Account for the movement of substances across membrane using the concepts of diffusion.
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
👆🏻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
👆🏻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 👏🏻
References
- Thomas, B. & Chris, H. (2013) Hong Kong DSE Biology Critical Guide. Yellowreef Limited.
- 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/
- 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