IB MYP 4-5 Biology-Digestion- Study Notes - New Syllabus
IB MYP 4-5 Biology-Digestion- Study Notes – New syllabus
IB MYP 4-5 Biology-Digestion- Study Notes – IB MYP 4-5 Biology – per latest IB MYP Biology Syllabus.
Key Concepts:
- Human digestive system structure
- Enzymatic breakdown of food molecules
- Absorption in small intestine
Digestion – Human Digestive System Structure
What is Digestion?
Components of the Digestive System
1. Digestive Tract (Alimentary Canal)
- Mouth: Chewing and salivary enzymes begin digestion.
- Pharynx: Shared food-air passage leads to esophagus.
- Esophagus: Pushes food to stomach via peristalsis.
- Stomach: Mixes food with enzymes and HCl to digest proteins.
- Small Intestine: Final digestion and nutrient absorption, lined with villi.
- Large Intestine: Absorbs water/minerals, forms feces.
- Rectum & Anus: Eliminate undigested waste.
2. Accessory Organs
- Salivary Glands: Secrete amylase for starch digestion.
- Liver: Produces bile and detoxifies blood.
- Gall Bladder: Stores and releases bile.
- Pancreas: Produces digestive enzymes and insulin. (In cystic fibrosis, thick mucus blocks the pancreatic duct, preventing digestive enzymes from reaching the small intestine, leading to poor absorption of nutrients and stunted growth.)
Digestive Processes
- Ingestion: Intake of food into mouth.
- Mechanical Digestion: Chewing and stomach churning.
- Chemical Digestion: Enzymatic breakdown of food.
- Absorption: Nutrient uptake into blood/lymph.
- Assimilation: Use of absorbed nutrients by cells.
- Egestion: Removal of undigested waste.
Specialized Structures and Functions
Mouth and Oral Cavity
- Teeth grind food, tongue forms bolus.
- Saliva contains amylase to digest starch.
- Epiglottis prevents food entering windpipe.
Esophagus
- Connects pharynx to stomach.
- Peristalsis moves food downward.
Stomach
- Secretes HCl and pepsin to digest proteins.
- Churns food into chyme.
- Has acidic pH (1.5–2) ideal for proteases.
Small Intestine
- Includes duodenum, jejunum, ileum.
- Bile emulsifies fats and neutralizes acid.
- Pancreatic and intestinal juices complete digestion.
- Villi and microvilli absorb nutrients into blood/lymph.
Large Intestine
- Absorbs water and salts.
- Hosts beneficial bacteria (e.g., Vitamin K production).
- Forms and stores feces.
Enzymes in Digestion
- Amylase: Breaks starch into sugars.
- Protease (pepsin, trypsin): Breaks proteins into amino acids.
- Lipase: Breaks fats into fatty acids and glycerol.
- Enzymes work best at specific pH and temperature ranges.
Summary Table: Digestive Organs and Functions
| Organ | Function |
|---|---|
| Mouth | Mechanical digestion, starch breakdown (amylase) |
| Esophagus | Moves food to stomach via peristalsis |
| Stomach | Protein digestion using pepsin and HCl |
| Small Intestine | Completes digestion and absorbs nutrients via villi |
| Large Intestine | Absorbs water, forms feces, houses bacteria |
| Liver | Produces bile, detoxifies blood |
| Gall Bladder | Stores and releases bile |
| Pancreas | Secretes digestive enzymes and insulin |
Enzymatic Breakdown of Food Molecules
What Are Enzymes?
Enzymes are biological catalysts made of proteins. They speed up chemical reactions without being used up. In digestion, enzymes break down large food molecules into smaller, absorbable forms.
Proteases digest proteins
Lipases digest fats
Carbohydrase’s (like amylase) digest carbohydrates
Enzymes need specific pH and temperature to work. Unfavorable conditions can slow or stop their activity.
Chemical Digestion of Carbohydrates
- Carbohydrates, such as starch, are broken down into glucose, a simple sugar your body uses for energy.
- The digestion begins in the mouth, where salivary amylase starts breaking down starch into maltose.
- As food enters the small intestine, enzymes like pancreatic amylase continue the breakdown. Finally, maltase converts maltose into glucose, which is then absorbed into the bloodstream.
Types of Digestive Enzymes and Their Actions
| Enzyme | Produced By | Acts on | End Products | Optimum pH |
|---|---|---|---|---|
| Amylase | Salivary glands, pancreas | Starch | Maltose → Glucose | 7 (saliva), 8 (intestine) |
| Protease | Stomach (pepsin), pancreas (trypsin) | Proteins | Amino acids | 2 (pepsin), 8 (trypsin) |
| Lipase | Pancreas | Fats | Fatty acids + Glycerol | 8 |
| Maltase | Small intestine | Maltose | Glucose | 8 |
| Lactase | Small intestine | Lactose | Glucose + Galactose | 6–7 |
| Sucrase | Small intestine | Sucrose | Glucose + Fructose | 6–7 |
Genetics of Enzyme Activity – AMY1 Gene
There’s a gene called AMY1 that tells your body how much salivary amylase to produce. It’s located on chromosome 1.
- Most people inherit two copies of this gene (one from each parent).
- Some people inherit extra copies due to tandem repeats – repeating chunks of DNA.
Why does this matter?
If someone has more copies of AMY1, they make more amylase, which helps them digest starch faster. People from regions with high-starch diets often have more AMY1 copies. This is a great example of gene-diet adaptation.
Where Enzyme Action Occurs
- Mouth: Salivary amylase begins starch digestion (neutral pH ~7)
- Stomach: Pepsin digests proteins in acidic conditions (pH ~2)
- Small Intestine: Pancreatic and intestinal enzymes complete digestion
- Bile: Emulsifies fats to help lipase work more efficiently
Summary of the Breakdown Process
| Food Type | Initial Breakdown | Enzyme(s) Involved | Final Products |
|---|---|---|---|
| Carbohydrates | Starch → Maltose | Amylase | Glucose |
| Proteins | Protein → Peptides | Pepsin, Trypsin | Amino acids |
| Fats | Fat globules → droplets | Bile (emulsification), Lipase | Fatty acids + Glycerol |
| Disaccharides | Maltose, Sucrose, Lactose | Maltase, Sucrase, Lactase | Simple sugars (monosaccharides) |
Key Features of Enzymes in Digestion
- Specificity: Each enzyme works on only one substrate
- Reusability: Enzymes are not used up in the reaction
- Optimal Conditions: Each enzyme needs a specific temperature and pH
- Denaturation: Extreme conditions can permanently deactivate enzymes
Why Enzymatic Digestion Is Important
Without enzymes, digestion would be too slow to sustain life. Enzymes allow the digestive system to:
- Break food down quickly and efficiently
- Operate at body temperature (~37°C)
- Ensure nutrients are absorbable and usable by body cells
Absorption in the Small Intestine
What Is Absorption?
Where Does Absorption Take Place?
Most absorption occurs in the small intestine, which is divided into:
- Duodenum: Primarily digestion
- Jejunum: Mainly absorption
- Ileum: Mainly absorption
Its 6–7 meter length gives it an extensive internal surface area ideal for nutrient uptake.
How Is the Small Intestine Perfect for Absorption?
The small intestine is the main site of absorption, and it’s built for this job. Here’s how:
- It’s very long, giving lots of space for absorption.
- It has villi and microvilli – tiny finger-like folds that massively increase surface area, so more nutrients can be absorbed.
- The lining is just one cell thick, which makes it easier for nutrients to move through.
- It has a rich blood supply, so absorbed nutrients are quickly carried away.
- It also contains lacteals, which absorb fats.
- Lastly, it keeps things moving with peristalsis, so food touches as much surface as possible.
All these features work together to make sure your body gets the most out of every bite of food.
What Is an Ileostomy Bag and Why Is It Needed?
Sometimes, part of the intestine is removed or bypassed due to disease. In such cases, a surgeon creates a small opening (called a stoma) and connects the end of the small intestine to a bag outside the body – called an ileostomy bag.
- This bag collects waste directly from the small intestine.
- Since it skips the large intestine, which usually reabsorbs water, people may lose more fluids.
- That’s why doctors recommend a low-fiber diet and extra hydration after this surgery.
- Urine output might increase slightly too, as the body tries to balance water loss.
What Is Absorbed in the Small Intestine?
| Nutrient | Absorbed as | Absorbed Into |
|---|---|---|
| Carbohydrates | Glucose | Blood Capillaries |
| Proteins | Amino Acids | Blood Capillaries |
| Lipids | Fatty acids + Glycerol | Lacteals (Lymph) |
| Vitamins | Soluble Forms | Blood |
| Minerals | Ions (e.g., Na⁺, Ca²⁺) | Blood |
| Water | H₂O | Blood |
Mechanisms of Absorption
- Diffusion: Passive movement from high to low concentration (e.g., glucose when blood is low).
- Active Transport: Requires energy; moves nutrients like glucose against concentration gradient.
- Osmosis: Water absorption through a semipermeable membrane.
- Facilitated Diffusion: Uses membrane proteins to transport certain molecules like fructose.
Role of Villi and Microvilli
- Capillaries: Absorb water-soluble nutrients like glucose and amino acids
- Lacteals: Absorb fat-soluble nutrients like fatty acids and glycerol
Microvilli on each epithelial cell further increase surface area and house enzymes that finish digestion at the cell surface.
How Water Is Handled During Digestion
Water is constantly moving in and out of your body – even during digestion.
| Process | What Happens |
|---|---|
| Saliva and digestive juice secretion | Water is added to help break down food |
| Absorption in intestines | Water is reabsorbed into the blood |
| Sweating, breathing out | Water is lost |
| Cell respiration | Water is actually produced inside cells |
| Urination | Water is lost as waste |
Summary: Why the Small Intestine Is Ideal for Absorption
- Huge surface area from villi and microvilli
- Thin epithelium for rapid diffusion
- Rich blood and lymphatic supply
- Multiple absorption mechanisms (diffusion, active transport, osmosis)