers) are known for having a high number of mitochondria, which supports aerobic respiration and endurance activities. They typically have higher glycogen levels compared to white fibers, which are more anaerobic. The statement about poorly formed sarcoplasmic reticulum is somewhat misleading, as red fibers have a functional SR, but they are not as extensive as in fast-twitch fibers.
- **Conclusion:** This statement is **partially correct** but misleading regarding glycogen levels.

### Final Conclusion
From the evaluation:
- The first and second statements are **correct**.
- The third statement is **incorrect**.
- The fourth statement is **partially correct**.

Thus, the correct statements are the first and second ones.