Biology (9610) | Digestion and abs

Digestion and Absorption: Fueling the Biological Machine (9610)

Hello Biologists! Welcome to the crucial chapter on Digestion and Absorption. This topic sits within the "Biological systems" section of your syllabus, and it’s all about how your body takes complex food molecules and breaks them down into tiny pieces that can be absorbed and used for energy, growth, and repair.

Think of your digestive system as the ultimate disassembly and logistics factory. Understanding its structure and the specific enzymes involved is vital, as it links directly back to the structure of biological molecules (Unit 1) and sets the stage for mass transport (how nutrients move around the body). Don't worry, we'll break down the chemistry step-by-step!

3.2.2.1 The Gross Structure of the Human Digestive System

The digestive system is essentially a long tube, known as the alimentary canal, running from the mouth to the anus, with various accessory organs dumping in essential chemicals.

The Alimentary Canal (The Main Tube)

Here are the main parts in order:

1. Oesophagus: A muscular tube that uses waves of contraction (peristalsis) to push food from the mouth to the stomach.

2. Stomach: A muscular bag that churns food, mixing it with strong acids (low pH) and protein-digesting enzymes.

3. Duodenum: The very first part of the small intestine. This is where most of the heavy-duty enzyme action takes place, as it receives secretions from the pancreas.

4. Ileum: The longest section of the small intestine. Its primary role is absorption of the digested end products (monomers).

5. Colon (Large Intestine): Absorbs water and minerals from the remaining indigestible material.

6. Rectum: Stores faeces before egestion (elimination of undigested waste).

Associated Glands (The Chemical Factories)

These glands produce and secrete substances (like enzymes and bile) into the alimentary canal:

• Salivary Glands: Found in the mouth, they produce saliva containing amylase to start carbohydrate digestion.
• Pancreas: This is the superstar factory. It produces and secretes large volumes of digestive enzymes (amylase, lipase, and proteases) into the duodenum.

Key Definitions

• Digestion: The process where large, insoluble molecules are broken down into small, soluble molecules by the addition of water (hydrolysis), a reaction catalysed by enzymes.
• Absorption: The movement of digested small molecules (like glucose and amino acids) from the lumen of the intestine, across the epithelial cells, and into the blood or lymph system.
• Assimilation: The process by which absorbed molecules are incorporated into the body's tissues or used for energy.

Key Takeaway: The digestive system is a specialised tube supported by glands (salivary, pancreas) designed to hydrolyse large polymers into absorbable monomers.

3.2.2.2 Digestion: The Role of Enzymes and Bile

All digestive processes involve hydrolysis—the breaking of chemical bonds using a water molecule. Since digestion must happen quickly at body temperature, specific enzymes are essential catalysts (remember Unit 1: Enzymes!).

1. Carbohydrate Digestion

Carbohydrates (like starch, a polysaccharide made of $\alpha$-glucose) are broken down in two main stages:

1. Amylases (Salivary and Pancreatic): These enzymes hydrolyse the glycosidic bonds within large starch molecules, breaking them down into smaller disaccharides, primarily maltose.
2. Membrane-bound Disaccharidases: These enzymes are embedded in the cell membranes of the ileum epithelial cells. They break down disaccharides (like maltose) into their final absorbable form: monosaccharides (like glucose).

2. Protein Digestion

Proteins are huge chains of amino acids held together by peptide bonds. They require a sequence of three different enzyme types to break them fully down into single amino acids:

1. Endopeptidases: Hydrolyse peptide bonds within the central region of the polypeptide chain, chopping the large protein into smaller peptides. (Think of them as cutting the middle of a ribbon).
2. Exopeptidases: Hydrolyse peptide bonds at the ends of the peptide chains, working inward to release single amino acids.
3. Membrane-bound Dipeptidases: These are located on the ileum cell surface membrane. They break the final dipeptide bonds into single amino acids, ready for absorption.

Memory Aid: P.E.D. (Protein $\rightarrow$ Endopeptidase $\rightarrow$ Exopeptidase $\rightarrow$ Dipeptidase).

3. Lipid Digestion

Lipids (fats and oils, mainly triglycerides) are non-polar and hydrophobic (don't mix with water), which makes digestion complicated.

• Bile Salts (The Emulsifier): Produced by the liver and stored in the gall bladder, bile salts do not contain enzymes. Their role is mechanical: they break large lipid droplets into tiny droplets called micelles. This process, called emulsification, massively increases the total surface area for the lipase enzymes to act upon.

• Lipases (Pancreatic): Hydrolyse the ester bonds in triglycerides, breaking them down into monoglycerides and fatty acids.

Key Takeaway: Digestion is sequential hydrolysis: amylase breaks starch, proteases (endo, exo, di) break protein, and lipase breaks lipids. Bile salts are essential helpers for lipids, increasing the surface area via emulsification.

3.2.2.3 Absorption: Getting Nutrients into the Bloodstream

Once broken down into monomers (monosaccharides, amino acids, monoglycerides, and fatty acids), these molecules must cross the intestinal lining (epithelium) of the ileum.

The Ileum: Structure for Absorption (Histology)

The ileum is perfectly adapted for maximum absorption:

• Large Surface Area: The inner wall is highly folded into finger-like projections called villi.
• Microvilli: The epithelial cells lining the villi have further tiny folds on their own surface membranes called microvilli, creating a "brush border." This dramatically increases the surface area for exchange.
• Short Diffusion Distance: The walls are only one cell thick.
• Blood/Lymph Supply: Each villus contains a dense capillary network and a central lymph vessel called a lacteal, ensuring rapid transport away from the intestine, maintaining a steep concentration gradient.

Absorption of Monosaccharides (e.g., Glucose) and Amino Acids

Glucose and amino acids are absorbed primarily using a transport mechanism that involves sodium ions: Co-transport. This is an example of secondary active transport.

Step-by-Step Glucose/Amino Acid Absorption:

1. Setting up the Gradient (Active Transport): Sodium ions (Na+) are actively pumped out of the epithelial cell and into the blood capillary by a carrier protein, requiring ATP. This creates a very low concentration of Na+ inside the cell compared to the intestine lumen.
2. Co-transport (Facilitated Diffusion): Na+ ions diffuse back into the epithelial cell, down their concentration gradient. They do this via a co-transport protein (a special carrier protein) that simultaneously carries a glucose molecule (or amino acid) *with* the Na+ ion.
3. Movement to Blood (Facilitated Diffusion): Once inside the epithelial cell, the concentration of glucose/amino acids rises. They then move out of the cell, across the opposite membrane, and into the blood capillary via specific carrier proteins (facilitated diffusion).

Did you know? Because the Na+ gradient was established using ATP (active transport), the absorption of glucose/amino acids is indirectly powered by active transport.

Absorption of Lipids (Monoglycerides and Fatty Acids)

Lipid products follow a unique pathway because they are non-polar.

1. Micelle Formation: Monoglycerides and fatty acids remain associated with bile salts, forming small droplets called micelles. These micelles ferry the lipid products towards the epithelial cell surface.
2. Diffusion: When the micelle reaches the brush border, the monoglycerides and fatty acids are released. Since they are small and lipid-soluble (non-polar), they simply diffuse directly across the cell surface membrane into the epithelial cell. This is passive movement (diffusion).
3. Reformation: Inside the epithelial cell, the fatty acids and monoglycerides are quickly recombined to form original triglycerides (fats).
4. Chylomicron Formation: These triglycerides are then packaged with proteins, cholesterol, and phospholipids inside the cell to form larger particles called chylomicrons.
5. Transport via Lymph: Chylomicrons are too large to enter the small blood capillaries. Instead, they leave the epithelial cell via exocytosis and enter the lacteal (the lymph vessel) found in the centre of the villus, eventually reaching the blood circulation via the lymphatic system.

Quick Review Box: Absorption Mechanisms
- Monosaccharides/Amino Acids: Co-transport with Na+ (indirectly active transport). Go into blood capillaries.
- Lipids: Passive diffusion into cell, packaged as Chylomicrons, enter the lacteal (lymph).