Nutrition & Enzymes

Introduction to Nutrition

Nutrition is the intake of food, considered in relation to the body’s dietary needs. All living organisms need food for three primary reasons: to provide energy for metabolic processes (measured in Joules, J, or kilojoules, kJ), to supply raw materials for growth and repair of tissues, and to provide substances to maintain health and prevent diseases.

The Components of a Balanced Diet

A balanced diet provides all essential nutrients in the correct proportions to maintain good health. In Pakistan, a typical meal of *roti*, *daal* (lentils), and some meat or vegetables can be balanced if portion sizes are controlled. The main food groups are:

1. Carbohydrates: The primary source of energy. They are large molecules broken down into simple sugars like glucose during digestion. Sources include staples like rice, roti, bread, and potatoes.
2. Proteins: Essential for growth, repairing tissues, and making enzymes. They are composed of long chains of amino acids. Rich sources include meat, fish, eggs, and pulses like *daal*.
3. Fats (Lipids): A concentrated energy source, providing more than twice the energy per gram compared to carbohydrates. They also provide insulation and are used to make cell membranes. They are digested into fatty acids and glycerol. Sources include ghee, butter, oils, and nuts.
4. Vitamins: Organic compounds required in small quantities. Vitamin C (from citrus fruits like *kinnow*) is crucial for healthy skin and gums, and its deficiency causes scurvy. Vitamin D is synthesized in the skin using sunlight and is vital for calcium absorption; its deficiency can cause rickets.
5. Minerals: Inorganic ions needed for various bodily functions. Calcium is essential for strong bones and teeth. Iron is a key component of haemoglobin in red blood cells, which transports oxygen; its deficiency leads to anaemia.
6. Dietary Fibre (Roughage): Indigestible plant material, primarily cellulose. It adds bulk to food, stimulating peristalsis (wave-like muscle contractions) in the intestines and preventing constipation. Found in whole grains, fruits, and vegetables.
7. Water: Makes up about 70% of the body. It acts as a solvent for chemical reactions, a transport medium in blood, and helps regulate body temperature.

Malnutrition is the condition resulting from an unbalanced diet. This can be undernutrition (e.g., Kwashiorkor from protein deficiency, Marasmus from overall energy deficiency) or overnutrition (e.g., obesity from excessive energy intake, which increases the risk of type 2 diabetes and coronary heart disease).

The Human Digestive System (Alimentary Canal)

Digestion is the breakdown of large, insoluble food molecules into small, water-soluble molecules that can be absorbed into the bloodstream.

1. Mouth: Ingestion occurs. Mechanical digestion by teeth (mastication) increases the surface area of food. Chemical digestion begins as salivary glands release salivary amylase, which breaks down starch into maltose.
2. Oesophagus: A muscular tube that pushes the bolus of food to the stomach via peristalsis.
3. Stomach: A muscular bag that churns food (mechanical digestion). Glands in the stomach wall secrete gastric juice containing hydrochloric acid (HCl) and the enzyme pepsin. HCl creates an acidic environment (pH 1.5-2.5) which kills harmful bacteria and provides the optimum pH for pepsin. Pepsin, a protease, begins the digestion of proteins into polypeptides.
4. Small Intestine (Duodenum & Ileum): The duodenum is the first part where most chemical digestion occurs. The pancreas secretes pancreatic juice containing amylase, trypsin (a protease), and lipase. The liver produces bile, which is stored in the gall bladder and released into the duodenum. Bile emulsifies fats—breaking large fat globules into smaller droplets, increasing the surface area for lipase to act upon. The ileum is the final, long section where digestion is completed and absorption occurs. Its wall is lined with millions of villi, which provide a massive surface area. Each villus has a one-cell-thick wall for a short diffusion path and a rich supply of blood capillaries and a lacteal to transport absorbed nutrients.
5. Large Intestine (Colon): Absorbs most of the remaining water and mineral salts from undigested food.
6. Rectum & Anus: Faeces (indigestible matter) are stored in the rectum and egested through the anus.

Enzymes: The Biological Catalysts

Enzymes are proteins that function as biological catalysts. They speed up the rate of chemical reactions without being chemically changed themselves. They are crucial for digestion.

• Mechanism of Action: Enzymes work via the lock-and-key hypothesis. Each enzyme has a specifically shaped active site that is complementary to its substrate (the molecule it acts on). The substrate fits into the active site, forming an enzyme-substrate complex, where the reaction occurs. The products are then released, and the enzyme is free to catalyse another reaction.
• Specificity: Due to the specific shape of the active site, each enzyme can typically only catalyse one type of reaction.

• Factors Affecting Enzyme Activity:
• Temperature: As temperature increases, the rate of reaction increases because enzymes and substrate molecules have more kinetic energy, leading to more frequent collisions. However, beyond the optimum temperature (around 37°C for human enzymes), the enzyme starts to denature. The high temperature breaks the bonds holding the protein in its specific 3D shape. The active site changes shape, and the substrate can no longer bind. This change is irreversible.
• pH: Each enzyme has an optimum pH at which it works best. For example, pepsin in the stomach works best at pH 2, while trypsin in the small intestine works best at pH 8. Extreme pH values away from the optimum cause the enzyme to denature.

Common Misconceptions & Exam Traps

* Trap: Saying enzymes are "killed" by high temperatures. Correction: Enzymes are not living. They are denatured. Use this precise term.

* Trap: Confusing bile with an enzyme. Correction: Bile is not an enzyme. It mechanically breaks down fats (emulsification); it does not chemically digest them.

* Trap: Mixing up absorption and assimilation. Correction: Absorption is the movement of digested food from the small intestine into the blood. Assimilation is the process where the body's cells take up and use these absorbed molecules (e.g., using glucose for respiration or amino acids for protein synthesis).