Structure and Function of Carbohydrates

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Monosaccharides

The backbones of common monosaccharides are unbranched carbon chains that range from three to seven carbon atoms; where the most common is known as glucose. They are linked by single bonds. When looking at the monosaccharide in an open chain there is a carbonyl group; where one of the carbon atoms is double-bonded to oxygen (C=O) and the other carbon atoms are a hydroxyl group (C-OH). The common suffix for monosaccharide names ends with -ose. A carbonyl group present at the end of the carbon chain is known as an aldehyde group and the monosaccharide is known as an aldose; whereas if the carbonyl group is within the chain any other position it is called a ketone group now and the monosaccharide is a ketose. The presence of an aldehyde is indicated by the prefix aldo- and a ketone has a prefix of a keto- (Figure 1).

Other common monosaccharides that only contain one sugar molecule are fructose and galactose. They have the same chemical formulae as glucose (C6H12O6)  however, vary in their structural formula because of the different arrangements of functional groups that surround the asymmetric carbon central atom (Figure 1). All three of these monosaccharides have more than one carbon that is asymmetric. Fructose is found in fruit sugars whereas, galactose is found in milk sugars. Common five-carbon sugars (n=5; C5H10O5)  are known as ribose and deoxyribose and are found in many important biological molecules such as ribonucleic acid (RNA) and ATP.

See Figure 1 for an illustration of the structure of common monosaccharides:

No. of carbons Class of Monosaccharide
3 triose
4 tetrose
5 pentose
6 hexose
Figure 1 Structure of the monosaccharides

Monosaccharides with four, five, six and seven carbon atoms are known as tetroses, pentoses, hexoses and heptoses which can be further divided with the addition of either aldoses to ketoses.

Disaccharides

Two monosaccharides joined together in a condensation reaction form a disaccharide. A disaccharide is any carbohydrate made up of two monosaccharides that are joined covalently by an O-glyosidic bond. During this condensation reaction, a hydroxyl group of one monosaccharide combines with another hydrogen atom, forming and releasing a water molecule (Figure 2). Common disaccharides include lactose, maltose and sucrose. The digestion of starch into two-glucose units by enzyme amylase forms maltose. Sucrose is a disaccharide that is the main transport sugar in plants and lactose is found only in mammalian milk. (Figure 3).

Figure 2 The formation of disaccharides, maltose
Figure 3 illustrates the structure of sucrose, lactose and maltose disaccharides

Polysaccharides

Polysaccharides also known as polymers are long complex chains of many monosaccharides that are joined together by glyosidic bonds. They are formed by a series of condensation reaction and yield more than 10 molecules monosaccharides on hydrolysis. Their general formula is (C6H10O5)n. Unlike both monosaccharides and disaccharides,  polysaccharides are insoluble and not sweet sugars. They are very large molecules (macromolecules) and the feature of them being insoluble makes them suited for storage. The structure of polysaccharides vary in length and a huge variety of polysaccharides can be created by:

  • changing the type of monosaccharide
  • changing of how the monosaccharides can be bonded together
  • its ability to be branched and folded in various ways

Polysaccharides as Energy Stores

  • α-glucose is the main source of energy in respiration
  • Excess chemical energy is stored in cells by forming polysaccharides of α-glucose. Instead of having too much glucose it can be added to polysaccharides to store away and be used when needed

In animals and plants α-glucose and polysaccharides are seen as a good storage energy and are well suited for energy storage for several reasons:

  • They are physically well compact and can pack lots of energy into small little spaces
  • They are insoluble in water and do not affect the osmatic balance of the cell; preventing cells from bursting as no water is able to enter a cell
  • They are extremely large molecules and do not diffuse readily in and out of a cell
  • They can be easily hydrolysed (broken down), making them readily accessible when energy is needed. For example when a high energy demand is needed the storage polysaccharides are broken down by enzymes that access the ends of the polysaccharides and essentially work by snipping of the α-glucose, in groups of two and are free to be used in respiration.

Important polysaccharides:

Starch is an insoluble storage polysaccharide found in plants and forms starch granules, or grains within plant cell; including roots and seeds. It also is an important component of foods that serves as the major energy source in many diets. Starch is not a pure substance and is a mixture of two substances; amylose and amylopectin (polymers of glucose). Around 80% of starch is made up of amylopectin and the remaining 20% is amylose. Starch is easily detected with its ability to turn (orange-yellow) iodine in potassium iodine solution into blue-black colour. The change is colour is due to iodine molecules becoming fixed into the centre of the helix of each starch molecule.

Amylose

Figure 4: Structure of amylose
  • is also known as poly-(1-4) glucose,
  • is a long, straight floppy chain of α-glucose molecules joined by 1,4 glyosidic bonds.
  • The chain has a tendency to coil up into a tight helix and makes the molecule more compact to allow for it be stored efficiently and not take up much space (Figure 4).
  • Each amylose molecule only has two accessible ends where the enzyme amylase can bind showing amylose can be only be broken down slowly

Amylopectin

Figure 5: Structure of amylopectin
  • is a larger polysaccharide
  • Amylopectin is also a long chain of α– glucose molecules that are joined together with 1, 4 glyosidic bonds
  • However, differs from amylose as it branched (4%) with a more open molecular structure that has the occasional 1, 6 glyosidic bonds (Figure 5). This makes it have more open side branches that can be easily accessible and broken at quicker rate
  • Amylopectin is more easily broken down by amylase enzymes when glucose is needed
  • A comparison of amylose and amylopectin structure is shown in Figure 6:
Figure 6: Comparison of amylose and amylopectin

Glycogen

  • It is the main energy storage polysaccharide found in fungi and animal cells and can be more rapidly broken down (to glucose for energy), compared to starch as animal cells are more active than plants.
  • Glycogen has a similar structure to amylopectin, it is a branched glucose polymer linked by α-1,4-glycosidic bonds and intersected by very frequent α-1,6-linked glucose residues resulting in a highly branched structure
  • Glycogen serves as glucose osmotically neutral storage, repository of energy and carbon.
  • Glycogen is found in cells with a high metabolic rate for example, mainly stored in the liver and skeletal muscle, although its presence has been also described in kidneys, brain, heart, as well as adipose tissue and red blood cells.
Figure 7: Glycogen structure – Branched glucose chains form a fractal structure around glycogen in, the small protein in the core.

Cellulose

  • Cellulose is an important structural component of the cell wall a plant cell
  • Cellulose is composed of many thousands of β – glucose monomers linked together by β – 1-4 glyosidic bonds (Figure 8).
  • To be able to form the 1,4 glyosidic bonds, each β-glucose monomer needs to be inverted by 180 from the previous molecule
  • Inversions keeps cellulose from coiling and allow for the chain to be long and straight. Chains are then able to run parallel to each other
  • The straight alignment up of the chains results in hydroxyl (-OH) groups to be in close proximity and allows for hydrogen bonds to form between them and adjacent chains
  • Cellulose is very strong due to the parallel chains that have cross-linking between the many thousands of hydrogen bonds that form into stronger fibres
  • The cellulose chains first bundle together to from microfibrils
  • Microfibrils then bundle together to from larger fibres called macro fibrils (fibres)
  • The macro fibrils is what wraps around plant cells in multiple layers at different angles and provides the plant cell wall extra strength
  • Cellulose in not easily digestible as a food source as it is very hard to break down by hydrolysis as animal cells lack the enzyme cellulase to break down the 1,4 glyosidic bonds between the beta glucose molecules
  • Humans also need cellulose as it provides fibre in their diet and keeps the digestive system healthy
Figure 8: Cellulose structure

References

[1]. Chemistry libreTextbooks: Carbohydrates Polysaccharides
[2]. Biochemistry (Campbell and Farell) Lehninger Principles of Biochemistry
[3]. Biology in Context for Cambridge International AS and A level.
[4]. https://openstax.org/books/biology-ap-courses/pages/3-2-carbohydrates
[5]. https://en.wikipedia.org/wiki/Glycogen