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.' : • You know that plants need water. Water carries nutrients throughout the plant so that it grows and reproduces. But did you know that water is what holds a plant upright, even if it is growing in the desert? Plants, just like all living things, are made up of cells. Cells contain a collection of tiny structures that keep plants alive and growing. In each cell, a wall surrounds these structures, giving the cell its shape. Water fills the cell and helps support the cell wall. If you look at a balloon, you will notice that when it is empty, it has almost no shape at all. If you stacked several empty balloons, you would have a flat pile of objects. If you threw them at a target, they would not fly far. If you tried to balance one on your head during a relay race, it would not provide much of a challenge. In short, an empty balloon is not very useful at all. The walls of a plant cell are supported by fluid inside. Suppose you were to fill the balloon with water. As more water enters the balloon, it begins to take a distinct shape. The water pushes against the inside, stretching the balloon tightly and making its shape more rigid. If you take several filled balloons and stack them in a container, the resulting structure will be taller than if you were to stack the same number of empty balloons. Plant cells are similar to water balloons. If the plant does not receive enough water, its cells cannot perform their functions. Without water, nutrients are not carried efficiently into the cells. and wastes are not carried away efficiently. In addition, without Discovery Education Science Page 1 of 2 © 2010 Discovery Communications, LLC • ÿ;ÿ Another example of pressure occurs when a trumpet player's lungs force air through the brass tubing of the trumpet--in this ease the pressure produces a tone. With a balloon, water pushes against its walls with enough pressure to ÿ " ....... u inflate it and give it shape. Pressure created by various forces affects the way plants live, grow, and reproduce. Wind, rain, animals, and people push and pull against plants. Gravity helps determine the direction in which they grow. The chemical bonds inside the plant's cells create forces that keep the cells attached to each other. |t . .... ,- ,.- ...... Yucca plants growing in the desert rely on turgor xessure to stay upright, Another important force is turgor pressure. Turgot pressure is the force of water pushing against the inside of cell walls. Turgor pressure enables each cell to perform its functions. The pressure makes the cell walls rigid, which supports the plant's structure and keeps it upright. Have you ever heard someone say, "I'm under too much • ,,, pressure"? When people use the word pressure in this way, they are probably not talking about physical forces such as turgor pressure. When people are "under pressure," they are usually experiencing stress that leaves them feeling tired and worn down. Plants are just the opposite! When plants are droopy and wilted, a little water increases the turgor pressure in their cells :[; ::1! '::,!:: and quickly helps them stand tall again. :, ...... Discovery Education Science L_ Page 2 of 2 © 2010 Discovery Communications, LLC Sir Isaac Newton studied the way objects move and developed laws of motion that today enable us to describe the forces at work around us. For instance, we know that inertia explains the motion of a football lying on the ground; the ball will remain on the ground until another force causes the ball to move. When the ball is thrown through the air, we know that the strength of the toss determines how quickly the ball moves. And when two helmets collide, we know that each player absorbs the same amount of force. These forces affect the motion of living things as well. When a football player leaps or dives into an end zone, Many forces are at work on the football field. gravity pulls him down toward the ground. Inertia acts ............................................................................................................... on his body, causing him to fall until his impact with the ground stops him. The larger the player is, the more force it takes to bring him down. And during a collision on the field, both players experience the impact. Newton also developed the concept of gravity, which is an attractive force that occurs between two objects, whether they are alive or not. The strength of gravity's pull depends on the mass of the objects--therefore, a large object (like Earth) exerts more gravitational pull than a smaller object (like a ball). This explains why a ball that is thrown into the air will always fall back toward Earth. Gravity keeps living things on Earth close to the planet's surface; gravity even acts on the cells within those living things. For example, the grass growing on the football field always grows up toward the sky, while the roots always grow down into the soil. This is partly because plant shoots, which need radiant energy to make their food, grow toward light. But most scientists believe that gravity also lends a hand. Plant cells contain a hormone called auxin that accumulates on the downward side of the cell. This hormone seems to permit the cells of roots and shoots to bend in a specific direction. Discovery Education Science Page 1 of 2 © 2010 Discovery Communications, LLC Roots grow in the direction of gravity's pull; scientists refer to this process as positive geotropism° Plant shoots and stems grow upward, opposite to the pull of gravity, and thereby exhibit negative geotropism. When grass first begins to sprout, it exerts a force of its own. A seedling must push through its seed easing and through the soil above it. The emergence force that a seedling uses to move toward the surface can be surprisingly strong, allowing plants to force their way through dense soil and sometimes even sidewalks or rocks. Have you noticed that healthy grass always stands tall and straight? Another force within the plant ensures this outcome. A flexible wall surrounds all plant cells, and water can move through this wall to deliver nutrients and remove waste. If the plant does not receive enough water, the cell's shape will lack definition, and the structure will not be supported. As water fills the cells, however, this water presses against the cell walls from the inside, strengthening the structure of the entire plant. This force of water is referred to as turgot pressure; this pressure is Turgor pressure supports cell walls, what enables a plant to stand upright. which in turn support the structure of the entire plant. Oeotropism, emergence force, and turgor pressure all support a plant's survival, but other forces can be devastating. Tension force is applied by pulling; if a plant is pulled too strongly, its roots will be pulled out of the ground or separated from the stem, and the plant will perish. Animals or humans walking through a field apply a force that can break or crush plant stems and leaves. The forces of wind, rain, snow, and hail can cause similar destruction and death. Every day, both beneficial and dangerous forces affect all living things. At times, harmful forces cause damage to organisms. When these forces are in balance, however, they provide an environment in which organisms can thrive. Discovery Education Science Page 2 of 2 © 2010 Discovery Communications, LLC