Science - Combined (0653) | Human nutrition

Hello Future Biologist!

Welcome to the fascinating world of Human Nutrition (Chapter B7)! This chapter is all about how your body takes in food, breaks it down, and uses the energy and materials to keep you running, growing, and healthy. Understanding nutrition isn't just great for your exams; it helps you make smart choices about what you eat every day. Let's dive in!

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Part 1: The Balanced Diet (B7.1)

What is a Balanced Diet?

A balanced diet is a diet that contains all the essential nutrients in the correct proportions to meet the body’s needs for energy, growth, and tissue repair. Think of your body like a car—it needs the right type of fuel (energy) and regular maintenance (nutrients for repair and growth).

The Seven Essential Dietary Components

We need seven main components in our diet. These fall into three groups: the big ones (Macronutrients), the small ones (Micronutrients), and the helpers.

1. Macronutrients (Needed in Large Amounts)

(a) Carbohydrates (Carbs)

• Sources: Bread, rice, pasta, potatoes, sugars (like glucose).
• Importance: They are the body's main and fastest source of energy.
• Did you know? Carbs break down into simple sugars (like glucose), which are then used in respiration (B12) to release energy.

(b) Fats and Oils (Lipids)

• Sources: Butter, oil, nuts, avocados, fatty meats.
• Importance:
  1. Provide a concentrated, long-term store of energy.
  2. Act as insulation (to keep you warm).
  3. Protect vital organs.
  4. Help dissolve and transport fat-soluble vitamins (A, D, E, K).

(c) Proteins

• Sources: Meat, fish, eggs, beans, lentils, cheese.
• Importance: Essential for growth and the repair of worn-out cells and tissues. They are also needed to make enzymes, hormones, and antibodies.

2. Micronutrients (Needed in Small Amounts)

(d) Vitamins (Limited to C and D)

• Vitamin C:
  • Sources: Citrus fruits (oranges, lemons), vegetables.
  • Importance: Helps maintain healthy skin, gums, and blood vessels. Lack of Vitamin C causes scurvy.
• Vitamin D:
  • Sources: Oily fish, eggs, and produced naturally in the skin when exposed to sunlight.
  • Importance: Necessary for the absorption of calcium. Lack of Vitamin D causes rickets (weak bones) in children.

(e) Mineral Ions (Limited to Calcium and Iron)

• Calcium (Ca):
  • Sources: Milk, cheese, leafy greens.
  • Importance: Essential for building strong bones and teeth, and necessary for blood clotting and muscle contraction.
• Iron (Fe):
  • Sources: Red meat, spinach, liver.
  • Importance: Essential component of haemoglobin in red blood cells (B9.4) for transporting oxygen. Lack of iron causes anaemia (tiredness, paleness).

3. The Helpers

(f) Fibre (Roughage)

• Sources: Whole grains, fruits, vegetables.
• Importance: Provides bulk to food, helping the muscles of the gut push food through the alimentary canal (peristalsis). This prevents constipation. Fibre itself is undigested.
• Analogy: Fibre acts like the scrubber brush, sweeping food along the pipe!

(g) Water

• Sources: Drinks (water, juice), fruits, and vegetables.
• Importance:
  1. Acts as a solvent for chemical reactions.
  2. It is the major component of blood (plasma - B9.4), helping transport substances.
  3. Helps regulate body temperature (through sweating).

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Part 2: The Digestive System (B7.2)

The digestive system is a group of organs that work together to process food. It’s essentially a long tube, the alimentary canal, with several associated organs attached to help the process.

The Alimentary Canal (The Main Tube)

This is the tube that runs from the mouth to the anus where food actually passes through.

1. Mouth: Where ingestion and initial digestion begin.
2. Oesophagus (Gullet): Muscular tube that pushes food from the mouth to the stomach via peristalsis (wave-like muscle contractions).
3. Stomach: A muscular bag where food is mixed with gastric juice (containing acid and enzymes).
4. Small Intestine: The site of the most intense digestion and absorption. It is very long and divided into:
   • Duodenum: Receives bile and pancreatic juice; completes digestion.
   • Ileum: Main area for nutrient absorption into the blood.
5. Large Intestine: Deals with undigested material. Divided into:
   • Colon: Where water is absorbed back into the body.
   • Rectum: Stores faeces temporarily.
   • Anus: Controls the removal of faeces (egestion).

Associated Organs (The Helpers)

These organs secrete chemicals that aid digestion but food does not pass through them.

• Salivary Glands: Produce saliva, containing amylase, to start starch digestion in the mouth.
• Pancreas: Secretes pancreatic juice (containing digestive enzymes) into the duodenum.
• Liver: Produces bile, which emulsifies fats (breaks large fat globules into smaller ones).
• Gall Bladder: Stores bile before releasing it into the duodenum.

The Five Stages of Food Processing

The whole process of getting energy and nutrients from food is broken down into five steps:

1. Ingestion: The simple act of taking substances (food and drink) into the body through the mouth.
2. Digestion: The breakdown of large, complex food molecules into small, simple, soluble molecules.
3. Absorption: The movement of these small, soluble nutrients (like glucose, amino acids) from the intestines (ileum) into the blood stream.
4. Assimilation: The uptake and use of these absorbed nutrients by the body's cells (e.g., using glucose for respiration or amino acids to build new proteins).
5. Egestion: The removal of undigested waste material (faeces) from the body through the anus. Note: This is NOT excretion (B1), which is the removal of metabolic waste like urea or CO₂.

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Part 3: Digestion: Breaking it Down (B7.3)

Digestion involves two types of action working together: physical and chemical.

Physical Digestion (Mechanical)

This is the breakdown of large pieces of food into smaller pieces without changing the chemical structure of the food molecules.

• Where: Primarily in the mouth (chewing) and stomach (churning).
• Function (Supplement): Physical digestion greatly increases the surface area of the food. This is vital because enzymes can only work on the surface of food particles. More surface area means faster chemical digestion!
• Analogy: Crushing a sugar cube into powder before adding it to water makes it dissolve faster—that's like physical digestion speeding up the work for the chemical digestion (enzymes).

Chemical Digestion

This is the breakdown of large, insoluble food molecules into small, soluble molecules using enzymes (B5).

• Role (Supplement): The goal is to produce molecules small enough (like glucose, amino acids, fatty acids, glycerol) to be absorbed into the blood through the walls of the small intestine.

The Key Digestive Enzymes (Supplement)

Enzymes are specific and only break down one type of nutrient:

1. Amylase

• Function: Breaks down starch (a large carbohydrate) into simple reducing sugars (e.g., maltose).
• Secretion and Action: Secreted by salivary glands (acts in mouth) and pancreas (acts in duodenum/small intestine).

2. Proteases

• Function: Breaks down proteins into small molecules called amino acids.
• Secretion and Action: Secreted by the stomach (acts in stomach) and pancreas (acts in duodenum/small intestine).

3. Lipase

• Function: Breaks down fats and oils (lipids) into fatty acids and glycerol.
• Secretion and Action: Secreted by the pancreas (acts in duodenum/small intestine).

The Role of Hydrochloric Acid (HCl) in the Stomach (Supplement)

The gastric juice produced by the stomach lining contains strong hydrochloric acid. It performs two critical functions:

1. Kills harmful microorganisms present in the food (acting as a defense mechanism).
2. Provides an acidic pH (around pH 2) which is the optimum pH for the protease enzymes (like pepsin) that work in the stomach.