Carbohydrates are made of three chemical elements: carbon, hydrogen, and oxygen, arranged in a ratio of 1:2:1. That simple formula, represented as (CH₂O)ₙ, is the foundation for everything from table sugar to the starch in a potato. But the way those three elements arrange themselves creates a surprising range of molecules with very different properties in your body.
The Three Elements and Their Ratio
Every carbohydrate molecule is built from carbon, hydrogen, and oxygen. In the simplest carbohydrates, for every one carbon atom there are two hydrogen atoms and one oxygen atom. The “n” in the formula (CH₂O)ₙ just refers to how many times that unit repeats. A sugar with six carbons, like glucose, has the formula C₆H₁₂O₆. A sugar with five carbons, like ribose (found in your DNA’s cousin, RNA), follows the same pattern with five of each unit.
Monosaccharides: The Smallest Units
The building blocks of all carbohydrates are monosaccharides, or single sugar molecules. The most important ones in human nutrition are glucose, fructose, and galactose, all of which contain six carbons. Glucose and fructose actually share the same chemical formula, C₆H₁₂O₆, but they differ in shape. Glucose has a reactive group at the end of its carbon chain, while fructose has that group on the second carbon. This small structural difference is why glucose forms a six-sided ring in solution while fructose typically forms a five-sided ring.
Sugars are also classified by their carbon count. Three-carbon sugars are called trioses, five-carbon sugars are pentoses (ribose is an example), and six-carbon sugars are hexoses. Most of the sugars you encounter in food are hexoses. In water, these molecules don’t stay as open chains. They fold into ring shapes, which is the form they take when they link together to build larger carbohydrates.
How Sugars Link Together
When two monosaccharides join, they form a bond called a glycosidic bond through a process known as dehydration synthesis. One sugar donates a hydrogen atom, the other donates a hydroxyl group (an oxygen bonded to a hydrogen), and those pieces leave as a water molecule. The oxygen left behind bridges the two sugars together. This is why building carbohydrates releases water, and breaking them apart requires water.
The specific carbons involved in this bond matter enormously. A bond between carbon 1 and carbon 4 creates a different molecule than a bond between carbon 1 and carbon 6. These seemingly minor differences determine whether a carbohydrate is digestible or not, whether it stores energy or provides structural support.
Disaccharides: Two Sugars Paired
Linking two monosaccharides produces a disaccharide. The three most common ones in your diet each have a distinct composition:
- Sucrose (table sugar): one glucose linked to one fructose
- Lactose (milk sugar): one galactose linked to one glucose
- Maltose (malt sugar): two glucose molecules linked together
These, along with the monosaccharides themselves, are what nutrition labels call “simple carbohydrates.” Common sources include table sugar, honey, fruit juice, and syrup. Your body breaks them down quickly because there are only one or two bonds to split.
Polysaccharides: Long Chains With Different Jobs
When many monosaccharides link together, they form polysaccharides, chains that can contain hundreds or thousands of sugar units. Three polysaccharides dominate biology, and all three are made entirely from glucose. What separates them is how the glucose units connect.
Starch is the energy reserve of plants. It comes in two forms: one is an unbranched chain of glucose units linked end to end, which coils into a helix. The other form adds occasional branch points, creating a bushy, tree-like structure. Starchy vegetables like potatoes, corn, peas, and legumes are rich sources, along with whole grains.
Glycogen is the animal equivalent of starch. It has the same type of bonds but branches far more frequently, creating a highly compact molecule that can release glucose rapidly. Your body stores roughly 600 grams of glycogen total, though this varies with your size, diet, and fitness level. Skeletal muscle holds the largest share, averaging about 500 grams (with a normal range of 300 to 700 grams). Your liver stores around 80 grams, and that supply constantly replenishes the approximately 4 grams of glucose circulating in your blood at any given time. Smaller amounts of glycogen sit in brain cells, heart cells, kidney cells, and even fat cells.
Cellulose is the structural backbone of plant cell walls, and it too is made entirely of glucose. The critical difference is the orientation of the bond. Starch and glycogen use one configuration of the glycosidic bond, while cellulose uses another. That single change means human digestive enzymes cannot break cellulose apart. Instead of being absorbed as fuel, cellulose passes through your gut as fiber. Hydrogen bonds between neighboring cellulose chains allow them to form strong, rigid fibers, which is why plants can stand upright without a skeleton.
Why Some Carbohydrates Are Indigestible
Dietary fiber is essentially carbohydrate (plus some non-carbohydrate material like lignin) that resists your digestive enzymes. Cellulose is the most abundant type, but it’s not the only one. Hemicellulose molecules are shorter and branched, containing a variety of sugars. Pectin, found in fruit, forms gels. Gums and mucilages are produced by plants as protective secretions.
What all these fibers share is a bond structure or molecular arrangement that human enzymes simply can’t cleave. Cellulose, for instance, resists breakdown because of both its bond type and the tight hydrogen bonding between its chains, which makes the molecule mechanically strong and nearly insoluble in water. This resistance to digestion is precisely what makes fiber useful: it adds bulk, feeds beneficial gut bacteria, and slows the absorption of other nutrients.
Energy From Carbohydrates
Digestible carbohydrates provide about 4 calories per gram, the same as protein and less than half the 9 calories per gram that fat provides. When you eat starch or sugar, your body breaks the glycosidic bonds, freeing individual glucose molecules that enter the bloodstream. That glucose either gets used immediately for energy or gets reassembled into glycogen for short-term storage. If glycogen stores are full, the excess can be converted to fat.
The speed of this process depends on the carbohydrate’s structure. A simple sugar like glucose enters the blood almost immediately. A complex, branched starch takes longer to disassemble. And fiber, of course, doesn’t deliver glucose energy directly at all, though gut bacteria can ferment some types into short-chain fatty acids that your body uses in other ways.