Learning Outcomes When you go to the dentist to get a tooth pulled, you really do not want to feel any pain. The dentist injects an anesthetic into your gum and it eventually becomes numb. One theory as to why anesthetics work deals with the movement of ions across the cell membrane. The anesthetic gets into the membrane structure and causes shifts in how ions move across the membrane. If ion movement is disrupted, nerve impulses will not be transmitted and you will not sense pain - at least not until the anesthetic wears off. PhospholipidsA phospholipid is a lipid that contains a phosphate group and is a major component of cell membranes. A phospholipid consists of a hydrophilic (water-loving) head and hydrophobic (water-fearing) tail (see figure below). The phospholipid is essentially a triglyceride in which a fatty acid has been replaced by a phosphate group of some sort. Following the rule of "like dissolves like", the hydrophilic head of the phospholipid molecule dissolves readily in water. The long fatty acid chains of a phospholipid are nonpolar and thus avoid water because of their insolubility. In water, phospholipids spontaneously form a double layer called a lipid bilayer in which the hydrophobic tails of phospholipid molecules are sandwiched between two layers of hydrophilic heads (see figure below). In this way, only the heads of the molecules are exposed to the water, while the hydrophobic tails interact only with each other.  Phospholipid bilayers are critical components of cell membranes. The lipid bilayer acts as a barrier to the passage of molecules and ions into and out of the cell. However, an important function of the cell membrane is to allow selective passage of certain substances into and out of cells. This is accomplished by the embedding of various protein molecules in and through the lipid bilayer (see figure below). These proteins form channels through which certain specific ions and molecules are able to move. Many membrane proteins also contain attached carbohydrates on the outside of the lipid bilayer, allowing it to form hydrogen bonds with water. Figure \(\PageIndex{3}\): The phospholipid bilayer of a cell membrane contains embedded protein molecules which allow for selective passage of ions and molecules through the membrane.Contributors and Attributions
Understanding: • Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules
Arrangement in Membranes:
Properties of the Phospholipid Bilayer:
Why is it important for phospholipid to be hydrophobic and hydrophilic?The amphipathic nature of the phospholipid bilayer allows for important properties of the cell membrane. For example, the hydrophobic nature of the tails promotes the selective permeability of the membrane. Large molecules or hydrophilic molecules are unable to pass through the bilayer without assistance.
Why is it important that membrane lipids have dual character?The dual character is critical for the formation of the lipid bilayer. The hydrophilic portions of the molecule are in contact with the aqueous environment of the cell, while the hydrophobic portion of the lipids is in the interior of the bilayer and provides a barrier to the passive diffusion of most molecules.
Why do both surfaces of your membrane model need to be hydrophilic?Hydrophobic and Hydrophilic
The interior and the exterior of cells is liquid, usually a solution or suspension of ions, small molecules and large molecules dissolved in water. Proteins must therefore be hydrophilic ("water loving") in order to be suspended in this environment.
Why does the membrane need two layers of phospholipids phospholipid bilayer?The phospholipid bilayer formed by these interactions makes a good barrier between the interior and exterior of the cell, because water and other polar or charged substances cannot easily cross the hydrophobic core of the membrane.
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Why is it important that the phospholipid bilayer be both hydrophobic and hydrophilic?
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