Carbohydrate Structure on the MCAT

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Carbohydrates are a very crucial part of your diet and physiological functioning. As you’ll see, these simple and complex sugars are a crucial energy source for the body to power the various cellular processes that keep us functioning!

This guide will introduce carbohydrate structure, some key terms, and definitions you should remember as you prepare for the MCAT

Let’s get started!

Carbohydrate Structure on the MCAT: What Do You Need to Know?

Carbohydrate structure is covered in the Biochemistry section of the MCAT

Introductory biochemistry accounts for 25% of the Biological and Biochemical Foundations of Living Systems section (Bio/Biochem) and 25% of the Chemical and Physical Foundations of Biological Systems section (Chem/Phys).

It’s hard to predict the exact number of questions about carbohydrates that will appear on the MCAT. However, you can expect it to appear in both the Bio/Biochem and Chem/Phys sections.

Important Sub-Topics – Carbohydrate Structure

We cannot stress enough the importance of carbohydrates during MCAT prep since they are closely related to basic organic chemistry, another important MCAT topic! 

Carbohydrates can be complicated, so let’s break them down into important sub-topics you can concentrate on.

1. Carbohydrates: Structural Description

Carbohydrates are biomolecules consisting of oxygen, hydrogen, and carbon atoms. Carbohydrates encompass all sugars, both simple and complex sugars. Simple sugars include monosaccharides and disaccharides while complex sugars include oligosaccharides and polysaccharides. 

As for carbon numbering, carbohydrates follow the same rules as carbons in organic chemistry!
Carbohydrates Structural Desc 1

There are two main ways carbohydrates can be represented on paper: the Fischer Projection and the Hawthorne Projection, which are used for depicting sugars in a planar and cyclic way, respectively.

Fisher D Glucose
Hawthorne D Glucose

In carbohydrate biochemistry, there are two important stereochemical topics: enantiomers in D and L Fisher projections and anomeric epimers in Hawthorne projections.

Carbohydrate sugars come in enantiomeric pairs also known as mirrored pairs, indicated by the D or L in front of the sugar name. The D indicates that the hydroxyl group on the highest numbered chiral carbon is ON THE RIGHT of the Fisher projection, while the L indicates that it’s ON THE LEFT. 

Anomeric epimers are stereoisomers differing only in the position of one stereocenter. For example, the alpha anomer of D-glucose has the anomeric hydroxyl group facing DOWN while it’s facing UP in the beta anomer. 

For more in-depth content review on structural description, check out these detailed lesson notes created by top MCAT scorers. 

2. Carbohydrates: Commonly Tested Sugars

We’ll break down the commonly tested carbohydrates into monosaccharides, disaccharides, and polysaccharides.

1. Monosaccharides

The simplest sugar formation; it is the building block for carbohydrates. Monosaccharides can be linked together to form disaccharides and polysaccharides. Examples include glucose, fructose, and galactose.


2. Disaccharides

These are composed of two monosaccharide subunits. Examples include sucrose, lactose, and maltose.

3. Polysaccharides

These are composed of many monosaccharide subunits. Examples include glycogen, starch, and cellulose.

For more in-depth content review on commonly tested sugars, check out these detailed lesson notes created by top MCAT scorers. 

3. Carbohydrates: Key Reactions

Carbohydrates' unique structural properties allow for their involvement in many different reactions!

There are two main carbohydrate reactions that we’ll cover: the presence of reducing sugars and the synthesis and hydrolysis of glycosidic linkages.

Reducing sugars exist when the anomeric carbon is free, meaning it is not involved in a glycosidic bond. Likewise, a nonreducing sugar refers to a sugar that DOES NOT contain a free anomeric carbon. You can detect the presence of a reducing sugar with Tollen’s or Benedict’s tests.

The second main reaction is the hydrolysis of glycosidic linkages. The formation of these linkages results in the formation of acetals and ketals, formed via a dehydration synthesis! Essentially, a hydroxyl group on one sugar will become the additional “-OR” group in the acetal and/or ketal. 

Likewise, the breakdown of glycosidic linkages involves hydrolysis, similar to other reactions, where the water molecule acts as the nucleophile. 

For more in-depth content review on carbohydrates key reaction, check out these detailed lesson notes created by top MCAT scorers. 

Key Terms and Definitions – Carbohydrates

Carbohydrates Keyterms

Here are some of the more important key terms and definitions to remember for this general guide to carbohydrates!




The most basic unit of carbohydrates.


The carbohydrate formed when two monosaccharides are bound by a glycosidic linkage.


The carbohydrate formed when more than two monosaccharides are bound by a glycosidic linkage.

Glycosidic linkage

A covalent bond that joins a carbohydrate group to another molecule.


A type of stereoisomer that differs in configuration at the anomeric carbon.

Anomers (type of epimer)

A type of stereoisomer that differs in configuration at the hemiacetal or acetal carbon.

Additional FAQs – Carbohydrates on the MCAT

What are the 4 structures of carbohydrates?

The 4 structures of carbohydrates are monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Should you memorize carbohydrate structure for the MCAT?

Unfortunately, you will need to memorize carbohydrate structure for the MCAT.

How do I remember the carbohydrate sugars?

Remembering the sugars can be tricky. Thankfully, there are many tried and tested strategies out there that you can use! One example is mnemonics. This method connects newly learned material to previously learned knowledge using visual and auditory cues. Basic mnemonic strategies emphasize the use of abbreviations, keywords, or rhymes.

What is the basic structure of carbohydrates?

The basic structure of carbohydrates contains carbon, hydrogen, and oxygen. Each carbon atom is bonded to at least one oxygen atom. All carbohydrates include an aldehyde or ketone group and a hydroxyl group. Carbohydrates may form long chains that are either straight or branched.

Additional Reading Links – Study Notes for Carbohydrates Structure on the MCAT

For more in-depth content review about carbohydrates structure on the MCAT, check out these detailed lesson notes created by top MCAT scorers!

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