Chemical digestion

Some food molecules are too large and insoluble to be absorbed from the digestive system. Chemical digestion breaks these molecules into smaller, soluble molecules. Digestive enzymes speed up the chemical reactions that break the large molecules down.

The small products can dissolve in the fluid in the digestive system and pass through its lining into the body. This transfer is called absorption. Digestion therefore makes the molecules small and soluble enough to be absorbed.

Amylase in digestion

Amylase is a digestive enzyme that breaks down starch, a large carbohydrate in food, into smaller sugars. These include maltose, a reducing sugar.

The salivary glands secrete amylase into the mouth. As food is chewed and mixed with saliva, the amylase begins to break down its starch.

The pancreas also secretes amylase into the small intestine. There, it continues to break down starch that remains in the food.

Proteases in digestion

Proteins are made of chains of amino acids. Proteases are digestive enzymes that break these chains down into amino acids.

Protein digestion begins in the stomach, where proteases are secreted and act on proteins. Digestion continues in the small intestine. The pancreas secretes proteases into the small intestine, and the small intestine also secretes proteases. These enzymes act in the small intestine to continue breaking down proteins into amino acids.

The digestive system

The digestive system is a group of organs that work together to break down food and absorb useful products into the body. Digestion breaks large food molecules into smaller, soluble molecules that can be absorbed.

The path of food

Food enters the mouth, where it is chewed and mixed with saliva. It then passes down the oesophagus, a muscular tube that moves it to the stomach.

The stomach churns food and contributes to its digestion. Food then passes into the small intestine, where digestion continues. The small intestine is the main site of absorption: soluble products of digestion pass through its wall into the bloodstream.

The remaining material moves into the large intestine, which absorbs water. Undigested material forms faeces, which are stored in the rectum before leaving the body through the anus.

Organs that assist digestion

Food does not pass through the liver, gall bladder or pancreas, but these organs assist digestion by supplying substances to the digestive tract:

• The liver produces bile, which helps with the digestion of fats.
• The gall bladder stores bile and releases it into the small intestine.
• The pancreas produces digestive enzymes and releases them into the small intestine, where they help break down food.

Enzyme action

Enzymes are proteins that speed up chemical reactions in living organisms. They are biological catalysts: they increase the rate of a reaction without being used up by it.

Each enzyme has a region called an active site, where a reacting substance, called a substrate, can bind. The active site has a shape complementary to a particular substrate, so the substrate fits and binds to it.

Once the substrate is bound, the enzyme catalyses its reaction. The products are then released from the active site. The enzyme remains available to bind another substrate and can be reused.

Enzyme action: the lock and key model

The lock and key model represents enzyme specificity: a substrate fits an active site of the correct shape, like a key fitting a lock. A substrate with a shape that does not fit cannot bind to that active site in the required way. This helps explain why each enzyme catalyses particular reactions.

Enzymes catalyse the reactions of metabolism, the sum of all the chemical reactions in an organism.

Effect of temperature on enzyme activity

Temperature changes the rate of an enzyme-catalysed reaction by affecting both how frequently enzyme and substrate molecules meet and, at high temperatures, the shape of the enzyme.

Increasing temperature up to the optimum

As temperature rises, enzyme and substrate molecules gain kinetic energy and move faster. They collide more frequently, so more enzyme–substrate complexes form each second and the reaction rate increases.

The optimum temperature is the temperature at which the enzyme-catalysed reaction has its highest rate under the conditions being measured.

Above the optimum temperature

At temperatures above the optimum, the enzyme’s structure changes. Its active site loses the shape that is complementary to the substrate, so the substrate can no longer bind effectively. The enzyme is described as denatured. Fewer enzyme–substrate complexes form, and the reaction rate falls rapidly.

Low temperatures

At low temperatures, molecules have less kinetic energy and enzyme–substrate collisions occur less frequently. The reaction is therefore slow. Low temperatures do not normally denature the enzyme, so its activity can increase again when it is warmed.

Effect of pH on enzyme activity

The pH of an enzyme’s surroundings affects how quickly it catalyses a reaction. Each enzyme has an optimum pH at which it works fastest. As the pH moves above or below this optimum, the enzyme’s activity decreases.

Enzymes have an active site where the substrate binds. At extreme pH values, the enzyme’s structure can change, altering the shape of its active site. The substrate can then no longer fit effectively, so the enzyme cannot catalyse the reaction effectively. This structural change is called denaturation.

Different enzymes can have different optimum pH values. A pH at which one enzyme works fastest is therefore not necessarily the optimum for another.

Calculating reaction rates

A reaction rate describes how quickly reactants are used up or products are formed. The average rate over a time interval is the change in a measured quantity divided by the time taken.

The amount used up or formed is a change, not necessarily the final measurement. To find it, compare the measurements at the start and end of the chosen interval:

• For a reactant, subtract the final amount from the initial amount.
• For a product, subtract the initial amount from the final amount.

Then divide the change by the length of that same time interval. The result is an average for the interval; the rate may not have been constant throughout it.

Reaction rate units

The rate unit combines the unit of the measured quantity with the unit of time. For example, if a volume is measured in cubic centimetres and time in seconds, the rate is in $$\text{cm}^3/\text{s}$$. Use consistent units for the two quantity measurements and express the time interval in the unit required for the rate.

Calculating an average reaction rate

A reaction produces gas. At the start of a $$20\text{ s}$$ interval, the collected volume is $$4\text{ cm}^3$$; at the end, it is $$12\text{ cm}^3$$. The volume formed during the interval is $$12 - 4 = 8\text{ cm}^3$$.

The gas was formed at an average rate of $$0.4\text{ cm}^3/\text{s}$$ over that interval.

Lipases in digestion

Lipases are digestive enzymes that break down lipids, including fats, into glycerol and fatty acids. The products are small molecules that can be absorbed through the wall of the small intestine.

Lipase is produced by the pancreas and the small intestine. It acts in the small intestine, where it digests lipids in food.

The breakdown can be represented by a word equation:

Bile in digestion

Bile is a fluid that helps lipase break down fat in the small intestine. It is made in the liver, stored in the gall bladder and released into the small intestine.

How bile helps lipase

Food arriving from the stomach contains hydrochloric acid. Bile is alkaline, so it neutralises this acid. This provides alkaline conditions in the small intestine that increase lipase activity.

Bile also emulsifies fat: it breaks large amounts of fat into small droplets. The smaller droplets have a greater total surface area, giving lipase more surface to act on. This increases the rate of fat breakdown.

Bile is not an enzyme. It helps lipase work by changing the conditions and dispersing fat; lipase is the enzyme that breaks down the fat.

Uses of digestion products

Digestion breaks large food molecules into small, soluble molecules that can be absorbed into the bloodstream. The blood carries these products to cells, where they can be used to build molecules or release energy.

Different food molecules produce different digestion products:

• Carbohydrates are broken down by carbohydrases into simple sugars, including glucose.
• Proteins are broken down into amino acids.
• Lipids are broken down into glycerol and fatty acids.

Cells use absorbed simple sugars to build new carbohydrates, amino acids to build new proteins, and glycerol and fatty acids to build new lipids. These new molecules can then be used by the body.

Some absorbed glucose is also used in respiration, which releases energy for cellular activities.