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Carbohydrates

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Carbohydrates as well as proteins are polymers and contain only a few different types of atom. In the case of carbohydrates, the basic molecular units are called monosaccharides - these are the monomers. (mono = single; poly = multiple; saccharide = sugar)

α (alpha) glucose is the most important monosaccharide to learn, as you need to be able to draw it:


The points where the lines intersect each symbolise a carbon (C) atom. You need not show those. The figure above is taken from the specification itself, so take it as a good guide. So the monosaccharide alpha glucose (commonly, just glucose) somehow becomes a polysaccharide, This is achieved by condensation reactions, and the bonds formed are called glycosidic bonds




You should be able to draw this. The resulting molecule, maltose, is a disaccharide (two monomers). If you keep adding glucose molecules to the chain, you get... *drum roll please* ...starch. Starch is made up of multiple (very many indeed) monomers, so it is a polymer i.e. it is made of multiple monosaccharides, so it is a polysaccharide.

Potatoes anyone?

You also need to know about two other disaccharides and their constituent monosaccharides - sucrose and lactose.

Sucrose is made of glucose and fructose.

Lactose is made of glucose and galactose.

It's easy enough to remember: they're both made of glucose, and lactose (MILK) is also made of galactose (galaxy - Milky Way). Both sucrose and lactose are formed similarly by the condensation of their monosaccharides.

During digestion, these large molecules must be broken back down into small molecules. Starch must be broken down to glucoseEnzymes produced by the salivary glands and the pancreas carry out this reaction in the mouth and stomach. One particular such enzyme is amylase. By the time food reaches the small intestine where nutrients are absorbed, glucose on its own must be made available for absorption in the epithelial cells lining the intestine. This is achieved by maltase, which breaks down... that's right, maltose, leaving behind single molecules of glucose.

Regarding lactose you need to be aware of lactose intolerance. Just as for starch or maltose, lactose too must be broken down successfully for its monomers (glucose and galactose) to be absorbed by the body. Not surprisingly, this is achieved by the enzyme lactase. People deficient of this enzyme will not properly digest, or break down lactose from milk, so lactose will go unchecked into the large intestine, where microorganisms will use it for respiration, leaving behind a lot of gas. This causes symptoms like bloating, abdominal pain and diarrhoea.

Sugars can be reducing or non-reducing, depending on their chemistry (you do not want to go into that. Unless you're into chemistry). Reducing sugars are ALL monosaccharides, and the disaccharides lactose and maltose. All else including sucrose is non-reducing. Benedict's reagent is a test for reducing and non-reducing sugars.

Benedict's reagent = light blue
You throw some glucose into the mix = orange/brick red due to it being reducing (reduces copper ions Cu2+  to Cu+)
You become adventurous and throw some sucrose into the mix = ends it utter disappointment when you get nothinglight blue, due to it not being reducing.

And finally, there is a test for starch. Chop a potato (they are practically made of the stuff), add iodine which is yellow. The starch in the potato will turn it blue. This is a very simple test for starch - if the solution stays yellow it's negative, if it goes blue, it's positive.


Starch

Basic unitα glucose


Function: the main storage molecule in plants

Structure: starch is made of two compounds - amylose and amylopectin. Both are, of course, made of α glucose, but their overall shapes differ. Amylose is a spiral, while amylopectin has branches. Combined, they give starch the appearance of a tightly wound molecule like a brush.

Crucially, starch is an excellent storage compound, so must satisfy certain requirements. Its size must be relatively big so that it is not soluble. This prevents it from causing an osmotic effect in cells whereby water floods in. The molecule must be compact in order to take up little space rather than a lot. This is achieved by the branches and spirals within starch. Finally, the branches also contribute to the readiness of the glucose molecules of being "nipped off" and quickly usable. This is because only glucose molecules at the ends of starch can be used in that way.
Glycogen

α glucose


the main storage carbohydrate in mammals

The structure of glycogen is essentially the same as that of amylopectin i.e. branched structure. The difference is that glycogen is even further branched compared to amylopectin. This enables a quicker build-up and breakdown of glycogen, hence meeting the superior energy demand of animals as compared with plants.
Cellulose

β glucose


confers structural strength in plant cell walls

Cellulose is the only molecule in this list which is solely made of β glucose. Despite beta glucose only having one chemical group different from alpha glucose, the result is a significant overall structural difference in cellulose. Unlike amylopectin and glycogen, cellulose has a structure based upon straight chains of beta glucose units, rather than spiraling chains of alpha glucose molecules.

These straight chains laid next to one another form hydrogen bonds which strengthen them into larger sub-units called microfibrils. Microfibrils are what cellulose is made of, and what gives plant cell walls their great strength.
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