Glucose, fructose, galactose, sucrose, lactose, maltose. If that list already reads like alphabet soup, you’re in good company! These are the commonly tested sugars on the MCAT, and the MCAT expects you to know how each one is built.
Some questions ask you to recall a structure outright. More often, a passage names a sugar and you’ll work faster if you can picture it. We’ll point out the patterns that link these MCAT sugars, so there’s less to memorize. Let’s get started!
What Are the Commonly Tested Sugars on the MCAT?
The AAMC’s content outline lists carbohydrates under both science sections, from common names and epimers to the glycosidic bond. First-semester biochemistry is about 25% of Bio/Biochem and 25% of Chem/Phys, so sugars can turn up in either one.
These are the sugars to know for the MCAT, sorted by size.
That’s the whole list! Each section below takes one group, with the structures drawn out.
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Monosaccharides: Glucose, Fructose and Galactose
Monosaccharides are the single sugar units that build every disaccharide and polysaccharide. On the MCAT, you’ll see them drawn as Fischer projections (straight chains) or Haworth projections (rings).
A sugar’s class name tells you two things. An aldose has an aldehyde at C1, and a ketose has a ketone, usually at C2. Add the carbon count, and glucose becomes an aldohexose!
Glucose structure on the MCAT
D-glucose is an aldohexose, C6H12O6. Read its hydroxyl groups from C2 down in a Fischer projection, and they point right, left, right, right. Drill that pattern until it’s automatic!
Learn glucose first, since the other hexoses here are small edits of it!
Galactose and mannose differ at one carbon
D-galactose matches glucose everywhere except C4, where its OH points left. Two sugars that differ at only one chiral carbon are epimers, so galactose is the C4 epimer of glucose.
D-mannose flips C2 instead, which makes it the C2 epimer of glucose!
Fructose is a ketose
D-fructose is also C6H12O6, with its carbonyl as a ketone at C2, so it’s a ketohexose. From C3 down, its hydroxyls match glucose (left, right, right), so there’s nothing new to learn there!
Ribose and deoxyribose
D-ribose is an aldopentose, a five-carbon sugar with every hydroxyl on the right in its Fischer projection. It’s the sugar in RNA.
Deoxyribose, the sugar in DNA, is ribose without the oxygen at C2. That’s why its full name is 2-deoxyribose. Our nucleic acid basics notes show both inside a nucleotide.
Memorize glucose as right, left, right, right. Galactose flips C4, mannose flips C2, and fructose shifts the carbonyl to C2. Our biochemistry mnemonics have more tricks like this one!
Every sugar on this page is a D sugar. In a Fischer projection, the OH on its highest-numbered chiral carbon points right. Our structural description notes cover D and L naming in more depth.
Alpha vs Beta: Anomers and Haworth Projections
In water, a monosaccharide spends most of its time as a ring. The hydroxyl on C5 of glucose attacks the aldehyde at C1, closing a six-membered ring called a pyranose.
Fructose can also close a five-membered ring, a furanose. That’s the form you’ll see in sucrose!
Ring closure turns C1 into a new chiral center, the anomeric carbon. Its hydroxyl can point down or up, which gives two anomers.
In a Haworth projection of a D sugar, the CH2OH at C5 points up. In the α anomer, the anomeric OH points down, on the opposite side. In the β anomer, it points up, on the same side.
To draw a Haworth from a Fischer projection, every group on the right points down and every group on the left points up. So glucose’s C2 and C4 hydroxyls sit below the ring, and its C3 hydroxyl sits above.
Try it once with the graphic open, and it clicks fast!
In water, the ring keeps opening and closing, so α and β glucose convert back and forth. That’s called mutarotation, and at equilibrium about two-thirds of glucose is the β form.
Anomers differ only at the anomeric carbon, like α and β glucose. Epimers differ at any one chiral carbon, like glucose and galactose at C4. Anomers count as epimers too, so read the answer choices closely!
Disaccharides: Sucrose, Lactose and Maltose
A disaccharide is two monosaccharides joined by a glycosidic bond. The anomeric carbon of one sugar bonds through an oxygen to a hydroxyl on the other, and a water molecule leaves.
Splitting that bond takes the water back, so digesting a disaccharide is a hydrolysis.
The bond’s name tells you how it’s built. The Greek letter gives the anomer at the first sugar’s anomeric carbon, and the numbers give the two carbons joined. In lactose, the β-1,4 bond links C1 of β-galactose to C4 of glucose.
Maltose
Two glucose units joined by an α-1,4 bond. Maltose comes from breaking down starch, and the enzyme maltase splits it.
Lactose
Galactose and glucose joined by a β-1,4 bond. It’s the sugar in milk, and lactase splits it. People who make too little lactase have lactose intolerance.
Sucrose
Glucose and fructose joined by an α-1,2 bond, also written α1→β2. It links C1 of α-glucose to C2 of β-fructose. Sucrose is table sugar, the same stuff in your coffee, and sucrase splits it :)
Acetals and ketals in disaccharides
If you’ve reviewed nucleophilic addition to aldehydes and ketones, these bonds will look familiar! A ring sugar’s anomeric carbon is a hemiacetal (in an aldose) or a hemiketal (in a ketose).
When a glycosidic bond swaps its OH for an OR group, it becomes an acetal or a ketal. Sucrose shows both, since glucose’s C1 becomes an acetal and fructose’s C2 becomes a ketal.
Reducing Sugars on the MCAT
A reducing sugar has a free anomeric carbon, one that isn’t tied up in a glycosidic bond. Its ring can open to an aldehyde, which gets oxidized, and the sugar reduces the test reagent in return.
Two lab tests pick them up. Tollens’ reagent leaves a silver mirror, and Benedict’s reagent forms a red precipitate of copper(I) oxide.
Here’s how the commonly tested sugars sort out!
- All monosaccharides, fructose included. Fructose is a ketose, and under basic conditions it tautomerizes into an aldose, so it tests positive too.
- Maltose and lactose. The glucose on one end keeps its anomeric carbon free.
- Sucrose isn’t one. Its glycosidic bond ties up both anomeric carbons, so neither ring can open.
Look at the glycosidic bond. If it joins two anomeric carbons, like sucrose’s α1→β2 bond, the sugar is non-reducing. If one anomeric carbon is left free, it’s reducing!
Our key reactions of carbohydrates notes cover sugar oxidation and reduction in more depth.
Polysaccharides: Glycogen, Starch and Cellulose
Polysaccharides are long chains of monosaccharides. Glycogen, starch and cellulose are all built from glucose. They differ in their bonds and in how often the chain branches.
Glycogen
Animals store glucose as glycogen, mostly in the liver and muscle. Its glucose units link by α-1,4 bonds along each chain, with α-1,6 bonds at the branch points.
Glycogen branches more often than starch does. All those branch ends give enzymes many places to add or remove glucose at once! Our glycogen storage notes cover how the body builds and breaks it down.
Starch
Plants store glucose as starch, and it’s a major carbohydrate in our diet (hello, potatoes and rice!). Starch is a mix of two polymers.
Amylose is an unbranched chain of α-1,4 bonds. Amylopectin adds α-1,6 branch points, though it branches less often than glycogen.
Cellulose
Cellulose builds plant cell walls. Its glucose units join by β-1,4 bonds into straight chains, and hydrogen bonds hold neighboring chains together.
We can’t digest it, because humans lack cellulase, the enzyme that breaks cellulose’s β-1,4 bonds. Cellulose is an insoluble fiber. It adds bulk and holds water in the stool, softening it and speeding transit.
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Sugar Terms to Know
Commonly Tested Sugars Practice Questions
Try these before you open the answers!
1. Which of the following is a homodisaccharide? (Look at the prefix for a hint!)
A. Sucrose
B. Lactose
C. Fructose
D. Maltose
Show the answer
D. Maltose. “Homo” means same and “hetero” means different, so a homodisaccharide is built from two identical monosaccharides. Maltose is two glucose units. Fructose is a monosaccharide, so it can’t be a disaccharide at all!
2. Which of the following best describes the relationship between D-glucose and D-galactose?
A. Epimers
B. Anomers
C. Enantiomers
D. Ketoses
Show the answer
A. Epimers. Epimers differ at only one chiral carbon, and glucose and galactose differ only at C4. They can’t be anomers, since that difference isn’t at the anomeric carbon. Both are aldoses, so D is out too.
3. A student adds Benedict’s reagent to four sugar solutions. Which one stays blue, with no red precipitate?
A. Glucose
B. Fructose
C. Maltose
D. Sucrose
Show the answer
D. Sucrose. Its glycosidic bond ties up both anomeric carbons, so it can’t open to reduce the copper. Fructose tests positive after it tautomerizes to an aldose, and maltose keeps one free anomeric carbon.
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MCAT Sugars FAQs
What sugars do I need to know for the MCAT?
Glucose, fructose and galactose (plus ribose and deoxyribose), the disaccharides sucrose, lactose and maltose, and the polysaccharides glycogen, starch and cellulose. Know how each one is built and which bond holds it together!
Do you need to memorize sugar structures for the MCAT?
Yes, for the main ones. Know glucose’s Fischer projection by heart, then learn galactose, mannose and fructose as small changes to it. You’ll also want to draw α and β glucose as Haworth projections!
How do you remember the structure of glucose?
Read the hydroxyls from C2 to C5 in a Fischer projection, and they go right, left, right, right. To turn it into a Haworth ring, send every right-side group down and every left-side group up!
Is sucrose a reducing sugar?
No. Its α1→β2 bond joins the anomeric carbons of both glucose and fructose, so neither ring can open. Maltose and lactose are reducing sugars.
What’s the difference between alpha and beta glucose?
Only the anomeric carbon, C1. In a Haworth projection, α glucose has that OH pointing down, opposite the CH2OH group, and β glucose has it pointing up.
Final Thoughts on MCAT Sugars
Nice work making it to the end! Once glucose is locked in, the rest of the commonly tested sugars come down to a few edits and three bond names. Draw them from memory a few times, and they’ll feel familiar fast!
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Supplemental Resources
Want to go deeper on this topic? Here's where to read, watch and practice it next.
Read
- Kaplan MCAT Biochemistry Review 2026-2027, §4.4 and §4.1
- The Princeton Review MCAT Biology and Biochemistry Review, sections 3.3
Watch
- “Carbohydrates- di and polysaccharides” (Khan Academy)
- “Carbohydrates, absolute configuration, epimers, common names” (Khan Academy)
Practice passages
- “The structure and function of glycogen” (question 3 of 5)
- “The structure and function of glycogen” (question 2 of 6)
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