Active and Passive Transport
The cell membrane, also called the plasma membrane, is selectively permeable, which means that
some substances can pass through it while others cannot. Oxygen, food molecules, and waste products all
must pass through the cell membrane. Substances that can move into and out of a cell do so by one of
two methods: passive or active transport.
The movement of dissolved materials through a cell membrane without using cellular energy is
called passive transport. Diffusion and osmosis are both types of passive transport. Diffusion is the main
method by which small molecules move across the cell membrane. Diffusion is the process by which molecules
tend to move from an area of higher concentration to an area of lower concentration. The concentration of a
substance is the amount of the substance in a given volume. Diffusion is caused by molecules moving and
colliding. The collisions cause the m olecules to push away from one another and spread out. Molecules diffuse
through the cell membrane into a cell when there is a higher concentration of the molecules outside the cell than
inside the cell.
The diffusion of water molecules through a selectively
permeable membrane is called osmosis. Because cells cannot
function properly without adequate water, many cellular processes
depend on osmosis. In osmosis, water molecules move by diffusion
from an area where they are highly concentrated through the cell
membrane to an area where they are less concentrated.
When a cell needs to take in materials that are in higher concentration inside the cell than outside
the cell, the movement of the materials requires energy. Active transport is the movement of materials
through a cell membrane using cellular energy (ATP). The main difference between passive transport and
active transport is that active transport requires the cell to use its own energy while passive transport
does not.
Cells have several ways of moving materials by active
transport. This mostly occurs because the molecule entering the cell is
too large to pass through the cell membrane without using energy. In
one method, transport proteins in the cell membrane "pick up"
molecules outside the cell and carry them in. Another method of active
transport is endocytosis, in which the cell membrane wraps
around, or engulfs, a particle and forms a vacuole within the cell.
This is known as engulfing. Engulfing occurs again when a vesicle
carrying something that needs to leave the cell comes to the cell
membrane, it will attach itself to the membrane and then the
materials are pushed out of the cell. This is known as exocytosis.
Most cells are very small. Remember that all materials move into
and out of cells through the cell membrane. The amount of cell
membrane limits the ability of cells to either get substances from the outside in or transport waste and other
materials out. Once a molecule enters a cell, it is carried to its destination by a stream of moving cytoplasm . As
a cell grows, its surface area/volume ratio eventually becomes too small to allow substances to enter or leave the cell
quickly. The larger it gets the longer it takes to move things to the center of the cell. At this point, the cell must
divide to form two smaller cells. This happens in a process called mitosis, or cell division.
Cell Growth -Your body, like that of most animals, is always growing. This
growth is the result of two processes: cell growth and cell division.
Cell growth causes a cell to become larger.
Cell division increases the number of cells. No one knows exactly how many cells make up an
adult human body, but scientists estimate the number to be between 10 trillion and 100 trillion cells.
Two factors limit the growth of cells.
1.) Cell volume, or the total amount of space a cell occupies.
2.) Surface area, or the total amount of the surface of the cell membrane that is exposed to
the environment.
Cell growth is limited by the ratio of a cell's surface area to its volume.
As a cell grows, its volume increases more quickly than its surface area. Thus, the ratio of the cell's
surface area to volume decreases as the cell grows. The following table illustrates how the surface
area/volume ratio becomes smaller as a cell grows larger. In this example, the cell is assumed to
have the shape of a cube.
Cell Surface Area to Volume Ratio (cube)
Length of Side
1 mm
3mm
Surface Area
Surface Area
Volume Ratio
Volume
.
1mm x 1mmx 6s =
6mm2
1mmx1mmx1mm=
1 mm3
6:1
3mm x3mm x 6s=
54 mm2
3mm x3mm x 3mm =
27 mm3
2:1
The cell membrane regulates what substances can enter and leave a cell. As a cell grows, its
surface area/volume ratio eventually becomes too small to allow substances to enter or leave the
cell quickly. At this point, the cell must divide to form two smaller cells. Each smaller cell will have a
surface area/volume ratio large enough to allow substances to enter and leave the cell quickly.
What is the correct cell surface area to volume ratio for the cell represented in the table?
Cell Surface Area to Volume Ratio (cube)
Length of Side
Surface Area
2cm
A. 8:1
24cm
2
Volume
8cm
B. 3:1
3
C. 6:1
Surface Area/Volume Ratio
?
D. 1:3
What is the surface area of a cell represented in the table?
Cell Surface Area to Volume Ratio
Surface Area
Volume
?
A. 1.5 mm2
1.5mm
B. 3 mm2
3
Surface Area/Volume Ratio
4:1
C. 4mm2
D. 6mm2
If the volume of a cell increases, the cell’s ability to move materials from the cell membrane
throughout the cell will be more difficult for the cell. Diffusion will not be able to take to move
materials in and out of the cell. The cell will not be able to perform the necessary tasks it needs
to in order to continue surviving, such as digestion, excretion, circulation and reproduction. The
cell will then take the next step and that is to divide, or reproduce itself.
Name _____________________________ Period__________ Date__________________
Active and Passive Transport
1.The cell membrane is ____________________
pass through it while others cannot.
_______________________ which means that some substances can
2. List 2 ways that substances can move into and out of a cell.
a. _______________________________________
b. ________________________________________
3. What is passive transport? _______________________________________________________________________
_____________________________________________________________________________________________
4. What are two types of passive transport? ____________________ and __________________________
5. What is diffusion? _________________________________________________________________________
6. In___________________________, water molecules diffuse through a selectively permeable membrane
7. Draw molecules on Part B of the diagram below to show how the molecules are distributed inside and outside
the cell after diffusion has occurred.
A. Before diffusion
B. After Diffusion
8. What is active transport? _____________________________________________________________________
9. Why would a cell have to use active transport? ______________________________________________________
10. List three ways that the cell moves things by active transport.
a. _______________________________
b. __________________________ c. ___________________________
11. What is exocytosis?____________________________________________________________________
_________________________________________________________________________________________
12. What is endocytosis? __________________________________________________________________________
____________________________________________________________________________________________________
13. True or False? As a cell gets larger, it takes longer for a molecule that to reach the middle of the cell.
14. If a cell gets too large for substances to travel through it, (a) what does it have to do? (b). What is the process called?
a. _________________________________________
b.______________________________________
Name_________________________
Active and Passive Transport
Understanding Main Ideas
Fill in the blank to identify the process illustrated in each of the following figures.
1.
3.
2.
96.5%
water
Water moves out of the cells of
a saltwater fish and into the
ocean.
1. _____ __________ _
155
units of
sodium
Oxygen moves from the
lungs into the bloodstream.
2.__________ _
Sodium is pumped out of a nerve cell.
3.________________________
___
Answer the following questions.
4. Explain how osmosis differs from diffusion.
________________________________________________________________________________
______________________________________________________________________________
5. Compare and contrast active and passive transport
_______________________________________________________________________________
_______________________________________________________________________________
6. Identify two methods of active transport.
________________________________________________________________________________
7. State one reason that cells are small.
________________________________________________________________________________
Building Vocabulary
If the statement is true, write true. If the statement is false, change the underlined word or words to make the statement
true.
_________________ 8. Selectively permeable means letting some but not all substances pass through.
_________________ 9. Osmosis is the process by which molecules tend to move from an area of
higher concentration to an area of lower concentration.
_________________ 10. The process by which water moves across a selectively permeable membrane
is called diffusion.
_________________ 11.Diffusion and osmosis are types of active transport.
_________________ 12.Passive transport requires the cell's own energy.