Human Biology

Digestion and Nutrition

5th Year · 6th Year (Leaving Cert)

✓By the end of this lesson students will be able to identify the main organs of the human digestive system and their associated secretions.
✓By the end of this lesson students will be able to describe the process of digestion for carbohydrates, proteins, and lipids, including the enzymes involved and their sites of action.
✓By the end of this lesson students will be able to explain the role of the small intestine in the absorption of digested nutrients.
✓By the end of this lesson students will be able to relate the structure of the villi to their function in absorption.

Key concepts

Organs and Secretions of the Digestive System

The human digestive system, also known as the alimentary canal, is a long tube extending from the mouth to the anus, with associated glands. Its primary function is to break down food into smaller molecules that can be absorbed and used by the body. This process involves both mechanical and chemical digestion. Key Organs and Secretions: * **Mouth:** Mechanical digestion (chewing) and chemical digestion begins. Salivary glands produce **saliva**, containing **salivary amylase** (for starch digestion) and mucus. * **Oesophagus:** Muscular tube connecting the mouth to the stomach. Food moves by **peristalsis** (wave-like muscular contractions). * **Stomach:** A muscular sac where food is churned (mechanical digestion) and mixed with **gastric juice**. Gastric juice contains **hydrochloric acid (HCl)** (denatures proteins, kills bacteria, activates pepsinogen), **pepsinogen** (inactive form of pepsin), and **mucus** (protects stomach lining). * **Small Intestine:** The primary site for chemical digestion and nutrient absorption. It is divided into the duodenum, jejunum, and ileum. Receives secretions from the pancreas and liver/gallbladder, as well as its own intestinal juice. * **Pancreas:** Produces **pancreatic juice** containing **pancreatic amylase** (starch), **lipase** (fats), and **trypsinogen** (inactive form of trypsin). * **Liver:** Produces **bile**, which is stored in the gallbladder. Bile emulsifies fats. * **Intestinal Glands:** Produce **intestinal juice** containing **peptidases** (proteins) and **disaccharidases** (carbohydrates). * **Large Intestine:** Primarily involved in water absorption and the formation of faeces. * **Rectum and Anus:** Storage and elimination of faeces.

Digestion of Carbohydrates

Carbohydrates are broken down into monosaccharides (simple sugars) for absorption. * **Mouth:** Salivary amylase begins the digestion of starch into maltose. Optimal pH is neutral (approx. 7). * Starch + Water --(Salivary Amylase)--> Maltose * **Stomach:** Salivary amylase is denatured and inactivated by the acidic pH (approx. 1.5-3.5) of the stomach. * **Small Intestine:** * **Pancreatic amylase** continues the breakdown of any remaining starch into maltose. Optimal pH is alkaline (approx. 7-8). * Starch + Water --(Pancreatic Amylase)--> Maltose * **Intestinal disaccharidases** (produced by the small intestine walls) break down disaccharides into monosaccharides: * **Maltase:** Maltose + Water --(Maltase)--> Glucose + Glucose * **Sucrase:** Sucrose + Water --(Sucrase)--> Glucose + Fructose * **Lactase:** Lactose + Water --(Lactase)--> Glucose + Galactose **End Products:** Glucose, Fructose, Galactose (monosaccharides).

Digestion of Proteins

Proteins are broken down into amino acids for absorption. * **Mouth:** No protein digestion occurs. * **Stomach:** Hydrochloric acid denatures proteins. Pepsinogen is activated by HCl into **pepsin**. Pepsin begins the digestion of proteins into smaller polypeptides. Optimal pH is acidic (approx. 1.5-3.5). * Proteins + Water --(Pepsin)--> Polypeptides * **Small Intestine:** * **Trypsinogen** (from the pancreas) is activated by enterokinase (an enzyme in the small intestine wall) into **trypsin**. Trypsin breaks down polypeptides into smaller peptides. Optimal pH is alkaline (approx. 7-8). * Polypeptides + Water --(Trypsin)--> Smaller Peptides * **Peptidases** (from the small intestine walls) break down smaller peptides into individual amino acids. * Smaller Peptides + Water --(Peptidases)--> Amino Acids **End Products:** Amino Acids.

Digestion of Lipids

Lipids (fats) are broken down into fatty acids and glycerol for absorption. * **Mouth/Stomach:** Very minor lipid digestion by lingual and gastric lipases, but not significant. * **Small Intestine:** This is the primary site for lipid digestion. * **Bile** (produced by the liver, stored in the gallbladder) is released into the duodenum. Bile does not contain enzymes but **emulsifies** fats, breaking large fat globules into smaller droplets. This increases the surface area for enzyme action. * **Pancreatic lipase** (from the pancreas) then digests the emulsified fats into fatty acids and glycerol. Optimal pH is alkaline (approx. 7-8). * Emulsified Fats + Water --(Pancreatic Lipase)--> Fatty Acids + Glycerol **End Products:** Fatty Acids and Glycerol.

Absorption in the Small Intestine

The small intestine is highly adapted for nutrient absorption, primarily in the jejunum and ileum. **Structural Adaptations for Absorption:** * **Length:** Very long (approx. 6 metres) providing a large surface area. * **Folds:** The inner lining has numerous circular folds. * **Villi:** The folds are covered with millions of tiny, finger-like projections called **villi** (singular: villus). Each villus is about 0.5-1.5 mm long. * **Microvilli:** The epithelial cells covering each villus have microscopic folds on their surface called **microvilli** (forming a 'brush border'). * These three features (folds, villi, microvilli) collectively increase the surface area for absorption by over 600 times. * **Thin Walls:** Each villus has a wall that is only one cell thick, allowing for rapid diffusion of nutrients. * **Rich Blood Supply:** Each villus contains a dense network of blood capillaries to transport absorbed nutrients away quickly, maintaining a steep concentration gradient. * **Lacteal:** Each villus also contains a central lymphatic vessel called a **lacteal**, which absorbs digested fats. **Mechanisms of Absorption:** * **Monosaccharides (glucose, fructose, galactose) and Amino Acids:** Absorbed into the blood capillaries of the villi by diffusion, facilitated diffusion, and active transport. They are then transported via the hepatic portal vein to the liver. * **Fatty Acids and Glycerol:** These diffuse into the epithelial cells of the villi. Inside the cells, they are re-esterified to form triglycerides, which are then packaged into tiny droplets called **chylomicrons**. Chylomicrons enter the lacteals and are transported via the lymphatic system, eventually entering the bloodstream. * **Water, Vitamins, and Minerals:** Absorbed throughout the small intestine, primarily by osmosis and active transport. **Role of the Large Intestine:** Primarily absorbs water and some vitamins, compacting indigestible material into faeces.

Key facts to remember

1Peristalsis is the wave-like muscular contraction that moves food through the alimentary canal.
2Hydrochloric acid in the stomach denatures proteins, kills bacteria, and activates pepsinogen to pepsin.
3Bile, produced by the liver, emulsifies fats in the small intestine, increasing the surface area for lipase action.
4Enzymes are specific in their action (e.g., amylase for starch, pepsin for protein, lipase for fat) and work optimally at specific pH levels.
5The small intestine's villi and microvilli dramatically increase its surface area, making it highly efficient for nutrient absorption.
6Monosaccharides (glucose, fructose, galactose) and amino acids are absorbed into the blood capillaries of the villi.
7Fatty acids and glycerol are absorbed into the lacteals (lymphatic capillaries) of the villi.
8Water is absorbed primarily in the large intestine.

Worked examples

Example 1

Describe the journey and chemical digestion of a starch molecule from the point of ingestion until its absorption into the bloodstream, naming the enzymes involved and their sites of action.

I**1. Mouth:** Upon ingestion, starch is mixed with saliva. Saliva contains **salivary amylase**, which begins the chemical digestion of starch into maltose. The pH is neutral (approx. 7).

II**2. Oesophagus:** The food (bolus) travels down the oesophagus by peristalsis. No chemical digestion of starch occurs here.

III**3. Stomach:** The acidic environment (pH 1.5-3.5) of the stomach denatures and inactivates salivary amylase, so no further starch digestion occurs here.

IV**4. Small Intestine (Duodenum/Jejunum):** As the chyme enters the duodenum, it mixes with pancreatic juice from the pancreas. **Pancreatic amylase** continues the breakdown of any remaining starch into maltose. The pH is alkaline (approx. 7-8).

V**5. Small Intestine (Jejunum/Ileum):** The walls of the small intestine produce **maltase**. Maltase breaks down maltose into two molecules of glucose. Other disaccharidases (sucrase, lactase) would break down sucrose and lactose into their respective monosaccharides.

VI**6. Absorption:** The resulting glucose molecules (monosaccharides) are then absorbed through the epithelial cells of the villi in the small intestine into the capillaries. This occurs via diffusion, facilitated diffusion, and active transport, depending on the concentration gradient. The glucose is then transported via the hepatic portal vein to the liver and subsequently to the general circulation.

Answer

The journey of a starch molecule involves initial breakdown in the mouth by salivary amylase, inactivation in the stomach, further breakdown in the small intestine by pancreatic amylase, and finally conversion to glucose by maltase before absorption into the bloodstream.

Remember to specify the enzyme, its substrate, its product, and the location/pH for each step of digestion.

Example 2

Explain how the structure of the small intestine is adapted for its primary function of absorption.

I**1. Length:** The small intestine is approximately 6 metres long, providing an extensive surface area over which absorption can occur.

II**2. Folds:** The inner lining of the small intestine has numerous circular folds, which further increase the surface area available for absorption.

III**3. Villi:** These folds are covered with millions of tiny, finger-like projections called villi. Each villus significantly increases the surface area, acting like a brush to maximise contact with digested food.

IV**4. Microvilli:** The epithelial cells covering each villus have microscopic folds on their surface called microvilli (forming a 'brush border'). These further amplify the surface area, allowing for efficient absorption.

V**5. Thin Walls:** Each villus has a wall that is only one cell thick. This provides a very short diffusion distance for nutrients to pass from the lumen of the intestine into the bloodstream or lymphatic system.

VI**6. Rich Blood Supply:** Each villus contains a dense network of blood capillaries. This ensures that absorbed nutrients (like glucose and amino acids) are quickly transported away, maintaining a steep concentration gradient for continuous absorption.

VII**7. Lacteal:** Each villus also contains a central lymphatic vessel called a lacteal. This vessel is specifically adapted for the absorption of digested fats (fatty acids and glycerol), which are then transported via the lymphatic system.

Answer

The small intestine's structure is highly adapted for absorption through its extensive length, circular folds, villi, and microvilli, all of which vastly increase the surface area. Its thin, one-cell-thick walls, rich blood supply, and the presence of lacteals ensure efficient and rapid transport of digested nutrients into the circulatory and lymphatic systems.

It's crucial to link each structural adaptation directly to how it aids the function of absorption.

Example 3

Outline the role of bile in lipid digestion and describe the subsequent chemical breakdown of lipids, naming the enzyme involved.

I**1. Role of Bile:** Bile is produced by the liver and stored in the gallbladder. It is released into the duodenum of the small intestine. Bile does not contain digestive enzymes. Its primary role is to **emulsify** fats. This means it breaks down large fat globules into smaller fat droplets, increasing their surface area. This physical process is crucial because it allows the fat-digesting enzyme to work more efficiently.

II**2. Chemical Breakdown:** Once fats are emulsified, **pancreatic lipase**, an enzyme secreted by the pancreas into the small intestine, acts on these smaller fat droplets. Pancreatic lipase hydrolyses the triglycerides (fats) into their constituent parts: **fatty acids and glycerol**.

III**3. Site of Action:** Both the emulsification by bile and the enzymatic digestion by pancreatic lipase occur in the **small intestine** (duodenum).

Answer

Bile, produced by the liver and stored in the gallbladder, emulsifies large fat globules into smaller droplets in the small intestine, increasing their surface area. Subsequently, pancreatic lipase, secreted by the pancreas, chemically digests these emulsified fats into fatty acids and glycerol in the small intestine.

Distinguish clearly between emulsification (physical breakdown by bile) and chemical digestion (by lipase).

Common mistakes

✗Confusing emulsification (physical breakdown by bile) with chemical digestion (enzymatic breakdown).
✗Incorrectly stating the site of production or action for specific enzymes (e.g., saying pepsin is produced in the small intestine).
✗Forgetting the role of bile in fat digestion or incorrectly stating that bile contains enzymes.
✗Not linking the structural adaptations of the small intestine (e.g., villi, microvilli) directly to their functional advantage in absorption.
✗Assuming all digestion occurs in the stomach, overlooking the crucial role of the small intestine.
✗Incorrectly identifying the end products of digestion for each food group (e.g., saying starch breaks down into maltose for absorption).

Exam tips

★Learn the names of key enzymes, their substrates, the products they form, and their optimal pH and site of action for each food group.
★Be able to draw and label a diagram of a villus, clearly indicating the blood capillaries and lacteal, and explain how its features aid absorption.
★Use correct biological terminology (e.g., 'peristalsis', 'emulsification', 'hydrolysis', 'denaturation') in your answers.
★Practice tracing the digestion and absorption of each major food group (carbohydrates, proteins, lipids) from ingestion to absorption, step-by-step.

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