modes of trans[port 2

A solution is composed of a solute dissolved in a solvent. In the sugar water described in Figure 5-1, the solute was sugar and the solvent was water, and the solute molecules diffused through the solvent. It is also possible for solvent molecules to diffuse. In the case of cells, the solutes are organic and inorganic compounds, and the solvent is water. The process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration is called osmosis (ahs-MOH-sis). Because water is moving from a higher to lower concentration, osmosis does not require cells to expend energy. Therefore, osmosis is the passive transport of water. Direction of Osmosis The net direction of osmosis depends on the relative concentra- tion of solutes on the two sides of the membrane. Examine Table 5-1. When the concentration of solute molecules outside the cell is lower than the concentration in the cytosol, the solution outside is hypotonic to the cytosol. In this situation, water diffuses into the cell until equilibrium is established. When the concentration of solute molecules outside the cell is higher than the concentration in the cytosol, the solution outside is hypertonic to the cytosol. In this situation, water diffuses out of the cell until equilibrium is established. Observing Diffusion Materials 600 mL beaker, 25 cm dialysis tubing, funnel, 15 mL starch solution (10 percent), 20 drops Lugol’s solution, 300 mL water, 100 mL graduated cylinder, 20 cm piece of string (2) Procedure 1. Put on your disposable gloves, lab apron, and safety goggles. 2. Pour 300 mL of water in the 600 mL beaker. 3. Add 20 drops of Lugol’s solution to the water. CAUTION: Lugol’s solution is a poison and eye and skin irritant. 4. Open the dialysis tubing, and tie one end tightly with a piece of string. 5. Using the funnel, pour 15 mL of 10 percent starch solution into the dialysis tubing. 6. Tie the other end of the dialysis tubing tightly with the second piece of string, forming a sealed bag around the starch solution. 7. Place the bag into the solution in the beaker, and observe the setup for a color change. Analysis What happened to the color in the bag? What happened to the color of the water around the bag? Explain your observations. Quick Lab www.scilinks.org Topic: Osmosis Keyword: HM61090 mb06se_homs01.qxd 11/27/07 8:52 AM Page 98 HOMEOSTASIS AND CELL TRANSPORT 99 When the concentrations of solutes outside and inside the cell are equal, the outside solution is said to be isotonic to the cytosol. Under these conditions, water diffuses into and out of the cell at equal rates, so there is no net movement of water. Notice that the prefixes hypo-, hyper-, and iso- refer to the relative solute concentrations of two solutions. Thus, if the solution outside the cell is hypotonic to the cytosol, then the cytosol must be hyper- tonic to that solution. Conversely, if the solution outside is hypertonic to the cytosol, then the cytosol must be hypotonic to the solution. Water tends to diffuse from hypo- tonic solutions to hypertonic solutions. How Cells Deal with Osmosis Cells that are exposed to an isotonic external environment usually have no difficulty keeping the movement of water across the cell membrane in balance. This is the case with the cells of ver- tebrate animals on land and of most other organ- isms living in the sea. In contrast, many cells function in a hypotonic environment. Such is the case for unicellular freshwater organisms. Water constantly diffuses into these organisms. Because they require a relatively lower concentration of water in the cytosol to function normally, unicel- lular organisms must rid themselves of the excess water that enters by osmosis. Some of them, such as the paramecia shown in Figure 5-2, do this with contractile vacuoles (kon-TRAK-til VAK-y ̄ ̄o ̄ ̄o-OL), which are organelles that remove water. Contractile vacuoles collect the excess water and then contract, pumping the water out of the cell. Unlike diffusion and osmosis, this pumping action is not a form of passive trans- port because it requires the cell to expend energy. Copyright © by Holt, Rinehart and Winston. All rights reserved. (a) (b) Vacuole filling with water Vacuole contracting TABLE 5-1 Direction of Osmosis Condition External solution is hypotonic to cytosol External solution is hypertonic to cytosol External solution is isotonic to cytosol Net movement of water into the cell out of the cell none H2O H2O H2O H2O H2O H2O The paramecia shown below live in fresh water, which is hypotonic to their cytosol. (a) Contractile vacuoles collect excess water that moves by osmosis into the cytosol. (b) The vacuoles then contract, returning the water to the outside of the cell. (LM 315) FIGURE 5-2 100 CHAPTER 5 (a) HYPOTONIC Cell walls (b) HYPERTONIC (a) ISOTONIC (b) HYPOTONIC (c) HYPERTONIC Other cells, including many of those in multicellular organisms, respond to hypotonic environments by pumping solutes out of the cytosol. This lowers the solute concentration in the cytosol, bring- ing it closer to the solute concentration in the environment. As a result, water molecules are less likely to diffuse into the cell. Most plant cells, like animal cells, live in a hypotonic environ- ment. In fact, the cells that make up plant roots may be surrounded by water. This water moves into plant cells by osmosis. These cells swell as they fill with water until the cell membrane is pressed against the inside of the cell wall, as Figure 5-3a shows. The cell wall is strong enough to resist the pressure exerted by the water inside the expanding cell. The pressure that water molecules exert against the cell wall is called turgor pressure (TER-GOR PRESH-er). In a hypertonic environment, water leaves the cells through osmosis. As shown in Figure 5-3b, the cells shrink away from the cell walls, and turgor pressure is lost. This condition is called plasmolysis (plaz-MAHL-uh-sis), and is the reason that plants wilt if they don’t receive enough water. Some cells cannot compensate for changes in the solute con-

Some substances, such as macromolecules and nutrients, are too large to pass through the cell membrane by the transport processes you have studied so far. Cells employ two other transport mecha- nisms—endocytosis and exocytosis—to move such substances into or out of cells. Endocytosis and exocytosis are also used to transport large quantities of small molecules into or out of cells at a single time. Both endocytosis and exocytosis require cells to expend energy. Therefore, they are types of active transport. Endocytosis Endocytosis (EN-doh-sie-TOH-sis) is the process by which cells ingest external fluid, macromolecules, and large particles, including other cells. As you can see in Figure 5-7, these external materials are enclosed by a portion of the cell’s membrane, which folds into itself and forms a pouch. The pouch then pinches off from the cell membrane and becomes a membrane-bound organelle called a vesicle. Some of the vesicles fuse with lysosomes, and their con- tents are digested by lysosomal enzymes. Other vesicles that form during endocytosis fuse with other membrane-bound organelles. Two main types of endocytosis are based on the kind of material that is taken into the cell: pinocytosis (PIEN-oh-sie-TOH-sis) involves the transport of solutes or fluids, and phagocytosis (FAG-oh-sie-TOH-sis) is the movement of large particles or whole cells. Many unicellular organisms feed by phagocytosis. In addition, certain cells in animals use phagocytosis to ingest bacteria and viruses that invade the body. These cells, known as phagocytes, allow lysosomes to fuse with the vesicles that contain the ingested bacteria and viruses. Lysosomal enzymes then destroy the bacteria and viruses before they can harm the animal. CYTOSOL EXTERNAL ENVIRONMENT During endocytosis, the cell membrane folds around food or liquid and forms a small pouch. The pouch then pinches off from the cell membrane to become a vesicle. FIGURE 5-7 vesicle from the Latin vesicula, meaning “bladder” or “sac” Word Roots and Origins www.scilinks.org Topic: Endocytosis Keyword: HM60505 mb06se_homs02.qxd 5/18/07 11:03 AM Page 105 106 CHAPTER 5 1. Explain the difference between passive trans- port and active transport. 2. What functions do carrier proteins perform in active transport? 3. What provides the energy that drives the sodium-potassium pump? 4. Explain the difference between pinocytosis and phagocytosis. 5. Describe the steps involved in exocytosis. 6. How do endocytosis and exocytosis differ? How can that difference be seen? CRITICAL THINKING 7. Analyzing Information During intense exercise, potassium tends to accumulate in the fluid surrounding muscle cells. What membrane protein helps muscle cells counteract this tendency? Explain your answer. 8. Evaluating Differences How does the sodium- potassium pump differ from facilitated diffusion? 9. Relating Concepts The vesicles formed during pinocytosis are much smaller than those formed during phagocytosis. Explain. SECTION 2 REVIEW Vesicle Cell membrane EXTERNAL ENVIRONMENT CYTOSOL During exocytosis, a vesicle moves to the cell membrane, fuses with it, and then releases its contents to the outside of the cell. FIGURE 5-8 INSIDE OF CELL Vesicle OUTSIDE OF CELL Exocytosis Exocytosis (EK-soh-sie-TOH-sis) is the process by which a substance is released from the cell through a vesicle that transports the sub- stance to the cell surface and then fuses with the membrane to let the substance out of the cell. This process, illustrated in Figure 5-8, is basically the reverse of endocytosis. During exocytosis, vesi- cles release their contents into the cell’s external environment. Figure 5-8 also shows a photo of a vesicle during exocytosis. Cells may use exocytosis to release large molecules such as pro- teins, waste products, or toxins that would damage the cell if they were released within the cytosol. Recall that proteins are made on ribosomes and packaged into vesicles by the Golgi apparatus. The vesicles then move to the cell membrane and fuse with it, deliver- ing the proteins outside the cell. Cells in the nervous and endocrine systems also use exocytosis to release small molecules that control the activities of other cells.

SYI-1.D: Describe the structure and/ or function of subcellular components and organelles. ★ SYI-1.E: Explain how subcellular components and organelles contribute to the function of the cell. ★ SYI-1.F: Describe the structural features of a cell that allow organisms to capture, store, and use energy. ★ ENE-1.B: Explain the effect of surface area-to-volume ratios on the exchange of materials between cells or organisms and the environment. ★ ENE-1.C: Explain how specialized structures and strategies are used for the efficient exchange of molecules to the environment. ★ ENE-2.A: Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell. ★ ENE-2.B: Describe the Fluid Mosaic Model of cell membranes. ★ ENE-2.C: Explain how the structure of biological membranes influences selective permeability. ★ ENE-2.D: Describe the role of the cell wall in maintaining cell structure and function. ★ ENE-2.E: Describe the mechanisms that organisms use to maintain solute and water balance. ★ ENE-2.F: Describe the mechanisms that organisms use to transport large molecules across the plasma membrane. ★ ENE-2.G: Explain how the structure of a molecule affects its ability to pass through the plasma membrane. ★ ENE-2.H: Explain how concentration gradients affect the ★ movement of molecules across membranes. ★ ENE-2.I: Explain how osmoregulatory mechanisms contribute to the health and survival of organisms. ★ ENE-2.J: Describe the processes that allow ions and other molecules to move across membranes. ★ ENE-2.K: Describe the membrane-bound structures of the eukaryotic cell. ★ ENE-2.L: Explain how internal membranes and membrane- bound organelles contribute to compartmentalization of eukaryotic cell functions. ★ EVO-1.A: Describe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cells. ★ EVO-1.B: Describe the relationship between the functions of endosymbiotic organelles and their free-living ancestral counterparts

7.03 Patients with Infectious Disease The center adheres to infection control policies to ensure the safety of patients, physicians, and staff members. Patients who are currently being treated for an infectious disease or condition that is transmitted via the contact route may not be treated at the center. Patients with disease spread via droplet (e.g. influenza) or airborne (e.g. tuberculosis, measles) routes will reschedule their procedures in consultation with their physicians. The infection control nurse, in consultation with the infection control consultant, will determine whether the patient requires isolation or other additional precautions. If a patient with current Clostridium difficile-associated diarrhea is treated at the center, all rooms where the patient was housed, even briefly, should be cleaned by housekeeping under supervision of the staff at the center. Physicians must document relevant information in their pre-admission documentation. Standard Precautions will be followed in the care of all patients. Cigarroa Interventional Institute establishes policies to ensure compliance with infection control policies for the care of patients with drug-resistant organisms. The physician will screen patients through the medical history review prior to scheduling a patient at the center. During the pre-operative (pre-op) phone call or interview, a pre-op screening of the patient will be conducted. Strict isolation policies are required to treat patients with active MRSA, VRSA, or tuberculosis. Since this facility has no provisions for strict isolation, those patients with active infections will be referred elsewhere for treatment. Patients who are undergoing or completed a course of antibiotic therapy, are colonized and/or are not actively infected may be treated at the center. Patients with active infections requiring transmission-based precautions are not appropriate candidates for this facility and will be referred to another facility or rescheduled. Procedures cannot be scheduled for patients currently undergoing an infection with transmission based precautions. The patient must finish the course, and they will be rescheduled. Definitions and Standards: The following definitions and standards are provided for informational purposes only: Airborne Transmission and Precautions: This mode of transmission occurs by the spreading of either airborne droplet nuclei (small particle residue of 5 microns or smaller), of evaporated droplets which contain microorganisms that remain suspended in the air for long periods of time) or dust particles containing microorganisms. Patients must be isolated in private rooms with special air handling and ventilation, and the door must remain closed. Patient transport should be restricted to essential transport only. Respiratory precautions must be taken when in the presence of patients with active tuberculosis, including respiratory masks. Droplet Transmission and Precautions: Droplets are transmitted from the host source by coughing, sneezing, talking, or during procedures such as suctioning or bronchoscopy. Patients must be isolated, and a distance of 3 feet maintained between the infected patient and others. Caregivers within 3 feet of the patient should wear a mask. Patient transport should be minimized. Contact Transmission and Precautions: Direct contact transmission involves direct body surface to body surface contact with physical transfer of microorganisms between a susceptible host and an infected person. Indirect contact transmission involves contact with an intermediate object (usually inanimate) and a susceptible host. Patients should be isolated as much as possible. Gloves and hand washing are essential for all contact with the patient and contact with objects, which come in contact with the patient. Gloves should be changed after each contact. Reportable Condition If the patient is determined to have a reportable condition at any time during pre-admission, the Director of Operations/Nurse Manager will be notified. The procedural physician will be contacted and the case will be canceled. The Department of Health will also be notified the same day following state regulations regarding Reportable Communicable Diseases. To report a disease or condition, contact: The City of Laredo Epidemiology 24/7 Reporting Line: (956) 763-2915, if unable to report locally, call The Texas Departmrent of Health Services epidemiology program: 24/7Number for Immediately Reportable – 1-800-705-8868

Comprehension Test: Superbook – Revelation Theme: Forgiveness, Salvation, and God’s Victory 🔹 Remembering (Items 1–4) What mistake did Chris make at the beginning of the episode? A. He lied to his parents B. He started a fire accidentally C. He broke a valuable item D. He skipped school Answer: B Who are the three main characters in the episode? A. Peter, Mary, and John B. Chris, Joy, and Gizmo C. David, Sarah, and Eli D. Paul, Ruth, and Micah Answer: B Where does Superbook transport the children? A. To the Garden of Eden B. To the time of Jesus’ birth C. To the end times D. To the Exodus Answer: C Who receives visions in heaven during the episode? A. Moses B. Apostle Paul C. Apostle John D. King Solomon Answer: C 🔹 Understanding (Items 5–8) Why does Chris believe his mistake is unforgivable? A. He was punished severely B. He hurt someone intentionally C. He fears the consequences are too great D. He doesn’t understand forgiveness Answer: C What does the descent of the New Jerusalem symbolize? A. The end of all prophecy B. The beginning of war C. Eternal life and God’s promise D. The destruction of the earth Answer: C What lesson does Chris learn from his father? A. That mistakes are permanent B. That forgiveness is earned C. That God’s mercy is unconditional D. That punishment is necessary Answer: C What theme is emphasized throughout the episode? A. Justice and revenge B. Power and control C. Forgiveness and salvation D. Wealth and prosperity Answer: C 🔹 Applying (Items 9–12) If a student feels guilty for a mistake, what lesson from the episode could help them? A. Avoid responsibility B. Seek revenge C. Trust in God’s forgiveness D. Hide the truth Answer: C How might Joy’s experience in heaven help her understand God’s plan? A. She sees the consequences of sin B. She learns about ancient history C. She witnesses God’s ultimate victory D. She meets famous prophets Answer: C What action could reflect the message of salvation in daily life? A. Ignoring others’ mistakes B. Offering forgiveness to someone who hurt you C. Avoiding difficult conversations D. Seeking personal gain Answer: B How could Gizmo’s role support the group’s understanding of Revelation? A. By distracting them with jokes B. By translating the visions C. By guiding them through spiritual truths D. By fixing technical problems Answer: C 🔹 Analyzing (Items 13–16) What contrast is shown between Chris’s fear and the final message of the episode? A. Fear leads to punishment; forgiveness leads to peace B. Fear is stronger than faith C. Mistakes are never forgiven D. God ignores human emotions Answer: A Why is the battle between angels and Satan significant? A. It shows the power of war B. It represents the struggle for heaven C. It symbolizes the victory of good over evil D. It predicts future disasters Answer: C What does the episode suggest about the nature of sin? A. It is always punished B. It can be forgiven through God’s mercy C. It is inherited D. It defines a person’s worth Answer: B How do Chris’s emotions evolve throughout the episode? A. From joy to anger B. From guilt to hope C. From confusion to pride D. From fear to rebellion Answer: B 🔹 Evaluating (Items 17–18) Do you agree that no sin is too great for God’s forgiveness? Why might this be important for viewers? A. Yes, it teaches hope and redemption B. No, some sins are unforgivable C. Yes, but only for believers D. No, forgiveness must be earned Answer: A How effective is the use of apocalyptic imagery in teaching moral lessons? A. It confuses young viewers B. It dramatizes the message of salvation C. It distracts from the core message D. It promotes fear Answer: B 🔹 Creating (Items 19–20) If you were to write a reflection based on this episode, what theme would you focus on? A. The importance of punishment B. The beauty of heaven C. The power of forgiveness D. The fear of judgment Answer: C What title would best capture the message of the episode? A. “The Fire Within” B. “Mercy Wins” C. “The End of Days” D. “Judgment and Justice” Answer: B

The plasma membrane (also called the cell membrane) has several functions. For example, it allows only certain molecules to enter or leave the cell. It separates internal metabolic reactions from the external environment. In addition, the plasma membrane allows the cell to excrete wastes and to interact with its environment. Membrane Lipids The plasma membrane, as well as the membranes of cell organelles, is made primarily of phospholipids. Phospholipids have a polar, hydrophilic (“water-loving”) phosphate head and two nonpolar, hydrophobic (“water-fearing”) fatty acid tails. Water molecules sur- round the plasma membrane. The phospholipids line up so that their heads point outward toward the water and their tails point inward, away from water. The result is a double layer called a phospholipid bilayer, as shown in Figure 4-10. The cell membranes of eukaryotes also contain lipids, called sterols, between the tails of the phospho- lipids. The major membrane sterol in animal cells is cholesterol. Sterols in the plasma membrane make the membrane more firm and prevent the membrane from freezing at low temperatures. SECTION 3 OBJECTIVES ● Describe the structure and function of a cell’s plasma membrane. ● Summarize the role of the nucleus. ● List the major organelles found in the cytosol, and describe their roles. ● Identify the characteristics of mitochondria. ● Describe the structure and function of the cytoskeleton. VOCABULARY phospholipid bilayer chromosome nuclear envelope nucleolus ribosome mitochondrion endoplasmic reticulum Golgi apparatus lysosome cytoskeleton microtubule microfilament cilium flagellum centriole Cell membranes are made of a phospholipid bilayer. Each phospholipid molecule has a polar “head” and a two-part nonpolar “tail.” FIGURE 4-10 Copyright © by Holt, Rinehart and Winston. All rights reserved. 78 CHAPTER 4 OUTSIDE OF CELL INSIDE OF CELL 1. Cell-surface marker: Glycoprotein that identifies cell type 3. Enzyme: Assists chemical reactions inside the cell 2. Receptor protein: Recognizes and binds to substances outside the cell 4. Transport protein: Helps substances move across cell membrane Carbohydrate portion Protein portion Phospholipid heads Phospholipid tails Phospholipid Cholesterol bilayer Membrane Proteins Plasma membranes often contain specific proteins embedded within the lipid bilayer. These proteins are called integral proteins. Figure 4-11 shows that some integral proteins, such as cell surface markers, emerge from only one side of the membrane. Others, such as receptor proteins and transport proteins, extend across the plasma membrane and are exposed to both the cell’s interior and exterior environments. Proteins that extend across the plasma membrane are able to detect environmental signals and transmit them to the inside of the cell. Peripheral proteins, such as the enzyme shown in Figure 4-11, lie on only one side of the membrane and are not embedded in it. As Figure 4-11 shows, integral proteins exposed to the cell’s external environment often have carbohydrates attached. These carbohydrates can act as labels on cell surfaces. Some labels help cells recognize each other and stick together. Viruses can use these labels as docks for entering and infecting cells. Integral proteins play important roles in actively transporting molecules into the cell. Some act as channels or pores that allow certain substances to pass. Other integral proteins bind to a mol- ecule on the outside of the cell and then transport it through the membrane. Still others act as sites where chemical messengers such as hormones can attach. Fluid Mosaic Model A cell’s plasma membrane is surprisingly dynamic. Scientists describe the cell membrane as a fluid mosaic. The fluid mosaic model states that the phospholipid bilayer behaves like a fluid more than it behaves like a solid. The membrane’s lipids and pro- teins can move laterally within the bilayer, like a boat on the ocean. As a result of such lateral movement, the pattern, or “mosaic,” of lipids and proteins in the cell membrane constantly changes.

Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. Once a rock has been broken down, a process called erosion transports the bits of rock and mineral away. No rock on Earth is hard enough to resist the forces of weathering and erosion. Together, these processes carved landmarks such as the Grand Canyon, in the U.S. state of Arizona. This massive canyon is 446 kilometers (277 miles) long, as much as 29 kilometers (18 miles) wide, and 1,600 meters (1 mile) deep. Weathering and erosion constantly change the rocky landscape of Earth. Weathering wears away exposed surfaces over time. The length of exposure often contributes to how vulnerable a rock is to weathering. Rocks, such as lavas, that are quickly buried beneath other rocks are less vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and water, As it smoothes rough, sharp rock surfaces, weathering is often the first step in the production of soils. Tiny bits of weathered minerals mix with plants, animal remains, fungi, bacteria, and other organisms. A single type of weathered rock often produces infertile soil, while weathered materials from a collection of rocks is richer in mineral diversity and contributes to more fertile soil. Soils types associated with a mixture of weathered rock include glacial till, loess, and alluvial sediments. Weathering is often divided into the processes of mechanical weathering and chemical weathering. Biological weathering, in whichliving or once-living organisms contribute to weathering, can be a part of both processes. Mechanical Weathering Mechanical weathering, also called physical weathering and disaggregation, causes rocks to crumble. Water, in either liquid or solid form, is often a key agent of mechanical weathering. For instance, liquid water can seep into cracks and crevices in rock. If temperatures drop low enough, the water will freeze. When water freezes, it expands. The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split. This specific process (the freeze-thaw cycle) is called frost weathering or cryofracturing. Figure 4.3 Frost Wedging Temperature changes can also contribute to mechanical weathering in a process called thermal stress. Changes in temperature cause rock to expand (with heat) and contract (with cold). As this happens over and over again. the structure of the rock weakens. Over time, it crumbles. Rocky desert landscapes are particularly vulnerable to thermal stress. The outer layer of desert rocks undergo repeated stress as the temperature changes from day Eventually, Lo outer night. layersflake off in thin sheets, a process called exfoliation. Exfoliation contributes to the formation of bornhardts, one of the most dramatic features in landscapes formed by weathering and erosion. Bornhardts are tall, domed, isolated rocks often found areas. in tropical Sugarloaf Mountain, an iconic landmark in Rio de Janeiro, Brazil, is bornhardt. a Salt also works to weather rock in a process called haloclasty. Saltwater sometimes gets into the cracks and pores of rock. If the saltwater evaporates, salt crystals are left behind. As the crystals grow, they put pressure on the rock, slowly breaking it apart. Plants and animals can be agents of mechanical weathering. The seed of a tree may sprout in soil that has collected in a cracked rock. As the roots grow, they widen the cracks, eventually breaking the rock into pieces. Over time, trees can break apart even large rocks. Even small plants, such as mosses, can enlarge tiny cracks as they grow. Animals that tunnel underground, such as moles and prairie dogs, also work to break apart rock and soil. Other animals dig and trample rock aboveground, causing rock to slowly crumble. Chemical Weathering Chemical weathering changes the molecular structure of rocks and soil.For instance, carbon dioxide from the air or soil sometimes combines with water in a process called carbonation. This produces a weak acid, called carbonic acid, that can dissolve rock. Carbonic acid is especially effective at dissolving limestone. When carbonic acid seeps through limestone underground, it can open up huge cracks or hollow out vast networks of caves. Carlsbad Caverns National Park, in the U.S. state of New Mexico, includes more than 119 limestone caves created by weathering and erosion. The largest is called the Big Room.. With an area of about 33,210 square meters (357,469 square feet), the Big Room is the size of six football fields. Another type of chemical weathering works on rocks that contain iron. These rocks turn to rust in a process called oxidation. Rust is a compound created by the interaction of oxygen and iron in the presence of water. As rust expands, it weakens rock and helps break it apart. Another familiar form of chemical weathering is hydrolysis. In the process of hydrolysis, a new solution (a mixture of two or more substances) is formed as chemicals in rock interact with water. In many rocks, for example, sodium minerals interact with water to form a saltwater solution. Hydration and hydrolysis contribute to flared slopes, another dramatic example of a landscape formed by weathering and erosion. Flared slopes are sometimes nicknamed "wave rocks." Their c-shape is largely concave rock formations a result of subsurface weathering, in which hydration and hydrolysis wear away rocks beneath the landscape's surfaceWeathering and People Weathering is a natural process, but human activities can speed it up. For example, certain kinds of air pollution increase the rate of weathering Burning coal, natural and petroleum releases chemicals such as nitrogen oxide and gas, sulfur dioxide into the atmosphere. When these chemicals combine with sunlight and moisture, they change into acids. They then fall back to Earth as acid rain. Acid rain rapidly weathers limestone, marble, and other kinds of stone. The effects of acid rain can often be seen on gravestones, making names and other inscriptions impossible to read. Acid rain has also damaged many historic buildings and monuments. For example, at 71 meters (233 feet) tall, the Leshan Giant Buddha at Mount Emei, China is the world's largest statue of the Buddha. It was carved 1,300 years ago and sat unharmed for centuries. An innovative drainage system mitigates the natural process of erosion But in recent years, acid rain has turned the statue's nose black and made some of its hair crumble and fall.

modes of transport

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