Download CK12 Passive Transport - Diffusion, Osmosis, and Facilitated Diffusion

Passive Transport: Diffusion,
Facilitated Diffusion &
Osmosis
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Printed: July 13, 2015
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Chapter 1. Passive Transport: Diffusion, Facilitated Diffusion & Osmosis
C HAPTER
1
Passive Transport:
Diffusion, Facilitated Diffusion &
Osmosis
•
•
•
•
Describe a concentration gradient.
Define diffusion.
Distinguish between diffusion and osmosis.
Differentiate between hypertonic, hypotonic, and isotonic solutions.
What happens if you put a few drops of food coloring in water?
Over time, the molecules of color spread out through the rest of the water. When the molecules are evenly spread
throughout the space, the water will become an even color. This process of molecules moving from an area where
there are lots of the same molecules to an area where there are fewer of these same molecules is known as diffusion.
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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. 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.
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). The amount of a substance in relation to the total volume
is the concentration. They will continue to move in this way until equilibrium is reached or when the particles
are equally disbursed throughout the area. In other words, there is no longer an area of high concentration or
low concentration. This is a natural process and does not require energy because molecules are always in motion.
Diffusion can occur across a semipermeable membrane, or a membrane that allows some things to cross such
as water and oxygen but not allow other things to cross such as sugar. In diffusion, molecules that are small and
uncharged like water and oxygen can freely diffuse across a cell membrane. They simply flow through the cell
membrane. Other larger or charged molecules that diffuse across a membrane may need assistance from a protein
doorway.
As stated, oxygen is a molecule that can freely diffuse across a cell membrane. For example, an oxygen molecule
can diffuse 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 in your lungs to the low concentration of oxygen in
your bloodstream.
FIGURE 1.1
The figure on top shows a high concentration of particles on the left and a low
concentration on the right. The particles
will move from left to right until they reach
equilibrium which is pictured on the bottom. In this case, the particles are evenly
spread across the area.
Facilitated Diffusion
As was stated in the previous section, a semipermeable membrane allows small particles like oxygen and water
to pass right through the cell membrane. Particles such as sugar and amino acids are too large to fit through the
semipermeable cell membrane and need to find a passage way through the membrane. In facilitated diffusion, a
protein doorway in the membrane allows these larger molecules to cross the membrane. Even though these larger
molecules move through a protein doorway, they still are moving from an area of higher concentration to lower
concentration so they are not using energy.
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Chapter 1. Passive Transport: Diffusion, Facilitated Diffusion & Osmosis
FIGURE 1.2
This is an example of facilitated diffusion.
On the left is a protein doorway that is
called a pore. The pore is large enough
to allow bigger molecules passageway
through the cell membrane. The one to
the right is a protein doorway that allows
one molecule to pass at a time.
Osmosis
The diffusion of water, and water only, across a membrane because of a difference in concentration is called osmosis.
Applications of Osmosis
How do marine animals keep their cells from shrinking? Marine animals live in salt water, which is an environment
where there is more salt in the water than in their cells. To prevent losing too much water from their bodies, these
animals intake large quantities of salt water and then secrete the excess salt. ( Figure below ).
Vocabulary
• Active Transport: The movement of molecules across a membrane which requires energy.
• Concentration: The amount of a substance in relation to the total volume
• Diffusion: The movement of molecules from an area of high concentration through a membrane to an area of
low concentration.
• Equilibrium: A situation where molecules are evenly distributed in an area.
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FIGURE 1.3
Osmosis causes fish cells to exchange
fresh water and salt by losing or gaining
water.
• Facilitated Diffusion: The movement of larger particles through a protein doorway from an area of high
concentration to low concentration.
• Osmosis: A special case of diffusion where water moves across a semi-permeable membrane.
• Passive Transport: The movement of molecules across a membrane without the input of energy
• Semi-permeable membrane: A membrane that allows certain things to cross and does not let other things to
cross.
Summary
• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.
• Osmosis is a special case of diffusion. When water crosses a semipermeable membrane from an area of high
concentration to an area of low concentration, this is called osmosis.
Explore More
Use the resource below to answer the following questions.
• Osmosis at http://www.youtube.com/watch?v=7-QJ-UUX0iY (5:07)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57351
1. What is osmosis?
2. How would a salt solution affect a cell? How could this affect cellular processes?
Review
1. Describe the process of diffusion.
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Chapter 1. Passive Transport: Diffusion, Facilitated Diffusion & Osmosis
2. Define osmosis.
3. Why doesn’t diffusion or osmosis require energy to occur?
4. If a plant cell is placed in a solution and the cell shrivels up, what type of solution was it placed in? How do
you know?
References
1. Mariana Ruiz Villarreal (LadyofHats). Fish cells and osmosis .
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