本期的內(nèi)容是第十一章第二節(jié)：膜通道與膜轉(zhuǎn)運蛋白。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Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分內(nèi)容來自khanacademy與維基百科。

11b Membrane Channels vs. Transporters

[1] Channels - continuous pore

????? Usually selectively allow small ions to pass through.?

????? Always passive, no energy involved. In passive transport, a solute always moves down its concentration gradient.?

????? Weak electrostatic interactions: Channels do not bind the solute that passes through them.

- Channel Pore is selective for size & charge

????Selectivity of a channel is achieved by the size (diameter) of the internal pore and by charged regions at the entrance of the pore that attract or repel ions of the appropriate charge.

- Channels are Gated - only open for some times.

????3 Basic Methods of Gating Channels: Voltage-Gated, Ligand-Gated, Mechanically Gated

????Ligand-gated channel can bind extracellular or intracellular ligands.

????Example in neurons: the opening of Ligand- or Mechanically gated channels triggers the opening of Voltage gated channels.

[2] Transporters - always closed at one side?

????? Selectively allow small or large molecules (sugar, amino acids) to pass through?

????? Passive or Active Transport (pumps)?

????? Tight binding sites for transported molecules.?

- Energy Sources for Active Transport: ATP, Light, Gradients (of others).?

????In the cases below, moving the yellow molecule against the gradient.

- Active Transport

????Primary Active Transport: Uses ATP or other direct energy sources;

????Secondary Active Transport:?

????????- Uses an electrochemical gradient of another ion / molecule.?

????????- Also known as coupled transport or cotransport;?

????????- Energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead it relies upon the electrochemical potential difference created by pumping ions in/out of the cell.?

????????- Permitting one ion or molecule to move down an electrochemical gradient, but possibly against the concentration gradient for another ion.

There are two sub-types of Coupled Transporters:

????Symport: The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time in the same direction.

????Antiport: An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in opposite directions, one into the cell and one out of the cell.

????Note that a symport binds two different solutes on the same side of the membrane. Turning the symport around would not change it into an antiport, which must also bind to different solutes, but on opposing sides of the membrane.

Transporters have Two Conformations:

????- Conformations face in or out, not a “switch” of shape;?

????- Tight binding sites for transported molecules;?

????- Never open both sides at the same time.

本次內(nèi)容到此結(jié)束，感謝閱讀！下一期內(nèi)容是一期案例研究：腸上皮細胞的膜運輸。

作者：離久-張所長