AbsorptionCambridge IGCSE Biology: Revision notes
Section 1
Where Does Absorption Happen?
Absorption is the movement of digested food molecules from the gut into the blood.
- The small intestine is the main region where nutrients (such as glucose, amino acids, fatty acids and glycerol) are absorbed
- Absorbed nutrients pass through the wall of the small intestine into the bloodstream, to be transported around the body
Key termsabsorption
Section 2
Water Absorption
Water is absorbed at more than one point along the gut:
- Most water is absorbed from the small intestine
- Some water is also absorbed from the colon (part of the large intestine)
Key termscolon
Common mistake
Do not say water is only absorbed in the large intestine - the small intestine absorbs the majority of it.
Section 3
Villi and Microvilli: Increasing Surface Area
The small intestine is highly adapted for efficient absorption because of villi and microvilli.
- Villi are tiny finger-like projections lining the small intestine wall
- Each villus is covered in even smaller projections called microvilli
- Together, villi and microvilli greatly increase the internal surface area of the small intestine
- A larger surface area allows more nutrients to be absorbed more quickly
Key termsvillimicrovilli
Exam tip
Whenever a structure question mentions villi or microvilli, the answer almost always links back to 'increased surface area for absorption'.
Section 4
Structure of a Villus
Each villus is adapted for efficient absorption:
- A thin wall (single layer of cells) provides a short diffusion distance
- Capillaries inside each villus absorb digested sugars and amino acids into the blood
- Lacteals (part of the lymphatic system) inside each villus absorb digested fats (fatty acids and glycerol)
- A good blood supply maintains a steep concentration gradient, speeding up absorption
Key termscapillarylacteal
Example
Glucose and amino acids enter capillaries; fatty acids and glycerol enter lacteals - remember the split by product type.
Must Know
- Absorption is the movement of digested food molecules from the gut into the blood
- The small intestine is the main site of nutrient absorption
- Most water is absorbed in the small intestine; some in the colon
- Villi and microvilli greatly increase the internal surface area of the small intestine
- Capillaries in villi absorb sugars and amino acids; lacteals absorb fatty acids and glycerol
That's the notes covered.
Carry on to the next subtopic.