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Selective Permeability: Definition & Overview

Selective Permeability: Definition & Overview
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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

This lesson will provide you with a definition of selective permeability. Also, an example and illustration will be provided to support your understanding of the process.

What Is Selective Permeability?

Because selective permeability is a complex topic, you will be learning the definitions of several terms in this lesson. Selective permeability usually refers to what can enter and exit a cell membrane. There are other processes that are selectively permeable, such as some plastics. However, for the purpose of this lesson we will refer to the cell membrane for consistency.

Let's describe a cell's membrane for you. It's a thin layer that may or may not allow particles, ions, and water to cross into and out of the cell. Semi-permeable means that some particles, ions, or water can cross the membrane. Therefore, selective permeability and semi permeable are similar terms. However, selective permeability is a more precise term because, as the name implies, the cell has some ability to select what can and cannot cross the cell membrane. This occurs based on the cell requirements and the makeup of the cell membrane.

To help you understand membranes, think of a small brook with flowing water. The water has small leaves and other debris in the water. There is a window screen in the brook. The screen represents the semi permeable or selectively permeable membrane. The leaves and other debris in the water represent particles and ions. In this case, small leaves, debris, and water can pass through the screen while larger leaves and debris cannot pass.

Context for Understanding Selective Permeability

If the cell needs a particular molecule inside the cell, that molecule is permitted to pass through the membrane. However, this depends on the size and charge of the molecule, either positive or negative, as well as the solubility of the molecule in water. Most cell membranes are semi-permeable, or selectively permeable. However, the cell membrane is made up of fatty acids and lipid layers which repel water.

Let's imagine a measuring cup in your kitchen. You pour ¾ cup of oil into the measuring cup. Then you add ½ cup of water. What happens? The water and oil don't mix together, do they? No, in fact the water is on the bottom and the oil is on the top. This is an example of the repelling that occurs at the fatty lipid cell wall when particles dissolved in water approach.

So what happens to allow particles and ions to cross the cell membrane? The process used is called facilitated diffusion. Facilitated diffusion is the process of transporting particles into and out of a cell membrane. Energy is not required because the particles move along the concentration gradient. This requires the use of a carrier.

Concentration gradient is the process of particles, which are sometimes called solutes, moving through a solution or gas from an area of higher number of particles to an area of lower number of particles. The carrier that assists the particle to cross the membrane is usually a special protein.

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