Lesson 3.4 Passive and active transport

3.4.3 Active transport

In sections 3.2 and 3.3, you learned about forms of passive transport, where no energy is required to move materials into or out of the cell. Active transport, in contrast, requires ATP. Recall that ATP (adenosine triphosphate) is the energy molecule of the cell.

 Energy is required for active transport because materials are being moved against the concentration gradient. This means that they are moving from areas of lower concentration to areas of higher concentration. Materials requiring active transport include: ions (charged particles), complex sugars, proteins and large cells

An example of such transport is the sodium ion/potassium ion pump. The term “pump” is sometimes used to refer to the protein involved in transport of the material and reinforces the fact that energy has to be used.

Activity:

Use Figure 3.4.4 to answer the questions below the figure.

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 Figure 3.4.4

Figure 3.4.4 illustrates the active transport system required for the movement of sodium ions and potassium ions across a cell membrane.

1. What is the brown structure embedded in the cell membrane?
2. What is the function of this brown structure?
3. Which direction is the sodium ion being transported? Explain your reasoning.
4. Which direction is the potassium ion being transported? Explain your reasoning.
5. What is ADP? What does Pi represent?
6. Is energy required for this transport system? Explain your reasoning.
7. Looking at the far right side of the diagram, is the concentration of the sodium ion higher inside or outside of the cell?
8. Looking at the far right side of the diagram, is the concentration of the potassium ion higher inside or outside of the cell?
9. Explain how your answers to numbers 7 and 8 support the fact that this is an example of active transport.
10. Check your answers as directed by your instructor. Be sure any questions you have are answered.

Protein pumps are not the only way to actively transport materials into or out of a cell. The processes of endocytosis and exocytosis also required ATP, but are not associated with proteins embedded in the cell membrane. Let’s take a closer look at these processes.

Both endocytosis and exocytosis involve the cell membrane. The prefix “endo” means “within” and the prefix “exo” means “external”. Endocytosis, then, can mean“to take within the cell” and exocytosis , “to remove from the cell”. It might be easier to remember if you recognize that the word “exit” and the prefix “exo” both start with “ex”.

Basically with the process of endocytosis, the cell is using energy in the form of ATP to help the cell membrane bring materials into the cell. The process of endocytosis is used for three specific functions as illustrated in Figure 3.4.5.

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 Figure 3.4.5

Figure 3.4.5 show three forms of the process of endocytosis in cells.

The term phagocytosis is used to describe endocytosis of large particles such as cell debris, dust-sized particles, and microorganisms. Phagocytosis occurs only in specialized cells, such as certain white blood cells in the human body. The process of phagocytosis begins when a particle comes in direct contact with a portion of the cell membrane, which then surrounds and engulfs it. A vacuole is formed and inside the cell this vacuole fuses to a lysosome. This fusion allows the enzymes from the lysosome to digest and destroy the materials in the vacuole. These digested materials are then released from the cell (Figure 3.4.6)

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 Figure 3.4.6

Figure 3.4.6 represents a white blood cell ingesting a bacterium through the process of endocytosis. The phagosome, a type of vacuole, that forms then fuses with a lysosome to destroy the bacterium.

Pinocytosis is the term used to describe endocytosis of a solution (liquid plus other materials). It is used to bring extracellular liquid into the cell. In pinocytosis, the cell membrane draws inward forming a pocket (referred to as invagination) bringing liquid with it. The cell membrane then pinches off, forming a vesicle containing the extracellular fluid. The materials within the cell may pass through the cytoplasm and be released from the cell or they may be used by the cell.

Receptor-mediated endocytosis is used to describe the endocytosis of specific materials. These materials will interact with specific receptors (usually proteins) on surface of the cell’s membrane. A vesicle forms and is then processed fusing with a lysosome, so that the materials are digested. Note that this is one way that certain types of viruses can infect human cells.

Exocytosis is essentially the opposite of endocytosis. This process does require energy for the release of materials from the cell. In some cases, this process is used to remove materials not needed by the cell, some of which may have been produced by cell organelles, such as the Golgi apparatus. In other cases, exocytosis is used to deliver lipids and other materials directly to the cell membrane. The vesicles that are created for exocytosis will fuse with the cell membrane and discharge their contents to the exterior of the cell (Figure 3.4.7).

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 Figure 3.4.7

Figure 3.4.7 illustrates the process of exocytosis in a cell.