Absorption — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 20Absorption of nutrients

Absorption

• The movement of digested food molecules from the digestive system into the blood and lymph.
• Glucose and amino acids go into the blood.
• Fatty acids and glycerol go into the lymph.

Site

• Nutrients are absorbed in the small intestine.

Assimilation

• Digested molecules are taken up by cells and used for growth, repair and energy production.

Section 2 of 20Absorption of water

Small intestine

• Most water absorption, around 80%, happens here.

Colon

• Water is also absorbed in the colon, leaving solid waste that forms faeces.

Section 3 of 20Surface area of the ileum (Supplement)

Length and folds

• The ileum is very long and has a highly folded surface.

Villi

• Millions of tiny finger-like projections cover the lining.

Microvilli

• Folds on the surface of villus cells increase surface area further.

Effect

• A large surface area makes absorption faster and more efficient.

Section 4 of 20Structure of a villus (Supplement)

Structure of a villus (Supplement)

Thin wall

• The wall of the villus is one cell thick, giving a short distance for diffusion and active transport.

Capillary network

• Blood capillaries carry glucose and amino acids away in the blood.

Lacteal

• A lymph vessel through the centre of the villus carries fatty acids and glycerol away in the lymph.

Section 5 of 20Adaptations compared

Adaptation	Effect on absorption
Long, folded ileum with villi	Increases surface area
Microvilli	Increase surface area further
Wall one cell thick	Shortens the diffusion distance
Capillary network	Removes glucose and amino acids, keeping a steep gradient
Lacteal	Removes fatty acids and glycerol

Section 6 of 20More from the specification

1

Nutritional support for physiological organs

• Nutritional support for physiological organs balanced dietary intake of macronutrients, vitamins,
  • and minerals ensures healthy metabolic function across all body organs.

2

Sequence of Digestion Prior to Absorption

• Mechanical mastication in the mouth and acidic proteolysis in the stomach convert food into chyme, followed by comprehensive alkaline enzymatic hydrolysis in the duodenum before monomer absorption.

3

Lipid Emulsification and Micelle Formation

• Bile produced by hepatocytes and concentrated in the gallbladder emulsifies fats into micro-droplets in the duodenum, enabling rapid lipase access and fatty acid uptake into intestinal lacteals.

4

Macromolecule Cleavage to Absorbable Units

• Salivary and pancreatic amylases break down starch into maltose and glucose,
• while endopeptidases and exopeptidases degrade dietary proteins into amino acids for active transport across the villus epithelium.

5

Scurvy

• A deficiency disease caused by lack of vitamin C.

6

Chemical digestion

• Breakdown of large insoluble molecules into small soluble molecules by enzymes.

7

Physical digestion

• Breaking food into smaller pieces without chemical change.

8

Dietary fibre

• Indigestible plant material giving bulk to the intestine contents.

9

Enzyme

• A protein that functions as a biological catalyst.

10

Vitamin

• A nutrient needed in small amounts to stay healthy.

11

Duodenum

• First part of the small intestine.

12

Starch

• An insoluble polysaccharide of glucose used by plants for energy storage.

13

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

14

Emulsification

• Breaking large fat drops into smaller ones by bile.

15

Organ

• A structure of different tissues working together for specific functions.

16

Pancreas

• Gland producing insulin and glucagon.

Section 7 of 20Absorption and assimilation

Absorption

Absorption movement of small soluble digested molecules, water, vitamins and mineral ions from the alimentary canal into the blood or lymph.

Blood and lymph

• Blood and lymph glucose and amino acids enter the blood; fatty acids and glycerol enter the lymph.

Site

• Site most absorption occurs in the ileum, the last part of the small intestine.

Assimilation

Assimilation the use of absorbed molecules by body cells for growth, repair and energy.

Why molecules can cross

• Why molecules can cross digestion has already broken starch to glucose, protein to amino acids and fat to fatty acids and glycerol.

Section 8 of 20Structure of a villus

Structure of a villus

Epithelium

• Epithelium one cell thick, giving a very short diffusion distance.
• Each villus is about 1 mm long and the ileum has millions of them.

Microvilli

• Microvilli on the surface cells, further increasing surface area.

Capillary network

• Capillary network carries glucose and amino acids away, keeping the concentration gradient steep.

Lacteal

• Lacteal a lymph vessel in the centre that absorbs fatty acids and glycerol.

Mitochondria

• Mitochondria many in the epithelial cells to supply energy for active transport.

Section 9 of 20How each substance is absorbed

Substance	Mechanism	Route
Glucose	Diffusion, and active transport when gut concentration is lower than blood	Capillaries, then hepatic portal vein to the liver
Amino acids	Diffusion and active transport	Capillaries, then hepatic portal vein
Fatty acids and glycerol	Diffusion	Lacteal (lymph), then into the blood
Water	Osmosis	Capillaries; about 80% in the small intestine, the rest in the colon
Mineral ions and vitamins	Active transport and diffusion	Capillaries

Section 10 of 20Glucose absorption and the concentration gradient

Early in digestion

• Early in digestion gut glucose concentration is high, so glucose diffuses into villus cells.

Later

• Later gut concentration falls below that of the blood, so glucose must be absorbed by active transport.

Gradient

• Gradient blood flowing through capillaries constantly carries glucose and amino acids away, keeping the concentration in the villus low.

Lymph

• Lymph the lacteal drains into a vein in the neck, and fat is carried in the blood to be used for energy or stored in adipose tissue.

Section 11 of 20Liver, large intestine and egestion

Hepatic portal vein

• Hepatic portal vein carries blood from the ileum to the liver, which regulates the level of nutrients.
• Excess glucose is stored as glycogen and amino acids are processed.

Alcohol and toxins

• Alcohol and toxins the liver is the first organ to meet absorbed toxins.

Large intestine

• Large intestine undigested material such as fibre moves in and water is absorbed.

Faeces

• Faeces the remaining solid waste is stored in the rectum and egested through the anus.

Section 12 of 20Absorption in numbers

Length

• Length the small intestine of an adult is about 6–7 m long;
  • villi and microvilli increase the absorbing area to about the size of a tennis court.

Fluid

• Fluid about nine litres enter the gut each day, including drinks and secretions such as saliva, gastric juice and pancreatic juice.

Water absorption

• Water absorption nearly all is absorbed, most in the small intestine by osmosis and the rest in the colon, so little leaves in faeces.

Section 13 of 20Cholera and failure of water absorption

Cause

• Cause a bacterium releases a toxin in the intestine that makes cells secrete chloride ions.

Effect

• Effect water follows by osmosis, giving large volumes of watery diarrhoea, dehydration and loss of mineral ions.

Treatment

• Treatment oral rehydration therapy, a drink of water, glucose and salts, because glucose helps co-transport sodium ions and water into cells.

Section 14 of 20Where absorption occurs

Absorption

Digestion breaks large food molecules into small soluble molecules. Absorption is the movement of these digested molecules across the wall of the small intestine into blood or lymph. Much absorption takes place in the ileum.

Villi

The ileum is long and its inner surface is folded into millions of finger-like villi.

• Each villus has even smaller projections, microvilli, on its surface cells.
• Together these features provide a very large surface area, allowing more nutrient molecules to cross at once.

Section 15 of 20Fast transfer through a villus

One cell thick

The epithelial wall of a villus is only one cell thick, so dissolved nutrients travel a short distance from the intestine into the transport vessels. A shorter diffusion distance increases the speed of transfer.

Glucose and amino acids

A dense network of blood capillaries carries absorbed glucose and amino acids away.

• Continuous removal helps maintain a concentration gradient between the intestine and blood.
• Some molecules cross by diffusion;
  • others can be taken up by active transport when energy is supplied by the cells.

Section 16 of 20Absorbing products of fat digestion

Lacteal

A lacteal runs through the centre of each villus. It is a lymph vessel that transports the products of fat digestion, including fatty acids and glycerol, away from the small intestine through the lymphatic system.

For an exam comparison, keep the two routes distinct: glucose and amino acids enter blood capillaries, while fat digestion products enter the lacteal. Both routes move products away from the absorption surface.

Section 17 of 20Structure linked to function

Structure linked to function

A complete answer pairs each feature with its effect: villi and microvilli → large surface area; thin epithelium → short transfer distance; capillaries → fast removal of water-soluble nutrients; lacteal → transport of fat digestion products.

Check:

Check Why would fewer villi reduce nutrient absorption? Answer: The total surface area would be smaller, so fewer digested molecules could cross the intestinal wall in a given time.

Section 18 of 20More from the specification

More from the specification

Metabolic demands of internal organs

• Metabolic demands of internal organs balanced dietary intake delivers essential amino acids, glucose,
  • and trace cofactors necessary to maintain healthy organ tissues.

Chemical digestion in the stomach and duodenum

• Pepsin in acidic gastric juice hydrolyses proteins into polypeptides; pancreatic enzymes continue digestion in the neutralised duodenum.
  • (amylase, trypsin, lipase)

Bile function in lipid absorption

• Bile salts produced by the liver and stored in the gallbladder emulsify large lipid droplets into micro-emulsions,
• increasing surface area for pancreatic lipase action.

Villi adaptations for nutrient absorption

• Microvilli on epithelial cells provide an extensive surface area with short diffusion distances into blood capillaries and central lacteals.

Mechanical versus chemical digestion

• Mechanical versus chemical digestion teeth break food physically while digestive enzymes in the stomach and duodenum chemically hydrolyse starch and protein polymers.

Dietary vitamin and mineral requirements

• Dietary vitamin and mineral requirements essential vitamins plus calcium build healthy teeth,
  • while growing children require elevated dietary protein for tissue synthesis.

Bile emulsification mechanism

• Bile emulsification mechanism bile salts tear fat globules into tiny droplets, providing an extended surface area for pancreatic lipase enzymes.

Scurvy

• A deficiency disease caused by lack of vitamin C.

Adaptation

• The process by which populations become better suited to their environment.

Growth

• A permanent increase in size and dry mass.

Chemical digestion

• Breakdown of large insoluble molecules into small soluble molecules by enzymes.

Physical digestion

• Breaking food into smaller pieces without chemical change.

Dietary fibre

• Indigestible plant material giving bulk to the intestine contents.

Enzyme

• A protein that functions as a biological catalyst.

Vitamin

• A nutrient needed in small amounts to stay healthy.

Duodenum

• First part of the small intestine.

Starch

• An insoluble polysaccharide of glucose used by plants for energy storage.

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Emulsification

• Breaking large fat drops into smaller ones by bile.

Organ

• A structure of different tissues working together for specific functions.

Pancreas

• Gland producing insulin and glucagon.

Section 19 of 20Key terms

Term	Meaning
Absorption	Movement of digested food molecules into the blood or lymph.
Assimilation	Uptake of digested molecules by cells for growth, repair and energy.
Ileum	Part of the small intestine where most absorption occurs.
Villus	A finger-like projection of the ileum lining.
Microvilli	Tiny folds on the surface of villus cells.
Lacteal	A lymph vessel in the centre of a villus.

Term	Meaning
Absorption	Movement of digested food, water and ions into the blood from the gut.
Villus	Finger-like fold of the ileum wall that increases surface area.
Lacteal	A lymph vessel in a villus that absorbs fatty acids and glycerol.
Hepatic portal vein	Vessel carrying absorbed nutrients from the gut to the liver.
Assimilation	Use of absorbed nutrients by body cells.
Microvilli	Tiny projections on the surface of epithelial cells.

Section 20 of 20Exam technique

1

Saying fatty acids go into the blood capillaries — fatty acids and glycerol are absorbed into the lacteal.

2

Saying most water is absorbed in the colon — most is absorbed in the small intestine.

3

Confusing absorption with assimilation — absorption moves nutrients into the blood, assimilation uses them in cells.

4

Forgetting microvilli — they increase surface area on top of the villi.

1

Saying fats enter the blood capillaries directly — fatty acids and glycerol enter the lacteal first.

2

Mixing up the large intestine (water absorption) with the ileum (nutrient absorption).

3

Saying absorption and assimilation are the same — assimilation is the use of nutrients by cells.

4

Describing diffusion as needing energy — it is passive.