Digestive System – NDA Biology Notes

What is the Digestive System?

The digestive system breaks food down into small particles. These small particles can then be absorbed into the blood and used by the body.

Food is made of carbohydrates, proteins, fats, and other substances. The body cannot absorb these in their original form. Enzymes (special proteins) break them down step by step.

The path food travels is called the alimentary canal. It goes from the mouth to the anus.

The Journey of Food — Step by Step

Step 1 — Mouth (Buccal Cavity)

Digestion begins in the mouth. [NDA tested via CDS]

This is important. Protein digestion begins in the stomach. But digestion overall begins in the mouth — because carbohydrate digestion starts here.

The mouth does two things:

• Teeth break food into smaller pieces (mechanical digestion).
• Saliva mixes with food (chemical digestion).

Saliva is secreted by the salivary glands. When we see or smell food we like, saliva is released automatically — this is a reflex. [NDA tested via CDS]

Enzyme in saliva: Ptyalin (Salivary Amylase)

Ptyalin is the enzyme in saliva. [NDA 2006-II]

It acts on starch and converts it into maltose (a simpler sugar). [NDA 2006-I]

Ptyalin is also the first enzyme to mix with food in the entire digestive tract. [NDA tested via CDS]

Lysozyme is another substance in saliva. It is an antibacterial enzyme — it destroys bacteria that enter with food. It is also found in tears. [NDA tested via CDS]

The inner lining of the mouth is made of squamous epithelium — flat cells suited to withstand friction. [NDA 2021-I] [NDA tested via CDS]

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Step 2 — Oesophagus (Food Pipe)

Food passes from the mouth through the oesophagus to the stomach. The oesophagus is a muscular tube.

No digestion happens in the oesophagus. It only transports food.

The inner lining of the oesophagus is also made of squamous epithelium. [NDA 2021-I] [NDA tested via CDS]

Step 3 — Stomach

The stomach is a J-shaped muscular bag. It stores food temporarily, mixes it with digestive juices, and releases it slowly into the small intestine.

The stomach does three things:

• Acts as a temporary reservoir for food. [NDA 2011-I]
• Mixes food with gastric juice. [NDA 2011-I]
• The rate of emptying depends on the type of food. [NDA 2011-I]

The stomach does NOT secrete lipase or amylase. This is an exam trap. [NDA 2011-I]

The stomach secretes gastric juice, which contains:

• Pepsin — the protein-digesting enzyme.
• HCl (Hydrochloric acid) — creates an acidic environment and activates pepsin.

Protein digestion begins in the stomach. [NDA 2006-I]

Enzyme in stomach: Pepsin

Pepsin is produced by the stomach wall. [NDA 2022-III]

Pepsin digests proteins. It breaks them into smaller chains called peptides.

Pepsin works at acidic pH — approximately 2.0. [NDA 2025-I]

If the acid-secreting cells of the stomach are damaged, HCl is not produced. Pepsin cannot work without acid. So protein digestion is specifically affected. [NDA 2018-I]

The opening of the stomach into the small intestine is called the pylorus. [NDA tested via CDS]

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Step 4 — Small Intestine

The small intestine is the most important part of the digestive system. It is here that:

• Complete digestion of all three nutrients happens — proteins, carbohydrates, and fats. [NDA 2013-I]
• Food is absorbed into the bloodstream.

The small intestine is also the longest part of the alimentary canal. [NDA tested via CDS]

Its inner lining has finger-like projections called villi. Villi increase the surface area for absorption. [NDA tested via CDS]

The small intestine receives digestive juices from two sources:

• Pancreas — secretes enzymes.
• Liver (via gall bladder) — secretes bile.

These juices enter the small intestine at the first part called the duodenum.

The acidic food coming from the stomach is neutralised by pancreatic juice when it enters the duodenum. [NDA 2018-III] This neutralisation is necessary because the intestinal enzymes work in alkaline conditions, not acidic ones.

Step 5 — Large Intestine

No significant digestion happens in the large intestine.

It absorbs water and some minerals from the undigested food. [NDA tested via CDS]

It compacts the remaining waste for removal.

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Key Enzymes — Complete Table

Enzyme	Source	Acts On	End Product	pH
Ptyalin (Salivary Amylase)	Salivary glands	Starch	Maltose	~7 (neutral)
Lysozyme	Salivary glands, tears	Bacteria	—	—
Pepsin	Stomach wall	Proteins	Peptides	~2.0 (acidic)
Pancreatic Amylase	Pancreas	Starch	Sugars	Alkaline
Trypsin	Pancreas	Proteins/Peptides	Amino acids	~7.9 (alkaline)
Lipase	Pancreas	Fats	Fatty acids + Glycerol	Alkaline
Lactase	Small intestine	Lactose (milk sugar)	Glucose + Galactose	—

Trypsin is an enzyme. Gastrin is a hormone. Keratin is a structural protein. Vasopressin is a hormone. Only trypsin is an enzyme among these. [NDA 2007-II]

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pH Along the Digestive Tract

Different parts of the digestive tract have different pH levels. Enzymes only work at their specific pH.

Location	pH	Nature
Gastric juice (stomach)	~2.0	Strongly acidic
Saliva (mouth)	~7.0	Neutral
Blood	7.5–8.0	Slightly alkaline

Correct increasing pH order: Gastric juice → Saliva → Blood. [NDA tested via CDS]

Pepsin works at acidic pH (~2.0) — stomach only. [NDA 2025-I]

Trypsin works at alkaline pH (~7.9) — small intestine. [NDA 2025-I]

Amylase works at neutral to alkaline pH.

Trypsin and amylase do NOT work at acidic pH. [NDA tested via CDS]

Any pair that does not include pepsin — for example, trypsin and amylase — does not work at acidic pH.

End Products of Digestion

This is a direct matching question in NDA exams. Each large molecule is broken down into simpler units that can be absorbed.

Original Molecule	End Product
Proteins	Amino acids
Carbohydrates	Monosaccharides (e.g., glucose)
Fats (Lipids)	Fatty acids + Glycerol
Nucleic acids	Nitrogenous bases + Pentose sugars

[NDA 2016-I]

In the digestive system, proteins are broken down into amino acids — not glycerol (that is from fat), not CO₂ and water (that is from cellular respiration). [NDA 2012-III]

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The Pancreas — Dual Function Organ

The pancreas is unique. It does two completely different jobs:

1. Exocrine function (digestive):
It secretes three enzymes into the small intestine:

• Lipase — digests fats
• Amylase — digests starch/carbohydrates
• Protease (Trypsin) — digests proteins

This is why the pancreas is the only organ that secretes both hormones and digestive enzymes. [NDA 2007-II]

2. Endocrine function (hormonal):
The Islets of Langerhans inside the pancreas produce insulin and glucagon, which control blood sugar.

The role of the pancreas in digestion is the secretion of lipase, amylase, and protease. [NDA 2024-I]

Bile is stored in the gall bladder — not the pancreas. Neutralising stomach acids is done by pancreatic juice as a medium, but the primary role of the pancreas is secreting enzymes.

The Liver and Bile

The liver is the largest gland in the body. In digestion, its job is to produce bile.

Bile is:

• Made in the liver
• Stored in the gall bladder
• Released into the small intestine (duodenum) when needed

What bile does:

• Emulsifies fats — breaks large fat droplets into small droplets. This increases the surface area for lipase to work on.
• Neutralises the acidic food coming from the stomach. Makes the environment alkaline for pancreatic enzymes.

What bile does NOT do:

• Bile contains NO digestive enzymes. [NDA tested via CDS]

Fat digestion in the intestine is done by bile and lipase together. Bile prepares the fat; lipase breaks it down. [NDA 2022-III]

Hormonal trigger for bile and pancreatic juice:

When acidic food enters the duodenum, the duodenal lining releases two hormones:

• Secretin — triggers the pancreas to release its juice.
• Cholecystokinin — triggers the gall bladder to release bile.
  [NDA tested via CDS]

Other Functions of the Liver

The liver does much more than produce bile. These functions are tested directly:

• Converts excess glucose into glycogen (storage form of sugar).
• Produces urea from breakdown of amino acids.
• Destroys old and worn-out red blood cells.
• Breaks down fat to produce cholesterol. [NDA 2011-I]

The liver does NOT absorb food and excess water. That is the function of the large intestine. This was listed as the “NOT a liver function” in NDA 2014. [NDA 2014-II]

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Absorption of Water

Water is absorbed in two places in the body:

• Renal tubules in the kidneys
• Large intestine

Both are correct. [NDA tested via CDS]

The liver does not absorb water. The pancreatic duct does not absorb water.

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Cellulose Digestion — Cattle and Rumen

Humans cannot digest cellulose. We have no enzyme for it.

Cattle can digest cellulose (present in grass). This is because their rumen (a part of their stomach) contains cellulose-degrading bacteria. The bacteria break down the cellulose for them. It is not a cattle-produced enzyme — it is microbial action. [NDA tested via CDS]

Special Facts — Tested Directly

Barium sulphate for X-ray:
To examine the alimentary canal by X-ray, a patient is given a barium sulphate meal. Barium is opaque to X-rays and outlines the canal. Not barium chloride (toxic). [NDA tested via CDS]

Fastest absorbed substance:
Raw alcohol is absorbed fastest through the digestive wall. It needs no digestion and passes directly from the stomach into the blood. [NDA tested via CDS]

Lactase and milk:
Lactose is the sugar in milk. The enzyme lactase breaks it down. Most adults lose lactase activity as they age — this causes lactose intolerance. Adults who can still digest milk continue to produce lactase. [NDA tested via CDS]

Extracellular digestion — fungi:
Yeast, mushroom, and bread mould digest food outside their bodies. They secrete enzymes into the surroundings and absorb the products. This is called extracellular digestion. [NDA tested via CDS]

Easiest protein to digest:
Egg albumin is the most easily digestible source of protein. [NDA tested via CDS]

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