The night before a Sunday morning parkrun in the Phoenix Park, Aoife eats a big bowl of pasta: mostly starch, the plant's compact store of glucose, which her body will later store as glycogen. This section analyses, from the simplest molecule to the polymer, how the same chemical unit —a monosaccharide— can serve as an immediate fuel or as a structural material, depending on how it is bonded and folded.
Monosaccharides: the basic unit
If you dissolve a spoonful of glucose in water and expose it to digestive enzymes or mild acids, the molecule does not break down into smaller carbohydrates: you have reached the indivisible limit of the sugars. What chemical features define this basic unit, and how does its structure determine the function it performs?
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What is a monosaccharide? Give two examples and state the main function of monosaccharides in the cell.
What is a monosaccharide? Give two examples and state the main function of monosaccharides in the cell.
Glucose and fructose have the same molecular formula, $\text{C}6\text{H}{12}\text{O}6$, but different structures. Explain what type of isomerism relates them based on…
Glucose and fructose have the same molecular formula, $\text{C}_6\text{H}_{12}\text{O}_6$, but different structures. Explain what type of isomerism relates them based on their linear forms and describe what happens to them when they dissolve in water: what forms do their molecules then adopt, what are α and β anomers, and how is this cyclisation related to the formation of the glycosidic bond?
Sucrose is formed by the joining of a glucose and a fructose molecule. What type of bond is formed between both monosaccharides, what is the reaction that forms it…
Sucrose is formed by the joining of a glucose and a fructose molecule. What type of bond is formed between both monosaccharides, what is the reaction that forms it called, and what molecule is released during it?
A cotton T-shirt and a spoonful of flour are made of exactly the same monomer —D-glucose—, yet the T-shirt neither dissolves nor can we digest it, while flour thickens when cooked and nourishes us. How do cells achieve such opposite properties from the same building block?
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State the biological function of glycogen in animals and in which organs it is mainly stored.
State the biological function of glycogen in animals and in which organs it is mainly stored.
Starch is a mixture of two polymers. Amylose is a long unbranched α(1→4) chain. Amylopectin (and glycogen, which is even more branched) has α(1→6) branch points every 20 to 30 glucose units. Every branch creates an extra non-reducing end. Branching creates more chain ends, so enzymes that act at the ends can work at many sites at once, and the compact branched molecule is hydrolysed faster than a linear one.
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The diagram shows how the chains of two polysaccharides made exclusively of glucose are arranged. State which corresponds to starch and which to cellulose, explain what…
The diagram shows how the chains of two polysaccharides made exclusively of glucose are arranged. State which corresponds to starch and which to cellulose, explain what difference exists in the bonding of their monomers (α or β bond), and reason how the shape of their chains justifies why one serves as an energy reserve and the other has a structural function.
Two insoluble structural polysaccharides are isolated: polymer A comes from the cell wall of a plant cell (oak) and polymer B from the exoskeleton of a crustacean…
Two insoluble structural polysaccharides are isolated: polymer A comes from the cell wall of a plant cell (oak) and polymer B from the exoskeleton of a crustacean (crab). Both present linear chains linked by beta-(1->4) glycosidic bonds, but elemental analysis reveals the presence of nitrogen only in polymer B.
a) Identify polymers A and B and state the name of the constituent monomer of each one.
b) Explain why the spatial conformation of the beta-(1->4) bond is ideal for the mechanical support function compared to the alpha-(1->4) bond of glycogen or starch, detailing the role of intra- and interchain hydrogen bonds in the formation of insoluble microfibrils.
c) If a solution of purified cellulase enzyme is added to a suspension of polymer B, reasonably predict whether hydrolysis will occur and justify it based on enzyme specificity.
A sports nutrition laboratory designs two energy recovery drinks with identical polysaccharide concentration and total mass: drink A contains highly branched…
A sports nutrition laboratory designs two energy recovery drinks with identical polysaccharide concentration and total mass: drink A contains highly branched amylopectin, while drink B contains linear amylose of similar molecular mass.
a) Reasonably predict which of the two drinks will induce a faster blood glucose spike after ingestion, based on the specificity of digestive enzymes (alpha-amylases) and the molecular structure of both polymers (type of glycosidic bonds and availability of non-reducing ends).
b) In myocytes, absorbed glucose is stored as glycogen and not in the form of free glucose or starch. Justify this biological adaptation taking into account cellular osmotic pressure and the speed with which the cell must mobilise energy in response to an urgent metabolic demand.
Benedict's test detects reducing sugars (all monosaccharides, and maltose and lactose). Add Benedict's solution to the sample in a test tube, heat in a hot water bath (about 80–100°C) and watch the colour change from blue to green, yellow, orange and finally brick red as the sugar concentration increases. Fehling's test works in the same way. Sucrose is a non-reducing sugar and stays blue unless it is first hydrolysed.
To estimate the amount of reducing sugar quantitatively, use a fixed volume of sample and an excess of Benedict's solution, heat in the water bath for a set time, then filter off, dry and weigh the red precipitate (more precipitate means more reducing sugar), or use a colorimeter to measure the colour (or the fading of the blue) and read the concentration from a calibration curve made with glucose solutions of known concentration. Keep the volumes, time and temperature the same for every sample so that the test is fair.
Iodine test: iodine solution turns from orange-brown to blue-black with starch.
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An infant dairy product advertised as "no added sugars" contains in its nutritional table 4.8 g of lactose and 12 g of cornstarch per 100 g of product. a) Refute the…
An infant dairy product advertised as "no added sugars" contains in its nutritional table 4.8 g of lactose and 12 g of cornstarch per 100 g of product.
a) Refute the claim that this food is free of carbohydrate or sugar intake for the body. Chemically classify lactose and starch (type of carbohydrate, monomers, and type of bond) and describe the monosaccharides that will pass into the blood after their complete digestion.
b) In the laboratory, a sample of the product is subjected to Fehling's test before and after incubating it with alpha-amylase. Predict and justify the result of the test on the initial sample (indicating to which component it is due) and explain with reasoning how the intensity of the reaction will vary after enzymatic action on the starch, based on the concept of anomeric carbon and reducing end.
Ribose is a pentose found in RNA; 2-deoxyribose, found in DNA, has H instead of OH on carbon 2. Glucose and fructose are hexoses that differ in their carbonyl group (aldose/ketose).
Back to the start: The athlete eats pasta because its starch has $\alpha(1\to 4)$ bonds that our enzymes readily hydrolyse into free glucose; this passes into the blood and is stored in the muscle as glycogen, a compact, branched reserve ready to release fuel quickly during the race.
Summary
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