Introduction
A three carbon sugar is the main product of photosynthesis
in most plants. This sugar, glyceraldehyde-3-phosphate, looks
like this:
CH2PO3-2
|
OH-C-H
|
O = C-0PO3-2
From this starting point, the plant can synthesize other sugars, including glucose, sucrose, and fructose, to name but a few. In this lecture, we review the terminology of sugars, and in particular, compounds made of several linked sugars, referred to as polysaccharides. All carbohydrates, which is the name for these types of compounds, have the basic structure of:
(CH2O)n
where the n stands for the number of carbons in the molecule. The most common sugars in plants are ribose, glucose, fructose, and sucrose. Many other compounds are composed of polymers of these and related sugars. Depending on whether the double bonded oxygen in these molecules is terminal or interior among the carbons determines if they are aldehyde (terminal) or ketone (interior) sugars. Aldehyde containing carbohydrates are known as aldoses, and ketone containing ones as ketoses.
Monosaccharides, Disaccharides and Polysaccharides
A carbohydrate that can not be hydrolyzed (have water added to break
it down into smaller molecules) is known as a monosaccharide.
If a carbohydrate can be hydrolyzed into two monosaccharides, it is a disaccharide.
If it hydrolyzes into more than two monosaccharides, it is a polysaccharide.
Sugars in plants most often have 3-7 carbon atoms. The terminology is:
Number of Carbons Group Classification
3 triose
4 tetrose
5 pentose
6 hexose
7 septose
The majority of plant sugars are either pentoses or hexoses. Carbohydrates are also classified as to whether they can reduce (give electrons to) certain reagents, such as Benedict's or Tollen's. Both aldehydes and ketones can reduce these reagents (they add electrons to a cupric ion and the solution changes color as a result). This will become important later when we discuss sugars in plants, and how certain reactions only work with reducing sugars.
The most common organic molecule in the entire world is probably glucose, a six carbon sugar. The formula is C6H12O6. Table 18.1, from another plant physiology textbook, shows the actual structures of glucose. Note that there can be two main forms, L and D. This is because they have on asymmetric carbon in them, and this means that stereoisomers can be formed. Living organisms only use the D form!
Conformation
Monosaccharides usually form a cyclic, rather than a linear form in
nature. The carbonyl oxygen (on carbon 1) reacts with hydroxyl groups elsewhere
on the molecule, resulting in the formation of either five-member (furanose)
or six-member (pyranose) rings. Because of rotation by some
of the carbon atoms, the rings differ in their atomic orientations, and
for a six-member ring, you can have both the and forms (see
Part B, molecules (1) and (2) - can you find where the difference is between
these two molecules?). Look at Figure 5.3 (from Campbell, Reece, and Mitchell,
Biology, 5th Edition, 1999), upper right box. Here the difference
between galactose and glucose is that the fourth carbon (count from the
top!) has rotated about its axis. In solution, monosaccarides can freely
rotate between the and forms, whereas if they become fixed as part of a
larger molecule, they attach as either the or form. Note in Figure 5.4
the various forms that a glucose molecule may take on.
Note particularly the numbering scheme for the carbons in a glucose
molecule - you begin numbering from the carbonyl end.
Dehydration Reactions
Two monosaccharides can be joined through a dehydration reaction (water
is lost). Figure 5.5a shows such a reaction. Here, two glucoses are joined
to form maltose (is maltose a disaccharide?). The number 1 carbon from
one glucose is joined to the number 4 carbon of the other glucose through
by a glycosidic bond (where an oxygen atom joins two carbon atoms). If
a glucose joins a galactose molecule, then you form lactose, the major
sugar in milk.
If you put two different hexoses together, like glucose and frutose, you can form the disaccharide sucrose (see Fig. 5.5b). Note, as the figure mentions, that fructose is one of those hexoses that forms a five-sided ring, rather than a six-sided one. Also take notice of the fact that in this molecule, the 1 and 2 carbons are connected, not the 1 and 4 carbons. Sucrose is the most common sugar in plants, and is the form in which carbohydrates are transported from one plant organ to another.
If you have a polymer of glucose, this compound would be a glucan. If attached to it were residues of another type of sugar, such as xylose, the compound's name would change to xyloglucan. If you have a polymer of xylose, with arabinose side chains, then you would have a compound called arabinoxylan. This will become more important later when the composition of materials making up the plant cell wall are discussed (see below).
Starch
Starch is a polysaccharide made up of glucose units joined by an (1-4)
bond (see Figure 5.6). The angle made by this bonding causes the
glucose units to coil in a tight helix. This is advantageous, since starch
is a storage molecule for excess glucose, and coiling allows a large number
of glucoses to be stored in a relatively small area. This form of starch
is known as amylose. Occasionally, a branch forms off a glucose
and where it does, the bond is an (1-6) bond, rather than
a (1-4) bond. When branches occur, the starch is called amylopectin.
Starch is insoluble, and therefore does not contribute to the osmotic potential
of a cell.
There are three enzymes that are the primary degraders of starch. -amylase breaks the bonds where branch points exist (the (1-6) bonds) which leaves long residues of (1-4) bonded glucoses. Then, -amylase attacks the ends of these residues, forming maltose (a disaccharide, remember?). Finally, maltase digests the maltose into individual glucose molecules.
Some algae synthesize a variation of typical plant starch, using (1-3) bonds, instead of (1-4) bonds. This shows the diversity of biochemical pathways available in the plant/alga groups.
Cellulose
Cellulose is a polysaccharide composed of glucose units joined by (1-4)
bonds. In this orientation, the glucoses arrange themselves in
long, straight chains, rather than coiled as in starch. This permits cellulose
to be synthesized as a fiber, and gives the molecule great strength, particular
with regards to tensile strength (ability to be stretched without breaking;
cellulose in this respect is as strong as steel!).
There are very few enzymes that can degrade cellulose - humans don't have such enzymes, and very few if any higher organisms can break these bonds. This means that plants that invest a lot of effort to make wood (which is mainly cellulose) don't will be relatively immune from attack by most organisms. Those organisms that do consume cellulose directly (like termites and grazing animals) use bacteria which contain cellulose digesting enzymes to get at the glucose molecules. We'll discuss these enzymes later.
The individual chains of cellulose can closely align and bond among themselves, making structures called microfibrils. These microfibrils are relatively stiff and can form a crystalline or semi-crystalline structure, about 4 nm in diameter. It is these crystalline fibers that are very resistant to enzymatic attack. Surrounding and covering these microfibrils are two major classes of compounds: hemicelluloses and pectins.
Hemicelluloses are flexible polysaccharides that often bind to the microfibrils (see Figure 15.4). They help bind microfibrils together, and may prevent adjacent microfibrils from joining with each other.
Pectins form a gel in which the microfibrils and hemicelluloses are embedded. Pectins are also hydrophilic (water can dissolve in them) and pectins are primarily responsible for determining porosity of the cell wall to macromolecules. Proteins are also found amidst the pectinaceous gel, but their function is not well known at present.
Composition of the Cell Wall
The primary wall (the wall material first laid down by the cell, more
later on this) is composed of about 25% cellulose, 35% hemicelluloses,
35% pectins, and maybe 1-8% protein. But there are large variations among
plants: some grasses have cell walls with up to 70% hemicelluloses, while
cereal endosperm tissue can be 85% hemicellulose. Secondary walls (which
are laid down later, and which give cells structural strength) contain
much higher amounts of cellulose.
Let us not forget that the wall also contains a great deal of water. The matrix, which consists of the hemicelluloses and pectins surrounding the microfibrils, can be 75-85% water by volume! Removal of water makes the wall stiffer, and may play a role in maintenance of turgor as plants dry out.