Why are the tails of phospholipids in the phospholipid bilayer in the center?

The main component of the cell membrane is a phospholipid bi-layer or sandwich. The heads (the phospho part) are polar while the tails (the lipid part) are non-polar. The heads, which form the outer and inner linings, are "hydrophilic" (water loving) while the tails that face the interior of the cell membrane are "hydrophobic" (water fearing). Water is attracted to the outsides (red) of the membrane but is prevented from going through the non-polar interior (yellow) layer.

Transport Across the Membrane

The membranes of the cell are semi-permeable. That means that while most things are effectively kept in (or out), some can pass through directly. So how do cells move things in and out? There are three methods.

1. Diffusion: If a molecule is very small, such as oxygen or carbon dioxide, diffusion does the trick. When the concentration of O2 outside the cell is higher than inside, O2 molecules diffuse in, passing through the membrane like it isn't even there. Similarly, when the concentration of the waste gas CO2 builds up inside the cell, it escapes naturally to the outside where the concentration is lower. Diffusion requires no expenditure of energy by the cell. It happens passively. While nature figured this out a long time ago, we now make fabrics and medical devices that copy this process. Gore Industries, one of the big employers in Flagstaff, makes a fabric called "Gore-Tex" which repels large water droplets but allows smaller air molecules to pass through, making the fabric "breathable."

The catch: While diffusion works well for the tiny single cell, it does not, by itself, get the job done in a multi-cellular organism where the tissues are buried deep inside the body. Imagine your bicep muscle while you are lifting weights. The tissue, comprised of millions of cells, will quickly run out of oxygen and build up carbon dioxide. Diffusion through the skin could not keep up. This is where the circulatory system helps out. The smallest blood vessels, the capillaries, run though these tissues. The blood from the lungs releases oxygen to the cells (because O2 is at lower concentration in the tissues), and picks up carbon dioxide (because CO2 is at higher concentration in the tissues) and carries it back to the lungs to be exhaled. This does require energy. It also explains why your breathing rate increases when you exert yourself, and is one of the costs of being multi-cellular.

2. Active Transport: Sometimes diffusion doesn't happen fast enough for the cell's needs, and there are times when nutrients need to be stockpiled or excreted at a higher concentration than would occur naturally by diffusion. In this case, the cell uses energy to pump good things in, and bad things out, through protein channels or gates. This process is called active transport.

3. Endocytosis: Sometimes, a large object needs to be moved in or out of the cell, but it's too big for the door. Think about moving a couch into your apartment and you will get the idea. But you can't just cut a hole in the cell membrane or all the good stuff inside would leak out, so how do you get something in without letting your interior be exposed to the exterior? The cell has a special trick that probably dates back to the days when all life was single celled, and this was how cells ate. The single celled Amoeba still consumes its food this way. It's called endocytosis, and it works like this. Note in particular that the engulfed food item is gradually enclosed in an "inside-out" section of the double-layered membrane, Pac-Man style! Once the food particle is completely surrounded, the exterior membranes fuse and the interior vacuole pinches off. By this method, the interior of the cell is never directly exposed to the exterior environment. The one side effect of this trick is that the membrane is now inside out, and that's interesting because it gives us a clue about the origins of the cellular organelles.

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Learning Outcomes

• Describe the structure of a phospholipid.
• Identify the polar (hydrophilic) and nonpolar (hydrophobic) regions of a phospholipid.
• Explain how the phospholipid molecules form the bilayer of the cell membrane.

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.

Phospholipids

A 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.

Figure \(\PageIndex{1}\): A phospholipid consists of a head and a tail. The "head" of the molecule contains the phosphate group and is hydrophilic, meaning that it will dissolve in water. The "tail" of the molecule is made up of two fatty acids, which are hydrophobic and do not dissolve in water.

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.

Figure \(\PageIndex{2}\): In a water solution, phospholipids form a bilayer where the hydrophobic tails point towards each other on the interior and only the hydrophilic heads are exposed to the water.

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

• Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)

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This page titled 14.3: Phospholipids in Cell Membranes is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation.

Why are the phospholipid tails facing inward toward each other in the phospholipid bilayer?

The non polar tails face inward toward each other. These ends do not like water, so they face each other in order to avoid contact with the water inside and outside of the cell.

What is important about the Centre of the phospholipid bilayer?

In this arrangement, the bilayer has a hydrophobic core that prevents the passage of polar molecules while allowing the relatively free diffusion of non-polar molecules. This is important because it allows the bilayer to select which molecules it will allow into and out of the cell.

Why the hydrophobic tails of phospholipid molecules are towards inner side of the cell membrane?

Hence, the correct answer is 'The non-polar or hydrophobic hydrocarbons tails of lipid, being on inner side ensure their protection from the aqueous environment.

Why is the phospholipid bilayer arranged the way it is?

Function of Phospholipids- In this bilayer, the phospholipids are arranged so that all the hydrophillic heads are pointing outward and the hydrophobic tails are pointing inward. This arrangement comes about because the areas both outside and inside your cell are mostly water, so the hydrophobic tails are forced in.