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CELLULAR PROCESSES reflect How would your body react if you stepped outside without a jacket on a cold day? Your muscles would tighten and you would feel very uncomfortable. If you stayed outside for more than a few moments, you would likely begin to shiver. What causes the tightening of muscles and the shivering? Why does your body react this way? Does it help you get warmer? Homeostasis In the example above, the body responded to cold air and reacted in a certain way in order to maintain homeostasis, which is the process of maintaining a constant state of balance within a normal range. The muscles tightened to conserve heat and the body shivered to help generate heat. All of this occurred in order to help maintain a fairly constant internal body temperature. While the word itself may sound complicated, homeostasis is a fairly simple concept. Cells and organisms must exist in a state of balance. All living cells are constantly working to maintain homeostasis. Your body is even working to maintain homeostasis right now! There are three general steps in a homeostatic response. When something in the body is out of balance, it is sensed by a biological “sensor.” This sensor sends a message to the “control center,” which is usually the nervous system. The control center then sends a command to an effector, a muscle or gland, to correct the imbalance. The regulation of thirst and the regulation of body temperature are examples of homeostasis in the human body. In order to maintain proper blood volume and fluid salinity (salt concentration), an animal must continually drink and excrete an appropriate amount of water. If body fluids are high in salinity (low in water), specific sensor cells in the brain are triggered. These cells can send commands that trigger a thirst response. They may also send commands in the form of hormones from a gland (effector) that reduces the further elimination of fluids. With respect to temperature, the human body must be maintained close to 98.6°F (37°C). Body temperature can rise, such as during physical exertion or fever. Body temperature can also decrease, such as when you step outside on a cold day. Temperature sensors detect the imbalance and the nervous system responds. On a warm day, a command could be sent to the sweat glands (effectors) to secrete sweat. This helps the body to cool and restore proper temperature. On a cold day, a signal might cause your muscles (effectors) to shiver, increasing your body temperature. © 2013-2014 Accelerate Learning - All Rights Reserved 1 CELLULAR PROCESSES The examples of homeostasis discussed here exist at the level of the organism. What about homeostasis at the cellular level? Individual cells must maintain their internal environment in a balanced state. This includes maintaining a constant supply of cellular energy as well as healthy cellular structures composed of biomolecules. Cells must also regulate the passage of materials across their membranes. The internal environment of a cell must maintain the proper salinity and pH. What are the cellular processes important for maintaining homeostasis? Energy Conversion in Cells All of the physical and chemical processes that take place inside a cell are referred to as cellular metabolism. Many of the metabolic reactions within a cell involve the conversion of energy between different forms. Cells require a constant supply of usable chemical energy to perform various functions. Some of these functions include building cellular structures, transporting materials, and eliminating wastes. The ATP provides cells with the energy molecule that supplies this cellular energy needed to perform essential functions. is adenosine triphosphate (ATP). ATP is considered a high-energy molecule because the bonds between the phosphate groups are unstable. When the bond between the second and third phosphate groups of an ATP molecule is broken, energy is released. This energy can be used to power a chemical reaction. The resulting molecule has two phosphate groups and is called ADP, or adenosine diphosphate. How does a cell obtain the ATP that it needs to perform cellular work? The process of cellular respiration breaks down larger molecules such as glucose (a sugar) into smaller molecules. Oxygen is commonly used during this process. Water, carbon dioxide, and energy are released when respiration occurs in the presence of oxygen. The energy released from this breakdown is used heterotroph: an to generate ATP from ADP and phosphate. For organisms organism that must like animals, the original chemical energy source must be consume its energy consumed. These organisms are called heterotrophs. sources C6H12O6 + 6O2 6CO2 + H2O + energy The formula for cellular respiration above illustrates that carbon dioxide, water, and energy are released. © 2013-2014 Accelerate Learning - All Rights Reserved 2 CELLULAR PROCESSES Autotrophs, such as plants, produce their own sugar autotroph: an molecules, generally during the cellular process of organism that photosynthesis. Sunlight is used to build glucose molecules is capable of during photosynthesis. Water and carbon dioxide are also producing its own required, and oxygen is released as a by-product. Therefore, energy sources via cellular respiration and photosynthesis are two complementary photosynthesis processes that convert energy between sunlight, sugar (glucose) molecules, and high-energy ATP molecules. Reactions that build larger molecules from smaller ones, such as photosynthesis, are called anabolic reactions. Catabolic reactions, on the other hand, break down larger molecules into smaller ones. Cellular respiration involves catabolic reactions. look out! Many people are familiar with the fact that plant cells undergo photosynthesis. However, it is important to remember that all living cells, including those that perform photosynthesis, also perform cellular respiration. Photosynthesis does not produce energy in the form of ATP. It yields the nutrient glucose, which is needed to generate ATP during cellular respiration. For this reason, photosynthetic autotrophs form the base of most food chains. Heterotrophs, such as animals, consume nutrients that are produced by autotrophs, either by consuming them directly or eating other organisms that consume them. Autotrophs, on the other hand, need not consume other species. In the case of plants, they photosynthesize their nutrients from water, carbon dioxide, and sunlight. But even these autotrophs must use cellular respiration to break down those nutrients in order to make ATP. Everyday Life: You Really Are What You Eat Living cells are made up of four major types of organic biomolecules—carbohydrates, lipids, proteins, and nucleic acids. Cells can synthesize these molecules from simpler building blocks. However, for the most part, those building blocks must be consumed in the food that organisms eat. Foods like bread, pasta, and fruit contain mostly carbohydrate molecules. These may be small (simple) carbohydrates such as sugars, or complex carbohydrates such as starch. As discussed in the previous sections, cells break down the simple sugar glucose to yield energy in the form of ATP. Some carbohydrates are also used for cellular structure, and others attach to the surface of cells and aid in cell recognition. The carbohydrate cellulose provides structural support for plant cells. Lipids are an important type of molecule consumed from fatty foods such as oils and animal fats. There are many different types of lipids. Like carbohydrates, lipids can be used as an energy source, and they can be broken down to generate ATP. Lipids also form the membranes around cells and around many internal cellular organelles. Some lipids are stored in the fat tissue of animals. © 2013-2014 Accelerate Learning - All Rights Reserved 3 CELLULAR PROCESSES The animal tissue provides cushioning and insulation. Lipids can also function as signaling molecules by moving into cells and triggering various processes to occur. The next major biomolecule, protein, comes from foods such as meat, dairy, nuts, and beans. After consumption, proteins are broken down into their constituent building Biomolecules are important components of blocks, called amino acids. Cells then use cell membranes. the amino acids to build the specific proteins needed for cellular work. There are many different kinds of proteins within cells. The functions of protein in a cell include structural support, signaling molecules, and catalyzing chemical reactions. Proteins that catalyze chemical reactions are called enzymes. Muscle tissue contains specific contractile proteins, while other proteins are found within cell membranes and regulate the import and export of cellular materials. Finally, nucleic acids are the biomolecule responsible for “information storage” within a cell. These molecules include DNA and RNA, the molecules that provide the genetic blueprint for building and maintaining living cells. Crossing the Cellular Membrane One of the primary strategies for maintaining cellular homeostasis is regulating materials that pass into and out of the cell. While some molecules can easily cross a cell membrane, the passage of many materials is tightly controlled. This variability in whether a certain substance can easily cross the membrane results from the fact that the cell membrane is selective, or semipermeable. Substances that are small and nonpolar are generally able to freely cross the cell membrane. These substances are able to squeeze through the nonpolar: lacking nonpolar lipids that comprise the membrane. Examples of such chemical polarity substances are the gases oxygen and carbon dioxide. © 2013-2014 Accelerate Learning - All Rights Reserved 4 CELLULAR PROCESSES When a substance can freely cross the membrane, it moves from an area of higher concentration to one of lower concentration. This type of movement is called diffusion. Like a spray of perfume spreading throughout a room, any dissolved substance will naturally diffuse in this manner. When a molecule goes freely through the lipid bilayer of the cell membrane it is called simple diffusion. If a substance is too large or polar, it may require the assistance of either a carrier protein or a channel protein to diffuse across the membrane. This type of movement is called facilitated diffusion. polar: having a slightly positive charge at one side of a molecule and a slightly negative charge at the other side carrier protein: a protein in the cell membrane that changes shape to allow a substance to pass through Neither simple diffusion nor facilitated diffusion requires the input of energy. They are thus considered to be forms of passive transport across membranes. The active transport of substances, on the other hand, requires the input of energy (from ATP). Why would energy be required to move substances across a membrane? Substances can only move passively (diffuse) down their concentration gradient. The movement of a substance from an area of lower concentration to one of higher concentration is in a direction against, or up, its concentration gradient. Movement in this direction requires energy. channel protein: a protein in the cell membrane that forms a channel through which a substance can pass through concentration gradient: a difference in the amount of substance on two sides of a barrier LOWER HIGHER Na+ Na + Sodium concentration gradient Potassium concentration gradient Na+ ADP + Pj P AT + K LOWER CYTOSOL K+ HIGHER Facilitated diffusion, pictured on the left, does not require an input of energy. Active transport, pictured on the right, requires energy to move substances against their concentration gradient. © 2013-2014 Accelerate Learning - All Rights Reserved 5 CELLULAR PROCESSES what do you think? Water (H2O) is a relatively small polar molecule. Water solute: a substance molecules, as with dissolved substances, must cross the that is dissolved in plasma membrane in order to regulate the tonicity of the another substance intracellular fluid. Tonicity refers to the relative concentration (called the solvent) of water and solutes in a solution. A hypertonic solution is one in which there is a higher solute concentration compared to another solution. A hypotonic solution has a lower solute solution: a mixture in concentration compared to another solution. Two solutions which the molecules that are isotonic have the same solute concentration. There of one substance are channel proteins within cell membranes through which (solute) are dissolved water molecules can pass. Osmosis refers to the diffusion in another substance of water molecules. Imagine that the fluid surrounding a cell (solvent) is hypertonic to the fluid inside the cell. Do you think water molecules will move into or out of the cell? What if the fluid outside the cell is hypotonic to the fluid inside? In each of these cases, is the movement of water (osmosis) passive or active? Tonicity of solution around the cell Hypertonic Solution around the cell (as compared to the cell) high (hyper) solute concentration low water concentration Solution (cytosol) in the cell low solute concentration high water concentration Hypotonic low solute (hypo) concentration high solute concentration high water concentration low water concentration Isotonic solute and water concentrations solute and water concentration are the same as that of the are the same as that of the cell solution © 2013-2014 Accelerate Learning - All Rights Reserved 6 CELLULAR PROCESSES what do you think? The chart below lists terms associated with cellular process in the left column. Match each term on the left with the related image or phrase on the right. Write the letter(s) of the matching image or phrase next to the term in the left column. Be sure to include all possible correct answers. Cellular Respiration Carbohydrate Autotroph Active transport Lipid B E D, H, K F J Facilitated Diffusion L Heterotroph Simple Diffusion K A, L Photosynthesis Passive transport ATP Homeostasis G, H A, L I C A. B. Process that yields ATP C. Balance of conditions within a cell or organism D. Performs photosynthesis E. F. Requires ATP to move substances against a concentration gradient G. Yields sugar (glucose) H. I. Releases energy when phosphate bonds are broken J. Olive oil K. Performs cellular respiration L. Involves a substance moving down its concentration gradient © 2013-2014 Accelerate Learning - All Rights Reserved 7 CELLULAR PROCESSES connecting with your child Exploring Food Labels To help students learn more about the major biomolecules, have them study food labels from some common foods in the house or the grocery store. Examples of food labels to research are peanut butter, packaged lunchmeat, baby carrots, honey, fruit jam, chocolate, vegetable oil, cheese, bread, packaged pasta, canned beans, etc. Have students guess the relative amounts of fat, protein, and carbohydrates in each food. Then, have them compare their guesses with the information on the food label. You may want to explain to students that a calorie is a unit of energy: one gram of fat contains nine calories of energy, whereas one gram of protein or one gram of carbohydrate contains four calories of energy. Here are some questions to discuss with students: • Why is it important to eat a balanced diet that contains sufficient carbohydrates, protein, and fat? How do living cells use each of these nutrients? • Why does a tablespoon of honey have so many fewer calories than a tablespoon of olive oil? • If protein, carbohydrates, and fat all contain energy, why must a cell undergo cellular respiration and break down sugars (or other nutrients) to produce ATP? © 2013-2014 Accelerate Learning - All Rights Reserved 8