Each rhodopsin molecule consists of an opsin protein covalently linked to a carotenoid chromophore, 11-cis-3-hydroxyretinal. Glycoproteins are proteins which contain oligosaccharide chains (glycans) covalently attached to amino acid side-chains. Biopolymers contain monomeric units that are covalently bonded to form larger structures. The backbones of these biological macromolecules consist of central chains of covalently bonded atoms. Nucleotides are nucleosides covalently linked to one or more phosphate groups. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials. In physical gels unlike covalently linked gels the polymers chains are not covalently linked together. Type V proteins are anchored to the lipid bilayer through covalently linked lipids. Macromolecules are large molecules composed of thousands of covalently connected atoms. Catalysis proceeds via a covalently bound intermediate. Heme is bound to the protein either covalently or noncovalently or both. The tetrahedron shape is seen in nature in covalently bonded molecules. As of 2007, no stable neutral molecules involving covalently bound helium or neon are known. Boron is similar to carbon in its capability to form stable covalently bonded molecular networks.