Mono and polysaccharides (B1.1.4,B1.1.5,B1.1.6)

• What are monosaccharides?


  Monosaccharides are the building blocks of carbohydrates. They are single sugar molecules, and are composed of carbon, hydrogen and oxygen, in a ratio of Cn H2n On (where n is 3-7, because monosaccharides have 3-7 carbon atoms).

  Based on how many carbon atoms they contain, a monosaccharide may be pentose or hexose. A pentose monosaccharide has five carbons, and a hexose has six carbon atoms. Pentoses and hexoses may exist in either straight-chain or ring form. To form disaccharides and polysaccharides, the monosaccharide must be in ring form.
• What is a pentose?

  Pentose monosaccharides contain five carbon molecules. Some examples are ribose (found in RNA) and deoxyribose (found in DNA, with one less oxygen molecule compared to ribose).
• What is a hexose?

  Hexose monosaccharides contain six carbon atoms. Some examples are glucose (the primary energy source in cells) fructose (the sugar found in fruits) and galactose (the sugar found in lactose).
• What is glucose?

  Glucose is a very common monosaccharide, and is a hexose (six carbon atoms.)

  Glucose is polar due to its hydroxyl groups that form hydrogen bonds with water. This means that it is also hydrophilic, and is soluble. Its soluble properties means that it can be easily transported, either within the bloodstream or within cells.

  Glucose needs to be transported for several reasons- dissolved in blood plasma for delivery to cells (to then be used for energy) and converted to sucrose for transport via phloem in plants.

  In aqueous environments, glucose is in ringed form, rather than chain form, meaning that it is more stable. As its structure is rather robust, it can resist spontaneous reactions, making it relatively stable. Because it is stable, it can be used as food storage. It is usually converted to glycogen or starch when stored, however, because in large quantities, glucose would cause osmotic issues when stored in cells.

  During respiration, the oxidation of one molecules of glucose yields around 38 ATP molecules. This means that glucose is an efficient and primary energy source for most organisms.
• What is a polysaccharide?

  Starch and glycogen are both polysaccharides, composed of large alpha glucose molecules that can be used as a substrate in cell respiration. They are also both used as energy stores, due to their stable structure, starch in plants and glycogen in animals. Polysaccharides are complex carbohydrates formed by the polymerization of many monosaccharide units linked by glycosidic bonds. Polysaccharides serve as energy storage molecules (e.g., starch, glycogen) or structural components (e.g., cellulose, chitin). They are typically large, insoluble, and ideal for long-term energy storage or support.
• What is starch?


  There are two main types of starch- amylose and amylopectin. (starch and glucose do not have a fixed molecular mass, so molar solutions cannot be made. Concentrations must be presented in percentages.)
• What is amylose?


  Amylose is an unbranched chain of alpha glucose that is linked by 1—>4 glycosidic bonds (the hydroxyl group on carbon 1 reacts with the hydroxyl group on carbon 4 of another). Alpha glucose exists in ring form when the hydroxyl group on the carbon 1 atom is below the plane of the ring (pointing downwards), in the same plane as carbon 2 and carbon 4. Due to the glycosidic bond angles, an amylose chain is not straight, but is helical.
• What is amylopectin?


  Amylopectin is a branched chain of alpha glucose that is linked by 1—>6 glycosylic bonds (the hydroxyl group on carbon 1 reacts with the hydroxyl group on carbon 6 of another). This glycosylic bond creates a branch point in the polysaccharide chain-and is also an alpha glucose so the hydroxyl group on the carbon 1 atom is below the plane of the ring (pointing downwards), in the same plane as carbon 2 and carbon 4. Every one in twenty glucose molecules are branched, and amylopectin chains are able to contain more than a hundred thousand alpha glucose subunits. The large size of amylopectin has a lower solubility than glucose so it does not contribute to osmolarity, making it an optimal store for glucose. Its branched nature also means that it is relatively compact, despite their large molecular mass. Branching also creates multiple ends where enzymes can act simultaneously to release glucose. This is particularly critical in animals for meeting sudden energy demands.(Below is amylopectin)
• What is glycogen?


  Glycogen is a branched chain of alpha glucose molecules linked by 1—>4 glycosidic bonds, and branched by 1—>6 bonding, every one in ten molecules (compared to amylopectin, which has only one in twenty glucose molecules branched). The molecules are alpha glucose so the hydroxyl group on the carbon 1 atom is below the plane of the ring (pointing downwards), in the same plane as carbon 2 and carbon 4. The large size of glycogen (up to tens of thousands) means that they have lower solubility than glucose, contributing little to the osmolarity of the cell, making them an effective store. Its branched nature also means that it is relatively compact, despite their large molecular mass. Branching also creates multiple ends where enzymes can act simultaneously to release glucose. This is particularly critical in animals for meeting sudden energy demands.
• What is polysaccharide coiling?

  Coiling allows polysaccharides to occupy minimal space, making them efficient for energy storage in cells, and their helical structure provides a stable, dense arrangement for energy storage, reducing the molecule's reactivity.
• What is polysaccharide branching?

  Branching, as seen in amylopectin and glycogen, creates multiple ends where enzymes can act simultaneously to release glucose. This is particularly critical in animals for meeting sudden energy demands. It also reduces the tendency of the polysaccharide to crystallize, enhancing its solubility in water and facilitating enzymatic access, and the molecule remains compact allowing efficient storage.
• How is alpha glucose condensed?

  During polymerization, alpha-glucose monomers are joined through condensation reactions, where a water molecule is removed as glycosidic bonds form. In plants, condensation reactions create starch (amylose and amylopectin), while in animals, glycogen is synthesized. This allows for long-term, stable storage of glucose in a compact, less reactive form (for all the reasons listed above).
• How is alpha glucose hydrolysed?

  Hydrolysis breaks glycosidic bonds using water, releasing glucose monomers for energy production. Glycogen in animals is hydrolysed by enzymes (e.g., glycogen phosphorylase) to release glucose-1-phosphate, which enters metabolic pathways like glycolysis. In plants, starch is broken down by amylase to provide glucose during periods of low photosynthetic activity. The controlled release of glucose provides a steady supply of energy for cellular processes.
• What is alpha glucose?

  In alpha glucose molecules, the -OH group faces downwards. They form structures that wind into a helix, such as starch and glycogen. They are linked via 1--->4 glycosidic bonds in amylose and 1--->6 glycosidic bonds in amylopectin and glycogen. They are used for energy and storage in both plants and animals.
• What is beta glucose?

  In beta glucose molecules, the -OH group faces upwards. They form structures that are straight chained, such as polysaccharides like cellulose. they are inked via 1-->4 glycosidic bonds in cellulose, and provide structural support in plant cell walls.
• What is cellulose?


  Cellulose is a structural polysaccharide found in plants, composed of beta glucose subunits. In beta glucose, the OH group on carbon 1 is angled upwards, and the OH group on carbon 4 is angled downwards. Therefore, the glucose subunits in a cellulose chain face alternately upwards and downwards. All of the links on a cellulose molecule are 1—>4 glycosylic bonds, meaning that the carbon 4 of one molecule is linked to the carbon 1 of another. Cellulose can contain more than ten thousand beta glucose molecules, with an overall length of more than ten micrometres.
• What are microfibrils?


  In cellulose, hydroxyl groups are evenly spaced along each chain, allowing several hydrogen bonds to form in-between hydroxyl groups of adjacent cellulose molecules, forming bundles of cellulose in a highly organised, crystalline structure. Those cellulose bundles are called microfibrils and are bundled into larger fibres and embedded in a matrix of hemicellulose and pectin within the plant cell wall. Due to the strong covalent bonds within each cellulose molecule, and the large number of molecules and hydrogen bonds between them, microfibrils have very high tensile strength.
• What is the function of microfibrils?

  Their high tensile strength prevents plant cells from bursting under turgor pressure, and allows them to grow tall and resist gravitational forces. Their tensile strength also allows plant cells to resist tearing and deformation, which is essential for plants exposed to wind, rain and other physical forces. The dense structure of the microfibrils also acts as a physical barrier to pathogens. Its orientation dictates the direction of cell expansion, influencing plant growth patterns, and ensures uniform growth and the formation of specialized structures like stems and leaves. Furthermore, most organisms lack enzymes to break beta-1→4 glycosylic bonds in cellulose, making it indigestible for many animals. This property makes cellulose an important component of dietary fibre for humans and a challenging substrate for herbivores, which rely on symbiotic microbes to digest it.