Specialised cells for diffusion

• Root hair cells?and?epithelial cells of the small intestine?are examples of cells that are?adapted?for the rapid transport of molecules across their membranes
• Root hair cells:
  • Are adapted for the absorption of?water?and?mineral ions?from soil
  • Have a specialised shape (the root ‘hair’) that?increases?the cell’s?surface area?so the rate of water uptake by osmosis is?greater?(can absorb more water and mineral ions than if the surface area was lower)
  • Have?thinner walls?than other plant cells so that water can move through easily (due to?shorter diffusion distance)
  • Have a permanent vacuole containing cell sap, which is more concentrated than soil water. This?ensures a high water potential gradient is maintained

   

• Epithelial cells of the small intestine:
  • Have?microvilli?(highly?folded?sections of the cell membrane), which?increases?the cell’s?surface area?so the rate of diffusion of the products of digestion is?greater?(more particles can be exchanged in the same amount of time)
  • Each villus of the small intestine has a?constant blood supply, which continually transports the products of digestion away from the epithelial cells. This?maintains a high concentration gradient?across the epithelial cell exchange surface (between the lumen of the small intestine and the interior of the epithelial cell)

   

Many cells that are adapted for diffusion have an increased surface area in some way – for example, root hair cells in plants and cells lining the ileum in animals

Specialised cells for facilitated diffusion

• Neurones,?muscle cells?and?some kidney cells?are examples of cells that are?adapted?for the rapid transport of molecules across their membranes via facilitated diffusion
• Certain kidney cells:
  • Have cell membranes that contain a very?high number of aquaporins
  • Aquaporins are special channel proteins that allow the?facilitated diffusion of water?through cell membranes
  • The aquaporins allow these kidney cells to reabsorb water, stopping it from being unnecessarily excreted by the body

   

• Neurones and muscle cells:
  • Are involved in the transmission of electrical impulses around the body
  • They have cell membranes that contain?channel proteins?for sodium, potassium and calcium?ions
  • The?opening?and?closing?of these channel proteins (and the resulting?facilitated diffusion?of these different ions), as well as the?number?of these channel proteins, plays an important role in the?speed of electrical transmission, both along the membranes of neurones (during nerve impulses) and in muscle cells (during muscle contraction)