Simplifying Microbiology One Post Every Saturday + Helpful Glossary

#5- The Macromolecules of Life (I): Carbohydrates

Trust me, everything exists because of these guys

To further understand the molecular workings that involve larger and specialised molecules, you will need to familiarise yourself with these essential macromolecules, starting with good ol’ carbs.

But first! Get your favourite carbohydrate meal before you read this 😄

What are Carbohydrates?

From biology’s point of view, they are the kind that provides energy for the organism, and upon digestion, they are used to build important body structures, like, the glycolipids and glycoproteins found on the plasma membrane (See Post #1).

On the chemistry side of things, carbohydrates are always made up of 3 elements- carbon (C), hydrogen (H), and oxygen (O).

Now let’s see some sugar!

Monosaccharides

Monosaccharides are the simplest sugars and it means ‘single sugar’. A general formula for them goes like this — (CH₂O)n. ‘n’ can be any number of carbon atoms from 3 to 7.

Here is the list where each of them is a different type of monosaccharide:

  • Trioses: n= 3
  • Tetroses: n= 4
  • Pentoses: n= 5
  • Hexoses: n= 6
  • Heptoses: n= 7

Okay, you must have heard of glucose, right? If not, you can find them as an ingredient on a food label, particularly in sports drinks or anything sweet. This monosaccharide is the most common hexose and has the chemical formula C₆H₁₂O₆.

The funny thing about glucose is that it has a variety of chemical structures, i.e., different forms known as isomers, despite having the same written chemical formula, C₆H₁₂O₆! The two most common isomers of glucose are α (alpha) and β (beta) isomers.

The structural formula of alpha (top) and beta (bottom) glucose isomers. The hydroxyl or OH group is positioned below carbon atom 1 for the alpha isomer, but above the carbon atom 1 in the beta isomer. Source in caption.
Figure 1- Structural formula of α glucose (top) and β glucose (bottom) isomers. Source- Advanced Biology, by Kent, 2nd Edn.

The difference between the alpha and beta isomers is the position of the hydroxyl group ( — OH) and hydrogen atoms when bound to the first carbon atom (See Fig. 1). You may say “They still look the same! What’s the big difference?” Oh boy! They actually do have a big difference which will be covered soon in this post. Other than glucose, fructose (fruit sugar) and galactose are essential hexoses, too.

Lastly, ribose and deoxyribose sugars are the most important pentoses present in RNA and DNA, respectively (See Post #3).

Fine, let’s make the sugars a bit big, shall we?

Disaccharides

When two monosaccharides join hands together (I mean bond), they form disaccharides or ‘double sugar’. Bonds just don’t happen without a sacrifice, there has to be a loss, a gain, or cooperation of sorts and that’s how two monosaccharides bind, by losing a single molecule of water (H₂O). This is called a condensation reaction.

Here are some well-known disaccharides:

  • Sucrose: Classic white sugar for your coffee, tea, desserts, etc.; glucose + fructose
  • Lactose: Natural sugar in milk; glucose + galactose
  • Maltose: Component of malt; glucose + glucose

Now that’s real sweet!: Fructose is the sweetest natural sugar and 1.5X sweeter than sucrose hence, a substitute for diabetics.

For my final trick, I am going to join some MORE monosaccharides because go big or go home, right?

Polysaccharides

Before we check out some polysaccharides, let’s start understanding what oligosaccharides are.

Oligosaccharides are definitely larger than disaccharides but are made with less than 10 monosaccharides.

Monosaccharides, disaccharides, and oligosaccharides are considered as sugars since they appear crystalline and can be dissolved in water.

In the case of polysaccharides, they are made of several repetitive monosaccharide units through a series of condensation reactions to form a much larger complex molecule. This phenomenon is known as polymerisation. The polysaccharide formed is a polymer. A familiar polymer we all know is cellulose, which is present in plants and it is the most abundant organic material in the world.

In contrast to sugars, polysaccharides do not dissolve in water, lack sweetness, and can’t be crystallised. They go by the general chemical formula (C₆H₁₀O₅)n, where ‘n’ is 2 or more.

Important tidbit!: Conjugated molecules involve chains of monosaccharides binding with lipids or proteins to form glycolipids or glycoproteins present in cell membranes.

The formation of disaccharides and polysaccharides by a condensation reaction (downward red arrows) and the reverse by hydrolysis reaction (upward blue arrows), i.e., the addition of water to get back the basic monomeric units. Source in caption.
Figure 2- Formation of disaccharides and polysaccharides (red arrows) and breakdown of them by hydrolysis reaction to give monosaccharides (blue arrows). Source- Advanced Biology, by Kent, 2nd Edn.

Easy to remember!: Add a spoonful of sugar to your cup of coffee (or beverage of choice) and watch it dissolve completely, that’s hydrolysis, the complete opposite of condensation (See Fig. 2).

Polysaccharides can appear straight and also in different shapes, like, branched, helical, or coiled (spiral), and that depends on these factors:

  • Type of monomers
  • Quantity of monomers
  • Type of bonds between monomers

Speaking of bonds between monomers, let's check out the two types of glycosidic bonds (See Fig. 3).

  • 1-4 Glycosidic links: Formed between — OH group of C1 (carbon atom 1) in one monosaccharide with C4 of another via condensation. Forms straight chains.
  • 1-6 Glycosidic links: Formed between — OH group of C1 in one monosaccharide with C6 of another via condensation. Forms branched chains.
A piece of a polymer named glycogen showing 1–4 glycosidic (highlighted in red) links forming straight chains, and a 1–6 glycosidic (also in red) link forming a branched chain. Source in caption.
Figure 3- A piece of a polymer named glycogen showing 1-4 glycosidic links forming a straight chain, and 1-6 glycosidic links forming a branched chain. Source- Advanced Biology, by Kent, 2nd Edn.

You now know some basics of carbohydrates so let’s see some examples of polysaccharides in a bit of detail.

Starch: Plants’ Energy Store

This plant polysaccharide consists of many α glucose monomers and has two parts:

  • Amylose: Mainly contains 1-4 glycosidic links for straight chains
  • Amylopectin: Has 1-6 glycosidic links for several highly branched chains
A chemical structure of starch. The highlighted orange  — OH groups bound to C2 and C3 face inwards in the helix. On the right of the chemical structure, a spiral in orange indicates the shape of starch. Source in caption.
Figure 4- Chemical structure of starch. The highlighted orange — OH groups bound to C2 and C3 face inwards in the helix. Source- Advanced Biology, by Kent, 2nd Edn.

Despite its complexity, it is a compact polymer which takes a helical shape, perfect for being stored as granules in amyloplasts or chloroplasts in plant cells.

Roundish starch granules stained purple most likely by an iodine-based stain in almost transparent potato cells. Source in caption.
Figure 5- Starch granules stained purple most likely by an iodine-based stain in potato cells. Source- By Ganímedes — Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=80613978

Fun Fact!: Eating a piece of plain white bread, or any kind of bread or starchy food with no sugar, you will get a sweet taste while chewing because the amylase enzyme in your saliva is digesting the starches and releasing the sugars.

Cellulose: Structural Plant Material

It is quite a tough polysaccharide always found in plants, providing structure, and as a major component in their cell walls. It’s made of β glucose isomers with 1-4 glycosidic bonds (See Fig. 6).

The plant’s outer cellulose wall is absolutely permeable to allow free movement of water and other substances, and prevent’s it from bursting when lots of water enter the cell by osmosis.

Cellulose cannot be hydrolysed easily by humans and some animals but can be fully digested by ruminants, like, cows and elephants, with special microbes in their guts that produce cellulase enzyme.

Chemical structure of cellulose showing beta glucose units bonding via 1–4 glycosidic bonds. Hydroxyl groups (in orange) alternate to make it a straight yet fibrous chain which links with neighbouring chains by hydrogen bonds to strengthen it. Source in caption.
Figure 6- Chemical structure of cellulose showing β glucose units bonding via 1-4 glycosidic bonds. Hydroxyl groups (in orange) alternate to make it a straight yet fibrous chain which links with neighbouring chains by hydrogen bonds to strengthen it. Source- Advanced Biology, by Kent, 2nd Edn.

Glycogen: The ‘Animal Starch’

If plants have that, so do we and other animals, too.

This one is also a polysaccharide, with almost similar structure and function to that of starch, however, it is more branched due to the large number of 1-6 glycosidic linkages. Hence, it is even less dense and more soluble than starch. Additionally, it is easily hydrolysed by enzymes and can be digested faster.

Glycogen is stored in animals and humans in the form of granules and is found mostly in liver cells and muscles.

A 2-dimensional cross-section of a glycogen molecule. It has a core glycogenin protein (as colourful helices) surrounded by many branched alpha glucose units. Source in caption.
Figure 7- A 2-dimensional cross-section of a glycogen molecule. It has a core glycogenin protein (colourful helices) surrounded by many branched α glucose units. Source- By Mikael Häggström. When using this image in external works, it may be cited as: Häggström, Mikael (2014). " Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002–4436. Public Domain.orBy Mikael Häggström, used with permission. — Own work (Original text: Own work by uploader, using following images:)Glucose (Public Domain license).Glycogenin structure (Public Domain license).Structure reference: Similar image on scientificpsychic.com — > Carbohydrates — Chemical Structure. By Antonio Zamora on May 27, 2009, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6880883

Okay! You have done a good job at reading about, and also appreciating the importance of carbohydrates in and around us. But of course, everything should be in moderation. These brilliant macromolecules are absolutely important as energy reserves whose complete elimination will prove detrimental to one’s health. So make sure to have an occasional bowl of pasta, your favourite fried rice, some fruits, beans, and lentils to maintain a balanced diet and enjoy a good workout! 👍

As always, do leave a comment if you have any questions and follow/subscribe to get more simplified microbiology in your life ✌😄

Glossary

Crystallised- Process of turning a chemical compound into a crystal

Amyloplasts- Modified, colourless plastid in plants for storing food

Chloroplasts- Modified plastid carrying green pigment in plants or other unicellular animal cells

Amylase- An enzyme found in saliva and pancreas

Enzyme- A protein that can speed up biochemical reactions

Osmosis- Movement of water from a region of low solute concentration to a region with high solute concentration

Sources

  • Chapter 2: The Chemicals of Life, Section 2.6: Carbohydrates: simple sugars. From the textbook Advanced Biology, Michael Kent, 2nd Edn.
  • Chapter 2: The Chemicals of Life, Section 2.7: Carbohydrates: polysaccharides. From the textbook Advanced Biology, Michael Kent, 2nd Edn.
  • Me remembering lecture material from school and university

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Treveni Mukherjee

Treveni Mukherjee

A University of Leeds alumna with an Integrated Masters degree in Microbiology taking a break from science 😄