Fiber Chemical Structure

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What is the difference between soluble and insoluble fiber
in terms of chemical structure

It depends on the fiber itself.

Insoluble fiber includes things like cellulose, lignan, some
hemicelluloses. Cellulose is nothing more than a linear
strand of glucose molecules, strung like pearls in a
necklace. Because it's linear, other cellulose molecules can
fit tightly against it, and those separate strands form a
variety of bonds with each other. (It actually forms
crystals.) Water has trouble getting in between the strands,
so cellulose is said to be insoluble. That's a grossly
simplified description. Lignan is not a polysaccharide
(polymer of sugar), but it is a repeating polymer. It's one
of the reasons wood is rigid. It also does not permit much
water into its structure. Hemicelluloses are more or less
linear chains of sugar molecules, but with side branches.
There are many different hemicelluloses, and they contain
other sugar molecules than just glucose, such as mannose,
galactose, etc. Some are insoluble, and some are soluble,
and some are in between.

Insoluble fiber tends to speed up the transit of food
through the gut. Your parents would use the word roughage to
represent this type of fiber. Wheat bran is a classic
example of a high-density insoluble fiber food. Insoluble
fiber gives structure to plant cells, so it is higher in
things like stems, peels, seed coats.

Soluble fiber interacts highly with water, to the extent of
20 to 200 times its dry weight. An example of a soluble
fiber is pectin. Like cellulose, pectin is composed of
strands of glucose molecules, but instead of being linear,
pectin is highly branched. When it is fully wetted by water,
the branches stand apart from one another, and you obtain a
gel. There are all kinds of soluble fiber, and fruits and
veggies are chock full of them. Cooking tomatoes thickens a
sauce because of the release of soluble fiber from within
the ruptured cells. Metamucil (psyllium fiber) is an
excellent source of soluble fiber.

Soluble fiber slows food transit through the gut, and
improves nutrient absorption. Because it holds water
well, despite the slower transit through the gut, fecal
passage is eased.

Soluble fiber may be partly digestible by humans, but
another consideration is whether it is fermentable. The
various bacteria in the gut can often efficiently digest
certain polysaccharides that we cannot, and byproducts
thereof include gases which have to get out somehow.

Lar