A corn tortilla, a plate of pasta before a race, or an ear of corn at the street market have something in common: they are carbohydrates, also called sugars (glucides). Often the same chemical unit, glucose, serves as immediate fuel, as storage, or as building material, depending on how it is bonded and folded. On this page you will see how.
Monosaccharides: the basic unit
If you dissolve a spoonful of glucose in water and add digestive enzymes or mild acids, the molecule does not break into smaller sugars: you have reached the indivisible limit. What defines this basic unit, and how does its structure determine what it does?
Monosaccharides are the cell's energy "currency": glucose is the main fuel that is oxidized in cellular respiration. Other examples are fructose (fruits and honey) and galactose (part of the lactose in milk).
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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 corn flour are made of the same monomer, D-glucose. But the T-shirt does not dissolve or get digested, while the flour thickens atole and nourishes us. How do cells achieve such opposite properties with the same building block?
Glycogen is the storage polysaccharide of animals: more branched than starch, it is stored mainly in the liver (it maintains blood glucose) and in muscles (fuel for their own work). Chitin, which forms the exoskeleton of insects and crustaceans, is a derivative of glucose with an amino group (it contains nitrogen).
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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.
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.
Branched carbohydrates, such as amylopectin or glycogen, have many ends where enzymes (amylases) can cut at the same time, so they release glucose faster than a linear chain like amylose. Amylase breaks α bonds, not β bonds.
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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.
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 the pentose of RNA; 2-deoxyribose in DNA has H instead of OH on carbon 2. Glucose, galactose, and fructose are hexoses. Glucose and fructose differ in the carbonyl group (aldose/ketose); glucose and galactose differ in the configuration of one asymmetric carbon (epimers).
Let's practice! 3 medium exercises in this section
Back to the beginning: someone who eats tortillas or pasta before a race gets starch with $\alpha(1\to 4)$ bonds, which our amylases easily hydrolyze to glucose; it passes into the blood and is stored in muscle and liver as glycogen, a compact, branched reserve ready to release energy quickly.
Summary
Let's practice! 3 medium exercises in this section