A linear polymer of amino acids linked together in a specific sequence and usually containing more than 50 residues. Proteins form the key structural elements in cells and participate in nearly all cellular activities.
A class of organisms composed of one or more cells containing a membrane-enclosed nucleus and organelles. This class constitutes one of three distinct evolutionary lineages (eubacteria, eukaryotes & archaea) of modern-day organisms. Includes all organisms except viruses and prokaryotes (mostly bacteria).