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Diffusion and Osmosis (Passive Transport)


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Introduction

Cells are found in all different types of environments, and these environments are constantly changing. One-celled organisms, like bacteria, can be found on your skin, or in the ground, or in all different types of water. The cells of your body interact with the food you eat, and also with other cells in your body. All cells need a way to protect themselves. This job is done by the cell membrane.

The cell membrane is semipermeable, or selectively permeable , which means that only some molecules can get through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. For example, if your body cells have a temperature of 98.6 degrees F, and it is freezing outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop.

How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell? The ways that cells control what passes through the cell membrane will be the focus of this lesson.

What is Transport?

Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a "bilayer") and proteins ( Figure below ). A single phospholipid molecule has two parts:

  1. A head that is hydrophilic , or water-loving.
  2. A tail that is hydrophobic , or water-fearing.

There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving and they want to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane because water is not found in this space. An interesting quality of the plasma membrane is that it is constantly moving, like a soap bubble. Water and small molecules such as oxygen and carbon dioxide can pass freely through the membrane, but larger molecules cannot easily pass through the plasma membrane. Some molecules need a special way to get across the membrane.

The cell membrane is made up of a phospholipid bilayer, two layers of phospholipid molecules.

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 molecules to an area where there are fewer molecules is known as diffusion.

Diffusion

Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance ( Figure below ). The amount of a substance in relation to the total volume is the concentration . During diffusion, molecules are said to flow down their concentration gradient , flowing from an area of high concentration to an area of low concentration. This is a natural process and does not require energy. Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration.

Diffusion is the movement of a substance from an area of a higher amount toward an area of lower amount. A concentration gradient initially exists across the cell membrane. Equilibrium is reached when there is an equal amount of the substance on both sides of the membrane.

Osmosis

The diffusion of water across a membrane because of a difference in concentration is called osmosis . Let's explore three different situations and analyze the flow of water.

  1. A hypotonic solution means the environment outside of the cell has a lower concentration of dissolved material than the inside of the cell. If a cell is placed in a hypotonic solution, water will move into the cell. This causes the cell to swell, and it may even burst.
  2. A hypertonic solution means the environment outside of the cell has more dissolved material than inside of the cell. If a cell is placed in a hypertonic solution, water will leave the cell. This can cause a cell to shrink and shrivel.
  3. An isotonic solution is a solution in which the amount of dissolved material is equal both inside and outside of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell.

Applications of Osmosis

How do marine animals keep their cells from shrinking? How do you keep your blood cells from bursting? Both of these questions have to do with the cell membrane and osmosis. Marine animals live in salt water, which is a hypertonic environment; 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. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would enter the blood cells, and they would swell and burst ( Figure below ).

Osmosis causes these red blood cells to change shape by losing or gaining water.

Passive Transport

Diffusion is called passive transport . This means it does not require energy to move molecules. 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. Sometimes, special proteins are needed to help molecules move across the membrane. These are called channel proteins or carrier proteins ( Figure below ).

Protein channels and carrier proteins are involved in passive transport.

Diffusion Across Cell Membranes: Passive Transport can be viewed at http://www.youtube.com/watch?v=JShwXBWGMyY (4:41).

Vocabulary

  • concentration : Amount of a substance in relation to the total volume.
  • concentration gradient : Gradual difference in the concentration of substances between two regions.
  • diffusion : Movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance.
  • equilibrium : State in which the concentrations of the diffusing substance are the same or become equal.
  • hypertonic solution : Environment (solution) outside of the cell has more dissolved material than the inside of the cell.
  • hypotonic solution : Environment (solution) outside of the cell has a lower concentration of dissolved material than the inside of the cell.
  • isotonic solution : Amount of dissolved material is equal, both inside and outside of the cell.
  • osmosis : Diffusion of water across a membrane.

Summary

  • Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.
  • The diffusion of water across a membrane because of a difference in concentration is called osmosis.

Practice

Use the resource below to answer the following questions.

  1. What is osmosis? What drives this process?
  2. What is tonicity? How is it similar to osmotic pressure?
  3. How can a hypotonic solution cause a cell to rupture? Describe this process as specifically as you can.
  4. How would a hypertonic solution affect a cell? How could this affect cellular processes?
  5. Do water molecules leave or enter a cell in an isotonic solution?

Review

  1. What is a concentration gradient?
  2. What happens at equilibrium?
  3. What size items would move through diffusion?  Give there examples of items.
  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?
  5. What item is moved during osmosis?

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