The Digestive System Knowledge Organiser

Definition: The digestive system is an organ system where multiple organs collaborate to digest and absorb food.
Function: Its primary role is to break down large, insoluble substances into small, soluble molecules that can be absorbed into the bloodstream.
Pathway of Food: The digestive system includes the following organs:
- Mouth
- Oesophagus
- Stomach
- Duodenum
- Small intestine
- Large intestine
- Rectum
Key Organs & Functions:
- Pancreas: Releases enzymes into the duodenum to aid digestion.
- Liver: Produces bile, which is stored in the gall bladder and released into the duodenum.
- Absorption Sites:
- Small intestine: Absorption of digestion products occurs here.
- Large intestine: Absorbs water into the bloodstream.
- Rectum: Stores undigested food before egestion through the anus.

Mechanical Digestion: Involves physical processes that break down food:
- Chewing: Initiated in the mouth; considered a mechanical process.
- Peristalsis: Movement of food through the oesophagus via smooth muscle contractions.
- Stomach Churning: Muscular contractions mix the food mechanically.
Chemical Digestion: Involves biochemical processes that break down food:
- Salivary Glands: Secrete enzymes into the mouth, starting chemical digestion.
- Hydrochloric Acid in Stomach: Destroys microorganisms in food and lowers pH for enzyme optimal activity.
- Enzymatic Action: Occurs in the small intestine to further digest remaining large molecules.
- Bile: Produced by the liver, alkaline to neutralize stomach acid and emulsifies fats, increasing surface area for lipid digestion.

Absorption Mechanism: Small, soluble molecules enter the bloodstream through diffusion or active transport.
Structural Adaptations:
- Villi and Microvilli: Highly folded structures that increase surface area for absorption.
- Length of Small Intestine: Its length prolongs the time available for absorption.
Villi Specialization:
- Thin Structure: Provides a short diffusion pathway for efficient absorption.
- Rich Blood Supply: Maintains a concentration gradient for absorption.
- Mitochondria Presence: Cells have many mitochondria for energy required for active transport of molecules.

Role of Digestive Products:
- Metabolism: The absorbed products are utilized for:
- Building new carbohydrates, lipids, and proteins.
- Some glucose is used in respiration.
Key Conversions:
- Glucose → Carbohydrates
- Amino Acids → Proteins
- Fatty Acids and Glycerol → Lipids

Balanced Diet Composition: A balanced diet includes carbohydrates, lipids, proteins, water, vitamins, and minerals in proper proportions.
Nutritional Functions:
- Proteins: Necessary for growth and repair.
- Carbohydrates: Function as a source of energy.
- Lipids: Provide insulation, energy, and are important for cell structure.
Testing Methods:
- Benedict's Test: Tests for sugars:
- Procedure: Add Benedict's reagent to sample and heat.
- Result: Color change from Blue → Brick-red indicates sugar presence (quantified by the depth of color).
- Iodine Test: Tests for starch:
- Procedure: Add iodine to sample.
- Result: Color change from Brown → Blue-black confirms starch presence.
- Biuret Test: Tests for protein:
- Procedure: Add Biuret reagent to sample.
- Result: Color change from Blue → Lilac (purple) suggests protein content.

Definition: Enzymes are biological catalysts that accelerate biochemical reactions in living organisms.
Nature of Enzymes:
- Type: Enzymes are proteins that catalyze specific reactions due to their unique three-dimensional structures.
- Active Site: Each enzyme has an active site that is structurally complementary to its substrate molecule.
- Substrate: A substrate is a reactant that an enzyme acts upon.
Enzyme-Substrate Interaction:
- Formation of enzyme-substrate complex occurs when a substrate binds to the enzyme's active site.
- Lock and Key Theory: A simplistic representation of enzyme action where the active site (lock) perfectly fits the substrate (key).
- Induced Fit Model: A more refined explanation stating that the binding of the substrate induces a change in both enzyme and substrate shapes, facilitating reaction.

Role of Enzymes in Digestion: Enzymes help convert large, insoluble molecules into smaller, soluble forms.
Types of Digestive Enzymes:
- Carbohydrases: Break down carbohydrates to simple sugars.
- Amylase: Specific carbohydrase that breaks down starch; produced in salivary glands, pancreas, and small intestine, acting in the mouth and small intestine.
- Proteases: Responsible for breaking proteins down into amino acids; produced in the stomach, pancreas, and small intestine, working in the stomach and small intestine.
- Lipases: Break down fats into glycerol and fatty acids; produced in pancreas and small intestine, acting within the small intestine.
Molecular Conversion Summary:
- Starch → Simple sugars (glucose)
- Protein → Amino Acids
- Lipids (fats) → Fatty Acids and Glycerol

Temperature Sensitivity: Enzymes function optimally at a specific temperature:
- Optimal temperature for human enzymes is 37 °C.
- At low temperatures:
- Enzyme activity declines as kinetic energy is low; substrates are less likely to bind.
- At high temperatures:
- Enzymes may denature, changing the active site's shape and rendering it nonfunctional.
pH Sensitivity: Enzymes are also sensitive to changes in pH:
- Low and high pH levels can cause denaturation, altering the active site's compatibility with substrates.
- Hydrochloric acid in the stomach affects the overall pH of the digestive system, influencing enzyme activity.
Continuous Sampling Technique for Amylase:
- A method to observe how long it takes for amylase to completely digest starch across varying pH conditions.
- Iodine reagent is employed to test for residual starch at 30-second intervals, ensuring controlled temperature through a water bath or electric heater.