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HKDSE Biology - Diffusion

Compulsory Part I Cells and Molecules of Life

· HKDSE,Biology,Diffusion,Simple diffusion,Cell membrane

Account for the movement of substances across membrane using the concepts of diffusion.

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Diffusion referring to the net movement of molecules from higher concentration region to lower concentration region ⏩ down the concentration gradient.

Besides, diffusion is one of the passive transport which do not require energy.

Mechanism of simple diffusion 

illustration of diffusion through cell membrane

👆🏻This photo illustrated how diffusion assist the transport of molecules through cell membrane.

Simple diffusion 👉🏻 transport of molecules that can be dissolved in lipid bilayer (non-polar).

  • Non-polar molecules transport from extracellular fluid to cytoplasm of cell through cell membrane by diffusion. 
  • Extracellular fluid consist of much more non-polar molecules than cytoplasm of cell. Herein, the non-polar molecules flow from higher concentration region (extracellular) to lower concentration region (intercellular).

Mechanism of facilitated diffusion

Diffusion occurred in channel protein

👆🏻This photo showing diffusion occurred in channel protein

Facilitated diffusion 👉🏻 transport of molecules that cannot be dissolved in lipid bilayer (polar).

  • Channel protein embedded in the cell membrane created an opening to facilitate diffusion.
  • Charged ions and polar molecules flow from higher concentration region (extracellular) to lower concentration region (intercellular).

Example of diffusion 🔅

  • The transport of carbon dioxide to chloroplast during photosynthesis in plant.
  • The transport of water to stomata  during respiration in plant.
  • The transport of oxygen from air sac to blood during respiration.
  • The transport of carbon dioxide from cell to blood during respiration .

Factors influencing the rate of diffusion 🧭

As a HKDSE Biology student, remember how to speed up the rate of diffusion ❗️ ❕

1. Concentration gradient

  • The steeper the concentration gradient, the higher the rate of diffusion.
  • Steeper the concentration gradient ➡️ greater concentration difference between two region.

2. Molecular size

  • The smaller the size of the molecule, the higher the rate of diffusion.

3. Solubility in lipid

  • The higher solubility of the molecule in lipid, the higher the rate of diffusion.

4. Sate of molecule

  • Gaseous state molecule diffuse faster than molecule in solid or liquid state.

5. Temperature

  • The higher the temperature, the higher the rate of diffusion.

6. Distance

  • The shorter the distance where diffusion occurred, the higher the rate of diffusion.

7. Surface area

  • The larger the surface area, the higher the rate of diffusion.

8. Nature of diffusion surface

  • The greater the number as well as size of the pores in cell membrane, the higher the rate of diffusion.

That's the end of this topic 👏🏻

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Written by Bella | HKDSE Biology Specialist @ TUTTEE

Want to improve your DSE Biology? Check out our study tips here ✒️

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References

  1. Thomas, B. & Chris, H. (2013) Hong Kong DSE Biology Critical Guide. Yellowreef Limited.
  2. Rae-Dupree, J., Odya, E., Anatomy & Physiology, & Carmel High School. (n.d.). The Cell Membrane: Diffusion, Osmosis, and Active Transport. Retrieved from https://www.dummies.com/education/science/anatomy/the-cell-membrane-diffusion-osmosis-and-active-transport/
  3. What is the role of protein channels in the cell membrane?: Socratic. (2018, May 15). Retrieved from https://socratic.org/questions/what-is-the-role-of-protein-channels-in-the-cell-membrane 
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