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Chapter 2, Lesson 3
Moving Cellular Material
How do materials enter and leave cells?

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A cell membrane is semipermeable. Some substances can pass
through it.
Materials have to pass through the cell membrane in one of these
ways:
1. Passive transport - 3 kinds
a) Diffusion
b) Osmosis
c) Facilitated diffusion
2. Transport requiring energy - 3 kinds
a) Active transport
b) Endocytosis
c) Exocytosis
1. Passive Transport
Passive transport is the movement of substances through a cell
membrane without using the cell’s energy.
• Small molecules, such as oxygen, carbon dioxide, and water
pass through membranes via passive transport.
• Passive transport depends on the amount of substance on
each side of a membrane.
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Diffusion is the movement of substances from an area of higher
concentration to an area of lower concentration.
 You can smell perfume at a distance because of diffusion of
molecules through the air.
o
Diffusion can take place through gases and liquids.
o
Diffusion can take place through a cell membrane.
o
Diffusion continues until the concentration of a substance is the
same on both sides of the membrane. The substance is then in
equilibrium.
Diffusion through a Membrane
This illustrates what happens in a beaker divided by a
semipermeable membrane.

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A red dye is added to the left size.
After time, the color on both sides is the same.
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Osmosis is defined as the diffusion of water molecules
through a membrane.
Semipermeable cell membranes allow water to pass
through them until equilibrium occurs.
Example: Water can diffuse out from a plant cell’s
central vacuole to the air if the air is dry. The plant
needs watering so it does not dry out.
Facilitated Diffusion
Some molecules are too large or chemically unable to pass
through a membrane by diffusion.
Facilitated diffusion occurs when molecules pass through
a cell membrane aided by special proteins called transport
proteins.
Facilitated diffusion does not require the cell to use energy.
(example - glucose)
There are 2 kinds of transport proteins.
o
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Carrier proteins carry large molecules, such as glucose,
through the cell membrane.
Channel proteins form pores through the cell
membrane.
 Channel proteins aid the diffusion of sodium &
other ions.
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Facilitated Diffusion through a Channel
(See p. 63.)
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Three kinds of Transport Requiring Energy:
Active transport is the movement of substances through a
membrane
only by using the cell’s energy.
 It is important for cells and organelles.
 Active
transport can move substances from areas of lower
concentration to areas of higher concentration.
o (This is the opposite of passive transport.)
 Cells
can take in needed nutrients from the environment
through active transport by using carrier proteins.
 Some molecules and wastes leave cells by active
transport.
Endocytosis is the process during which a cell takes in a
substance by surrounding it with a cell membrane.
Occurs with things that are too large for diffusion or
active transport.

A white blood cell can envelop and take in a bacterium.
This type of endocytosis is called phagocytosis.
Exocytosis is the process during which a cell’s vesicles
release their contents outside the cell. Cells use
exocytosis to release proteins & other substances.
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Active Transport:
See illustration on p. 64
Active Transport, Endocytosis, and Exocytosis
Note how vesicles are involved in endocytosis and exocytosis.
Cell Size and Transport
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The area of the cell membrane is the cell’s surface area.
For a cell to survive, its surface area must be large compared
to its volume because the amount of transport is limited by its
surface area.
As a cell grows, its volume increases faster than its surface
area.