What Happened to the Dinosaurs

Amy: Hi Tom! You weren't on the school bus today. What happened? 嗨，Tom！你今天不在校车上。怎么了？ Tom: I was picked up by my uncle. 我被我叔叔接走了。 Amy: Oh, I see. I was picked up by my mom, as usual. 哦，我明白了。我像平常一样被妈妈接走了。 Tom: On the way home, we saw the police call at Mr. Lee’s house. 回家的路上，我们看到警察去李先生家了。 Amy: Really? A doctor called at our house last night to see my sister. 真的吗？昨晚医生来我们家给我妹妹看病。 Tom: I expected it to rain today, so I brought an umbrella. 我以为今天会下雨，所以带了雨伞。 Amy: Me too! I expected the weather to be cold, but it’s so hot. 我也是！我以为会冷，但今天好热。 Tom: I used to eat carrots, but not anymore. 我以前爱吃胡萝卜，但现在不吃了。 Amy: Haha, I used to love dinosaurs, but not anymore. 哈哈，我以前喜欢恐龙，但现在不再喜欢了。 Tom: My toy car was lost, but now it’s to be found in my backpack! 我的玩具车丢了，但现在在我书包里找到了！ Amy: That’s lucky! I hope my missing pen is to be found soon. 真幸运！我希望我丢的钢笔也能被找到。

What happened to the raw materials that slaves produced? GRADE 7

Numbers of slaves that were taken to America and What happened to the raw materials that slaves produced?

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-

Out of the Darkness Nearly a week passed before the girl was able to explain what had happened to her. One afternoon she set out from the coast in a small boat and was caught in a storm. Towards evening, the boat struck a rock and the girl jumped into the sea. Then she swam to the shore after spending the whole night in the water. During that time she covered a distance of eight miles. Early next morning, she saw a light ahead. She knew she was near the shore because the light was high up on the cliffs. On arriving at the shore, the girl struggled up the cliff towards the light she had seen. That was all she remembered. When she woke up a day later, she found herself in hospital.

Riding with Rosa Parks On the Bus. Marissa and her mother were riding the bus. They were on their way to Grandma's house. They had to sit in the back seats. Marissa didn't like the back seats. There was a law that black people had to sit at the back of the bus. Her mother said it was unfair. Some white people came on the bus. So some of the black people stood up and gave them their seats. That was another part of the law. Refusing to Stand. But one black woman didn't stand up. Her name was Rosa Parks. She didn't want to move from her seat. The driver told Rosa she had to move, but she said, "No." Then the driver called the police! The police came and arrested Rosa Parks. Marissa was very confused. Why didn't Rosa Parks get up? Everybody knew the law, even if it wasn't fair. Taking Steps. When other people heard what happened to Rosa, they got very upset. They decided to stop riding the city buses! Marissa asked her mom why they had to walk. Her mother said people stopped riding the buses to show that the law was unfair. For a long time they walked everywhere. Marissa's legs were sometimes very tired. Still, they didn't ride the bus. Making a Difference. Finally, an amazing thing happened. The law was changed! Marissa and her mom could now sit at the front of the bus. Marissa learned that one person could make a difference. She learned people could join together to change things. Today, people of all colors in the United States have the same rights. It all started with one woman saying no to something that was unfair.

John wants to build a house next to a forest. In the forest, there are lots of trees, flowers and animals. There is a lake too.John takes his axe and goes to the forest. He talks to a tree. “I need wood to build my house,” he says. So, the tree gives John some wood.John’s friends visit him and they like his house. John and his friends go to the forest and cut down more and more and more trees, to build more and more houses.Soon there are not more trees. There aren’t any flowers. There are not any animals. The lake is dirty. The people are sad and thirsty.John goes to the forest.” What happened?” John asks the tree.“You cut down the trees in the forest. Now the animals do not have food to eat or a place to live. The lake is dirty so there aren’t any fishes”, says the tree.“What can I do?” asks John.“Plant trees and clean the lake” says the tree.John and his friends plant trees and they clean the lake. Now the trees are tall and have leaves.There are flowers. The animals are back in the forest. There are fishes in the lake. The forest is cool again. Everyone is happy.

What do an ancient Greek philosopher and a 19th century Quaker have in common with Nobel Prize-winning scientists? Although they are separated over 2,400 years of history, each of them contributed to answering the eternal question: what is stuff made of? It was around 440 BCE that Democritus first proposed that everything in the world was made up of tiny particles surrounded by empty space. And he even speculated that they vary in size and shape depending on the substance they compose. He called these particles "atomos," Greek for indivisible. His ideas were opposed by the more popular philosophers of his day. Aristotle, for instance, disagreed completely, stating instead that matter was made of four elements: earth, wind, water and fire, and most later scientists followed suit. Atoms would remain all but forgotten until 1808, when a Quaker teacher named John Dalton sought to challenge Aristotelian theory. Whereas Democritus's atomism had been purely theoretical, Dalton showed that common substances always broke down into the same elements in the same proportions. He concluded that the various compounds were combinations of atoms of different elements, each of a particular size and mass that could neither be created nor destroyed. Though he received many honors for his work, as a Quaker, Dalton lived modestly until the end of his days. Atomic theory was now accepted by the scientific community, but the next major advancement would not come until nearly a century later with the physicist J.J. Thompson's 1897 discovery of the electron. In what we might call the chocolate chip cookie model of the atom, he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons. Thompson won a Nobel Prize in 1906 for his electron discovery, but his model of the atom didn't stick around long. This was because he happened to have some pretty smart students, including a certain Ernest Rutherford, who would become known as the father of the nuclear age. While studying the effects of X-rays on gases, Rutherford decided to investigate atoms more closely by shooting small, positively charged alpha particles at a sheet of gold foil. Under Thompson's model, the atom's thinly dispersed positive charge would not be enough to deflect the particles in any one place. The effect would have been like a bunch of tennis balls punching through a thin paper screen. But while most of the particles did pass through, some bounced right back, suggesting that the foil was more like a thick net with a very large mesh. Rutherford concluded that atoms consisted largely of empty space with just a few electrons, while most of the mass was concentrated in the center, which he termed the nucleus. The alpha particles passed through the gaps but bounced back from the dense, positively charged nucleus. But the atomic theory wasn't complete just yet. In 1913, another of Thompson's students by the name of Niels Bohr expanded on Rutherford's nuclear model. Drawing on earlier work by Max Planck and Albert Einstein he stipulated that electrons orbit the nucleus at fixed energies and distances, able to jump from one level to another, but not to exist in the space between. Bohr's planetary model took center stage, but soon, it too encountered some complications. Experiments had shown that rather than simply being discrete particles, electrons simultaneously behaved like waves, not being confined to a particular point in space. And in formulating his famous uncertainty principle, Werner Heisenberg showed it was impossible to determine both the exact position and speed of electrons as they moved around an atom. The idea that electrons cannot be pinpointed but exist within a range of possible locations gave rise to the current quantum model of the atom, a fascinating theory with a whole new set of complexities whose implications have yet to be fully grasped. Even though our understanding of atoms keeps changing, the basic fact of atoms remains, so let's celebrate the triumph of atomic theory with some fireworks. As electrons circling an atom shift between energy levels, they absorb or release energy in the form of specific wavelengths of light, resulting in all the marvelous colors we see. And we can imagine Democritus watching from somewhere, satisfied that over two millennia later, he turned out to have been right all along.

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