Can any molecule move freely through your cell membranes?
The cell regulates most molecules that pass through the cell membrane. If a molecule is charged
or very big, it won't make it through the cell membrane on its own. However, small, non-charged
molecules like oxygen, carbon dioxide, and water, can pass through the cell membrane freely.
Passive Transport
Recall that the cell membrane is semipermeable. It does not allow everything to pass through.
Some molecules can pass easily through your cell membranes, while others have more difficulty.
Sometimes molecules need the help of special transport proteins to move across the cell
membrane. Some molecules even need an input of energy to help get them across the cell
membrane. The movement of molecules across a membrane without the input of energy is
known as passive transport. When energy is needed, the movement is known as active
transport.
Simple Diffusion
One example of passive transport is diffusion, when molecules move from an area of high
concentration (large amount) to an area of low concentration (low amount). Molecules are said to
flow down their concentration gradient. This type of diffusion proceeds without an input of
energy. In simple diffusion, molecules that are small and uncharged can freely diffuse across a
cell membrane. They simply flow through the cell membrane. Simple diffusion does not require
energy or need the assistance of a transport protein. Other larger or charged molecules that
diffuse across a membrane may need assistance from a protein.
Oxygen is a molecule that can freely diffuse across a cell membrane. For example, oxygen
diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more
concentrated in your lungs than in your blood. Oxygen moves from the high concentration of
oxygen in your lungs to the low concentration of oxygen in your bloodstream.
Passive Transport using Membrane Proteins
Sometimes, molecules cannot move through the cell membrane on their own. These molecules
need special transport proteins to help them move across the membrane, a process known as
facilitative diffusion. These special proteins are called channel proteins or carrier proteins
(Figure below), and they are attached to the cell membrane. In fact, they go through the cell
membrane, from the inside of the cell to the outside. Channel proteins provide an open channel
or passageway through the cell membrane for molecules to move across. Many channel proteins
allow the diffusion of ions. Carrier proteins bind and carry the molecules across the cell
membrane. These proteins bind a molecule on one side of the membrane, change shape as they
carry the molecule across the membrane, and deposit the molecule on the other side of the
membrane. Even though a protein is involved in both these methods of transport, neither method
requires energy. Therefore these are still types of passive transport.
Protein channels and carrier proteins are involved in passive transport.
Vocabulary
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active transport: Movement across a membrane during which molecules move from an
area of low concentration to an area of high concentration.
carrier protein: Transport protein that aids in diffusion by carrying a molecule across
the membrane.
channel protein: Transport protein that aids in diffusion by creating a passageway
through the membrane.
diffusion: Movement of molecules from an area of high concentration to an area of low
concentration.
facilitative diffusion: Diffusion in which assistance by a transport protein is required.
ion: Charged atom; atom that has gained or lost one or more electrons.
passive transport: Movement of molecules across a membrane without the input of
energy.
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simple diffusion: Diffusion in which molecules freely move across the membrane; no
assistance by proteins is necessary.
Summary
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Passive transport does not require energy input.
An example of passive transport is diffusion, the movement of molecules from an area of
high concentration to an area of low concentration.
Carrier proteins and channel proteins are involved in facilitated diffusion.
Practice
Use the resource below to answer the questions that follow.
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Cell Membrane Passive Transport at
http://www.youtube.com/watch?v=JShwXBWGMyY (4:41)
1. What does selectively permeable mean?
2. Can a membrane control the direction of diffusion? Explain your reasoning fully.
3. Give two examples of phospholipid soluble molecules? How can these molecules move
across a cell membrane? What affects the direction of their movement?
4. What is the difference between simple and facilitated diffusion? What are two types of
facilitated diffusion?
5. What characteristics do all channel proteins have? How do channel proteins differ? How
do the differences in channel proteins give cells more control over what enters and exits a
cell?
Review
1. Explain two ways materials can enter the cell through passive transport.
2. Does passive transport involve an expenditure of much energy? Why or why not?