AQA A Level Biology复习笔记2.4.12 Specialised Cells

Explaining Adaptations in Specialised Cells

 

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)

     

Cell-Adaptations-for-Diffusion

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)

     

Exam Tip

In the case of the kidney cells described above, water is transported across the cell membrane via facilitated diffusion through channel proteins. Don’t forget, however - water can also diffuse through cell membranes (this can occur even though it is a polar molecule because it is a relatively small molecule).

 

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