14.3 Chemistry of Aquatic Cellulose
315
the algae cell wall. Group 1 comprises of algae which have mainly native cellulose
within their cell walls. Algae belonging to this group are the Cladophorales and some
Siphonocladales. Most algae fall into Group 2 in terms of their cell wall structure.
Group 2 comprises of algae whose cell walls are made up of mercerized cellulose
derived from native cellulose. This form of cellulose has low crystallinity. Example
of such is the Spongomorpha. The third group in this classification is Group 3 algae.
These are characterized by heterogeneous cell wall structure in which cellulose is
not the major component of the cell walls. Spirogyra and Vaucheria fall into this
group. The variation in crystallinity in cell walls is attributed to the cellulose synthase complex responsible for determining the structure of the cellulose microfibrils
(Brown and Saxena 2000). Selectivity of species for cellulose extraction is therefore
necessary in order to obtain cellulose of desired crystallinity.
Hornification occurs to a much lower degree in crystalline cellulose (Fernandez
Diniz et al. 2004) due to the high level of orderliness in the microfibrils which hinder
water absorption. Such property aids formation of uniform dispersion in micro- and
nanoformulations of cellulose. This further adds to the appeal of cellulose from
green algae. Cellulose from Cladophora has a surface area of 95 m
2 /g, a relatively
high surface area compared to that of industrial adsorbents which could be around
100 m
2 /g. This surface area is about 100 times higher than that of pharmaceutical
grade microcrystalline cellulose which is around 1 m
2 /g. This high surface area
coupled with their very low level of hornification makes green algae sourced cellulose
a potentially better candidate for production of cellulose-based aerogels that are more
resistant to humidity than those produced from native cellulose.
Cellulose has particularly excellent mechanical properties due to its linear orientation, microfibrillar structure and high degree of polymerizations. Linear polymers
are able to form well-ordered structures resulting in densely packed secondary structure which prevents penetration of heat, moisture or other molecules. The superior
mechanical properties of starch fit well with the structural role it plays in the plant
cell wall. Cellulose provides rigidity and strength to the plant. The strong hydrogen
bonds present within the cellulose structure also contribute to the strong mechanical
structure of cellulose. This property also makes the hydrolysis of cellulose much
more difficult than that of starch.
Such high mechanical strength of cellulose does not seem so pertinent in algae
compared to land plants which need to grow higher above the ground and require
strong cell walls for strength. Hypothesis developed to explain the need for such
crystalline cellulose in the Cladophora algae is to retain turgor pressure where salinity
fluctuates, as is common in aquatic environment and/or withstand the drag flow
of water (Johnson and Shivkumar 2004). The cellulose from aquatic source can
therefore be said to have similar chemical structure as terrestrial cellulose but for
differences in secondary structure in some cases such as in Cladophora.
315
the algae cell wall. Group 1 comprises of algae which have mainly native cellulose
within their cell walls. Algae belonging to this group are the Cladophorales and some
Siphonocladales. Most algae fall into Group 2 in terms of their cell wall structure.
Group 2 comprises of algae whose cell walls are made up of mercerized cellulose
derived from native cellulose. This form of cellulose has low crystallinity. Example
of such is the Spongomorpha. The third group in this classification is Group 3 algae.
These are characterized by heterogeneous cell wall structure in which cellulose is
not the major component of the cell walls. Spirogyra and Vaucheria fall into this
group. The variation in crystallinity in cell walls is attributed to the cellulose synthase complex responsible for determining the structure of the cellulose microfibrils
(Brown and Saxena 2000). Selectivity of species for cellulose extraction is therefore
necessary in order to obtain cellulose of desired crystallinity.
Hornification occurs to a much lower degree in crystalline cellulose (Fernandez
Diniz et al. 2004) due to the high level of orderliness in the microfibrils which hinder
water absorption. Such property aids formation of uniform dispersion in micro- and
nanoformulations of cellulose. This further adds to the appeal of cellulose from
green algae. Cellulose from Cladophora has a surface area of 95 m
2 /g, a relatively
high surface area compared to that of industrial adsorbents which could be around
100 m
2 /g. This surface area is about 100 times higher than that of pharmaceutical
grade microcrystalline cellulose which is around 1 m
2 /g. This high surface area
coupled with their very low level of hornification makes green algae sourced cellulose
a potentially better candidate for production of cellulose-based aerogels that are more
resistant to humidity than those produced from native cellulose.
Cellulose has particularly excellent mechanical properties due to its linear orientation, microfibrillar structure and high degree of polymerizations. Linear polymers
are able to form well-ordered structures resulting in densely packed secondary structure which prevents penetration of heat, moisture or other molecules. The superior
mechanical properties of starch fit well with the structural role it plays in the plant
cell wall. Cellulose provides rigidity and strength to the plant. The strong hydrogen
bonds present within the cellulose structure also contribute to the strong mechanical
structure of cellulose. This property also makes the hydrolysis of cellulose much
more difficult than that of starch.
Such high mechanical strength of cellulose does not seem so pertinent in algae
compared to land plants which need to grow higher above the ground and require
strong cell walls for strength. Hypothesis developed to explain the need for such
crystalline cellulose in the Cladophora algae is to retain turgor pressure where salinity
fluctuates, as is common in aquatic environment and/or withstand the drag flow
of water (Johnson and Shivkumar 2004). The cellulose from aquatic source can
therefore be said to have similar chemical structure as terrestrial cellulose but for
differences in secondary structure in some cases such as in Cladophora.