Welcome to the World of Nutrition and Digestion!
Have you ever wondered what happens to that sandwich after you take a bite? Or why you are always told to "eat your greens"? In this chapter, we are going to explore how living organisms get the nutrients they need and how the human body turns food into fuel. Nutrition is about taking in nutrients, and digestion is the breakdown of large food molecules so they can be absorbed and used by the body. Let's take it one step at a time!
1. What’s in Our Food? (The 7 Nutrients)
To stay healthy, the human body needs seven different types of nutrients from a balanced diet:
1. Carbohydrates: The body’s main and quickest source of energy.
Examples: Bread, rice, pasta, potatoes, and fruits.
2. Lipids (Fats and Oils): Provide a concentrated store of energy, cushion organs, and provide thermal insulation (keep you warm).
Examples: Butter, cooking oil, oily fish, and nuts.
3. Proteins: Needed for growth and repair of body tissues.
Examples: Meat, fish, eggs, beans, and lentils.
4. Vitamins: Organic substances needed in tiny amounts to maintain healthy functioning of tissues and immune defences.
Examples: Vitamin C (in citrus fruits) prevents scurvy; Vitamin D (in dairy products and made using sunlight) helps absorb calcium.
5. Minerals: Inorganic elements needed in small amounts for specific roles.
Examples: Calcium (for strong bones and teeth) and iron (needed to make red blood cells).
6. Dietary Fibre: Plant material (mainly cellulose) that cannot be digested. It provides bulk to keep food moving through the gut and prevents constipation.
Examples: Wholemeal cereals, vegetables, and fruit skins.
7. Water: Vital for cellular chemical reactions, dissolving substances, and transporting nutrients around the body.
Quick Mnemonic: The 7 Nutrients
Cats Love Playing Very Messy Funny Whisker-games! (Carbohydrates, Lipids, Proteins, Vitamins, Minerals, Fibre, Water).
2. Energy Requirements and Dietary Imbalance
A balanced diet provides the right amounts of all seven nutrients to meet an individual's energy and physiological needs. Energy in food is measured in kilojoules (kJ) or joules (\(\text{J}\)).
Daily energy requirements depend on:
1. Age: Growing teenagers generally require more energy per day than older adults.
2. Activity level: Athletes or manual workers need significantly more energy than someone sitting at a desk.
3. Body size and sex: Biological males typically have a higher proportion of muscle mass, often requiring slightly more energy.
Consequences of Dietary Imbalance
When a diet contains too much or too little of certain nutrients or energy, health problems arise:
- Obesity: Caused by regularly consuming more energy than the body expends. The excess energy is stored as body fat, which increases the risk of heart disease, type 2 diabetes, and high blood pressure.
- Starvation: Caused by severe and prolonged lack of energy intake. The body breaks down its own fat and muscle stores, leading to extreme weight loss, weakness, and damage to vital organs.
- Deficiency Diseases: Caused by a chronic lack of a specific vitamin or mineral in the diet:
• Scurvy: Lack of Vitamin C, causing bleeding gums, poor wound healing, and joint pain.
• Rickets: Lack of Vitamin D or Calcium, causing weak, soft, and deformed bones.
• Anaemia: Lack of Iron, meaning fewer red blood cells can be made, causing tiredness and shortness of breath.
3. The Journey of Food: The Human Digestive System
Digestion breaks down large, insoluble food molecules into small, soluble molecules that can be absorbed across the intestinal wall into the bloodstream.
Organs and Adaptations:
1. The Mouth: Teeth chew and break food down mechanically. The salivary glands produce saliva, which moistens food and introduces digestive enzymes.
2. The Oesophagus: A muscular tube connecting the mouth to the stomach. Contractions of circular and longitudinal muscles push food downwards in a process called peristalsis.
3. The Stomach: A muscular bag that churns food into a liquid mixture. It secretes hydrochloric acid (which kills harmful microorganisms and creates an acidic environment for stomach enzymes) and starts the digestion of proteins.
4. The Small Intestine: Further digestive enzymes breakdown carbohydrates, proteins, and lipids. Soluble food molecules are absorbed into the blood.
5. The Large Intestine: Water is reabsorbed into the body from the remaining undigested material, leaving solid waste known as faeces.
6. The Rectum and Anus: Faeces are stored in the rectum and expelled through the anus. This removal of undigested food is called egestion (not to be confused with excretion, which is the removal of cellular metabolic wastes like urea and carbon dioxide).
4. Biological Catalysts and Gut Bacteria
Enzymes act simply as biological catalysts: they speed up chemical reactions in living things without being consumed by the reaction.
- Carbohydrases (such as amylase) break down complex carbohydrates (like starch) into simple sugars (like glucose).
- Proteases break down proteins into amino acids.
- Lipases break down lipids into fatty acids and glycerol.
The Importance of Bacteria in Digestion
The human large intestine contains trillions of non-pathogenic bacteria. These helpful microorganisms perform crucial roles: they ferment dietary fibre that human enzymes cannot digest, produce useful vitamins (such as vitamin K and certain B vitamins), and help prevent harmful bacteria from multiplying.
5. Absorption in the Small Intestine
The small intestine is specially adapted for efficient absorption of digested food molecules into the bloodstream:
1. Villi and Microvilli: Millions of tiny, finger-like projections cover the inner surface, providing a massive surface area for diffusion.
2. Thin Walls: The epithelial lining is only one cell thick, providing a very short diffusion pathway.
3. Rich Blood Supply: A dense network of blood capillaries maintains a steep concentration gradient between the intestine and the blood.
6. Nutrition in Plants
Unlike humans and other animals, green plants do not eat food; they make their own nutrients:
- Making Carbohydrates: Plants make carbohydrates (glucose) in their leaves by the process of photosynthesis, using light energy absorbed by chlorophyll, carbon dioxide from the air, and water.
- Minerals and Water: Plants take up water and essential dissolved mineral nutrients (such as nitrates for making proteins and magnesium for making chlorophyll) from the soil through specialised root hair cells via their roots.
7. Summary
- 7 Nutrients: Carbohydrates, lipids, proteins, vitamins, minerals, dietary fibre, and water.
- Energy Balance: Energy intake (in kJ) must match activity levels and growth needs; imbalances lead to obesity, starvation, or deficiency diseases (e.g. scurvy, rickets, anaemia).
- Digestive System: Mouth → Oesophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus.
- Helpers: Enzymes act as biological catalysts to break down food; gut bacteria ferment undigested fibre and produce vitamins.
- Plant Nutrition: Plants synthesise carbohydrates in leaves using photosynthesis and absorb water and mineral nutrients from soil through roots.