Physical digestion
The key principle here is that no chemical change occurs to the food molecules during this process. The chemical structure of the nutrients (carbohydrates, proteins, fats) remains exactly the same; only the physical size of the food particles changes.
This process is essential because it prepares the food for chemical digestion. Chemical digestion relies on enzymes to break chemical bonds in large insoluble molecules into small soluble molecules. Enzymes can only work effectively on the surface of the food particles. By breaking food into smaller pieces, physical digestion dramatically increases the total surface area available for enzymes to act upon, speeding up the rate of chemical digestion.
The Correction: This description belongs to chemical digestion. Physical digestion only breaks large pieces of food into smaller pieces. It does not alter the molecular structure. If you mention breaking bonds or changing molecules, you are describing chemical digestion, not physical digestion.
Why examiners accept this: The markscheme specifically looks for two distinct points: (1) the breakdown into smaller pieces, and (2) the absence of chemical change. Omitting either point loses a mark.
Correct phrasing example: "Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules."
Common pitfall: Do not just say 'breaking down food.' You must explicitly state that there is no chemical change or that the molecules remain unchanged.
Building on the concept of surface area, different teeth create different types of cuts to maximize this effect.
| Function in Physical Digestion |
|---|
| Biting, cutting, or slicing food. They act like knives to remove pieces from larger items. |
| Tearing or piercing food (especially tough meat). Their pointed shape concentrates force to puncture. |
| Crushing and grinding food. They help break down food into smaller pieces. |
| Grinding and chewing food. Their large surface area allows them to crush food effectively into a paste. |
Canines: Pointed teeth for tearing/piercing.
Premolars: Intermediate teeth for crushing/grinding.
Molars: Back teeth with broad surfaces for grinding/chewing.
The Correction:
- Incisors are for cutting/biting, NOT grinding. They have a sharp edge, not a flat surface.
- Molars are the primary grinding teeth due to their large, flat surface area.
- Premolars also grind/crush but are smaller than molars.
If asked to identify a tooth used for grinding in a diagram, look for the tooth with the broadest, flattest top surface (usually the molar).
Why examiners accept this: Examiners look for specific functional keywords linked to the correct anatomical feature. For example, linking 'grinding' to 'flat surface' or 'cutting' to 'sharp edge'.
Correct phrasing example: "This is a molar because it has a large flat surface suitable for grinding food."
Common pitfall: Do not just say 'it is big.' You must link the shape (e.g., pointed, flat) to the function (e.g., tearing, grinding).
The syllabus limits the description to specific layers and components:
| Description and Function |
|---|
| The hardest substance in the body. It covers the crown (the visible part) of the tooth. Its function is to protect the inner layers from wear and tear during chewing. |
| A hard, bony tissue located beneath the enamel. It makes up the bulk of the tooth structure and supports the enamel. |
| The soft inner core of the tooth. It contains living tissue that nourishes the tooth. |
| Located within the pulp. They transmit sensations such as pain (e.g., when enamel is worn down and dentine is exposed) or temperature changes to the brain. |
| Located within the pulp. They supply nutrients and oxygen to the living cells of the tooth, keeping it alive. |
| A hard layer covering the root of the tooth (the part embedded in the bone). It anchors the tooth to the jawbone. |
Dentine: Hard layer beneath enamel.
Pulp: Soft inner core containing nerves and blood vessels.
Cement: Hard layer covering the root.
The Correction:
- Enamel is only on the crown (top part).
- Cement is only on the root (bottom part).
- Nerves and blood vessels are located together in the pulp at the very center.
Do not describe cement as covering the crown, or enamel as covering the root.
Why examiners accept this: The markscheme requires precise location terms (crown vs. root) and functional links (e.g., enamel = protection/hardness).
Correct phrasing example: "The enamel covers the crown and is the hardest part of the tooth, protecting it from wear. The pulp contains nerves and blood vessels."
Common pitfall: Avoid vague terms like 'inside' or 'outside.' Use specific anatomical terms: crown, root, enamel, dentine, pulp.
The walls of the stomach contain strong muscles. These muscles contract (squeeze) and relax rhythmically. This action churns the food, mixing it with gastric juices.
This churning breaks the food into even smaller pieces and mixes it thoroughly with digestive enzymes, creating a semi-liquid substance called chyme. This further increases the surface area for chemical digestion to occur efficiently.
The Error: Students often say the stomach 'produces bile' or 'digests proteins physically.'
The Correction:
- The stomach performs physical digestion via muscle contraction (churning).
- The stomach produces gastric juices (containing pepsin and HCl) for chemical digestion, but it does not produce bile.
- Bile is produced by the liver and stored in the gall bladder.
Why examiners accept this: The key concept is muscle contraction leading to churning/mixing. Mentioning 'breaking into smaller pieces' is also required.
Correct phrasing example: "The muscles in the stomach wall contract to churn (or mix) the food, breaking it into smaller pieces."
Common pitfall: Do not mention enzyme production here unless asked about chemical digestion. For physical digestion, focus on the mechanical action of the muscles.
While bile does not contain enzymes, it plays a crucial role in physical digestion of fats through a process called emulsification.
Fats and oils are insoluble in water and tend to form large globules (droplets) in the digestive tract. Bile salts break these large fat globules into many tiny droplets. This process is called emulsification.
This dramatically increases the surface area of the fats, allowing lipase enzymes to work more efficiently on the surface of the droplets for chemical digestion.
The Correction: Bile does not digest fats chemically. It does not break chemical bonds. It only breaks large globules into smaller droplets physically.
Also, do not say bile is produced in the gall bladder. It is produced by the liver and stored in the gall bladder.
Why examiners accept this: The markscheme requires the link between bile, surface area, and fat globules. You must mention that large globules become smaller droplets.
Correct phrasing example: "Bile emulsifies fats, breaking large fat globules into smaller droplets, which increases the surface area for lipase to act upon."
Common pitfall: Never say bile 'digests' or 'breaks down molecules.' Use words like 'breaks up,' 'emulsifies,' or 'increases surface area.'