Nutrition and Digestion: Enzymes, Macronutrients, and the Human Digestive System

Sample Answer

Nutrition and Digestion

Enzyme Function: Lock and Key and Induced Fit Models

Enzymes are biological catalysts that speed up chemical reactions in the body without being used up. They work by lowering the activation energy required for reactions to occur.

The lock and key model explains enzyme specificity. In this model, the enzyme has an active site that is exactly complementary to the substrate. The substrate fits into the active site like a key fits into a lock, forming an enzyme-substrate complex. This leads to the reaction taking place and the product being released.

The induced fit model provides a more accurate explanation. It suggests that the active site is flexible rather than rigid. When the substrate approaches, the enzyme changes shape slightly to fit the substrate more closely. This improves the efficiency of the reaction and explains why enzymes are highly specific but also adaptable.

Enzyme Inhibitors and Coenzymes

Enzyme inhibitors are substances that reduce or stop enzyme activity.

There are two main types of inhibitors. Competitive inhibitors bind to the active site of the enzyme, competing with the substrate. This prevents the substrate from binding. Non-competitive inhibitors bind to a different part of the enzyme, causing a change in the enzyme’s shape so the substrate can no longer bind effectively.

Coenzymes are small organic molecules that assist enzymes in carrying out reactions. They often act as carriers for chemical groups or electrons. For example, NAD is a coenzyme involved in respiration, helping transfer energy within cells. Without coenzymes, some enzymes would not function properly.

Factors Affecting Enzyme Activity

Several factors influence how well enzymes work.

Temperature affects enzyme activity by increasing kinetic energy. As temperature rises, enzyme activity increases until an optimum temperature is reached. Beyond this point, the enzyme denatures and loses its shape, reducing activity.

pH also affects enzyme function. Each enzyme has an optimum pH. For example, enzymes in the stomach work best in acidic conditions, while those in the small intestine prefer alkaline conditions. Extreme pH levels can denature enzymes.

Substrate concentration influences the rate of reaction. Increasing substrate concentration increases enzyme activity until all active sites are occupied. At this point, the reaction rate levels off.

LO2: Lipids, Carbohydrates and Proteins

Structure and Function of Lipids

Phospholipids are made of two fatty acids, a glycerol molecule, and a phosphate group. They form the structure of cell membranes, creating a bilayer that controls what enters and leaves the cell.

Cholesterol is another type of lipid that helps maintain membrane fluidity and stability. It also acts as a precursor for hormones.

Saturated fatty acids contain no double bonds and are usually solid at room temperature. Polyunsaturated fatty acids contain multiple double bonds and are typically liquid. Polyunsaturated fats are considered healthier because they help reduce cholesterol levels.

Carbohydrates and Glycosidic Bonds

Carbohydrates are made up of sugar units. Monosaccharides such as glucose are the simplest form and provide immediate energy.

Disaccharides are formed when two monosaccharides join together through a glycosidic bond, which is created by a condensation reaction. For example, glucose and fructose combine to form sucrose.

Polysaccharides are long chains of monosaccharides. Examples include starch, glycogen, and cellulose. Starch and glycogen are used for energy storage, while cellulose provides structural support in plant cell walls.

Proteins and Peptide Bonds

Proteins are made from amino acids linked by peptide bonds. These bonds form through condensation reactions, where water is removed.

Proteins have four levels of structure. The primary structure is the sequence of amino acids. The secondary structure involves folding into shapes such as alpha helices and beta sheets. The tertiary structure is the overall three-dimensional shape of the protein. The quaternary structure occurs when multiple protein chains join together.

The structure of a protein determines its function. For example, enzymes rely on their specific shape to bind to substrates.

Biochemical Tests

Biochemical tests are used to identify nutrients in food samples.

The Benedict’s test is used for reducing sugars. When heated, a colour change from blue to orange or red indicates the presence of reducing sugars.

The iodine test is used for starch. A blue-black colour indicates starch is present.

The Biuret test is used for proteins. A purple colour indicates peptide bonds.

The ethanol emulsion test is used for lipids. A milky white appearance indicates the presence of fats.