Required Reading

Nelson Biology 12: Chapter 2.2, p.81-86

The Fluid Mosaic

Did You Know?

The cell membrane is so thin that if the cell were the size of car, the cell membrane would be as thick as a sheet of paper.

How could something so thin maintain the integrity and the function of the cell? Well, it must be really good at regulating the borders of the cell.

The composition of the cell membrane makes it particularly good at regulating passage. The cell membrane is thus referred to as semi-permeable; only some molecules are able to pass across it.

After much research, scientists have learned that the cell membrane is not a rigid structure, but is, in fact, a fluid collection of proteins, lipids and other molecules that float freely among each other. As such, the cell membrane is often referred to as a fluid mosaic (Figure 1).

This model, the Fluid Mosaic Model, is the basis of our current understanding of the cell membrane.

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Figure 1: The cell membrane is a fluid mosaic of proteins, lipids and carbohydrates. (Nelson Biology 12, p.82)

Phospholipids

Phospholipid molecules make up the majority of the cell membrane. When there are many of these molecules in aqueous environment, their unique structure (Figure 2a) allows them to spontaneously arrange themselves in a bilayer (Figure 2b), with their hydrophobic tails facing each other and their polar heads facing the environment.

This creates the basic structure of the cell membrane. Sometimes the phospholipids can spontaneously arrange themselves in a sphere-like structure called a vesicle or a micelle (Figure 2c).

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Figure 2: Phospholipid bilayer (Nelson Biology 12, p.83)

Lipids and Carbohydrates

Some of the lipid and protein components of the cell membrane are attached to carbohydrate groups or lipid groups that extend into the extracellular environment. Lipids that are attached to carbohydrate groups are called glycolipids and proteins that are attached to carbohydrate groups are called glycoproteins (Figure 1).

These structures are often involved in cell-cell recognition or cell-cell interactions.

Cholesterol

While the kinky tails of the phospholipid molecules already lend a certain fluidity to the cell membrane, other lipid molecules, such as cholesterol, further contribute to maintaining the proper fluidity of the membrane. The more closely that lipids are packed, the less fluid the membrane will be.

If the membrane is too fluid, too many molecules can diffuse in and out. If the membrane is not fluid enough, too many molecules would not be able to pass through.

Cholesterol acts as a membrane stabilizer (Figure 3). At higher temperatures, it prevents the membrane from becoming too fluid. At lower temperatures, it prevents the membrane from becoming too rigid.

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Figure 3: Cholesterol interacts with the phospholipid tails to maintain proper fluidity across different temperatures. (Nelson Biology 12, p.84)

Proteins

The cell membrane serves many functions for the cell, in addition to simply containing the organelles. The different proteins in the cell membrane facilitate this. There are proteins that sit on either side of the membrane (peripheral proteins) and there are proteins that sit within the membrane (integral proteins). There are four functional groups of proteins associated with the cell membrane (Figure 4), and some proteins serve only one of these functions, while others serve more than one of these functions.

transport (Figure 4a)
enzymatic activity (Figure 4b)
triggering signals (Figure 4c)
attachment and recognition (Figure 4d)

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Figure 4: Functions of cell membrane proteins (Nelson Biology 12, p.85)

ePortfolio Submission: Learning Check

Record your answers to the following question in your ePortfolio.

1. List examples for each of the types of membrane proteins. You can use the following table to organize your answer.

Table 1: Examples of membrane proteins

Type of Membrane Protein	Example
transport
enzymatic activity
triggering signals
attachment and recognition

Record your answers to the following questions (Nelson Biology 12, p.86) in your ePortfolio. You can find the answers in your textbook, Appendix C.

2. a) What is meant by membrane fluidity?
2. b) Explain how the chemical makeup of a membrane gives it fluidity.
2. c) Which components affect the fluidity of a membrane?
2. d) How is their movement related to this fluidity?

3. Describe what is meant by integral and peripheral membrane proteins. How do they differ in their chemical makeup and their arrangement of amino acids?