chapter 3 Cell Processes section 2 Moving Cellular Materials What You ll Learn how selectively permeable membranes work about diffusion and osmosis the differences between passive transport and active transport Before You Read On the lines below, describe the purpose of window screens. Think of what they keep out and what they allow to pass through. Study Coach Make Flash Cards Think of a quiz question for each paragraph. Write the question on one side of the flash card and the answer on the other side. Keep quizzing yourself until you know all of the answers. C Describe Make a two-tab book, as shown below. Use the Foldable to take notes about active and passive transport. Read to Learn Passive Transport Window screens keep unwanted things, such as bugs, leaves, and birds, outside. But screens do let some things, such as air and smoke, pass through. Cells get food, oxygen, and other substances from their environments. They release waste materials into their environments. The membrane around the cell works like a window screen works for a room. A window screen is selectively permeable (PUR mee uh bul). It lets things like air come into the room and keeps some things like bugs out of the room. A cell s membrane also is selectively permeable. It lets some things come into or leave the cell. It also keeps other things from entering or leaving the cell. Things move through a cell membrane in several ways. The movement depends on the size of the molecules, the path the molecules take, and whether energy is needed. When substances move through the cell membrane without using energy, this movement is known as passive transport. Three types of passive transport are diffusion, osmosis, and facilitated diffusion. The type of transport depends on what is moving through the cell membrane. 40 Cell Processes
How does diffusion create equilibrium? Molecules move constantly and randomly. You might smell perfume when you walk past someone who is wearing it. The perfume molecules move freely throughout the air. This random movement of molecules from an area where there are more of them into an area where there are fewer of them is called diffusion. Diffusion is a type of passive transport. Molecules will keep moving from one area to another until the number of these molecules is equal in the two areas. When this occurs, equilibrium is reached and diffusion stops. All cells in your body use oxygen. Oxygen moves through your body in the red blood cells. When your heart pumps blood to your lungs, your red blood cells contain few oxygen molecules. Your lungs have many oxygen molecules. Oxygen molecules move, or diffuse, from your lungs into your red blood cells. The blood continues its journey through your body. When the blood reaches your big toe, there are more oxygen molecules in your red blood cells than in the cells of your big toe. The oxygen diffuses from your red blood cells to your big toe s cells. The process is shown in the figure below. 1. Determine How does diffusion create equilibrium? Picture This 2. Explain Use the figure to explain to a partner how diffusion works. Air sac in lung Oxygen Red blood cell In your lungs, oxygen diffuses into red blood cells. Nucleus In your big toe, oxygen diffuses out of red blood cells. Oxygen Red blood cell Toe cell Reading Essentials 41
3. Describe What do transport proteins do? Picture This 4. Explain Why does a plant wilt? What is facilitated diffusion? Some substances pass easily through the cell membrane by diffusion. Larger substances may need help passing through the cell membrane. Transport proteins in the cell membrane help these substances enter the cell. This process is called facilitated diffusion. Transport proteins are similar to the gates at a stadium. Gates are used to move people into and out of the stadium. Similarly, transport proteins are used to move substances into and out of a cell. What is osmosis? Remember that water makes up a large part of living matter. Water molecules move by diffusion in and out of cells. The diffusion of water through the cell membrane is called osmosis. What happens when you do not water plants? As a plant cell loses water, its cell membrane pulls away from the cell wall. This reduces pressure against the cell wall, and the plant cell becomes limp, as shown on the left in the figure below. The plant wilts because more water leaves the plant s cells than enters them. When you water the plant, the water moves through the cell membranes and fills the cells with water. The plant s cell membranes push against their cell walls, and the cells become firm, as shown on the right in the figure below. The carrot stick becomes limp when more water leaves each of its cells than enters them. Equilibrium occurs when water leaves and enters the cells at the same rate. 42 Cell Processes
Osmosis in Animal Cells Osmosis also takes place in animal cells. If animal cells were placed in pure water, they too would swell up. However, animal cells are different from plant cells. Just like an overfilled balloon, animal cells will burst if too much water enters the cell. Active Transport Suppose you have just left a theater at the end of a movie when you remember that you left your jacket inside. You have to move against the crowd to enter the theater and get your jacket. Which takes more energy leaving the theater with the crowd or moving against the crowd to get back into the theater? Something similar to this happens in cells. Active transport takes place when energy is needed to move substances through a cell membrane. For example, root cells require minerals from the soil. The root cells already have more molecules of the minerals than the surrounding soil. Normally, the mineral molecules would move out of the root into the soil until equilibrium is reached. But the root cells need to take in the minerals from the soil. Like facilitated diffusion, active transport uses transport proteins. In active transport, transport proteins bind with the needed substance and cellular energy is used to move it through the cell membrane. Endocytosis and Exocytosis Some molecules are too large to move through the cell membrane by diffusion or by using transport proteins. Large protein molecules, for example, can enter a cell when they are surrounded by the cell membrane. The cell membrane folds around the molecule, completely surrounding it. The sphere created is called a vesicle. The sphere pinches off and moves the molecule into the cell. The process of taking substances into a cell by surrounding it with the cell membrane is known as endocytosis (en duh si TOH sus). Some one-celled organisms take in food this way. Exocytosis (ek soh si TOH sus) is the process in which the contents of a vesicle are moved outside a cell. A vesicle s membrane joins with a cell s membrane, and the vesicle s contents are released. Exocytosis occurs in the opposite way that endocytosis does. Cells in your stomach use exocytosis to release chemicals that help digest food. 5. Explain What must be used with transport proteins to move a substance through a cell membrane in active transport? 6. Explain What happens during exocytosis? Reading Essentials 43
After You Read Mini Glossary active transport: takes place when energy is needed to move substances through a cell membrane; uses transport proteins diffusion: random movement of molecules from an area where there are more of them into an area where there are fewer of them endocytosis (en duh si TOH sus): process of taking substances into a cell by surrounding them with the cell s membrane equilibrium: the number of molecules in two areas are the same exocytosis (ek soh si TOH sus): process in which the contents of a vesicle are moved outside a cell osmosis: the diffusion of water through the cell membrane passive transport: movement of substances through the cell membrane without using energy 1. Review the terms and their definitions in the Mini Glossary. Choose one term that explains how substances move into and out of cells and write a sentence explaining how the process works. 2. Complete the Venn diagram below to help you compare active and passive transport. Active Transport Both Active and Passive Transport Passive Transport End of Section 44 Cell Processes Visit booka.msscience.com to access your textbook, interactive games, and projects to help you learn more about the movement of cellular materials.