Sugar (Glucose)

Glucose is the simplest sugar your body uses for energy. Its chemical formula is C6H12O6: six carbon atoms, twelve hydrogens and six oxygens. Glucose is the main fuel of nearly every living thing, from the bacteria in your gut to the brain inside your skull. Plants make it from sunlight, animals eat it (or eat each other to get it), and inside cells it is "burned" with oxygen to release energy. The sugar in fruits, honey, bread, rice and pasta all ends up as glucose in your bloodstream after digestion.

  • FormulaC6H12O6A medium-sized molecule
  • TypeSimple sugar (monosaccharide)6 carbon atoms in a ring
  • Made byPlants (photosynthesis)From CO2 + water + sunlight
  • Energy released15 kJ per gramWhen fully burned in cells
  • SweetnessReference 100Other sugars compared to it
  • Brain usesAround 120 g/day60 per cent of total in some studies

What is glucose?

Glucose is the smallest sugar molecule of any importance to biology. Six carbon atoms link in a ring, with extra hydrogen and oxygen atoms attached. Both the open-chain and ring forms exist in solution, with the ring being more common.

Glucose dissolves easily in water and is mildly sweet. It is a monosaccharide, meaning a single sugar unit. Larger sugars are built by joining glucose to other sugar molecules:

  • Sucrose (table sugar) = 1 glucose + 1 fructose
  • Lactose (milk sugar) = 1 glucose + 1 galactose
  • Maltose (malt sugar) = 2 glucoses joined
  • Starch = hundreds of glucose units in long chains (the way plants store food)
  • Glycogen = thousands of glucose units in branched chains (the way animals store food)
  • Cellulose = thousands of glucose units in a different chain pattern (the structure of plant cell walls and wood)

Where glucose comes from

Plants make glucose from carbon dioxide and water using sunlight, in a reaction called photosynthesis:

6 CO2 + 6 H2O + sunlight -> C6H12O6 + 6 O2

Almost every other source of glucose you can think of traces back to plants:

  • Sugar in fruit is made by the fruit plant.
  • Sucrose in table sugar comes from sugar cane or sugar beet.
  • Honey is plant nectar processed by bees.
  • Lactose in milk is made by mammals from glucose in their food.
  • Bread, pasta, rice, potatoes contain starch, which is broken down to glucose during digestion.
Fact Your brain runs almost entirely on glucose. It is only about 2 per cent of your body weight but uses around 20 per cent of your total energy. That works out at roughly 120 grams of glucose a day, enough to fill a small mug. When you skip meals and feel grumpy or fuzzy-headed, it is partly because your brains glucose supply has dropped.

How your body uses glucose

When you eat carbohydrates (bread, rice, fruit, sugary drinks), enzymes in your saliva and gut break them down into glucose. The glucose passes through your gut wall into your bloodstream.

From there, your body has three options:

  • Use it now: cells take in glucose and break it down (in a controlled way) with oxygen, releasing energy as ATP molecules. This is called cellular respiration.
  • Store it short-term: extra glucose is joined into chains of glycogen in the liver and muscles, ready to be used later in the day.
  • Store it long-term: if even more glucose is left, the body converts it into fat for long-term storage.

The hormone insulin tells cells to take in glucose from the blood when blood sugar is high. The hormone glucagon tells the liver to release stored glucose when blood sugar is low. Together they keep your blood sugar in a narrow safe range.

What if glucose control breaks?

Diabetes is the most common condition where the body cannot manage glucose properly.

  • Type 1 diabetes: the immune system attacks the cells in the pancreas that make insulin. Without insulin, glucose cannot get into cells, and blood sugar rises dangerously high. People with type 1 diabetes inject insulin every day.
  • Type 2 diabetes: the bodys cells stop responding well to insulin, often related to lifestyle, weight and genetics. It usually develops in adulthood and can be managed with diet, exercise and medicines.
Did you know? Different sugars have very different sweetness. If glucose has a sweetness score of 100, then fructose (in fruit) is 173, sucrose (table sugar) is 130, lactose (milk) is only 16, and the artificial sweetener saccharin is around 30,000. That is why a tiny pinch of saccharin can make a whole cup of coffee sweet.

How sweet is sugar?

Different sugars have different sweetness levels, which is why fruit sugar tastes sharper and brighter than regular table sugar. Manufacturers blend different sugars to balance sweetness, texture and shelf life in everything from cakes to fizzy drinks. Some foods (like ketchup) contain surprising amounts of added sugar.

Sugar in industry

Glucose has uses beyond the dinner plate:

  • Fermentation: yeasts turn glucose into alcohol and CO2, used in brewing beer, making wine and baking bread.
  • Medicines: hospitals use glucose drips to treat patients who cannot eat. The pure glucose goes straight into the bloodstream.
  • Biofuels: glucose from corn or sugar cane can be fermented into bioethanol, used as a vehicle fuel.
  • Industrial chemistry: glucose can be turned into a wide range of useful chemicals, from vitamin C to plastic precursors.
Try this Watch sugar fuel yeast. Half-fill a small bottle with warm water, stir in two teaspoons of sugar and one packet of dried yeast. Stretch a balloon over the top of the bottle. Within 20 minutes the yeast will start to ferment, eating the sugar and producing CO2. The balloon will slowly inflate. The same chemistry makes bread dough rise, beer fizz and wine alcoholic.
Deeper dive: how too much sugar started a global health crisis

For most of human history, sugar was rare and expensive. Honey was hard to gather. Fruit was seasonal. People got most of their carbohydrate from grains, root vegetables and beans: foods that release glucose slowly into the bloodstream.

That changed in the 1800s when industrial machinery made sugar cane and sugar beet processing cheap. Refined white sugar suddenly became affordable for everyone. Sugar consumption per person rose 10-fold between 1700 and 2000. Today the average person eats around 100 grams of added sugar every day, compared with under 10 grams in 1800.

Most of this extra sugar is hidden in processed food and drink. A typical can of cola contains 35 grams of sugar (about 9 teaspoons). A pot of fruit yoghurt can contain 20 grams. Even foods that do not taste sweet, like ketchup, baked beans and bread, contain plenty of added sugar.

The health effects are widespread. Diets high in sugar are linked to tooth decay, obesity, type 2 diabetes and heart disease. The world now has over 500 million people with diabetes and more than 1 billion who are overweight or obese.

Many governments have responded with new laws. In 2018 the UK introduced a sugar tax on sugary drinks. Manufacturers were given a choice: reformulate to use less sugar, or pay extra tax. Most chose to reformulate. Within a few years, the sugar in soft drinks had dropped by nearly 30 per cent across the UK market. Other countries have followed with similar taxes and clearer food labels. The story of sugar is now also a story of public health, food labelling and chemistry put to work for a healthier world.

For more, see photosynthesis and water.