4.14. Active Transport

Other channels through the plasma membrane are closed doors. These channels open only when energy is provided. They are designed to enable the cell to maintain high or low concentrations of certain molecules, much more or less than exists outside the cell. Like motor-driven turnstiles, the channels operate to move a certain substance up its concentration gradient. The operation of these one-way, energy-requiring channels results in active transport, the movement of molecules across a membrane to a region of higher concentration by the expenditure of energy.

The Sodium-Potassium Pump The most important active transport channel is the sodium-potassium (Na⁺-K⁺) pump, which expends metabolic energy to actively pump sodium ions (Na⁺) in one direction, out of cells, and potassium ions (K⁺) in one direction, into cells. More than one-third of all the energy expended by your body’s cells is spent driving Na⁺-K⁺ pump channels. This energy is derived from adenosine triphosphate (ATP), a molecule we will learn more about in chapter 5. The transportation of two different ions in opposite directions happens because energy causes a change in the shape of the membrane protein carrier. The Key Biological Process illustration below walks you through one cycle of the pump. Each channel can move over 300 sodium ions per second when working full tilt. As a result of all this pumping, there are far fewer sodium ions in the cell. This concentration gradient, paid for by the expenditure of considerable metabolic energy in the form of ATP molecules, is exploited by your cells in many ways. Two of the most important are (1) the conduction of signals along nerve cells (discussed in detail in chapter 28) and (2) the pulling of valuable molecules such as sugars and amino acids into the cell against their concentration gradient!

We will focus for a moment on this second process. The plasma membranes of many cells are studded with facilitated diffusion channels, which offer a path for sodium ions that have been pumped out by the Na⁺-K⁺ pump to diffuse back in. There is a catch, however; these channels require that the sodium ions have a partner in order to pass through—like a dancing party where only couples are admitted through the door—which is why these are called coupled channels. Coupled channels won’t let sodium ions across unless another molecule tags along, crossing hand in hand with the sodium ion. In some cases the partner molecule is a sugar (see the last entry in table 4.3), in others an amino acid or other molecule. Because the concentration gradient for sodium is so large, many sodium ions are trying to get back in, and this diffusion pressure drags in the partner molecules as well, even if they are already in high concentration within the cell. In this way, sugars and other actively transported molecules enter the cell—via special coupled channels.

Key Learning Outcome 4.14. Active transport is energy-driven transport across a membrane toward a region of higher concentration.

TABLE 4.3. MECHANISMS FOR TRANSPORT ACROSS CELL MEMBRANS

Inquiry & Analysis

Why Does a Cell's Disposal of Damaged Proteins Consume Energy?

Much of modern biology is devoted to learning how cells build things—how the information encoded in DNA is used by cells to manufacture the proteins that make us what we are. The Nobel Prize in Chemistry was awarded in 2004 to researchers for their discovery of how the opposite, less glamorous process works: how cells break down and recycle proteins that are damaged or have outlived their usefulness.

It turns out that a cell's recycling of proteins is much more than just "taking out the trash.” Particular proteins are removed, often quite quickly, and cells use such targeted removals to control a lot of their activities, timing when a cell carries out particular functions, when it divides, and even when it dies. Of the 25,000 genes in your DNA, about 1,000 take part in this protein recycling system.

Our understanding of how this system works begins with a puzzle first noted in the 1950s. Most enzymes that break down proteins, including those that digest food, do not need energy to work. But a cell's recycling of its own proteins does consume energy. Researchers had no idea why energy was needed.

The answer to this puzzle came from an unexpected direction. In 1975 scientists discovered a small protein in calves' brains consisting of just 76 amino acids. Soon they realized that exactly the same protein is found in all eukaryotes, from yeasts to humans. They called this ubiquitous ("found everywhere”) protein ubiquitin. In the early 1980s researchers worked out that ubiquitin was a label that the cell attaches to proteins to mark them for destruction, a sort of molecular "kiss of death.” The process of attaching ubiquitin takes energy, solving the puzzle of why protein recycling requires energy. The tagged proteins are taken to a barrel-shaped chamber in the cell's cytoplasm called a proteasome, which slices the proteins into bits that are then recycled by the cell into new protein.

The graph above displays the sort of protein recycling experiment that revealed ubiquitin's key role. The experiment monitors levels of a particular protein involved in cell division (the "target” protein) within human cells growing in culture in a laboratory flask. Two cultures are monitored in side-by-side experiments: In the culture indicated by red dots, cells contain functional copies of the ubiquitin gene (ubi⁺); in the culture indicated by blue dots the ubiquitin gene has been deleted from the DNA (ubi^-). After 20 minutes, energy in the form of ATP is made available to the growing cells, which until then had been energy-starved.

1. Applying Concepts

a. Variable. In the graph, what is the dependent variable?

b. Concentration. After 100 minutes, which of the two cultures represents the higher concentration of target protein?

2. Interpreting Data Does the addition of ATP affect the level of target protein in either culture? Which one?

3. Making Inferences How does this culture differ from the other? Why might ATP stimulate removal of target protein from this culture, but not the other?

4. Drawing Conclusions Using the information in the graph, suggest why the functioning of ubiquitin requires ATP energy for the effective removal of the target protein.

Test Your Understanding

1. Cell theory includes the principle that

a. cells are the smallest living things; nothing smaller than a cell is considered alive.

b. all cells are surrounded by cell walls that protect them.

c. all organisms are made up of many cells arranged in specialized, functional groups.

d. all cells contain membrane-bounded structures called organelles.

2. The plasma membrane is

a. a carbohydrate layer that surrounds groups of cells to protect them.

b. a double lipid layer with proteins inserted in it, which surrounds every cell individually.

c. a thin sheet of structural proteins that encloses cytoplasm.

d. composed of proteins that form a protective barrier.

3. Organisms that have cells with a relatively uniform cytoplasm and no organelles are called _____ , and organisms whose cells have organelles and a nucleus are called _____.

a. cellulose, nuclear

b. eukaryotes, prokaryotes

c. flagellated, streptococcal

d. prokaryotes, eukaryotes

4. Within the nucleus of a cell you can find

a. a nucleolus.

b. a cytoskeleton.

c. mitochondria.

d. All of these.

5. The endomembrane system within a cell includes the

a. cytoskeleton and the ribosomes.

b. prokaryotes and the eukaryotes.

c. endoplasmic reticulum and the Golgi complex.

d. mitochondria and the chloroplasts.

6. It was once thought that only the nucleus of each cell contained DNA. We now know that DNA is also found in the

a. cytoskeleton and the ribosomes.

b. prokaryotes and the eukaryotes.

c. endoplasmic reticulum and the Golgi bodies.

d. mitochondria and the chloroplasts.

7. Which of the following statements is true?

a. All cells have a cell wall for protection and structure.

b. Eukaryotic cells in plants and fungi, and all prokaryotes, have a cell wall.

c. There is a second membrane composed of structural carbohydrates surrounding all cells.

d. Prokaryotes and all cells of eukaryotic animals have a cell wall.

8. If you put a drop of food coloring into a glass of water, the drop of color will

a. fall to the bottom of the glass and sit there unless you stir the water; this is because of hydrogen bonding.

b. float on the top of the water, like oil, unless you stir the water; this is because of surface tension.

c. instantly disperse throughout the water; this is because of osmosis.

d. slowly disperse throughout the water; this is because of diffusion.

9. When large molecules, such as food particles, need to get into a cell, they cannot easily pass through the plasma membrane, and so they move across the membrane through the processes of

a. diffusion and osmosis.

b. endocytosis and phagocytosis.

c. exocytosis and pinocytosis.

d. facilitated diffusion and active transport.

10. Active transport of specific molecules involves

a. facilitated diffusion.

b. endocytosis and phagocytosis.

c. energy and specialized pumps or channels.

d. permeability and a concentration gradient.