What is the functional unit of muscle contraction?

Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units, primarily glucose. In humans, these molecules play a major role in energy storage and structural support. The body must maintain a readily available energy reserve to sustain metabolic demands between meals and during physical activity. Glycogen is the primary storage polysaccharide in humans, synthesized in liver and skeletal muscle cells and tightly regulated by hormonal control.

A. Lactose: Lactose is a disaccharide composed of glucose and galactose and is primarily found in milk and dairy products. It is not a storage polysaccharide in human tissues. Instead, it functions as a dietary carbohydrate that is digested in the small intestine by lactase. Since it is not synthesized for long-term energy storage in liver or muscle, it is incorrect.

B. Glucose: Glucose is a monosaccharide and serves as the primary immediate energy source for cellular metabolism. It is not a polysaccharide and therefore cannot be stored in long polymerized form as glycogen. Excess glucose in the body is converted into glycogen for storage or into fat for long-term energy reserves.

C. Glycogen: Glycogen is the storage polysaccharide synthesized in humans and stored mainly in the liver and skeletal muscles. It consists of highly branched chains of glucose molecules linked by α-1,4 and α-1,6 glycosidic bonds. In the liver, glycogen helps maintain blood glucose levels during fasting, while in muscle, it provides a rapid energy source for contraction. Its structure allows rapid mobilization when energy is required.

D. Cellulose: Cellulose is a structural polysaccharide found in the cell walls of plants and is composed of β-glucose units linked by β-1,4 glycosidic bonds. Humans lack the enzyme cellulase required to digest cellulose, making it a dietary fiber rather than a storage molecule. It is not synthesized or stored in human tissues.

Skeletal muscle is organized into a hierarchical structure that allows coordinated contraction and force transmission. Each muscle is composed of individual muscle fibers grouped into fascicles, which are further bundled to form the entire muscle. Connective tissue layers surround each structural level to provide support, protection, and pathways for blood vessels and nerves. These layers are arranged from the deepest level surrounding individual fibers to the most superficial layer covering the entire muscle and separating it from surrounding structures.

A. Fascia → epimysium → perimysium → endomysium: This reverses the correct anatomical order from superficial to deep rather than deep to superficial. Fascia is the most superficial connective tissue layer, but epimysium lies directly over the muscle, not beneath fascia. Perimysium surrounds fascicles, and endomysium surrounds individual muscle fibers at the deepest level. Therefore, this sequence incorrectly inverts the hierarchical organization of muscle connective tissue layers.

B. Endomysium → perimysium → epimysium → fascia: This reflects the anatomical organization from deepest to most superficial layer. The endomysium surrounds individual muscle fibers, providing support and a pathway for capillaries and nerve fibers. The perimysium encloses bundles of muscle fibers called fascicles, while the epimysium surrounds the entire muscle. Finally, fascia lies most superficially, separating and encasing muscles within compartments.

C. Endomysium → epimysium → perimysium → fascia: This disrupts the correct sequence of connective tissue organization. While endomysium is correctly placed as the deepest layer, epimysium and perimysium are reversed. The perimysium must surround fascicles before the epimysium encloses the entire muscle.

D. Fascia → endomysium → perimysium → epimysium: This option begins with fascia, which is the most superficial layer rather than the deepest. It then incorrectly places endomysium beneath fascia without proper intermediate organization. Additionally, it reverses the correct order of perimysium and epimysium. This sequence does not reflect the anatomical layering of skeletal muscle connective tissues.