U1L3 - Carbohydrates
Monosaccharides, Polysaccharides, and Key Definitions
Monosaccharide: a single sugar molecule
mono- refers to one, saccharide refers to sugars.
Monosaccharides are the monomers of polysaccharides (many sugars).
Example shown: glucose.
Polysaccharides: long chains formed
linking many monosaccharide units (poly = many).
Glucose: a canonical hexose sugar with the molecular formula .
Structurally depicted in a ring form in practice, though the linear form exists.
In diagrams, not all carbons are drawn to avoid clutter
carbons are present and bound to hydrogens and hydroxyls (–OH).
Ring concept:
shows carbons in a cyclic arrangement with an oxygen in the ring
the exocyclic CH₂OH group is on carbon 6.
Important functional group context:
hydroxyl groups (–OH) location
determines identity and classification of the sugar.
The ring is typically drawn to
illustrate stereochemistry at the hydroxyl-bearing carbons
the actual structure contains a carbon framework with each carbon attached to H, OH, and/or other carbons.
The hydroxyl groups and their orientation determine whether glucose is classified as:alpha or beta (see below).
Names and placement basics you need to know:
Carbon numbering in the ring
starts at the oxygen atom and runs clockwise
C1, C2, C3, C4, C5
C6 as the exocyclic carbon attached as CH₂OH.
position of the hydroxyl on carbon 1 determines alpha vs beta:
If:
hydroxyl on C1 is in the down position, it is alpha;
hydroxyl on C1 is in the up position, it is beta.
The carbons on the end-group that the carbonyl exists (in the linear form)
classifies the sugar as an
aldose (aldehyde) or ketose (ketone) when considered in the extended form;
in the ring, glucose is typically an aldose sugar (due to an aldehyde group in the linear form).
Hexose sugars: six carbons in the molecule; hex stands for six.
Aldose sugars:
have an aldehyde group (C=O at the end in linear form)
ketose sugars have a ketone group (C=O within the molecule, not at the end).
In glucose
the end carbonyl group in the linear form is an aldehyde,
so glucose is an aldose.
In fructose:
the carbonyl group is a ketone,
so fructose is a ketohexose (a ketose).
Isomerism: Molecules with the same chemical formula can have different arrangements of atoms
glucose, galactose, and fructose are all structural isomers.
Isomers of (three key examples):
Glucose: aldose hexose; common in biology
primary product of photosynthesis &key substrate in cellular respiration
In ring form, the OH orientation on C1 determines alpha vs beta.
Galactose: an aldose hexose isomer of glucose
same molecular formula but OH arrangement differs notably on C4.
Galactose is the monomer of lactose (the disaccharide).
Fructose: a ketohexose; same molecular formula as glucose and galactose
carbonyl occurs at C2 (a ketone)
gives a different structural arrangement.
Summary identity check approach (practice strategy):
Step 1: Count atoms to confirm formula .
Step 2: Look at carbon 1 to determine alpha vs beta
Alpha - C1 OH @bottom
Beta = C1 OH @ top
Step 3: Look at carbon 4 to distinguish glucose vs galactose
glucose = C4 OH down
galactose = C4 OH up
Step 4: If the carbonyl is on C2 (ketone), as in fructose,
it is a keto sugar.
Fructose (ketose) details:
Molecular formula:
same as glucose/galactose - structural isomer
Ketose classification due to a ketone group (C=O)
not on the terminal (main) carbon.
Often drawn with a different arrangement
(two side chains) compared to glucose/galactose.
Pentose sugars: five carbons total
Ribose: ; a pentose sugar.
Found in RNA and ATP (adenosine triphosphate).
Deoxyribose: ; missing one oxygen on carbon 2 compared to ribose (hence “deoxyribo”).
The sugar in DNA (deoxyribonucleic acid).
Both ribose and deoxyribose are pentose sugars; the key difference is the presence/absence of an oxygen on C2.
Nomenclature and functional groups recap
Aldose vs Ketose:
Aldose: aldehyde group at the end of the chain (e.g., glucose, galactose) → ring form often described as aldose sugars.
Ketose: ketone group within the chain (e.g., fructose) → ring form described as a keto sugar.
Hexose vs Pentose:
Hexose: six carbons (e.g., glucose, galactose, fructose).
Pentose: five carbons (e.g., ribose, deoxyribose).
Alpha vs Beta (anomeric orientation):
Alpha: C1 hydroxyl down in the ring form.
Beta: C1 hydroxyl up in the ring form.
Structural isomerism within hexoses:
Glucose and galactose are both aldose hexoses with the same formula but different hydroxyl orientations (notably at C4).
Lactose and other disaccharides: lactose is a disaccharide composed of glucose and galactose.
Quick practice points and study tips from the lecture
Create a quick-reference table of sugars with definitions and classifications (alpha/beta, aldose/ketose, hexose/pentose).
Use flashcards to memorize the definitions and key features of glucose, galactose, fructose, ribose, and deoxyribose.
Practice counting carbons, hydrogens, and oxygens to identify sugars: count C, then H, then O; for the example sugar, you should verify counts like and for ribose , etc.
Remember that the oxygen in the chain can be described as an alkoxy group (an oxygen between two carbons). In contrast, a carbonyl with a hydrogen at the end denotes an aldehyde.
The practical significance of these classifications: different sugars have different roles in biology (e.g., glucose in energy metabolism; ribose in RNA/ATP; deoxyribose in DNA).
Connections to broader biology and chemistry
Glucose, galactose, and fructose are common monosaccharides that form part of larger carbohydrates; they are the building blocks for disaccharides and polysaccharides.
The same chemical formula can describe multiple sugars that differ in spatial arrangement, highlighting the importance of stereochemistry in biochemistry.
The ring forms and anomeric configuration (alpha/beta) influence the way sugars cyclize and participate in glycosidic bond formation, which is central to carbohydrate chemistry and energy storage.
Real-world relevance and ongoing study notes
Glucose’s role as a primary energy source in cellular respiration and its production during photosynthesis.
Lactose as a common disaccharide composed of glucose and galactose; an example of how sugar monomers combine to form polymers.
Ribose in RNA and ATP and deoxyribose in DNA illustrate how small changes in sugar composition alter the structure and function of essential biomolecules.
Quick glossary (for quick memorization)
Monosaccharide: single sugar unit.
Hexose: 6-carbon sugar (e.g., glucose, galactose, fructose).
Pentose: 5-carbon sugar (e.g., ribose, deoxyribose).
Aldose: sugar with aldehyde group (end-of-chain carbonyl).
Ketose: sugar with ketone group (internal carbonyl).
Alpha (α): C1 hydroxyl down in the ring form.
Beta (β): C1 hydroxyl up in the ring form.
Anomer: stereoisomer differing at the anomeric carbon (C1 in cyclic form).
Isomer: same molecular formula but different arrangement of atoms.
Quick worked example (from the lecture practice)
Given a ring sugar with C6, H12, O6, where C1 OH is down and C4 OH is down:
Count carbons: 6 → hexose.
C1 OH down → alpha.
C4 OH down → glucose.
Therefore the molecule is alpha-glucose (an aldose hexose).
If C1 OH is up and C4 OH is up:
C1 OH up → beta.
C4 OH up → galactose if C4 differs accordingly; but if C4 is up in a glucose-like framework, you’d classify accordingly as beta-galactose.
Final takeaway
Sugar identities are determined by carbon count (pentose vs hexose), carbonyl type (aldose vs ketose), and stereochemical orientation (alpha/beta at C1, and C4 orientation for glucose vs galactose).