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Transcript
Name: _________________________________________ Block: _______ Date: _______________
Chemical Activity in the Cell
Introduction to the Cell Membrane
The inside of a single-celled organism is very much alive. However, the physical environment outside
the cell is the opposite—a nonliving place where many changes occur. What stands between a cell and
the potentially hostile environment that surrounds
it? An ultrathin, extremely important layer
separates the living world inside a cell from the
nonliving world outside. This is the cell
membrane, or plasma membrane.
The cell membrane performs two primary, yet very
different, functions: it separates the cell from its
environment and it enables communication and
movement of materials between the cell and its
environment. Without a cell membrane, there
could be no cell. Protein molecules, which float
within lipids in the membrane, enable much of the
movement of materials across the cell membrane.
(See Figure 6-1.) These protein molecules often extend from one side of the membrane to the other. In
multicellular organisms, some cell membrane proteins function as receptor molecules to which
chemical signals may attach as one cell communicates with another cell.
Transport Across the Cell Membrane
For a cell to remain alive, it must have a very special collection of chemicals inside it. These chemicals
may be quite different from the chemicals located in the outside environment. Some substances that are
abundant outside the cell are not found inside the cell. Other substances that are scarce outside the cell
are present in larger quantities inside the cell. The cell membrane creates and maintains this special
environment inside the cell. How does it do this?
The cell membrane allows some substances—that is, molecules—to pass through but keeps other
substances out. This ability to determine which molecules can pass through is called selective
permeability. The cell membrane is selectively permeable—it determines which molecules move
through it and whether the molecules go into or out of the cell. It also makes possible the rapid
transport of some molecules across it, while other molecules pass through slowly.
Passive Transport
Typically, there is an overall or net movement of molecules from an area of high concentration — a
place where molecules are crowded together — to an area of low concentration. This kind of
movement is called diffusion. Molecules are constantly in motion and they naturally move from where
they are more concentrated to where they are less concentrated. This movement happens automatically
with a cell if its membrane is permeable to the molecules and if there is a difference in concentration
of the molecules on either side of the membrane. This is called passive transport, because no energy is
used by the cell and no work is done. For example, one of the basic needs of most cells is oxygen.
There are few oxygen molecules inside a cell, but there is usually an abundance of oxygen molecules
Adapted from AMSCO Chem–Cells Revised Edition
Page 1
in the water or other liquid that surrounds the cell. Thus, oxygen molecules diffuse across the cell
membrane into the cell by passive transport. (See Figure 6-2.)
The diffusion of water molecules across a cell
membrane—so important for living cells—is
given a special name, osmosis. When plant cells
are put in a strong salt solution, the abundant
fresh water inside the plant cells automatically
moves out of the cells, to where there is more
salt and relatively fewer water molecules. Plant
cell membranes can be seen pulling away from
the cell walls as the cells lose water. The reverse
happens when limp celery stems are put in fresh
water. The celery stems are limp because their
cells have too little water in them. When the celery is put in the water, osmosis occurs and water
molecules move into the cells. The cells expand, the cell membranes push against the cell walls, and
the cells—and thus the celery stems—become firm again
Active Transport
The movement of a substance against the concentration gradient is known as active transport. When
substances are moved from an area of low concentration to an area of high concentration, energy is
used and work is done. This kind of transport of materials across the cell membrane is one of the most
important activities of cells. Other than using energy from your food to keep you warm, the most
important use of energy in your body is to help pump substances across the membranes of your cells
by active transport—a process that goes on all the time. Cells get the energy for active transport from
ATP molecules.
A Tour of the Cell: Organelles
Some living things are made up of a single cell, but
most have many cells that work together on behalf
of the organism. Yet, almost everything an organism
does to stay alive is accomplished by each individual
cell: getting food, using food for energy,
transporting substances, growing, reproducing, and
eliminating wastes. Each of these activities involves
a large number of chemical reactions. Organization
is needed for all of these reactions to take place
under the precise control of so many enzymes. In
many cells, these reactions take place in special
internal structures called organelles.
Organelles are dispersed throughout the cytoplasm, which fills the cell and transports materials within
it. Cytoplasm is a thin gel, made up mostly of water, with many other chemicals dissolved in it. The
cell membrane encloses the cytoplasm. In a typical eukaryotic cell (that is, a cell with a nucleus), there
are many ribosomes, the organelles at which proteins are built. The lysosomes, which are scattered
throughout the cell, are the structures involved in the breaking down of food. Complicated-looking
Adapted from AMSCO Chem–Cells Revised Edition
Page 2
mitochondria, shaped like tiny kidney beans,
are the organelles in which the cell’s energy is
released. (See Figure 6-7.)
By far the largest structure in a cell is the
nucleus, which is responsible for information
storage. The nucleus often fills the entire
central portion of the cell. In addition, other
specialized structures may be present in a cell
to handle other functions, such as: the Golgi
complex, which packages many materials;
vacuoles, which store materials such as
wastes; and, in plants, chloroplasts, the
organelles in which plants convert energy from
the sun into food. (See Figure 6-8.)
Review Questions
1. Which letter indicates the cell structure that directly controls the movement of molecules into
and out of the cell?
A.
B.
C.
D.
A
B
C
D
2. Which statement about the functioning of the cell membrane of all organisms is not correct?
A. The cell membrane forms a boundary that separates the cell’s contents from the outside
environment.
B. The cell membrane forms a barrier that keeps all substances that might harm the cell
from entering it.
C. The cell membrane is capable of receiving and recognizing chemical signals.
D. The cell membrane controls the movement of molecules into and out of the cell.
Adapted from AMSCO Chem–Cells Revised Edition
Page 3
3. What happens during diffusion?
A. Molecules move automatically from an area of higher concentration to an area of lower
concentration.
B. Molecules are pumped from an area of lower concentration to an area of higher
concentration.
C. An enzyme joins with a particular molecule.
D. A catalyst speeds up the rate of a chemical reaction.
Base your answer to question 4 on the following diagram, which represents a cell in water. Formulas
of molecules that can move freely across the cell membrane are shown. Some molecules are located
inside the cell and others are in the water outside the cell.
4. Based on the distribution of these molecules, what would most likely happen to them after a
period of time has passed?
A. The concentration of O2 will increase inside the cell.
B. The concentration of O2 will remain the same outside the cell.
C. The concentration of CO2 will remain the same inside the cell.
D. The concentration of CO2 will decrease outside the cell.
5. A plant cell shrinks when placed in salt water due to the osmosis of
A. water molecules out of the cell
B. water molecules into the cell
C. salt into the cell
D. salt out of the cell
6. Placing limp celery in water will make the celery stalk firm again due to
A. diffusion
B. osmosis
C. active transport
D. a catalyst
7. A high concentration gradient means that the concentration of a substance is
A. low on both sides of the cell membrane
B. high on both sides of the cell membrane
C. about the same on both sides of the cell membrane
D. high on one side of the cell membrane and low on the other side
Adapted from AMSCO Chem–Cells Revised Edition
Page 4
Base your answer to question 8 on the diagram below, in which the dark dots represent small
molecules. These molecules are moving out of the cells, as indicated by the arrows. The number of
dots represents the relative concentrations of the molecules inside and outside of the two cells.
8. ATP is being used to move the molecules out of
A. cell A only
B. cell B only
C. both cell A and cell B
D. neither cell A nor cell B
Refer to the set of diagrams below, which shows the movement of a large molecule across a cell
membrane, to answer question 9.
9. Which process is best represented by this set of diagrams?
A. active transport
B. protein building
C. diffusion
D. gene transfer
10. While viewing a slide of rapidly moving sperm cells, a student concludes that these cells
require a large amount of energy to maintain their activity. The organelles that most directly
provide this energy are known as
A. vacuoles
B. chloroplasts
C. ribosomes
D. mitochondria
11. The organelle that stores wastes for the cell is the
A. vacuole
B. chloroplast
C. ribosome
D. Golgi complex
Adapted from AMSCO Chem–Cells Revised Edition
Page 5
12. Chloroplasts are important because they
A. are necessary to release stored energy
B. store wastes in both plants and animals
C. use energy from the sun to make food
D. are in the nuclei of both plant and animal cells
13. Discuss the meaning of “selective permeability” for a cell membrane by explaining the
following:
 why the cell membrane is said to be selectively permeable - _________________________
_______________________________________________________________________
_______________________________________________________________________

why this characteristic is important to the health of a cell - __________________________
_______________________________________________________________________
_______________________________________________________________________
Base your answers to questions 14 through 16 on the diagram below, which represents a unicellular
organism in a watery environment. The small triangles represent molecules of a specific substance.
14. What kind of transport is represented by arrow A? ___________________________________
15. State two ways in which active transport is different from diffusion (passive transport).
(1) _____________________________________________________________________
(2) _____________________________________________________________________
16. In the cells of multicellular organisms, structure B often contains special proteins known as
receptor molecules. What specific function do these protein molecules carry out for the cell?
________________________________________________________________________
________________________________________________________________________
17. What, specifically, would happen to a cell if its mitochondria were removed? Explain why.
________________________________________________________________________
________________________________________________________________________
Adapted from AMSCO Chem–Cells Revised Edition
Page 6
Refer to the diagrams below to answer questions 18 and 19.
18. The diagrams represent the movement of molecules from an area of
A. low concentration to an area of high concentration
B. high concentration to an area of low concentration
C. low concentration to an area of equal concentration
D. high concentration to an area of equal concentration
19. The diagrams could be used to illustrate all of the following types of transport except
A. diffusion
B. osmosis
C. active
D. passive
20. Just like complex organisms, cells are able to survive by coordinating various activities.
Complex organisms have a variety of systems, and cells have a variety of organelles that work
together for survival. Describe the roles of two organelles. In your answer be sure to include:

the names of two organelles and the function of each
(1) _______________________________________________________________________
(2) _______________________________________________________________________

an explanation of how these two organelles work together
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Adapted from AMSCO Chem–Cells Revised Edition
Page 7