Lesson 6 Membrane Transport
Passive Transport Through the Cell Membrane
How do materials, compounds, elements, and particles move through the cell membrane?
There are two main classes of transportation through the cell membrane, and we will look at both. These are
- passive transport—does not require energy to move particles
- active transport—requires energy to move particles
A6.8 Passive vs. active transport
Diffusion uses the concentration gradient to move small particles across the cell membrane. This means the particles move from where there is a
higher concentration to where there is a lower concentration. For example, if there is more oxygen molecules surrounding the cell than on the inside, then the outside of the cell has a high concentration of oxygen. The oxygen will then move
down the concentration gradient, moving from the outside of the cell to the inside of the cell.
Did You Know?
Concentration does not just mean there is more of one kind of a particle than another. It is the amount of a particle in a given space. For example, if you and your friends are waiting for the school bus outside your school, there is a high concentration of students on the sidewalk. If you missed your bus and are now waiting for someone to come and pick you up, you are the only student left. That would be a low concentration of students on the sidewalk. The given space is the sidewalk, and the students are the particles!
© OpenStax, via Wikimedia Commons
A6.9 Molecules diffusing across the cell membrane
A6.9 Molecules diffusing across the cell membrane
Oxygen is a very small particle, so it can slip between the constantly moving phospholipids in the membrane to get into the cell. This is possible because of the particle model of matter. Remember, the particle model of matter states all
matter is made up of tiny particles and these particles are always moving. It also states particles have spaces between them, which the oxygen uses to slip past the membrane.
The oxygen will stop moving into the cell once the concentration inside the cell is equal to the concentration outside of the cell. In reality, this will never happen, as the cell uses oxygen almost as fast as the oxygen crosses the membrane.
The oxygen will stop moving into the cell once the concentration inside the cell is equal to the concentration outside of the cell. In reality, this will never happen, as the cell uses oxygen almost as fast as the oxygen crosses the membrane.
Did You Know?
A6.10 High concentration of students
Concentration does not just mean there is more of one kind of a particle than another. It is the amount of a particle in a given space. For example, if you and your friends are waiting for the school bus outside your school, there is a high concentration of students on the sidewalk. If you missed your bus and are now waiting for someone to come and pick you up, you are the only student left. That would be a low concentration of students on the sidewalk. The given space is the sidewalk, and the students are the particles!
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A6.11 Osmosis
Osmosis is the movement of water across a concentration gradient. Often the particle dissolved in water is too large to fit through the membrane. Instead of the large dissolved particle moving across the cell membrane, the water moves
across it to try to even out the concentration. This process works the same as diffusion, only it is the water moving across the membrane. One thing to note is that water is always moving across the membrane. If the water is mostly moving
from outside the cell to the inside, then there is a higher concentration of the large dissolved particle on the inside of the cell. If water is mostly moving from the inside of the cell to the outside, then there is a higher concentration
of the large dissolved particle on the outside. A good rule of thumb is that water moves to where the concentration of the large dissolved particle you are looking at is highest. It does this to try to dilute the
concentration, or make the concentration smaller.
If water is moving the same amount into and out of the cell, then the concentration of the large dissolved particle is equal on both sides. This is called being in a state of equilibrium.
If water is moving the same amount into and out of the cell, then the concentration of the large dissolved particle is equal on both sides. This is called being in a state of equilibrium.
We have special names for each of the solutions described above:
Hypotonic:
This is when there is more of the dissolved particle inside the cell, so water moves into the cell to try to even out the concentration. The water moves to the higher concentration of dissolved particles. “Hypo” means smaller
or less, so there is a smaller concentration of the dissolved particle outside the cell. This will cause the cell to swell or burst.
A6.12 Hypotonicity
Hypertonic:
This is when there is more of the particle outside the cell, so water moves out of the cell to try to even out the concentration. The water moves to the higher concentration of dissolved particles. “Hyper” means more. This
will cause the cell to shrink or shrivel up.
Isotonic:
This is when the concentrations are equal, so water moves into and outside the cell at equal rates. “Iso” means the same. The shape of the cell will not change.
A6.14 Isotonicity
A6.15 What happens to a cell in different solutions
Large starch molecules cannot move through the cell membrane. If there was a higher concentration of starch on the outside of the cell, water would move from the inside of the cell to the outside. By doing this, it is lowering the concentration of the starch outside of the cell. This is a hypertonic solution since there is more starch outside the cell.
Now let’s say we are looking at a plant cell that creates starch. There would be a higher concentration of starch on the inside of the cell. Water would move into the cell to try to lower the concentration inside the cell. This is a hypotonic solution since there is a lower concentration of starch on the outside of the cell.
Watch This
Osmosis © YouTube Amoeba Sisters
Watch this video for an overview of osmosis, the three different kinds of solutions, and real-world applications of this type of transport.
A6.16
Facilitated diffusion using carrier proteins
Facilitated diffusion is very similar to diffusion except instead of the particles passing through the membrane, they pass through
special proteins in the membrane called channel proteins or carrier proteins.
Channel proteins go through the entire membrane and act as a passageway from one side of the membrane to the other. Due to their size and shape, they only allow certain molecules to pass through.
Carrier Proteins also go through the entire membrane, but they have a more complex structure. They only allow certain molecules to bind to them due to their shape. Once that molecule has bound to the carrier protein, the carrier protein changes shape and opens up on the other side of the cell. This allows the molecule to pass to the other side.
Facilitated diffusion works the same way as diffusion: The particles move along the concentration gradient. For example, glucose (a sugar used for energy) is too big to fit through the membrane without help. If there is a higher concentration of glucose on the outside of the cell, then the glucose will move from the outside of the cell to the inside of the cell using the carrier or channel protein that is designed for it. The glucose will continue to move into the cell until the concentrations inside and outside of the cell are the same.
Channel proteins go through the entire membrane and act as a passageway from one side of the membrane to the other. Due to their size and shape, they only allow certain molecules to pass through.
Carrier Proteins also go through the entire membrane, but they have a more complex structure. They only allow certain molecules to bind to them due to their shape. Once that molecule has bound to the carrier protein, the carrier protein changes shape and opens up on the other side of the cell. This allows the molecule to pass to the other side.
Facilitated diffusion works the same way as diffusion: The particles move along the concentration gradient. For example, glucose (a sugar used for energy) is too big to fit through the membrane without help. If there is a higher concentration of glucose on the outside of the cell, then the glucose will move from the outside of the cell to the inside of the cell using the carrier or channel protein that is designed for it. The glucose will continue to move into the cell until the concentrations inside and outside of the cell are the same.
Read This
Please read page 275 to 278, stopping at the “Active Transport” heading in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the different kinds of passive transport and how they work. Remember,
if you have any questions or you do not understand something, ask your teacher!
Practice Questions
Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.- Draw a diagram explaining how each of these types of passive transport work.
Your diagrams should be a variation of these:
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A6.11 Osmosis
A6.13 Facilitated diffusion using carrier proteins
A6.17 Facilitated diffusion using channel
proteins
- Explain how the particle model of
matter relates to diffusion, osmosis, and facilitated diffusion.
The particle model of
matter states these four things:
Diffusion: Points i, ii, and iv are used here. Diffusion uses the movement of the particles to move them through the spaces between the particles in the membrane. This only works if the particle is small enough to fit in those spaces.
Osmosis: Points i, ii, and iv are used here. Osmosis uses the movement of the particles in water to move the water through the spaces between the particles in the membrane. Water particles are small enough to fit through the spaces in the membrane.
Facilitated Diffusion: Points i, ii, iii are used here. Facilitated diffusion uses the movement of particles and the attraction particles have for one another to move them through channel or carrier proteins The particles have to be of a particular size and shape to fit through the channel or carrier proteins..
i. All
matter is made of particles, but the particles in different substances may be different in size and composition.
ii. The particles of matter are constantly moving or vibrating.
iii. The particles of matter are attracted to one another or are bonded together.
iv. Particles have spaces between them. These spaces may be occupied by particles by another substance.
ii. The particles of matter are constantly moving or vibrating.
iii. The particles of matter are attracted to one another or are bonded together.
iv. Particles have spaces between them. These spaces may be occupied by particles by another substance.
Diffusion: Points i, ii, and iv are used here. Diffusion uses the movement of the particles to move them through the spaces between the particles in the membrane. This only works if the particle is small enough to fit in those spaces.
Osmosis: Points i, ii, and iv are used here. Osmosis uses the movement of the particles in water to move the water through the spaces between the particles in the membrane. Water particles are small enough to fit through the spaces in the membrane.
Facilitated Diffusion: Points i, ii, iii are used here. Facilitated diffusion uses the movement of particles and the attraction particles have for one another to move them through channel or carrier proteins The particles have to be of a particular size and shape to fit through the channel or carrier proteins..