6.2 Applications and skills

6.2.1 Membrane transport in the small intestine

Absorption in the small intestine involves nutrients moving from the lumen of the small intestine, through the epithelium cell, into the interstitial fluid, and finally into the capillary.

  • All types of membrane transport are involved when the small intestine is at work. However, it is important to note that all absorption into the bloodstream is passive.
  • Active transport, in the form of sodium and potassium pumps, maintains concentration gradients that allow passive transport, including osmosis, to occur in the epithelium of the small intestine (see Figure 6.2.1a). 

Figure 6.2.1a – Glucose loadingFigure 6.2.1a – Glucose loading
Glucose loading is an example illustrating how energy-requiring processes maintain concentration gradients for passive transport.

Nutrient General method Detail
Water (and electrolytes, like Na+, K+) Osmosis and simple diffusion
  • Water moves in and out of the epithelium in response to osmolarity differences maintained by sodium pumps.
  • Water and electrolytes diffuse into the capillary from the interstitial fluid.
Glucose (and other monosaccharides)

Facilitated diffusion using:

  1. sodium co-transporter proteins
  2. glucose channel proteins
  • Sodium diffuses into the cell from the lumen through the co-transporter protein, bringing glucose with it.
  • Protein channels allow glucose to diffuse from cell to interstitial space and into the capillary.
Amino acids

Facilitated diffusion using:

  1. sodium co-transporter proteins
  2. amino acid channel proteins

Uses four different sodium co-transport proteins.

  • The proteins change shape when in contact with sodium, allowing for specific amino acids to bind and be transported into the epithelium from the lumen (mechanism similar to glucose absorption).
Fatty acids

Simple diffusion  

Facilitated diffusion  

Exocytosis

Small monoglycerides and fatty acids diffuse directly through the epithelial plasma membrane as micelles.

  • Longer fatty acids use specific transporter proteins to get through the plasma membrane
  • Once inside the cell, smaller components are assembled into chylomicrons (Figure 6.2.1b).
  • Chylomicrons exit the cell by exocytosis, then enter the lacteal and travel through the lymph.
  • Fats are deposited into the bloodstream by the lymphatic vessels (Figure 6.2.1c).


Figure 6.2.1b - Chylomicrons
Figure 6.2.1b – Chylomicrons
Chylomicrons are assembled by the Golgi apparatus of epithelial cells and are made of many components in addition to triglycerides.

Application: Modelling the small intestine with dialysis tubing

Dialysis tubing is a semi-permeable membrane that is used in laboratories to separate molecules from a solution based on differential diffusion. It is often used as a teaching tool to model absorption in the small intestine.

Figure 6.2.1d – A model gut using dialysis tubingFigure 6.2.1c – A model gut using dialysis tubing

Figure 6.2.1c shows the initial set-up of a demonstration of a model gut. At the beginning of the experiment, the dialysis tubes contain different substances, and the height of the column in each capillary tube is identical.

  • Predict what will happen to the contents of the tubing and the level in the capillary tubes after one hour. Explain your predictions.
  • Outline the tests you would perform to determine the contents of the dialysis tubing after one hour.
  • Describe how this set-up models the working of the small intestine.
  • Discuss limitations of this model.

Key questions

  • Outline digestion and absorption of starch, proteins and lipids.
  • Describe three differences between the mechanisms for absorption of lipids, and monosaccharides/amino acids.

Course link

  • Review the different methods of membrane transport in Page 1.1.4.
  • Review the structure of micelles in Page 1.1.5.

Language help

  • A co-transporter protein is called a symport protein in some texts.
  • A membrane protein that facilitates diffusion in opposite directions is called a counter-transporter or an antiport protein.

support proteinFigure 6.2.1d – Symport protein 

Did you know?

  • Iodine solution turns blue/black when in contact with starch.

Lab extension

Change the concentration of the solution, the type of solution or the temperature of the set-up.

Figure 6.2.1c – Fatty acid absorptionFigure 6.2.1e – Fatty acid absorption
The pathway of absorption for fatty acids is different from that for glucose and amino acids. Click image to reveal steps in the process