Absorption of glucose and amino acids
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of active transport, co-transport mechanisms and the structure of the small intestine. You can test your knowledge on these below.
What is co-transport?
Co-transport is a type of facilitated diffusion where the movement of a molecule across a membrane is coupled with the movement of another molecule.
Explain the role of sodium ions in the co-transport of glucose in the ileum.
- Sodium ions are actively transported out of epithelial cells into the blood, lowering their concentration inside the cell.
- This creates a concentration gradient for sodium ions to diffuse back in from the ileum, bringing glucose with them.
Why can't glucose diffuse directly through the phospholipid bilayer of epithelial cells?
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Glucose is a large, polar molecule and cannot pass through the phospholipid bilayer. (It is water soluble, not lipid soluble).
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Instead, it requires transport proteins to facilitate its movement.
Topic Explainer Video
Structure and Function of the Ileum in Nutrient Absorption
- The ileum is the final part of the small intestine where absorption of glucose, amino acids, fatty acids and monoglycerides takes place.
- The ileum is lined with folds called villi.
- There are further folds called microvilli on the epithelial cells that line each villus.
- These provide a large surface area for absorption.
- The epithelial cells also contain many mitochondria which produce ATP for active transport.
- The wall of the villi consists of a single layer of epithelial cells, providing a short diffusion pathway.
- The epithelial cells have many transport proteins for the absorption of glucose and amino acids, such as carrier proteins and co-transport proteins.
- The way in which these transport proteins are used is described below.
The Importance of Co-Transport
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The epithelial cells of the ileum absorb glucose and amino acids from digested food.
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Since their concentration in the intestine may be lower than in the blood, active transport and co-transport are needed.
The Role of Sodium Ions in Glucose and Amino Acid Absorption
- The sodium-potassium pump actively transports Na⁺ ions out of epithelial cells into the blood. This requires energy from ATP.
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This creates a concentration gradient whereby there is a lower concentration of Na⁺ in the epithelial cells and a higher concentration of Na⁺ in the lumen of the ileum.
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Na⁺ ions diffuse back into the epithelial cells from the lumen of the ileum via a co-transport protein.
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As Na⁺ moves in, glucose or an amino acid is transported in with it.
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The glucose or the amino acid is moving against its concentration gradient.
Movement into the Bloodstream
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Once inside the epithelial cell, glucose and amino acids move into the bloodstream by facilitated diffusion through specific carrier proteins.
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This ensures efficient absorption and maintains blood glucose levels.
Key Terms
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Co-transport: A type of facilitated diffusion where two substances move together across a membrane via a carrier protein, we can also call this a co-transport protein.
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Sodium-potassium pump: A carrier protein that actively transports sodium ions out of a cell and potassium ions in, using ATP.
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Concentration gradient: The difference in concentration of a substance between two areas.
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Facilitated diffusion: The passive movement of molecules across a membrane via a transport protein.
No answer provided.
Exam Tip
Common Mistake: Students often confuse co-transport with active transport.
Remember:
- The sodium-potassium pump is using active transport (needs ATP).
- Co-transport itself does not use ATP directly it relies on the sodium ion gradient created by active transport.
No answer provided.
Explain how glucose is absorbed in the ileum via co-transport. (5 marks)
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The sodium-potassium pump actively transports Na⁺ out of epithelial cells into the blood, creating a low Na⁺ concentration inside the cell.
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Sodium ions diffuse back into the epithelial cells from the lumen of the ileum via a co-transport protein.
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As sodium moves in, glucose (or an amino acid) is transported into the cell alongside it.
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Glucose (or an amino acid) is moving against it’s concentration gradient
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Glucose then moves from the epithelial cell into the bloodstream by facilitated diffusion via a carrier protein.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!