Welcome to the Journey of Food!
Have you ever wondered what happens to that sandwich after you take a bite? Your body is like a highly efficient factory. It takes the food you eat, breaks it down into tiny pieces, and absorbs the "fuel" into your blood to keep you running. This process is called digestion and absorption.
In this chapter, we will explore the Alimentary Canal (the tube food travels through), the enzymes that act as chemical scissors, and how we finally get those nutrients into our cells. Note: For details on what makes a balanced diet or the structure of teeth, see the other chapters in this section.
1. The Alimentary Canal: The Path of Food
The alimentary canal is a long, continuous muscular tube that runs from your mouth to your anus. Each part has a specific job to do.
The Main Organs and Their Functions:
- Mouth: This is where it all starts! Mechanical digestion happens as teeth grind food, and chemical digestion begins as saliva mixes with food.
- Oesophagus: A muscular tube that connects the mouth to the stomach.
- Stomach: A muscular bag that churns food. It produces hydrochloric acid to kill bacteria and provides the right pH for protease enzymes to work.
- Small Intestine: This is where most chemical digestion happens and where nearly all absorption of nutrients occurs.
- Pancreas: This is a "gland" (not a tube) that creates a "cocktail" of digestive enzymes and squirts them into the small intestine.
- Large Intestine: Its main job is to absorb water from the remaining indigestible food, turning it into solid waste.
How Food Moves: Peristalsis
Food doesn't just fall down into your stomach by gravity—you could actually swallow while standing on your head! This is because of peristalsis. Peristalsis is the contraction and relaxation of muscles in the wall of the alimentary canal that pushes food along.
Quick Tip: Think of squeezing a tube of toothpaste from the bottom to the top. That's exactly how your muscles push food through your gut!
Dietary Fibre: Fibre is essential because it gives the muscles of the gut something to "grip" against, keeping peristalsis efficient and preventing constipation.
2. Digestive Enzymes: The Chemical Scissors
Food molecules like starch and protein are far too large to pass through the walls of your intestine into your blood. Enzymes are biological catalysts that break these large, insoluble molecules into small, soluble ones.
The Three Main Enzyme Groups You Need to Know:
A. Carbohydrases (Breaking down Starch)
- Amylase: Breaks Starch down into Maltose. (Produced in salivary glands and pancreas).
- Maltase: Breaks Maltose down into Glucose. (Produced in the small intestine).
- Overall: \(Starch \rightarrow Maltose \rightarrow Glucose\)
B. Proteases (Breaking down Proteins)
- Pepsin: Works in the stomach (very acidic).
- Trypsin: Works in the small intestine.
- Overall: \(Proteins \rightarrow Amino \ Acids\)
C. Lipases (Breaking down Fats)
- Lipase: Breaks Lipids (fats and oils) into Fatty Acids and Glycerol. (Produced in the pancreas).
- Overall: \(Lipids \rightarrow Fatty \ Acids + Glycerol\)
Key Takeaway: Remember that enzymes are specific. Amylase only breaks down starch; it cannot break down protein!
3. Bile: The Multi-Tasker
Bile is a green-yellow fluid that plays two vital roles in digestion. It is produced in the liver and stored in the gall bladder before being released into the small intestine.
- Neutralising Acid: Food coming from the stomach is full of hydrochloric acid. Enzymes in the small intestine don't like acid! Bile is alkaline, so it neutralises the acid to create the perfect slightly alkaline pH for enzymes like trypsin to work.
- Emulsifying Lipids: Bile breaks large drops of fat into thousands of tiny droplets. This is called emulsification. It increases the surface area for the lipase enzymes to work on, making fat digestion much faster.
4. Absorption: Getting Nutrients into the Blood
Once the food is broken down into small, soluble molecules (like glucose and amino acids), it must get into the blood. This happens in the small intestine.
The Villus (Singular: Villi)
The inside of the small intestine is covered in millions of tiny, finger-like projections called villi. These are perfectly adapted for absorption:
- Huge Surface Area: There are so many villi (and even smaller microvilli on them) that the total surface area is massive, allowing more nutrients to pass through at once.
- Thin Walls: The wall of a villus is only one cell thick, so nutrients have a very short distance to travel (short diffusion path).
- Rich Blood Supply: Each villus has a network of capillaries to carry away glucose and amino acids quickly. This maintains a steep concentration gradient.
- Lacteal: This is a special tube in the center of the villus that absorbs fatty acids and glycerol into the lymphatic system.
5. Required Practicals
In your exam, you may be asked about these specific investigations:
Investigating Enzyme Activity (Temperature and pH)
You can investigate how fast amylase breaks down starch at different temperatures or pH levels using iodine. Iodine turns blue-black if starch is present and stays orange-brown if the starch has been digested.
Common Mistake: Students often say enzymes are "killed" by high heat. This is wrong! Use the word denatured. The enzyme's active site changes shape, so the substrate no longer fits.
Immobilised Enzymes
Sometimes we trap enzymes in alginate beads. These are called immobilised enzymes.
Why use them?
1. They can be reused.
2. The product (like lactose-free milk) isn't contaminated with the enzyme.
Quick Review Box:
- Ingestion: Taking food in.
- Digestion: Breaking food down (Mechanical and Chemical).
- Absorption: Small molecules moving into the blood via villi.
- Peristalsis: Muscle contractions moving food.
- Bile: Neutralises acid and emulsifies fats.