Objectives
Describe the structure of cellular membranes.
Identify functions of proteins in cellular membranes.
Key Term
phospholipid bilayer
The plasma membrane can be thought of as the edge of life—it is the
boundary that separates the interior of a living cell from its surroundings.
The membrane is a remarkable film so thin that you would have to stack
8,000 of them to equal the thickness of the page you are reading. Yet the
plasma membrane can regulate the traffic of chemicals into and out of the
cell. The key to how a membrane works is its structure.
Membrane Structure
Membranes help keep the functions of a eukaryotic cell organized. As
partitions, the membranes isolate teams of enzymes within a cell's
compartments. But membranes are more than cellular room dividers.
Membranes, unlike walls, regulate the transport of substances across the
boundary, allowing only certain substances to pass. In this way, a
membrane maintains a specific chemical environment within each
compartment it encloses.
The plasma membrane and other membranes of a cell are composed
mostly of proteins and a type of lipid called phospholipids. A
phospholipid molecule (Figure 6-7) is structured much like the fat
molecules you learned about in Chapter 5 but has only two fatty acids
instead of three. The two fatty acids at one end (the tail) of the
phospholipid are hydrophobic (not attracted to water). The other end (the
head) of the molecule includes a phosphate group (PO43-), which is
negatively charged and hydrophilic (attracted to water). Thus, the tail end
of a phospholipid is pushed away by water, while the head is attracted to
water.
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Wednesday, October 19, 2011 9:14:03 AM CT
Figure 6-7
A space-filling model of a phospholipid depicts
the hydrophilic head region and the
hydrophobic fatty acid tails. The simplified
representation of phospholipids used in this
book looks something like a lollipop (the head)
with two sticks (the tails).
The structure of phospholipids enables them to form boundaries, or
membranes, between two watery environments. For example, the plasma
membrane separates a cell's aqueous cytoplasm from the aqueous
environment surrounding the cell (Figure 6-8). At such boundaries, the
phospholipids form a two-layer "sandwich" of molecules, called a
phospholipid bilayer, that surrounds the organelle or cell. In the bilayer
membrane, the phosphate ends face the watery inside and watery outside
of the cell. The hydrophobic fatty acid tails are tucked inside the
membrane, shielded from the water. These hydrophobic tails play a key
role in the membrane's function as a selective barrier. Nonpolar molecules
(such as oxygen and carbon dioxide) cross with ease, while polar
molecules (such as sugars) and many ions do not.
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Wednesday, October 19, 2011 9:14:03 AM CT
Figure 6-8
A cell's plasma membrane contains a diversity of proteins that drift about
in the phospholipid bilayer. Even the phospholipid molecules themselves
can move along the plane of the fluid-like membrane. Some membrane
proteins and lipids have carbohydrate chains attached to their outer
surfaces.
Together, the phospholipids, proteins, and other membrane components
form a dynamic structure. Membranes are fluid-like, rather than sheets of
molecules locked rigidly in place. Most of the proteins drift about freely
in the plane of the membrane, much like "icebergs" floating in a "sea" of
phospholipids.
The Many Functions of Membrane Proteins
Many types of proteins are embedded in the membrane's phospholipid
bilayer. Other molecules, such as carbohydrates, may be attached to the
membrane as well, but the proteins perform most of the membrane's
specific functions. For example, sets of closely placed enzymes built into
the membrane carry out some of a cell's important chemical reactions
(Figure 6-9a). In Chapters 7 and 8, you will learn more about how such
membrane-bound enzymes contribute to cellular processes.
Another function of membrane proteins is to help cells—especially cells
that are part of a multicellular organism—communicate and recognize
each other (Figure 6-9b and c). For example, chemical signals released by
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Wednesday, October 19, 2011 9:14:03 AM CT
one cell may be "picked up" by the proteins embedded in the membrane
of another cell.
Still other membrane proteins, called transport proteins, help move
certain substances such as water and sugars across the membrane (Figure
6-9d). Although small nonpolar molecules such as carbon dioxide and
oxygen pass freely through the membrane, many essential molecules need
assistance from proteins to enter or leave the cell. You will read more
about how molecules move across a membrane in Concept 6.3.
Figure 6-9
Many functions of the plasma membrane involve its embedded proteins. a.
Enzymes catalyze reactions of nearby substrates. b. Molecules on the
surfaces of other cells are "recognized" by membrane proteins. c. A
chemical messenger binds to a membrane protein, causing it to change
shape and relay the message inside the cell. d. Transport proteins provide
channels for certain solutes.
Concept Check 6.2
1. Describe how phospholipid molecules are oriented in the plasma
membrane of a cell.
2. What is the function of a transport protein?
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Wednesday, October 19, 2011 9:14:03 AM CT