CHAPTER 3-B: HEMICELLULOSE

Question 1

hemicellulose also known as __________

Answer 1

polyose

Question 2

defined as cell wall polysaccharides that have the capacity to bind strongly to cellulose micro fibrils by hydrogen bonds

Answer 2

hemicellulose

Question 3

degree of polymerization of glucose units in hemicelluloses is in the range of ___________, which is much lower than cellulose

Answer 3

100–200 units

Question 4

usual DP values in wood are reported to be between ___________

Answer 4

300 and 1700

Question 5

in nature, cellulose has a DP of approximately ___ glucopyranose units in wood cellulose and of ______ units in plant cellulose

Answer 5

10,000 ; 15,000

Question 6

What subunits does hardwood hemicellulose consist of?


Answer 6

Pentose, specifically xylose

Question 7

_____ is a monosaccharide with five carbon atoms.The chemical formula of many pentoses is _______.

Answer 7

pentose; C ₅H ₁₀O ₅.

Question 8

What subunits does softwood hemicellulose consist of?

Answer 8

Hexose subunits like glucose

Question 9

__________ any of the class of simple sugars whose molecules contain six carbon atoms, such as glucose and fructose. They generally have the chemical formula ______.

Answer 9

Hexose; C6H12O6

Question 10

What percentage of hemicellulose do non-wood fibers contain?


Answer 10

Up to 40%

Question 11

What percentage of hemicellulose do wood fibers contain?


Answer 11

25% to 35%

Question 12

What are the classifications of hemicellulose?


Answer 12

a. β-(1→4)-linked backbones
b. Includes xyloglucans, xylans, mannans, glucomannans
c. β-(1→3,1→4)-glucans in some groups

Question 13

Xyloglucan has been found in every land plant species that has been analyzed, including mosses EXCEPT in ______

Answer 13

Charophytes

Question 14

Xyloglucan the most abundant hemicellulose in primary walls of spermatophytes except for ______

Answer 14

grasses

Question 15

What is the main component of xylans?


Answer 15

Xylose residues

Question 16

What percentage of dry weight in plants can xylans constitute?


Answer 16

More than 30%

Question 17

mainly constituted by _____ as the monomeric unit, and traces of_________ are also present

Answer 17

D-xylose; L-arabinose

Question 18

Xylansof several wood species, particularly of hardwoods, are ______

Answer 18

acetylated

Question 19

What are glucomannans primarily composed of?


Answer 19

β-(1→4)-linked polysaccharides containing mannose

Question 20

What role do glucomannans play in plants?


Answer 20

Energy reserves and structural roles

Question 21

What components make up the plant cell wall?


Answer 21

a. Cellulose
b. Hemicellulose
c. Pectin
d. Glycoproteins

Question 22

What is the purpose of hemicelluloses in the cell wall?


Answer 22

Cross-link glycans and interlock cellulose fibers and form a mesh like structure to deposit other polysaccharides.

Question 23

What are the methods for extracting hemicellulose from lignocellulosic biomass?


Answer 23

1. Hydrothermal extraction
2. Steam explosion
3. Acidic pretreatments
4. Alkaline pretreatments
5. Solvent extraction
6. Ultrasonic-assisted processes
7. Microwave-assisted processes

Question 24

Why is pretreatment of lignocellulosic biomass necessary?


Answer 24

To improve bioconversion for sugar production

Question 25

What requirements must an effective pretreatment for lignocellulosic biomass meet?


Answer 25

1. Improve sugar production 2. Avoid degradation of cellulose and hemicelluloses 3. Minimize inhibitor production 4. Be cost-effective

Question 26

What is hydrothermal extraction also known as?


Answer 26

Autohydrolysis

Question 27

What is a key advantage of hydrothermal extraction?


Answer 27

It uses only water as a reaction medium

Question 28

can be considered to be a green technology, because of its ability to prevent corrosion problems and not need chemical recycling

Answer 28

Hydrothermal Extraction (Autohydrolysis)

Question 29

What is the typical temperature range for hydrothermal treatments?


Answer 29

160-240 °C

Question 30

What happens to hemicelluloses during hydrothermal treatment?


Answer 30

They are solubilized and converted to monomers

Question 31

What pH should be maintained during hydrothermal treatment?

Answer 31

Between 4 and 7

Question 32

What is the purpose of steam explosion in biomass pretreatment?


Answer 32

To destroy cell wall structure and separate components

Question 33

a common process for separating and extracting hemicellulose from lignocellulosic biomass

Answer 33

Alkaline extraction

Question 34

What is the most common alkali used in alkaline extraction?

Answer 34

NaOH solution

Question 35

What does alkaline extraction promote in hemicellulose?

Answer 35

Swelling of cellulose and hydrolysis of hemicelluloses

Question 36

What is the main outcome of acid extraction of hemicellulose?

Answer 36

Separation of main components of biomass

Question 37

What is a significant drawback of acid hydrolysis?


Answer 37

Low yield and molecular weight of extracted hemicellulose

Question 38

What are the by-products formed during acid extraction?

Answer 38

- Furfural - 5-hydroxymethylfurfural - Acetic acid