# Lignocellulosic Biomass as a Substrate for Oleaginous Microorganisms: A Review
## 2 Oleaginous microorganisms
### Lignocellulose Structure and Chemical Composition  
Lignocellulose provides mechanical support and tolerance to biotic and abiotic stresses in plants. Its structure consists mainly of a network of cellulose, hemicelluloses (which together form the holocellose) and lignin polymers and in less quantities starch, pectin, extractives, ashes, etc. [162; 163; 164; 165]. The content of each of the above compounds depends on the species, tissues and age of the cells [161].  
Cellulose is the most abundant polymer, representing 34-50% of the dry biomass in softwood species and 41-50% in hardwood species, while in agricultural residues ranges from 17 to 42% [161]. It is an amphiphilic and linear homopolymer composed of more than 10,000 glucose molecules linked by \(\beta\)(1-4) glycosidic bonds. Specifically, between two glucose molecules the second unit rotates around the C1-C4 axis, forming a dimer known as cellobiose that binds to each other forming linear glucose chains [166].  
Hemicelluloses are amorphous, branched heteropolysaccharides formed by a main chain and other lateral ones. The main chain is composed of the same (homopolymer) or different (heteropolymer) sugar units and is shorter in length than cellulose. The side chains are short and linked mainly by \(\beta\)(1-4) glycosidic bonds and occasionally by \(\beta\)(1-3), which are readily hydrolysable. Sugars can be hexoses (glucose, mannose, and galactose), pentoses (xylose and arabinose), uronic acids and deoxyhexoses (such as rhamne and fucose) [165]. Hardwoods (i.e., angiosperms) contain mainly glucuronoxylans, while softwoods (i.e., gymnosperms) contain mainly galactoglucmannans, which represent approximately 18-35% of the lignocellulosic biomass [161; 167]. In agricultural biomass, xylan is the main hemicellulose and represents between 12% and 37% of the lignocellulosic biomass [161].  
Lignin is the most abundant organic polymer after cellulose, representing 19-35% of the lignocellulosic biomass of woods and 3.5-30% of grasses [161; 168; 169]. It is mainly formed in vascular tissues and provides mechanical resistance to xylem. Lignin is classified into three types of phenylpropane units (or monolignols), guaicyl (G), syringyl (S) and p-hydroxyphenyl (H), derived from alcohols, coniferyl, sinapyl and p-counaryl, respectively. In softwoods, the predominant lignin is guaicyl, in hardwoods guaicyl and syringyl in a ratio of 1:1 to 1:3, while all the lignin monomers are present in the grasses [170].  
In addition, other biomolecules are also present in the cell wall of plants in smaller quantities. Compounds such as tannins, phenols and lignans are mostly connected to lignin and represent between 1% and 5% of lignocellulosic biomass [166]. It is also possible to find pectins and pectin-like compounds (such as galacturonans, galactans and arabians), starch, and proteins, although they decrease as plants age. Free amino acids or alkaloids that contribute to the nitrogen content, inorganic components (e.g., Ca, K, Mg, Mn, Na, P and Cl) and trace elements (e.g., Ba, Al, Fe, Zn, Si, Pb, Ni, V, Co, Ag and Mo) essential for growth are also present [166].  
Most of the highly diverse compounds found in the lignocellulosic biomass, can be metabolized by oleaginous microorganisms, but not all of them can lead to an important production of lipids. For this reason, it is essential to analyze the effect of the different lignocellulosic compounds on the metabolism of oleaginous microorganisms in order to design an efficient production process. In this sense, there is still much to be investigated experimentally.