Digestion & absorption 6.1

This subsection looks at the human digestive system, its parts and functions are in this topic. This includes digestive enzymes, their optimum conditions, substrates and products, as well as detailed structure of villi and microvilli in the small intestine. 

Key concepts

Learn and test your biological vocabulary for 6.1 digestion and absorption using these flashcards.


These slides summarise the essential understanding and skills in this topic. 
They contain short explanations in text and images - good revision for all students.

Read the slides and look up any words or details you find difficult to understand.


Summary list for 6.1 Digestion and absorption

  • The outline structure of the digestive system (skill - annotated diagram).
  • Food is mixed with enzymes and moved along by the contraction of circular and longitudinal muscle of the small intestine.
  • Enzymes (amylase, lipase and an endopeptidase) are secreted by the pancreas into the small intestine where they digest macromolecules ( starch, glycogen, lipids and nucleic acids) in food into monomers.
  • Tissue layers in the intestines include longitudinal and circular muscles, mucosa and epithelium.
  • The presence of Villi in the small intestines increases the surface area of epithelium and helps absorb the monomers formed by digestion and mineral ions and vitamins.
  • Different nutrients are absorbed into epithelial cells by different membrane transport mechanisms.


  • Apply knowledge of the process or digestion and absorption in the small intestine to the digestion of starch and the transport of maltose and glucose to the liver.
  • Describe your experience of the use of dialysis tubing as a model of the intestines.
  • Draw an annotated diagram of the digestive system.
  • Identify tissue layers in transverse sections of the small intestine.


Exam style questions

Video tutorial

Watch the Screencast of a teacher drawing the diagram below.
This shows how to draw a simple diagram of the digestive system that can be drawn quickly and used in IB exams.

Extra exam question on digestive enzymes.

This question tests an understanding of enzymes involved in digestion.

Answer the question below on a piece of paper, then check your answer with the model answer.

Compare and contrast the digestion of starch and lipids in the small intestine. (5 marks)

Compare and contrast – similarities and differences. Put both comparators in the same sentence (e.g. a mammal has fur; a bird has feathers. Both mammals and birds have pentadactyl limbs.)

Click the + to see a model answer.

Extra exam question on absorption in the intestine.

This question tests an understanding of structure and function of the villi of the small intestine.

Answer the question below on a piece of paper, then check your answer with the model answer.

Outline the functions of the villi of the small intestine. (5 marks)

Examiner hint; outline the main points in short sentences.

Click the + to see a model answer.

Test yourself

This is a self marking quiz containing questions covering the topic outlined above.
Try the questions to check your understanding


Drag and drop activities

Test your ability to construct biological explanations using the drag and drop questions below.

The intestinal epithelia uses different methods of transport to move substances into the epithelial cells from the gut lumen.

Active co-transport is used to absorb hydrophilic organic molecules into the gut epithelium from the lumen.

Lacteals in the small intestine

Drag and drop the correct word or phrase into the gap to describe the absorbtion of glucose and amnio acids by the intestinal epithelium.



absorbed lipid area facilitated specific glucose transporter protein membrane channels sodium ions bind each active hydrophilic

Glucose is so cannot pass through the of the membrane. Glucose is absorbed by a combination of transport and diffusion by the sodium-glucose co-transporter protein. The sodium pump situated in the epithelia actively pumps Na+ into gut lumen. These with glucose and then to the in the membrane of the villus epithelium. Sodium ions coupled to glucose diffuse back into the cell through the protein . The glucose then moves through pores between the capillary cells to be very rapidly into the blood.

Amino acids are also absorbed in this way but by a variety of transporter proteins to amino acid.

Active transport moves sodium ions, once these combine with glucose or amino acids, they pass passively through channel proteins, hence co-transport. Transporter proteins are specific to particular amino acids or to glucose.

Click the '+' symbol to open the next explanation.

Extra drag and drop explanation on absorption of water, minerals, vitamins and lipids in the small intestine  click to open.

Water, vitamins, minerals and lipids are also absorbed in the small intestine.

Drag and drop the correct word or phrase into the gap to outline how these molecules are absorbed into the intestinal epithelial cells.



channels minerals lipid osmosis diffusion vitamins specific epithelial hydrophilic energy pinocytosis vesicles

Protein in the epithelial membrane allow substances to diffuse into the intestinal cells by facilitated diffusion without the use of . Some monosaccharides, ions and are absorbed this way.

The pores are to the substance they transport.

Water moves into the intestine cells by aided by the absorption of .

Lipids and fatty acids are hydrophobic and can pass by through the layer of the intestinal mucosa membrane.

Liquid droplets are also taken into cells by using invagination of the membrane and the formation of .


Hydrophilic molecules cannot pass through the intestinal cell membranes. Protein channels are used, the inside of the channel is hydrophilic.

Just for fun

A little bit of fun to revise the digestion topic. Can you make it to the leader board?
If you can't see the content below, please click this link to Digestion card match game