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2.7.5Unit 2 — Multicellular Organisms

Nutrient Absorption — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 5 parts Lesson 39 of 39

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Part 1 of 5The Small Intestine

2.7.5 Nutrient Absorption
The Small Intestine
Position
  • The small intestine is a long tube in the abdomen between the stomach and the large intestine.
Digestion
  • Enzymes in the small intestine complete the digestion of food into small soluble molecules.
Absorption
  • Absorption is the movement of small digested food molecules from the digestive system into the blood.
Methods
  • Small soluble molecules are absorbed by diffusion and active transport.

Part 2 of 5Surface Area of the Intestine

1
The small intestine is very long, which gives a large internal area for the absorption of nutrients.
2
Its inner surface is lined with millions of finger-like projections called villi.
3
Each villus has a surface covered with microvilli, which add even more area for absorption.
4
The collective surface area is therefore very large, so more food molecules can be absorbed at the same time.

Part 3 of 5Adaptations of a Villus

Large area
  • There are many villi, and microvilli on each villus further increase the area available for absorption.
Short distance
  • The wall of a villus is only one cell thick, so absorbed molecules have a very short distance to travel.
Capillaries
  • A network of blood capillaries carries glucose and amino acids away, which keeps the concentration gradient steep.
Lacteal
  • A lacteal in the centre of each villus carries away fatty acids and glycerol, which also maintains a steep gradient.
Product of digestionWhere it goes
GlucoseBlood capillaries
Amino acidsBlood capillaries
Fatty acids and glycerolLacteal

Part 4 of 5Linking to Transport

Linking to Transport
To the cells
  • Absorbed glucose and amino acids are carried in the blood plasma to respiring cells and to growing tissues.
Concentration gradient
  • Removing absorbed molecules quickly means more can diffuse in from the intestine.
Active transport
  • Active transport allows absorption even when the concentration of nutrients in the intestine is lower than in the cells.
Exam wording
  • Say thin wall or one cell thick, and always link each villus feature to a faster rate of absorption.

Part 5 of 5Exam Checklist & Tips

2.7.5 – Key Points for the Exam

  • Villi Finger-like projections lining the small intestine that absorb digested food.
  • Microvilli Tiny projections on the surface of villus cells that add surface area.
  • Lacteal A vessel in a villus that transports fatty acids and glycerol.
  • Absorption Movement of digested food molecules into the blood.
  • Active transport Movement of substances against a concentration gradient using energy.

Common Mistakes

  • Saying that digestion takes place in the villi; digestion is by enzymes and the villi absorb.
  • Stating that fatty acids go into the blood capillaries, when they pass into the lacteal.
  • Describing villi as having thick walls to protect the lining.
  • Forgetting that capillaries maintain the concentration gradient by removing absorbed nutrients.

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