Human Digestive System – Chapter 14

Food contains different types of nutrients.
The three common types of nutrients are carbohydrates, proteins and fats.
Besides the 3 common types of nutrients, the body also requires other nutrients such as vitamins, water, fibre and mineral salts.
- Food needs to be broken down so that nutrients in it can be absorbed by our body and used by the body cells.
- Large organisms like humans needs a digestive system to break food down into small soluble molecules.
- Only then can the digested food
  be absorbed by the body cells
  to release energy.

Physical (mechanical)
- Food is broken up to smaller pieces by physical or mechanical means such as crushing, chewing and mixing
- E.g. chewing a biscuit
- Increases surface area for enzymes to act.
Chemical
- Food is broken down into simpler molecules through the action of biological molecules known as enzymes
- Specific enzymes act on specific substrates only.

Carbohydrase → acts on carbohydrates → simple sugars.
Protease → acts on proteins → amino acids.
Lipase → acts on fats/lipids → fatty acids + glycerol.
Enzymes are biological catalysts; they speed reactions without being consumed.
Commercial application: some laundry detergents use carbohydrases, proteases, lipases to remove stains.
The end products of digestion (simple sugars, amino acids, fatty acids and glycerol) are able to be absorbed into the blood and get transported to other parts of the body

Physical: Strong teeth and jaw muscles in the mouth cut and crush food into smaller pieces (chewing)
Chemical: Saliva in the mouth contains carbohydrase. Partial digestion of starch (a type of carbohydrate) into partially digested carbohydrates
Other Functions: Saliva lubricates + softens food for easier swallowing.
Fun fact: Tooth enamel is the body’s hardest substance—harder than steel—but cannot regenerate.

Physical: None
Chemical: The gullet does not release any digestive juice. However, starch from the mouth continues to be digested by carbohydrase that are mixed with the food.
Other Functions: The muscles of the gullet pushes the food down towards the stomach

Physical: The stomach churns food into smaller pieces
Chemical: The stomach releases gastric juice that contains protease to digest proteins
Other Functions:
- The gastric juice contains hydrochloric acid which kills harmful microorganisms and provides the right condition for protease to act.
- The stomach has a thick layer of mucus to protect its tissue from damage caused by hydrochloric acid and protease.
- The muscles at both ends of the stomach control the food entering and leaving the stomach

Physical: None
Chemical: The small intestine is the main site for chemical digestion due to the numerous enzymes released. Fats are digested in small intestine due to presence of lipase in digestive juice carbohydrates, proteins and fats are fully digested in small intestine
Fun Fact:
- Length: $6\text{–}7\,\text{m}$ ; internal folds + villi give total absorptive area ≈ size of a tennis court.

Physical: None
Chemical: None
Other Functions:
- Some of the remaining water and mineral salts in the food are absorbed here

Physical: None
Chemincal: None
Other Functions:
- Indigestible food and waste form faeces, which is temporarily stored in rectum before it leaves the body.

Definition:
- The end products of digestion are small enough to be absorbed into the blood.
- Blood carries these molecules all around the body where they are used for various purposes.
- For e.g, cells use simple sugars during respiration to release energy.

Simple sugars
- Transported by blood to cells → used in cellular respiration → energy.
Amino acids
- Build/repair tissues, synthesize enzymes & hormones, support growth.
Fatty acids + glycerol
- Re-assembled into body fats for insulation, protection, long-term energy storage.

Over-consumption of carbs ⇒ persistently high blood glucose
Diabetes: Blood sugar too high due to
- Insufficient insulin production
- Body’s cells not responding to insulin (insulin resistance)
Complications: Obesity, hypertension, stroke, blindness, slow wound healing, frequent urination, unexplained weight loss, hunger, tingling/numbness, blurred vision
Monitoring tools
- Conventional glucometer
- Continuous glucose monitor linked to smartphone.
Risk reduction
- Exercise $\ge$ 30 min/day, 5 days/week.
- Balanced diet: more whole grains, veg, fruit, lean meat.
- Limit salt, sugar, saturated fat.
- Avoid smoking & alcohol.
- Regular health screening.

The food we consume and our lifestyle choices can affect the function of our digestive system
We should eat only in moderation and what we need and exercise regularly
We can use the healthy plate to plan a balanced and healthy meal

Constipation
- Slow transit through large intestine ⇒ excessive water reabsorption ⇒ hard/dry faeces.
- Causes: stress, medications, low fibre, inadequate water
- Remedies: high-fibre diet, more water, regular exercise.
Gastric pain (gastritis)
- Inflammation of stomach lining.
- Symptoms: nausea, bloating, upper abdominal pain
- Causes: bacterial/viral infection, skipping meals, excess alcohol, oily food
Food poisoning
- Pathogens: cholera, salmonella, etc
- Symptoms: vomiting, diarrhoea
- Not common with modern sanitation but vigilance needed.

Handling food safely
- Cover food before refrigeration.
- Store cooked/ready‐to‐eat food above raw food; raw food
- Use separate utensils for raw vs cooked items.
Choosing food carefully
- Bottled drinks sealed; check expiry dates.
- Discard dented/bloated cans.
- Avoid chilled foods stored outsideb or frozen foods above
- Reject mouldy products.

Beneficial roles
- Gut microbiota ferment certain carbohydrates → produce vitamins, aid digestion.
- Used industrially to make yogurt, cheese, sourdough, coffee, etc.
Harmful potential
- Pathogenic strains cause infections (e.g.
  salmonella, cholera).
- Balance of microbiome influenced by diet & antibiotics.