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The Phospholipid Bilayer Advanced
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
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Printed: March 12, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
www.ck12.org
C HAPTER
Chapter 1. The Phospholipid Bilayer - Advanced
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The Phospholipid Bilayer Advanced
• Describe the structure and function of the plasma membrane.
Why a bilayer?
What’s on the inside of the cell and on the outside? Mostly water. As you can see here, the water-based interior of
the cell has lots of components. These need to be kept inside the cell. And it is the nature of the phospholipid bilayer
to keep the inside of the cell separate from the outside.
Phospholipids
The cell membrane (or plasma membrane) is composed mainly of phospholipids with embedded proteins. The
membrane is a lipid bilayer, with the phospholipids oriented in a distinct manner to provide qualities necessary to
maintain a cell in a water-based environment.
A phospholipid is made up of a polar, phosphorus-containing head, and two long fatty acid (hydrocarbon), non-polar
"tails." That is, the head of the molecule is hydrophilic (water-loving), and the tail is hydrophobic (water-fearing).
Cytosol and extracellular fluid - the insides and outsides of the cell - are made up of mostly water. In this watery
environment, the water loving heads point out towards the water, and the water fearing tails point inwards, and push
the water out. The resulting double layer is called a phospholipid bilayer. A phospholipid bilayer is made up of
two layers of phospholipids, in which hydrophobic fatty acids are in the middle of the plasma membrane, and the
hydrophilic heads are on the outside. An example of a simple phospholipid bilayer is illustrated in Figure 1.1.
The cell membrane also decides what may enter or leave a cell. The membrane is said to be semipermeable
or selectively permeable, allowing only certain ions and organic molecules to cross the membrane. The plasma
membrane contain many proteins, as well as other lipids called sterols. The proteins have various functions, such
as channels (channel proteins) that allow certain molecules into the cell, and receptors (receptor proteins) that bind
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FIGURE 1.1
Phospholipid Bilayer.
The phospholipid
bilayer consists of two layers of phospholipids, with a hydrophobic, or waterhating, interior and a hydrophilic, or waterloving, exterior. The hydrophilic (polar)
head group and hydrophobic tails (fatty
acid chains) are depicted in the single
phospholipid molecule. The polar head
group and fatty acid chains are attached
by a 3-carbon glycerol unit.
The hy-
drophobic fatty acids point towards the
middle of the plasma membrane, and the
hydrophilic heads point outwards.
The
membrane is stabilized by cholesterol
molecules (green). This self-organization
of phospholipids results in a semipermeable membrane which allows only certain
molecules in or out of the cell.
to signal molecules. In Figure 1.1, the smaller (green) molecules shown between the phospholipids are cholesterol
molecules. Cholesterol helps keep the plasma membrane firm and stable over a wide range of temperatures. At least
ten different types of lipids are commonly found in plasma membranes. Each type of cell or organelle will have a
different percentage of each lipid, protein and carbohydrate.
Vocabulary
• cholesterol: A steroid alcohol that is present in animal cells and body fluids, regulates membrane fluidity, and
functions as a precursor molecule in various metabolic pathways.
• hydrophilic: Characteristic of the phospholipid head group; water-loving.
• hydrophobic: Characteristic of the phospholipid tails; water-hating.
• phospholipid: A major component of the cell membrane; consists of two hydrophobic tails and a hydrophilic
phosphate head group.
• phospholipid bilayer: A bilayer (2 layers) of phospholipids that surrounds and encloses a cell; physical
boundary between the intracellular space and the extracellular environment.
• plasma membrane: Thin coat of lipids (phospholipids) that surrounds and encloses a cell; physical boundary
between the intracellular space and the extracellular environment; also called the cell membrane.
Summary
• A phospholipid is a lipid molecule with a polar head group ( a phosphate group) and two non-polar hydrocarbon tails.
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Chapter 1. The Phospholipid Bilayer - Advanced
• The plasma membrane is a selectively permeable lipid bilayer that contains mostly lipids and proteins. These
lipids and proteins are involved in many cellular processes.
Explore More
Use this resource to answer the questions that follow.
• What Is the Phospholipid Bilayer? at http://www.wisegeek.com/what-is-the-phospholipid-bilayer.htm .
1. What is the phospholipid bilayer?
2. Describe the structure of a phospholipid.
3. What are the phospholipid bilayer’s problems?
Review
1. Why can hydrophobic (“water-hating”) molecules easily cross the plasma membrane, while hydrophilic (“waterloving”) molecules cannot?
2. Describe the composition of the plasma membrane.
3. Describe the orientation of the phospholipids in the cell membrane.
4. What is the role of cholesterol in the plasma membrane?
References
1. Mariana Ruiz Villarreal (LadyofHats) for the CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.0
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