Sciences - Co-ordinated (Double) (0654) | Human nutrition

Study Notes: B7 Human Nutrition (Co-ordinated Sciences 0654)

Hello future scientists! This chapter is all about how your body takes in, processes, and uses the food you eat. Nutrition is the foundation of life, providing the energy you need to run, study, and grow, so let's dive into the fascinating journey of food through your body!

B7.1 The Importance of a Balanced Diet

What is a Balanced Diet?

A balanced diet is a diet that provides all the necessary nutrients in the correct amounts and proportions to maintain health. Think of it like tuning a car—it needs exactly the right amount of fuel, oil, and coolant to run perfectly.

If your diet is unbalanced (either too much or too little of certain nutrients), you can suffer from malnutrition.

Principal Dietary Sources and Importance (The Seven Essential Classes)

We classify nutrients into seven main groups. You need to know their primary sources and why they are essential:

1. Carbohydrates (Carbs)

• Sources: Cereals (rice, wheat), potatoes, bread, pasta, sugar.
• Importance: They are the body’s main source of energy (fuel for respiration).

2. Fats and Oils (Lipids)

• Sources: Butter, oil, nuts, meat, cheese.
• Importance: Provide a concentrated store of energy, insulation, and protection for vital organs. They also help absorb certain vitamins.

3. Proteins

• Sources: Meat, fish, eggs, dairy, beans, lentils.
• Importance: Essential for growth and repair of tissues (like muscles, hair, and skin). They are also used to make enzymes and some hormones.

4. Vitamins (The tiny regulators)

Vitamins are needed in small quantities to help the body function correctly. We focus on two specific vitamins:

• Vitamin C (Ascorbic acid):
  • Sources: Citrus fruits, fresh vegetables.
  • Importance: Essential for maintenance of healthy tissues, especially gums, and helps fight infections.
• Vitamin D:
  • Sources: Oily fish, eggs, and produced in the skin when exposed to sunlight.
  • Importance: Essential for the absorption of calcium and phosphorus, leading to strong bones and teeth.

Did you know? Unlike carbs, fats, and proteins, vitamins and minerals do not provide energy directly, but they are crucial for releasing energy from other foods!

5. Mineral Ions (The chemical helpers)

These are inorganic substances needed in small amounts. We focus on two:

• Calcium (Ca²⁺):
  • Sources: Milk, cheese, leafy green vegetables.
  • Importance: Required for strong bones and teeth, and essential for blood clotting and muscle contraction.
• Iron (Fe²⁺):
  • Sources: Red meat, liver, dark green vegetables.
  • Importance: Needed to make haemoglobin, the red pigment in blood that transports oxygen.

6. Fibre (Roughage)

• Sources: Wholemeal bread, fruit and vegetable skins, cereals.
• Importance: Provides bulk (roughage) to the food, helping the muscles of the gut push the food along (peristalsis). This prevents constipation. Fibre is mostly undigested.

7. Water

• Sources: Drinks, fruits, vegetables.
• Importance: Essential solvent for all chemical reactions in the body, transport medium (blood), and crucial for temperature regulation.

Common Deficiency Diseases

• Scurvy: Caused by a lack of Vitamin C. Symptoms include bleeding gums, tiredness, and poor wound healing.
• Rickets: Caused by a lack of Vitamin D or Calcium. Symptoms include soft, weak bones, leading to skeletal deformities (especially bowed legs).

B7.2 The Human Digestive System

The Five Stages of Food Processing

The digestive system handles food through five main steps:

1. Ingestion: This is simply taking substances (like food and drink) into the body, usually through the mouth.
2. Digestion: The breakdown of large, insoluble food molecules into small, soluble molecules. This happens through physical (chewing, churning) and chemical (enzyme action) means.
3. Absorption: The movement of the small, soluble nutrient molecules from the intestines into the blood.
4. Assimilation: The uptake and use of absorbed nutrients by the body’s cells. For example, glucose is assimilated by muscle cells for respiration.
5. Egestion: The removal of undigested food (faeces) from the body via the anus. (Note: This is NOT excretion, which is the removal of metabolic waste.)

Mnemonic for Stages: I Dip And Act Eventually (Ingestion, Digestion, Absorption, Assimilation, Egestion)

Main Organs of the Digestive System

1. The Alimentary Canal (The Food Tube)

This is the long tube through which food passes:

• Mouth: Ingestion and physical digestion (chewing).
• Oesophagus: Muscular tube pushing food from mouth to stomach via peristalsis.
• Stomach: Muscular sac for churning (physical digestion) and chemical digestion of proteins.
• Small Intestine: The main site for chemical digestion and absorption. It has two parts:
  • Duodenum: Where most chemical digestion occurs.
  • Ileum: Where most absorption occurs due to its large surface area.
• Large Intestine: Primarily for absorbing water and mineral ions. It includes:
  • Colon: Water absorption.
  • Rectum: Stores faeces temporarily.
  • Anus: Outlet for egestion.

2. Associated Organs (The Helpers)

These organs secrete substances used in digestion, but food does not pass through them:

• Salivary Glands: Secrete saliva, containing amylase, into the mouth.
• Pancreas: Secretes pancreatic juice (containing amylase, protease, and lipase) into the duodenum.
• Liver: Produces bile.
• Gall Bladder: Stores bile before releasing it into the duodenum.

B7.3 Digestion (The Breakdown Process)

Physical vs. Chemical Digestion (Core)

1. Physical Digestion (Mechanical)

• Definition: The breakdown of food into smaller pieces without changing the chemical structure of the food molecules.
• Examples: Chewing (mouth), churning (stomach).
• Role: Physical digestion increases the surface area of the food. This larger surface area means chemical enzymes can access the food more easily and rapidly.

2. Chemical Digestion

• Definition: The breakdown of large, insoluble molecules (like starch, protein) into small, soluble molecules (like glucose, amino acids) using digestive enzymes.
• Role: This process is essential because large molecules cannot pass through the gut wall into the blood. Chemical digestion produces the small soluble molecules that can be absorbed.

Enzymes: The Biological Cutters (Core & Supplement)

Enzymes are protein molecules that function as biological catalysts, speeding up the specific breakdown reactions of digestion.

Functions of Key Digestive Enzymes (Supplement)

Location of Enzyme Action (Supplement)

• Amylase is secreted by salivary glands (acts in mouth/oesophagus) and the pancreas (acts in small intestine).
• Protease is secreted by the stomach lining and the pancreas (acts in stomach and small intestine).
• Lipase is secreted by the pancreas (acts in small intestine).

The Role of Hydrochloric Acid and Bile (Supplement)

Hydrochloric Acid (HCl) in the Stomach

Gastric juice in the stomach contains powerful hydrochloric acid (HCl), which has two main functions:

1. Killing Harmful Microorganisms: The low, acidic pH (around pH 1.5–3.5) kills most bacteria and pathogens consumed with food, protecting the body from illness.
2. Optimum pH for Proteases: It provides the acidic pH necessary for the protease enzyme found in the stomach (like pepsin) to work effectively. If the pH was neutral, pepsin would be inactive.

Bile (Produced by Liver, Stored in Gall Bladder)

Bile is an alkaline (basic) mixture that enters the duodenum from the gall bladder. It performs two critical roles:

1. Neutralisation: It neutralises the highly acidic mixture of food and gastric juices (called chyme) entering the duodenum from the stomach. This neutralization creates a suitable alkaline pH for the pancreatic enzymes (amylase, protease, lipase) to work optimally in the small intestine.
2. Emulsification: Bile physically breaks down large fat droplets into tiny droplets (a process called emulsification). This massively increases the surface area of the fat, making it much easier for lipase enzymes to carry out chemical digestion.