Facilitated Diffusion

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What is facilitated diffusion? In membrane, there are protein channels (pores) that allow polar molecules (soluble in water but not soluble in lipids) to move through them. The molecules that move through these channels never come into contact with the hydrophobic layer or the membrane's polar surface because these channels provide a continuous pathway for molecules that are moving through them. In this type of diffusion, like simple diffusion, no energy is consumed. Large and polar molecules require help to pass through the membrane. This help is done by a type of protein called carrier (transport) protein . To pass through the membrane, a molecules binds (temporarily) with carries protein in lipid bilayer and is transported from an area of higher concentration to an area of lower concentration.  Related topics: Simple Diffusion
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Definition of polyspermy/Fast block anf slow block polyspermy-The Knowledge

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POLYSPERM

Polyspermy refers to the egg that has been fertilized by more than one sperm.

Types of polyspermy

There are two types of polyspermy.

1.Fast block to polyspermy 2.Slow block to polyspermy

Fast block to Polyspermy

The Fast block polyspermy is achieved by changing the electric potential of the egg plasma membrane. This membrane provides a selective barrier between the egg cytoplasm and the outside environment, and the ionic concentration of the egg differs greatly from that of its surroundings.

This concentration difference is especially significant for sodium and potassium ions. Seawater has a particularly high sodium ion concentration, whereas the egg cytoplasm contains relatively little sodium. The reverse is the case with potassium ions.

Slow block to polyspermy

The eggs of sea urchins (and many other animals) have a second mechanism to ensure that multiple sperm do not enter the egg cytoplasm. The fast block to polyspermy is transient, since the membrane potential of the sea urchin egg remains positive for only about a minute. This brief potential shift is not sufficient to prevent polyspermy, which can still occur if the sperm bound to the vitelline envelope are not somehow removed . This type of polyspermy is slow block polyspermy.

 

 

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