Proteins are a type of macromolecule made of different combinations of just 20 amino acids. Proteins have very specific shapes that help determine their function. When proteins are denatured (their shape is altered), they no longer function correctly. Heat, changes in pH, and changes in salt concentration can all denature proteins. The primary structure of proteins is their amino acid sequence. The secondary structure is different folds (pleated sheets) and coils (alpha helices) that happen in the polypeptide. Depending on what type of protein it is, it will have different amounts of helices and pleated sheets (for example, globular proteins have a lot of pleated sheets while fibrous proteins have many alpha helices). The third level of protein structure is the tertiary structure. This is the overall, three-dimensional shape of a polypeptide. Most tertiary structures can be described as either globular or fibrous. This level of structure is a result of the interactions of the R groups of the amino acids making up the chain. The polypeptide is folded to that the hydrophobic R groups are away from water and the hydrophilic R groups are closer to the water. The last level of protein structure is not found in all proteins. When two or more polypeptide chains join, the protein is said to have quaternary structure.
The egg shown in the picture has a type of storage protein known as ovalbumin. Ovalbumin is the protein found in egg whites and supplies amino acids to developing embryos. Milk and plant also have storage proteins for offspring- Milk proteins supply amino acids for baby mammals while plants have storage proteins for the embryo in the seed.