本期的內(nèi)容是第十章第三節(jié)：細(xì)胞膜脂動力學(xué)。如果有不太明白的或者有錯誤的地方隨時來找UP主喔~ 文集本部分的參考文獻(xiàn)Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分內(nèi)容來自khanacademy與維基百科。

11c Membrane Lipid Dynamics

????Bilayers are two-dimensional liquids: although the hydrophobic tails pointing inwards, there are only weak, transient interactions (dispersion forces), they are not ordered and in rapid movements - keep forming and breaking bonds.

Lipid Movements in a Bilayer:

????Lateral diffusion: common;

????Rotation: common;

????Flexion: tails bending & straightening

????Flip-Flop: rarely happens, the hydrophilic head needs to pass through the hydrophobic layer of the membrane.

????Flippases can actively move lipids between bilayers. The movement is directional, establishing & maintaining the asymmetry in the leaflets. Note that many cells maintain asymmetric distributions of phospholipids between their cytoplasmic and exoplasmic membrane leaflets.

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Lipid Movements in a Bilayer make the membrane fluid - intermediate fluidity.

????- If too stiff - membrane proteins cannot diffuse to find binding partners; cells cannot change shape (eg. as it crawls around or divides);

????- If too fluid - membrane is not a good barrier.

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Membrane fluidity - Affected by Temperature, Tail Length & Saturation, Cholesterol.

????[1] Temperature:

????????Heat - breaks interactions, more fluid;

????????Cold: less fluid.

?????[2] Tail Length & Saturation:

????????Long, Saturated (straight) tails: more tightly packed, increased surface area that can interact with neighboring molecules, more dispersion force, less fluid;

????????Short, Unsaturated (kinked) tails: more loosely packed, less dispersion forces, more fluid - more mobile (more double bonds make it more mobile).

????????Cells use lipid structure to counteract temperature effects: High heat: increase the number of long, saturated chains; Cold: increase the number of short, unsaturated chains.

?????[3] Cholesterol

????????Cholesterol is an organic molecule. It is a sterol, a type of lipid. On average about 50% of eukaryotic plasma membrane lipids are Cholesterol.

????????Shown in the picture below in green. Cholesterol is amphipathic, it assembles with bilayer.

????????Polar group + hydrophobic tail, same orientation as the phospholipid. Gaps between unsaturated fatty acids are filled with this short, rigid lipid - often decreasing fluidity and permeability. So Cholesterol makes Eukaryotic plasma membranes less fluid and less mobile.

? ? ????Cholesterol is essential for Eukaryotes. It is used as a precusor to synthesize many other molecules such as Bile acid, Steroid hormone (testosterone, estrogen), Vitamin D3, etc.

本次內(nèi)容到此結(jié)束，感謝閱讀！下一期內(nèi)容將開始講述膜蛋白。

作者：離久-張所長