分子与细胞生物学 30 - L11a Membrane Transport (1)

本期的内容是第十一章第一节：膜传输的能量。如果有不太明白的或者有错误的地方随时来找UP主喔~ 文集本部分的参考文献Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分内容来自khanacademy与维基百科。

11a Membrane Transport Energetics

Membrane Transport - movement across the bilayer

    - Size: small molecules diffuse into small gaps between lipids;

    - Polarity: charged molecules must break contacts with water first;

What can cross a bilayer by itself?

    - Hydrophobic / non-polar molecules: can always enter, can be any size; Eg: O2, CO2, N2, steroid hormones, etc.

    - small uncharged polar molecules: can diffuse but slowly; Eg: H2O, ethanol, glycerol, etc.

    - Large uncharged polar molecules: too big to enter; Eg: sugars (C6H12O6), etc.

    - Ions: cannot enter, regardless of the size.

Electrochemical Gradient

    Electrical: different amounts of charges

    Chemical: different concentrations

    In the process shown in the picture below:

    Start with sodium ions on the left side of the container and not on the right. In the end, the sodium ions will be evenly distributed throughout the container. 

    Net movement follows the gradient, but random movement for each molecule; 

    Entropy is increasing throughout the process, and this is a downhill reaction with a negative ∆G.

Ion flow can be used as an Energy Source for the cell.

    Ion gradients in human cells: unequal distributions, see the picture below:

    - The cell is more negative inside; 

    - There are about 30 times more of Na+ ions outside of the cell; when Na+ ions move into the cell, they move down the concentration & electrical gradients 

    - very favorable to move in Na+, can be used to power other reactions. 

    - There are about 30 times more of K+ ions inside of the cell; when K+ ions move out of the cell, they move down the concentration gradient but up the electrical gradient;

    - little favorable to move out K+, cannot be used to power other reactions. 

    - The pH outside of the cell is higher, so there are more H+ inside the cell.

    - Ca2+ is very low concentration in the cell cytoplasm. It has high concentrations outside the cell and stored in the ER. 

    - Anions: to balance cations, but not balanced out: voltage across the membrane - attracts positive ions towards the inside of the cell & attracts negative ions towards the outside of the cell.

本次内容到此结束，感谢阅读！下一期内容将开始讲述膜通道与膜转运蛋白。

作者：離久-張所長