C4 in the context of amino acids usually refers to C4 plants and their specific biochemical pathway, not directly to a type of amino acid. Amino acids are organic compounds that serve as the building blocks of proteins. There are 20 standard amino acids in proteins, and they do not include a category known as "C4 amino acids." The confusion may stem from the mention of C4 in discussions related to plant biology and photosynthesis. C4 refers to a carbon fixation pathway that some plants use, which is highly efficient in hot, dry environments. This pathway is named C4 because the first product of carbon dioxide fixation is a four-carbon compound, as opposed to the three-carbon compound in the more common C3 pathway. While this process is crucial for the synthesis of organic compounds in plants, including those that can be converted into amino acids, "C4 amino acids" as a term does not directly refer to a specific type of amino acid.

When referring to resin in the context of smoking or cigarettes, it's important to clarify the type of resin being discussed. Synthetic resins used in crafting or industrial applications can indeed melt or become damaged when exposed to the heat of a lit cigarette. These types of resins have specific melting points and can be affected by direct heat. However, if talking about natural resin buildup from smoking cannabis, for example, this resin can also be affected by heat but in a different way. It won’t exactly "melt" like a synthetic resin because it's already in a somewhat sticky, tar-like state. Applying heat from a cigarette could potentially soften or combust it, depending on its consistency and the amount of heat applied. It’s advisable to keep lit cigarettes or any open flame away from synthetic resins to avoid damage or unwanted chemical reactions.

Polypropylene plastic, denoted as #5 PP (Polypropylene), is indeed recyclable and is becoming increasingly common in recycling programs. It's a thermoplastic polymer used in a variety of applications including packaging, textiles, automotive parts, and reusable containers due to its resistance to heat, chemicals, and fatigue. Recycling PP reduces the demand for virgin plastic, conserves resources, and minimizes environmental impact. However, the recycling rate for PP is lower compared to some other plastics like PET and HDPE, mainly due to collection and sorting challenges. Advances in sorting technology and a growing emphasis on sustainable practices are improving PP recycling rates. Consumers can contribute by checking local recycling guidelines and properly sorting their waste.