The Molecules of Food: Carbohydrates, Lipids, Proteins and More

What's actually in a granola bar?

Pick up any packaged food in a Canadian grocery store and turn it over. The Nutrition Facts table lists fat, carbohydrate (including fibre and sugars), protein, sodium, potassium, calcium and iron. Those words name the molecules your digestive system must take apart and your cells must rebuild.

Before we follow food through the gut, we need to know what we are digesting. This lesson is the chemistry of food.

Six classes of nutrients

• Carbohydrates: the body's main quick energy source; fibre keeps the gut working.
• Lipids (fats and oils): long-term energy storage, insulation, cell membranes, some hormones.
• Proteins: enzymes, muscle, hemoglobin, antibodies, transport molecules, structure (hair, skin).
• Vitamins: small organic molecules needed in tiny amounts, often helping enzymes work.
• Minerals: elements such as calcium, iron, sodium and potassium.
• Water: the medium for almost every reaction in the body; about 60% of an adult's body mass.

The first three are macronutrients, needed in large amounts and supplying energy. Vitamins and minerals are micronutrients. Food also contains nucleic acids (DNA and RNA) from the cells of whatever you eat.

Building up and breaking down

Large food molecules are polymers: long chains of smaller monomers.

• Cells build polymers by dehydration synthesis: two monomers join and a water molecule (H₂O) is released.
• Hydrolysis is the reverse: water is added, and the bond breaks. (Hydro = water, lysis = splitting.)

Chemical digestion is hydrolysis. Every digestive enzyme you will meet in the next lessons speeds up the hydrolysis of one kind of bond.

Carbohydrates

Carbohydrates are made of carbon, hydrogen and oxygen, usually in the ratio 1 : 2 : 1 (glucose is C₆H₁₂O₆).

• Monosaccharides (single sugars): glucose, fructose (in fruit and honey), galactose. These are small enough to be absorbed.
• Disaccharides (two sugars): maltose = glucose + glucose; sucrose (table sugar) = glucose + fructose; lactose (milk sugar) = glucose + galactose.
• Polysaccharides (many sugars):
• Starch: how plants store glucose (potatoes, wheat, rice).
• Glycogen: how animals store glucose, in the liver and muscles.
• Cellulose: the material of plant cell walls. Humans make no enzyme that can break its bonds, so it passes through as insoluble fibre, which adds bulk and helps keep the gut moving.

Lipids

Lipids do not dissolve in water. The main types:

• Triglycerides (fats and oils): one glycerol joined to three fatty acids. They store about twice as much energy per gram as carbohydrates or proteins (about 37 kJ/g, compared with about 17 kJ/g).
• Saturated fatty acids have only single carbon–carbon bonds, so their tails are straight and pack tightly. They tend to be solid at room temperature (butter, lard).
• Unsaturated fatty acids have one or more C=C double bonds, which bend the tail. They pack loosely and tend to be liquid (canola oil, olive oil).
• Phospholipids: the double layer that forms every cell membrane.
• Steroids: such as cholesterol (part of cell membranes, and used to make some hormones and bile salts).

Proteins and nucleic acids

Proteins are chains of amino acids joined by peptide bonds. There are 20 kinds of amino acids. Your body can make some of them, but nine are essential amino acids that must come from food. A short chain is a peptide; a long one is a polypeptide, which folds into a precise 3-D shape. The shape decides the job: an enzyme's shape lets it grab one particular molecule, and hemoglobin's shape lets it hold oxygen. Heat, strong acid or strong base can denature a protein (unfold it), and it stops working.

Nucleic acids (DNA and RNA) are chains of nucleotides. Your gut breaks them down too. (You will study DNA's role in the Genetic Processes unit.)

Vitamins, minerals and water: small amounts, big jobs

• Vitamin C is needed to make collagen, a protein in skin, blood vessels and gums. A severe lack causes scurvy.
• Vitamin D helps the intestine absorb calcium. Our skin makes it in sunlight, but in a Canadian winter there is little strong sunlight, which is one reason cow's milk sold in Canada has vitamin D added.
• Vitamin K is needed to make blood-clotting proteins (you'll see it in the blood lesson).
• Iron is part of hemoglobin, which carries oxygen. Too little iron can cause anemia: tiredness and shortness of breath.
• Calcium builds bones and teeth and is needed for muscle contraction and blood clotting.
• Sodium and potassium carry the electrical signals of nerves and muscles, including the heart.
• Water dissolves and transports substances, takes part in hydrolysis, and helps control body temperature.

Testing foods for nutrients (lab)

In a school lab you test foods with indicators that change colour when a nutrient is present. Always compare with a negative control (water) and a positive control (a known sample).

• Iodine solution → starch: yellow-brown turns blue-black.
• Benedict's solution (heated in a hot-water bath) → reducing sugars such as glucose, fructose, maltose and lactose: blue turns green, yellow, orange or brick red as the amount of sugar increases. Sucrose is not a reducing sugar, so it stays blue.
• Biuret reagent → protein: blue turns violet (purple).
• Sudan IV (a red dye) or the grease-spot test → lipids: Sudan IV stains lipids red; a fat leaves a translucent spot on brown paper that does not dry away.

Lab safety: wear goggles. Iodine stains skin and clothing; Biuret reagent contains sodium hydroxide, which is corrosive; Benedict's test uses a boiling-water bath. Follow WHMIS labels and never try these reagents at home.