Download HERE

33
Cell Processes
2
section ●
Moving Cellular Materials
What You’ll Learn
how selectively
permeable membranes
work
■ about diffusion and
osmosis
■ the differences between
passive transport and
active transport
■
Study Coach
Make Flash Cards Think of
a quiz question for each
paragraph. Write the question
on one side of the flash card and
the answer on the other side.
Keep quizzing yourself until you
know all of the answers.
C Describe Make a two-tab
●
book, as shown below. Use the
Foldable to take notes about
active and passive transport.
40
Cell Processes
Before You Read
On the lines below, describe the purpose of window screens.
Think of what they keep out and what they allow to pass
through.
Read to Learn
Passive Transport
Window screens keep unwanted things, such as bugs,
leaves, and birds, outside. But screens do let some things,
such as air and smoke, pass through.
Cells get food, oxygen, and other substances from their
environments. They release waste materials into their
environments. The membrane around the cell works like a
window screen works for a room. A window screen is
selectively permeable (PUR mee uh bul). It lets things like
air come into the room and keeps some things like bugs out
of the room. A cell’s membrane also is selectively permeable.
It lets some things come into or leave the cell. It also keeps
other things from entering or leaving the cell.
Things move through a cell membrane in several ways.
The movement depends on the size of the molecules, the
path the molecules take, and whether energy is needed.
When substances move through the cell membrane without
using energy, this movement is known as passive transport.
Three types of passive transport are diffusion, osmosis, and
facilitated diffusion. The type of transport depends on what
is moving through the cell membrane.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
chapter
How does diffusion create equilibrium?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Molecules move constantly and randomly. You might
smell perfume when you walk past someone who is
wearing it. The perfume molecules move freely throughout
the air. This random movement of molecules from an area
where there are more of them into an area where there are
fewer of them is called diffusion. Diffusion is a type of
passive transport. Molecules will keep moving from one
area to another until the number of these molecules is
equal in the two areas. When this occurs, equilibrium is
reached and diffusion stops.
All cells in your body use oxygen. Oxygen moves through
your body in the red blood cells. When your heart pumps
blood to your lungs, your red blood cells contain few
oxygen molecules. Your lungs have many oxygen molecules.
Oxygen molecules move, or diffuse, from your lungs into
your red blood cells. The blood continues its journey
through your body. When the blood reaches your big toe,
there are more oxygen molecules in your red blood cells
than in the cells of your big toe. The oxygen diffuses from
your red blood cells to your big toe’s cells. The process is
shown in the figure below.
1.
Determine How does
diffusion create
equilibrium?
Picture This
2.
Explain Use the figure to
explain to a partner how
diffusion works.
In your big toe,
oxygen diffuses out
of red blood cells.
Air sac in lung
Toe cell
Oxygen
Oxygen
Red blood
cell
In your lungs, oxygen
diffuses into red blood cells.
Nucleus
Red
blood cell
Reading Essentials
41
What is facilitated diffusion?
Some substances pass easily through the cell membrane
by diffusion. Larger substances may need help passing
through the cell membrane. Transport proteins in the cell
membrane help these substances enter the cell. This process
is called facilitated diffusion. Transport proteins are similar
to the gates at a stadium. Gates are used to move people
into and out of the stadium. Similarly, transport proteins
are used to move substances into and out of a cell.
Describe What do
transport proteins do?
What is osmosis?
Remember that water makes up a large part of living
matter. Water molecules move by diffusion in and out of
cells. The diffusion of water through the cell membrane is
called osmosis.
What happens when you do not water plants? As a plant
cell loses water, its cell membrane pulls away from the cell
wall. This reduces pressure against the cell wall, and the
plant cell becomes limp, as shown on the left in the figure
below. The plant wilts because more water leaves the plant’s
cells than enters them.
When you water the plant, the water moves through the
cell membranes and fills the cells with water. The plant’s cell
membranes push against their cell walls, and the cells
become firm, as shown on the right in the figure below.
Picture This
4.
Explain Why does a
plant wilt?
The carrot stick becomes
limp when more water
leaves each of its cells
than enters them.
42
Cell Processes
Equilibrium occurs
when water leaves
and enters the cells
at the same rate.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
3.
Osmosis in Animal Cells Osmosis also takes place in
animal cells. If animal cells were placed in pure water, they
too would swell up. However, animal cells are different from
plant cells. Just like an overfilled balloon, animal cells will
burst if too much water enters the cell.
Active Transport
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Suppose you have just left a theater at the end of a movie
when you remember that you left your jacket inside. You
have to move against the crowd to enter the theater and get
your jacket. Which takes more energy—leaving the theater
with the crowd or moving against the crowd to get back
into the theater? Something similar to this happens in cells.
Active transport takes place when energy is needed to
move substances through a cell membrane. For example, root
cells require minerals from the soil. The root cells already
have more molecules of the minerals than the surrounding
soil. Normally, the mineral molecules would move out of
the root into the soil until equilibrium is reached. But the
root cells need to take in the minerals from the soil.
Like facilitated diffusion, active transport uses transport
proteins. In active transport, transport proteins bind with
the needed substance and cellular energy is used to move it
through the cell membrane.
5.
Explain What must be
used with transport
proteins to move a
substance through a cell
membrane in active
transport?
Endocytosis and Exocytosis
Some molecules are too large to move through the cell
membrane by diffusion or by using transport proteins.
Large protein molecules, for example, can enter a cell when
they are surrounded by the cell membrane. The cell
membrane folds around the molecule, completely surrounding
it. The sphere created is called a vesicle. The sphere pinches
off and moves the molecule into the cell. The process of
taking substances into a cell by surrounding it with the cell
membrane is known as endocytosis (en duh si TOH sus).
Some one-celled organisms take in food this way.
Exocytosis (ek soh si TOH sus) is the process in which
the contents of a vesicle are moved outside a cell. A vesicle’s
membrane joins with a cell’s membrane, and the vesicle’s
contents are released. Exocytosis occurs in the opposite way
that endocytosis does. Cells in your stomach use exocytosis
to release chemicals that help digest food.
6.
Explain What happens
during exocytosis?
Reading Essentials
43
After You Read
Mini Glossary
active transport: takes place when energy is needed to
move substances through a cell membrane; uses
transport proteins
diffusion: random movement of molecules from an area
where there are more of them into an area where there
are fewer of them
endocytosis (en duh si TOH sus): process of taking
substances into a cell by surrounding them with the
cell’s membrane
equilibrium: the number of molecules in two areas are the
same
exocytosis (ek soh si TOH sus): process in which the
contents of a vesicle are moved outside a cell
osmosis: the diffusion of water through the cell membrane
passive transport: movement of substances through the
cell membrane without using energy
1. Review the terms and their definitions in the Mini Glossary. Choose one term that
explains how substances move into and out of cells and write a sentence explaining how
the process works.
Active Transport
Passive Transport
Both Active
and Passive
Transport
End of
Section
44
Cell Processes
Visit booka.msscience.com to access your textbook,
interactive games, and projects to help you learn more about
the movement of cellular materials.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
2. Complete the Venn diagram below to help you compare active and passive transport.