The human digestive system
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.
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.
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.
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:
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 human digestive system
The digestive system is a group of organs that work together to break down food and absorb useful substances. Digestion breaks large food molecules into smaller, soluble molecules that can be absorbed into the blood.
The path of food through the digestive system
Food passes through the following organs in order:
- Mouth: Digestion begins as food is chewed and mixed with saliva.
- Oesophagus: Food passes down this tube from the mouth to the stomach.
- Stomach: Food is mixed and digestion continues.
- Small intestine: Digestion is completed, and soluble digestion products are absorbed into the bloodstream.
- Large intestine: Water is absorbed from the remaining material.
- Rectum and anus: Waste is stored in the rectum and leaves the body through the anus.
Organs that support digestion
Some organs supply substances that help digestion, although food does not pass through them. The salivary glands supply saliva to the mouth, and the pancreas supplies digestive substances to the small intestine.
The liver produces bile, which is stored in the gall bladder. The gall bladder releases bile into the small intestine, where it helps digestion.
Enzyme action
Enzymes speed up chemical reactions in living organisms. They are biological catalysts: they increase the rate of reactions without being used up. Most enzymes are proteins.
How an enzyme acts on a substrate
Each enzyme has an active site, a region with a particular shape. A molecule called the substrate binds to the active site if its shape is complementary to it.
At the active site, the substrate is converted into a product or products. The products leave the active site, and the enzyme can be used again to catalyse the same reaction.
The shape of the active site helps explain enzyme specificity. Only substrates with a complementary shape can bind to the active site in the required way, so an enzyme catalyses a specific reaction.
The lock and key model
The lock and key model is a simplified way of representing enzyme specificity. It compares the fit between a substrate and an enzyme’s active site to the fit between a key and a lock. A substrate with a complementary shape fits the active site; one with a different shape does not fit in the required way.
Enzymes and metabolism
Metabolism is the sum of all the chemical reactions in an organism. Enzymes catalyse metabolic reactions that build larger molecules as well as reactions that break molecules down.
Effect of temperature on enzyme activity
Temperature affects how quickly an enzyme catalyses a reaction. As temperature rises, the reaction rate generally increases until it reaches a maximum. Above this temperature, the rate falls because the enzyme’s structure changes.
Increasing temperature
Enzyme and substrate molecules gain kinetic energy as temperature rises. They move faster and collide more frequently, so substrate molecules bind to enzyme active sites more often. This increases the reaction rate up to the optimum temperature, where the rate is highest.
Above the optimum temperature
Above the optimum, heat can disrupt the enzyme’s structure and change the shape of its active site. The enzyme is then denatured: the substrate no longer fits the active site effectively, so fewer reactions are catalysed and the reaction rate falls.
Low temperatures
At low temperatures, enzyme and substrate molecules move more slowly and collide less frequently. The reaction is therefore slower. Low temperatures do not normally denature the enzyme, so its activity can increase again when the temperature rises.
Effect of pH on enzyme activity
An enzyme works fastest at a particular pH, called its optimum pH. As the pH moves away from this value in either direction, the enzyme’s activity decreases: it catalyses the reaction more slowly.
Changes in acidity or alkalinity can disrupt the interactions that maintain the enzyme’s shape. This can change the shape of its active site, the region where the substrate binds. If the active site is no longer as well matched to the substrate, the substrate binds less effectively, so the enzyme catalyses fewer reactions.
At an extreme pH, the enzyme can become denatured: its shape changes so much that the substrate can no longer bind effectively to the active site.
Different enzymes can have different optimum pH values. A pH that gives one enzyme its greatest activity may reduce the activity of another.
Calculating reaction rates
A reaction rate describes how quickly reactants are used up or products are formed. A larger rate means a greater change in the amount of reactant or product per unit of time.
The mean rate is the average rate over a chosen time interval:
To find the amount used or formed, compare the measurements at the beginning and end of the interval:
- For a reactant, subtract the final amount from the initial amount.
- For a product, subtract the initial amount from the final amount.
The time taken is the difference between the times of the two measurements. The rate is a mean for that interval; the reaction may not have proceeded at the same rate throughout it.
Calculating a mean reaction rate
Suppose the mass of a product increases from $$2\ \text{g}$$ to $$8\ \text{g}$$ over $$3\ \text{s}$$. The amount formed is $$8 - 2 = 6\ \text{g}$$, so:
The mean rate is 2 grams per second. Rate units depend on both the quantity measured and the time unit used.
Interpreting word equations
A word equation represents a chemical reaction using the names of the substances involved. It shows which substances are changed and which substances are produced.
The starting substances, called reactants, are written on the left. The substances formed, called products, are written on the right. The arrow shows that the reactants are converted into the products. A plus sign separates two or more substances on the same side of the equation.
For example:
This equation shows that lipid is broken down into glycerol and fatty acids. Lipid is the reactant; glycerol and fatty acids are the products. The plus sign separates the two products.
An enzyme may catalyse a reaction, meaning it speeds up the reaction. The enzyme is not a reactant that is used up in the reaction.