Co-transport
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of active transport and concentration gradients. You can test your knowledge on these below.
What is facilitated diffusion?
- Diffusion of molecules down their concentration gradient (from a higher to a lower concentration)
- Using a channel or carrier protein
What is active transport?
- The movement of molecules against their concentration gradient (from a lower to a higher concentration)
- Using a carrier protein
- Requiring energy from ATP
Which type of molecules can cross the phospholipid bilayer?
- Small / non-polar / lipid soluble molecules
- Such as oxygen and carbon dioxide
Topic Explainer Video
Check out this @LauraDoesBiology video that explains co-transport or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Co-Transport Overview
Co-transport
Glucose and other polar molecules, such as amino acids are absorbed by a combination of facilitated diffusion and active transport along with a second substance. This is known as co-transport.
For example, special carrier proteins in the cell surface membranes of epithelial cells in the ileum transport glucose in the presence of sodium ions.
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Sodium ions (Na+) are actively transported out of the epithelial cells and into the blood using a carrier protein called the sodium-potassium pump. This requires ATP.
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This establishes a concentration gradient for sodium ions whereby their concentration is lower inside the epithelial cell than the lumen of the ileum.
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Sodium ions will then move into the epithelial cell from the lumen by facilitated diffusion, down their concentration gradient. As they move in, they carry with them a molecule of glucose or an amino acid, against their concentration gradient. This uses a co-transport protein.
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The concentration of glucose/ amino acids is now higher in the epithelial cell than the blood in the capillary. They are therefore able to move via facilitated diffusion from the epithelial cell into the blood.
This process describes how glucose and amino acids are absorbed into the bloodstream.
Key Ideas
Why are sodium ions actively transported out of the cell at step 1?
- To establish a concentration gradient for sodium ions between the epithelial cells and the ileum.
Name two things needed or active transport.
- ATP and a carrier protein.
Why can’t glucose move into the epithelial cell by simple diffusion?
- Glucose is polar and is too large to diffuse through the phospholipid bilayer of the cell surface membrane. It also needs to move from a lower concentration in the ileum to a higher concentration in the epithelial cell.
What would happen if the ATP supply from respiration stopped?
- Sodium ions would not be actively transported out of the epithelial cells by active transport (as active transport requires energy from ATP) This means there would be no concentration gradient established for sodium ions. As such, sodium ions would not move into the cells via facilitated diffusion and would therefore not carry glucose or amino acids into the cells with them.
How are the epithelial cells of the ileum adapted for co-transport?
- They have a large number of carrier proteins for active transport.
- They have a large number of co-transport proteins for facilitated diffusion.
- They have a large number of mitochondria to make more ATP by aerobic respiration. (Remember, active transport requires energy from ATP).
- They have microvilli to provide a large surface area for absorption.
Key Terms
- Polar: a molecule which has an uneven distribution of electrons and therefore an uneven distribution of charge.
- Facilitated diffusion: diffusion of a molecule down its concentration gradient using a channel or carrier protein. Molecules that use facilitated diffusion may be too large to cross the phospholipid bilayer or may be charged/ polar.
- Active transport: the movement of a molecule against its concentration gradient using a carrier protein and requiring energy from ATP.
- Ileum: the final part of the small intestine where absorption of nutrients into the blood takes place.
- Epithelial cells: cells that line the body’s surfaces.
- Lumen: the open space inside a tube or cavity in the body.
- Microvilli: folds of the cell surface membrane that increase the surface area.
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Exam Tip
When describing the process of co-transport, always start with the active transport of sodium ions from the epithelial cells into the blood. This needs to happen to maintain the diffusion gradient for sodium ions between the lumen of the ileum and the epithelial cells.
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Cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions. Explain how. (3 marks)
- Sodium ions actively transported from ileum cell to blood.
- Maintains / forms diffusion gradient for sodium ions to enter cells from ileum (and with it, glucose).
- Glucose enters by facilitated diffusion/ co-transport with sodium ions.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!