Carbohydrates
Before a Licey baseball game, a player eats rice, yuca, or bread. Why do those foods give him energy, while a cotton fiber or wood, also made of glucose, cannot be digested? It all depends on how the molecules are joined.
Monosaccharides: the basic unit
If you dissolve a spoonful of glucose in water, the molecule does not split into smaller sugars: you have reached the limit. That is the basic unit of carbohydrates.
Glucose has the carbonyl group at the end of the chain (aldehyde); fructose has it at carbon 2 (ketone). They have the same formula, $\text{C}_6\text{H}_{12}\text{O}_6$, but different structures: they are isomers. Glucose and fructose are abundant in fruits such as bananas and passion fruit, and in honey.
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.
Disaccharides: the glycosidic bond
Classify the following carbohydrates as monosaccharides, disaccharides or polysaccharides: glucose, sucrose, starch, fructose, lactose and cellulose.
Classify the following carbohydrates as monosaccharides, disaccharides or polysaccharides: glucose, sucrose, starch, fructose, lactose and cellulose.
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?
Polysaccharides: storage and structure
A cotton T-shirt and a spoonful of flour are made of the same monomer, glucose, but you can't eat the T-shirt and you can eat the flour. How do cells achieve such opposite properties?
Starch accumulates in yuca, potatoes, rice, and plantains. Glycogen is the storage carbohydrate of animals and is stored mainly in the liver and the muscles. Cellulose forms the wall of plant cells and is made only of glucose. Chitin forms the exoskeleton of shrimp, crabs, and insects: it is a chain with β(1→4) bonds whose monomer is N-acetylglucosamine, a nitrogen-containing derivative of glucose.
Amylose and amylopectin: two forms of starch
Starch is not a single molecule: it is a mixture of two glucose polymers with α bonds.
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Amylose is a linear chain, with α(1→4) bonds and coiled into a helix. It has only two ends.
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Amylopectin is a branched chain: in addition to the α(1→4) bonds, every so many glucose units there is an α(1→6) bond that creates a branch. It has a great many ends. Glycogen is similar to amylopectin, but with even more frequent branches.
Why does branching matter? α-amylase (from saliva and the pancreas) breaks α(1→4) bonds inside the chains and produces short fragments (maltose and dextrins). Then enzymes in the intestine release glucose from the ends of those fragments, and others cut the α(1→6) bonds at the branches. A branched molecule offers many ends and many points of attack at once, so many enzymes work at the same time and glucose is released faster. In amylose, with only two ends, release is slower. That is why glycogen is mobilized quickly when the muscle needs energy.
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.
Carbohydrates in the cell membrane
Carbohydrates don't only provide energy or form walls: they are also in the "skin" of your cells. On the outer face of the plasma membrane, short chains of sugars are attached to proteins (glycoproteins) and to lipids (glycolipids). All together they form a layer called the glycocalyx.
These are short chains, not long storage polysaccharides. You will see the complete structure of the membrane (phospholipid bilayer and proteins) in the section MEMBR-ESTRUCM.
Classify the following carbohydrates as monosaccharides, disaccharides or polysaccharides: glucose, sucrose, starch, fructose, lactose and cellulose.
Classify the following carbohydrates as monosaccharides, disaccharides or polysaccharides: glucose, sucrose, starch, fructose, lactose and cellulose.
Carbohydrates in the diet and metabolism
A typical laboratory test is Fehling's test, which detects reducing sugars such as glucose: it is positive with glucose and negative with starch. After treating starch with amylase, which hydrolyzes it to maltose and glucose, the test turns positive.
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.
Extension: isomerism and ring formation in sugars
You already saw that glucose and fructose are isomers: they have the same formula $\text{C}_6\text{H}_{12}\text{O}_6$ and different structures. Since the difference lies in the carbonyl group (aldehyde in glucose, ketone in fructose), they are called functional isomers.
In aqueous solution, these molecules do not stay as a linear chain: they cyclize and form a ring. The carbon of the carbonyl (the anomeric carbon) can end up with its OH pointing to one side or the other of the ring, and so two variants form, the α and β anomers.
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?
Back to the beginning: the player performs well on rice or yuca because their starch has α bonds that our enzymes hydrolyze to glucose, which enters the blood and is stored in the muscle as glycogen. Cotton and wood contain cellulose, with β bonds that we cannot break.