分子与细胞生物学 27 - L10c Cell Membrane (2)

本期的内容是第十章第三节：细胞膜脂动力学。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。

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.

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.

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.

本次内容到此结束，感谢阅读！下一期内容将开始讲述膜蛋白。

作者：離久-張所長