Which of the following is chemically stable (unreactive)?

A. Transport proteins: Transport proteins are essential for moving ions, nutrients, and other molecules across the plasma membrane. While they play an important role in cardiac muscle function, they do not address mechanical stress caused by repeated contractions.

B. Glycolipids: Glycolipids contribute to cell recognition, signaling, and stability of the cell membrane. However, they are not structural elements that resist the pulling forces and stress cardiac muscle cells experience during contraction.

C. Tight junctions: Tight junctions create a seal between adjacent cells to prevent leakage of extracellular fluid. In cardiac muscle, their role is less about mechanical strength and more about maintaining selective permeability, so they do not counteract contractile stress effectively.

D. Desmosomes: Desmosomes are strong anchoring junctions that mechanically bind cardiac muscle cells together. They provide structural integrity and prevent separation during forceful contractions, making them the key feature that resists mechanical stress in the myocardium.

A. Contact: Contact signaling involves direct physical interaction between cell surface receptors and ligands on adjacent cells. This allows cells to recognize one another, which is crucial for tissue development, immune responses, and maintaining cellular organization.

B. Chemical: Chemical signaling involves the release of signaling molecules (like hormones or neurotransmitters) that travel to target cells, often over a distance. While important, it does not involve direct cell-to-cell recognition as contact signaling does.