Carbohydrates
Every time a soccer player eats a plate of pasta the night before a big game, she is eating a store of glucose that the plant packaged as starch and that her body will later store as glycogen. This section analyzes, 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.
Macromolecules: monomers and polymers
When you eat a bowl of oatmeal, your body breaks big molecules into small ones, then builds the big molecules it needs. How do cells put together and take apart large biological molecules?
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 limit of the sugars. What chemical features define this basic unit, and how does its structure determine the function it performs?
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 cyclization related to the formation of the glycosidic bond?
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 molecule and a fructose molecule. What type of bond is formed between the two monosaccharides, what is the reaction that…
Sucrose is formed by the joining of a glucose molecule and a fructose molecule. What type of bond is formed between the two monosaccharides, what is the reaction that forms it called, and what molecule is released during it?
Polysaccharides: energy storage and structure
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?
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 have 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 intrachain 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 diet and metabolism
A sports nutrition lab designs two energy recovery drinks with identical polysaccharide concentration and total mass: drink A contains highly branched amylopectin, while…
A sports nutrition lab 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 cause 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 muscle cells, 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 mobilize energy in response to an urgent metabolic demand.
Pentoses, isomerism and recognizing structures
Ribose is a pentose found in RNA; 2-deoxyribose, found in DNA, has H instead of OH on carbon 2. Glucose, galactose and fructose are hexoses. Glucose and fructose differ in their carbonyl group (aldose/ketose); glucose and galactose differ in the configuration of one asymmetric carbon (epimers).
Back to the start: The soccer player eats pasta because its starch has $\alpha(1\to 4)$ bonds that our enzymes readily hydrolyze 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.