Download Cell processes Membranes

Student:
Class:
Date:
Cell processes
Student Activity Sheet 1; use with Exploring, “Osmosis moves water molecules”
Membranes
In some ways, a cell is similar to a carton of milk. The membrane around a cell is like the walls of a
milk carton. This membrane forms a boundary that separates the inside of the cell from the outside. Like
a carton of milk, a cell contains a fluid substance with a variety of molecules in it.
The cell membrane is more complicated, however, than the walls of a milk carton. The cell membrane
only lets some things pass through it. To act as an effective boundary, the membrane must prevent
molecules from moving freely into or out of the cell. The membrane must be impermeable to most
substances; in other words, it must prevent most substances from being able to pass into or out of the
compartment. However, the molecules that the cell needs for its activities must be able to enter the cell
through the membrane. Because cell membranes are only permeable to certain molecules, they are said
to be selectively permeable. A membrane’s selective permeability (its ability to regulate the passage of
molecules) depends on its structure.
As Figure 1 shows, a typical cell membrane is made up primarily of fat (lipid) molecules. The fat
molecules are not locked rigidly in place. They drift randomly in the plane of the membrane. An
important property of lipid molecules is that they do not mix well with water. In the cell membrane, the
fat (lipid) molecules are arranged into two layers (called the lipid bilayer). The parts of the fat
molecules that have the highest tendency to interact with water are oriented toward the outside and
inside of the cell. The parts of the fat molecules with the lowest tendency to interact with water are
oriented toward the interior of the
membrane. The interior part of the
membrane prevents molecules that are
water soluble from easily passing through
it. By contrast, fat-soluble molecules
move through the membrane with ease.
Even though water molecules are not fat
soluble, they can move through the
membrane, however. This is because they
are very small and their electrical charges
are very slight.
Figure 1: Diagram of a section of the lipid
bilayer of a cell membrane. Here, the lipids are
oriented in such a way that the interior of the
membrane repels water. Protein molecules may
span the membrane or be exposed on the inner or
outer surface. What structures facilitate selective
permeability?
Copyright © 2008 Agile Mind, Inc.™ Content from
BSCS Biology: A Human Approach, 3rd ed.
Copyright © 2006 by BSCS. All rights reserved.
Activity Sheet 1, Page 1 of 2
Biology
Student:
Class:
Date:
Cell processes
Student Activity Sheet 1; use with Exploring, “Osmosis moves water molecules”
Look at Figure 1 again. Notice that the membrane contains many protein molecules positioned among
the fat molecules. Proteins are large molecules made up of many smaller molecules called amino acids.
The amino acids are linked together to form a long, folded chain. Some of these protein molecules,
particularly those on the outer surface of the membrane, act as receptors. Receptors bind specifically to
molecules such as hormones. The hormones act as chemical messengers. By binding to their receptors,
hormones trigger the cell to respond to some body condition in an appropriate way.
In addition to receptor proteins, cell membranes contain many other proteins. These proteins allow
specific molecules to move into and out of the cell. In other words, some membrane proteins allow the
membrane to be selectively permeable. For example, certain proteins form channels through which
glucose and other sugars move. Molecules like these are too big to pass directly through the lipid bilayer
part of the membrane. The only way that they can enter the cell is through a specific protein channel.
Other proteins form channels through which ions (molecules that have an electrical charge) may pass.
For example, when table salt (NaCl) is dissolved in water, it forms the ions Na+ and Cl–. Ions like these
are relatively small. Nonetheless, they cannot pass directly through the lipid bilayer because the fat
molecules repel ions. As a result, they must travel into and out of the cell by way of protein-based ion
channels.
Thus, biological membranes are much more complex than the walls of a milk carton or even an insulated
beverage container. They also perform a far more complex and important role in regulating internal
conditions.
Copyright © 2008 Agile Mind, Inc.™ Content from
BSCS Biology: A Human Approach, 3rd ed.
Copyright © 2006 by BSCS. All rights reserved.
Activity Sheet 1, Page 2 of 2
Biology