Welcome to the study notes for Physical Digestion! This is an important part of the Human Nutrition topic. Don't worry, this chapter is very logical. We are learning how your body acts like a giant, super-efficient food processor before the chemical breakdown even starts.
Understanding physical digestion is key because it sets the stage for the crucial step that follows: chemical digestion by enzymes. If you don't break the food down physically first, the enzymes can't work fast enough!
1. Defining Physical Digestion
What exactly is Physical Digestion? (Core 7.3.1)
Physical digestion is simply the process of breaking down large pieces of food into smaller pieces. Crucially, this process involves no chemical change to the food molecules themselves.
- Analogy: Imagine shredding a sheet of paper. You've changed its size and shape, but it's still chemically paper. Physical digestion is just like shredding the food.
- This is also called mechanical digestion.
The Main Goal: Increasing Surface Area (Core 7.3.2)
The whole point of physical digestion is to increase the surface area of the food particles. Why? To prepare the food for chemical digestion.
- Enzymes (which carry out chemical digestion) can only attack the surface of the food particles.
- If you have a big lump of food, the enzyme can only work on the outside.
- If you break that lump into many tiny pieces, the total surface area exposed to the enzymes increases dramatically, allowing the enzymes to work much faster and more efficiently.
Quick Review Box:
Physical Digestion = Big pieces to Small pieces.
Main Effect = Increased surface area.
Primary Benefit = Faster enzyme action during chemical digestion.
2. Physical Digestion in the Mouth: Teeth
The mouth is where physical digestion starts, mainly through chewing (known scientifically as mastication). Your teeth are perfectly designed for this job.
Structure of a Human Tooth (Core 7.3.4)
Teeth are specialized organs embedded in the jawbone and gums. They have several distinct layers:
- Enamel: This is the hardest substance in the human body. It forms the outer protective layer, shielding the tooth from damage.
- Dentine: A bone-like material found beneath the enamel. It forms the bulk of the tooth structure.
- Pulp: The soft center of the tooth. This vital area contains the tooth's living tissues: the nerves (which sense pain, pressure, and temperature) and blood vessels (which supply nutrients and oxygen).
- Cement: A thin layer that covers the root (the part embedded in the jaw) and helps anchor the tooth to the surrounding **bone** and gums.
Did you know?
Because enamel is the hardest biological substance, it is highly resistant to wear and tear. However, it can be dissolved by acids produced by bacteria if dental hygiene is poor, leading to decay.
Types and Functions of Human Teeth (Core 7.3.3, 7.3.5)
Humans have four main types of teeth, each specialized for a specific action:
1. Incisors (I)
- Structure: Sharp, flat edges (like chisels or scissors).
- Function: Used for cutting and slicing food, often the first teeth used when biting into an apple or a sandwich.
2. Canines (C)
- Structure: Pointed and sharp (like fangs).
- Function: Used for tearing tough foods, especially meat.
3. Premolars (P)
- Structure: Broader surfaces with ridges or cusps.
- Function: Used for grinding and crushing food. They act as a transition zone between the tearing teeth and the heavy grinding teeth.
4. Molars (M)
- Structure: Largest teeth, very broad surfaces with many cusps.
- Function: The primary teeth for heavy grinding and mashing the food into a soft ball (a bolus) ready for swallowing.
Memory Trick: Think of the action:
In = Incisors (cut into food)
Canines = Carnivorous (tearing)
Premolars / Molars = Grinding and Mashing
3. Physical Digestion in the Stomach (Core 7.3.6)
After being swallowed, food travels down the oesophagus into the stomach. While the stomach is famous for its acid and enzymes (chemical digestion), it also performs crucial physical digestion.
Stomach Function in Physical Digestion
The stomach walls are made of thick, strong muscle layers. These muscles perform rhythmic contractions, a process related to peristalsis (the wave-like muscular movements of the alimentary canal).
- The muscular contractions vigorously churn and mix the food.
- This mixing action ensures that the food is thoroughly combined with the gastric juice (which contains hydrochloric acid and enzymes).
- By churning the food, the stomach physically breaks down the remaining large pieces even further, producing a thick, semi-liquid mixture called chyme.
Key Takeaway: The stomach uses powerful muscular contractions to churn food, maximizing contact between food particles and gastric juice.
4. Supplement Content: The Role of Bile (Supplement 7.3.7)
If you are studying the Extended syllabus, you need to know about the role of bile in preparing fats and oils for digestion.
What is Bile?
- Bile is an alkaline liquid produced by the liver and stored in the gall bladder.
- Bile is released into the duodenum (the first part of the small intestine).
The Process of Emulsification
Bile does not contain digestive enzymes, so it is involved in physical, not chemical, digestion. Its role is emulsification:
- Large fat globules present in the food enter the duodenum.
- Bile salts break these large fat globules into tiny droplets. This action is called emulsifying fats and oils.
Why is Emulsification Important?
Just like chewing food increases the surface area for enzymes, emulsification significantly increases the surface area of the fats.
- This massive increase in surface area allows the fat-digesting enzyme, lipase, to attack the fat molecules much more quickly and efficiently.
Analogy: Imagine trying to break down a huge block of butter (fat globule). It's hard! Bile breaks that block into thousands of tiny spreadable dots (droplets), making it easy for the lipase (the enzyme) to work on every surface area.
Key Takeaway (Supplement): Bile physically breaks large fat globules into tiny droplets (emulsification), greatly increasing the surface area for lipase action.
Summary of Physical Digestion
Physical digestion is the mechanical preparation that ensures chemical digestion is rapid and complete. It happens in three main places:
| Location | Mechanism | Purpose |
| Mouth | Chewing (Teeth action) | Break food into smaller pieces to swallow easily and increase surface area. |
| Stomach | Churning (Muscular contraction) | Mix food thoroughly with gastric juice and form chyme. |
| Small Intestine | Emulsification (Bile action) | Break down large fat globules into tiny droplets to increase the surface area for lipase. |