Lecture 27: Absorption Flashcards
What is absorption?
▪ Movement of solutes and water from the GIT lumen
▪ across the epithelium
▪ into the interstitial fluid
▪ and then into the blood or lymph
Where does bulk absorption occur?
Bulk absorption in “leaky” epithelium
Small intestine:
▪ Main site of absorption
▪ 90% of water and sodium
▪ All nutrients
Where does fine tuning absorption occur?
Regulated/ fine tuning absorption in “tight” epithelium
Large intestine:
▪ 9% of water and sodium
What are the five factors affecting absorption?
- Reduction in nutrient size by chemical digestion
- Motility
- Transport across GIT epithelium
- Surface area available for absorption
- Removal from interstitial fluid
What are the three main driving forces for particles to move across a membrane?
▪ ATP: primary active transport
▪ Electrochemical gradient: secondary active transport, facilitated diffusion & passive diffusion
▪ Osmotic gradient: movement of water
Is the paracellular pathway (between cells) selective?
Relatively non-selective (tight junctions only barrier)
▪ If the tight junctions are “leaky” AND the particle is small enough it can move via the paracellular pathway
▪ If the tight junctions are “tight” particles are unable to
move via the paracellular pathway
NOTE:
Requires a gradient as the driving force
How do lipid soluble particles cross the transcellular pathway (two cell membranes)?
Lipid soluble particles:
▪ Diffuse through the membrane
▪ Requires a gradient as the driving force:
- Chemical /concentration gradient
How do water soluble particles cross the transcellular pathway (two cell membranes)?
Water soluble particles
▪ Require a channel or transporter membrane protein to cross the membrane
▪ Requires a driving force:
- ATP: primary active transport
- Osmotic gradient: for movement of water
- Electrical, chemical (concentration) or electrochemical (both) gradient
How does surface area effect absorption?
Greater the surface area the faster the rate of
absorption
Achieved by:
▪ Length of intestine (6 m)
▪ Circular folds (plicae circulares)
▪ Villi
▪ Microvilli
How does removing particles from interstitial fluid effect absorption and how is this achieved?
Prevent build up of particles in the interstitial fluid after absorption:
Achieved by
▪ High blood flow to intestines
▪ Blood vessels and lacteals close to the basolateral side of
epithelial cells
Reason?
To maintain a driving force for absorption of nutrients from the GIT
Which of the following statements is NOT correct?
1. Particles can only be absorbed via the paracellular pathway in epithelia with “tight” tight junctions.
2. Segmentation increases absorption in the small intestine.
3. Lipid soluble particles can be absorbed via the transcellular pathway.
4. The greater the surface area in the GIT the faster the rate of absorption.
1
What are the two pathways for carbohydrate and proteins absorption?
- PASSIVE absorption: via paracellular pathway (concentration gradient)
- ACTIVE absorption: via transcellular pathway
▪ Driving force = Sodium gradient generated by
Na+/K+-ATPase
How are monosaccharides transported across the cell membranes?
Apical membrane:
▪ Secondary active transport via sodium-glucose cotransporter
Basolateral membrane:
▪ Facilitated diffusion via glucose carrier/transporter
How are amino acids transported across the cell membranes?
Apical membrane:
▪ Secondary active transport via
sodium-amino acid cotransporter
Basolateral membrane:
▪ Facilitated diffusion via amino acid carrier/transporter
How are di and tri protein peptides absorbed?
Only ACTIVE absorption via the transcellular pathway
Driving force:
▪ Negative membrane potential generated by Na+/K+-ATPase creates electrical gradient for H+
Apical membrane:
▪ Secondary active transport via H+- peptide cotransporter
▪ Inside the cytoplasm peptidases digest small peptides to amino acids
Basolateral membrane:
▪ Facilitated diffusion via amino acid carrier/transporter
