Change in movement

Severe Weather

Push and Pull

Types of Movement

Seasons

In many cases, cells must move materials from an area of lower concentration to an area of higher concentration, or “up” their concentration gradient. Such movement of materials is known as active transport. Unlike passive transport, active transport requires a cell to expend energy. CELL MEMBRANE PUMPS Ion channels and carrier proteins not only assist in passive trans- port but also help with some types of active transport. The car- rier proteins that serve in active transport are often called cell membrane “pumps” because they move substances from lower to higher concentrations. Carrier proteins involved in facilitated diffusion and those involved in active transport are very similar. In both, the molecule first binds to a specific kind of carrier protein on one side of the cell membrane. Once it is bound to the molecule, the protein changes shape, shielding the molecule from the hydrophobic interior of the phospholipid bilayer. The protein then transports the molecule through the membrane and releases it on the other side. However, cell membrane pumps require energy. Most often the energy needed for active transport is supplied directly or indirectly by ATP. Sodium-Potassium Pump One example of active transport in animal cells involves a carrier protein known as the sodium-potassium pump. As its name sug- gests, this protein transports Na ions and K ions up their con- centration gradients. To function normally, some animal cells must have a higher concentration of Na ions outside the cell and a higher concentration of K ions inside the cell. The sodium- potassium pump maintains these concentration differences. Follow the steps in Figure 5-6 on the next page to see how the sodium-potassium pump operates. First, three Na ions bind to the carrier protein on the cytosol side of the membrane, as shown in step . At the same time, the carrier protein removes a phosphate group from a molecule of ATP. As you can see in step , the phos- phate group from the ATP molecule binds to the carrier protein. Step shows how the removal of the phosphate group from ATP supplies the energy needed to change the shape of the carrier pro- tein. With its new shape, the protein carries the three Na ions through the membrane and then forces the Na ions outside the cell where the Na concentration must remain high. 3 2 1 SECTION 2 OBJECTIVES ● Distinguish between passive transport and active transport. ● Explain how the sodium-potassium pump operates. ● Compare endocytosis and exocytosis. VOCABULARY active transport sodium-potassium pump endocytosis vesicle pinocytosis phagocytosis phagocyte exocytosis www.scilinks.org Topic: Active Transport Keyword: HM60018 mb06se_homs02.qxd 5/18/07 11:02 AM Page 103 104 CHAPTER 5 K+ K+ K+ K+ K+ K+ INSIDE OF CELL OUTSIDE OF CELL Carrier protein Cell membrane P P P P Na+ Na+ Na+ ATP ADP Na+ Na+ Na+ Na+ Na+ Na+ 1 2 3 4 5 6 At this point, the carrier protein has the shape it needs to bind two K ions outside the cell, as step shows. When the K ions bind, the phosphate group is released, as indicated in step , and the carrier protein restores its original shape. As shown in step this time, the change in shape causes the carrier protein to release the two K ions inside the cell. At this point the carrier protein is ready to begin the process again. Thus, a complete cycle of the sodium-potassium pump transports three Na ions out of the cell and two K ions into the cell. At top speed, the sodium-potassium pump can transport about 450 Na ions and 300 K ions per second. The exchange of three Na ions for two K ions creates an electrical gradient across the cell membrane. That is, the outside of the membrane becomes positively charged relative to the inside of the membrane, which becomes relatively negative. In this way, the two sides of the cell membrane are like the positive and nega- tive terminals of a battery. This difference in charge is important for the conduction of electrical impulses along nerve cells. The sodium-potassium pump is only one example of a cell membrane pump. Other pumps work in similar ways to transport important metabolic materials across cell membranes.

Demographic trends The annual rate of natural increase in Southeast Asia averages slightly higher than the annual world rate. Considerable variation exists, however, among the region’s countries. The Philippines, Laos, Malaysia, Vietnam, and Brunei are characterized by higher growth; Singapore, Thailand, and Indonesia, on the other hand, have considerably lower rates, primarily because of the implementation of effective family-planning programs in these countries. In general, the pace of fertility decline is accelerating, although it is being offset by declining infant mortality and increasing life expectancy. Infant mortality for the region approximates the world average. In the more developed nations—especially Singapore, Malaysia, and Thailand—health care programs for infants and children have helped bring about mortality rates well below world averages, while the scarcity of these programs in such countries as Cambodia and Laos has contributed to continued high rates. Life expectancy in the region is somewhat below the world average, with Cambodia having the lowest average and Singapore the highest. Population change also is directly related to internal and external migration. As noted above, rural-to-urban migration continues to be a major aspect of change in nearly all Southeast Asian nations. In certain countries, considerable evidence exists for movements between rural areas (e.g., Thailand) and mobility between urban areas (Indonesia). Internal migration in the Philippines is dominated by movements to Manila and to the frontier areas in the south. Perhaps most significant, given the increasing mobility of the population and access to transport services, is the growth of nonpermanent population movements. Seasonal and other forms of circular migration for limited periods of time are conspicuous, especially in Malaysia, Indonesia, and Thailand. The growth in transport access also has created greater commuting ranges for individuals who in the past often had to leave their homes and fields for extended periods to take up work. Refugee movements have been conspicuous in the region, particularly since the mid-1970s. The Vietnamese out-migration to Malaysia, Thailand, and Indonesia, as well as to Hong Kong, is noteworthy. Cambodian and Laotian peoples also have experienced displacement. In addition, there have been numerous instances of religious minorities fleeing persecution, such as the departure of Muslim Burmans in the early 1990s.

SC.8.P.9.2 • Students will differentiate between physical and chemical changes. • Students know the characteristics of physical (change in state of matter) and chemical changes. (i.e.: new odor, change in color, new substance, change in temperature) • Students will identify a change as physical, or chemical given evidence. • Students will explain that mass is conserved when substances undergo physical and chemical changes, according to the law of conservation of mass. • Students will summarize the Law of Conservation of mass, explaining that mass cannot be created or destroyed, only transferred. • Students will describe how temperature influences chemical changes. • Students will know thermal energy increases the movement of atoms/molecules. • Students will explain that scientific explanations are based on empirical evidence, logical reasoning, predictions, and modeling.

Related quizzes

Related quizzes