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In microorganisms, the glyoxylate cycle allows cells to use two carbons (C2 compounds), such as acetate, to satisfy cellular carbon requirements when simple sugars such as glucose or fructose are not available. [2] The cycle is generally assumed to be absent in animals, with the exception of nematodes at the early stages of embryogenesis.
The major polymers that make up wood (largely secondary cell walls) include: cellulose, 35-50%; xylan, 20-35%, a type of hemicellulose; lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.
These XTH enzymes need to diffuse into the cell wall and form complexes with, or act on, xyloglucans in the non-load bearing outer region of the cell wall. After a lag period, the enzymes will reach a concentration threshold in the load-bearing region of the cell wall and the activity of the enzyme will then be apparent. [2]
The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding. [1] It is most prevalent in the Ground tissue found in vascular plants, with Collenchyma having little to no lignin, and Sclerenchyma having lignified secondary cells walls. [2] [3]
Given that mixed-linkage glucan is a non-branched homopolymer of glucose, there is no side-chain synthesis, only the addition of glucose to the backbone in two linkages, β1-3 and β1-4. [9] Backbone synthesis is mediated by enzymes in cellulose synthase-like protein families F and H (CSLF and CSLH), specifically glucan synthase.
Microfibrils in the primary cell wall are approximately 36 chains long while those of the secondary cell wall are much larger, containing up to 1200 β-(1→4)-glucan chains. [ 15 ] [ 10 ] Uridine diphosphate-glucose (UDP), which is produced by the enzyme sucrose synthase (SuSy) that produces and transports UDP-glucose to the plasma membrane is ...
Xyloglucan binds to the surface of cellulose microfibrils and may link them together. It is the substrate of xyloglucan endotransglycosylase, which cuts and ligates xyloglucans, as a means of integrating new xyloglucans into the cell wall. It is also thought to be the substrate of alpha-expansin, which promotes cell wall enlargement.
Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in vascular and support tissues: xylem tracheids, vessel elements and sclereid cells. [citation needed] Lignin plays a crucial part in conducting water and aqueous nutrients in plant stems.