CHAPTER 5 The Working Cell Key Terms activation energy energy coupling metabolism active site entropy noncompetitive inhibitor active transport enzyme osmoregulation aquaporin exergonic reaction osmosis ATP exocytosis passive transport cellular respiration facilitated diffusion phagocytosis chemical energy feedback inhibition phosphorylation coenzyme first law of thermodynamics potential energy cofactor fluid mosaic model receptor-mediated endocytosis competitive inhibitor heat second law of thermodynamics concentration gradient hypertonic substrate diffusion hypotonic thermal energy endergonic reaction isotonic thermodynamics endocytosis kinetic energy tonicity energy metabolic pathway Lecture Outline I. Introduction A. The plasma membrane and its proteins enable cells to 1. survive and 2. function. B. Aquaporins are membrane proteins that function as water channels. C. This chapter addresses how working cells use 1. membranes, 2. energy, and 3. enzymes. II. Membrane Structure and Function A. 5.1 VISUALIZING THE CONCEPT: Membranes are fluid mosaics of lipids and proteins with many functions 1. Biologists use the fluid mosaic model to describe a membrane’s structure, a patchwork of diverse protein molecules embedded in a phospholipid bilayer. 2. The plasma membrane exhibits selective permeability. 3. The proteins embedded in a membrane’s phospholipid bilayer perform various functions. Copyright © 2015 Pearson Education, Inc. 55 B. 5.2 EVOLUTION CONNECTION: The spontaneous formation of membranes was a critical step in the origin of life 1. Phospholipids, the key ingredient of biological membranes, spontaneously self-assemble into simple membranes. 2. The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells. C. 5.3 Passive transport is diffusion across a membrane with no energy investment 1. Diffusion is the tendency of particles to spread out evenly in an available space. a. Particles move from an area of more concentrated particles to an area where they are less con- centrated. b. This means that particles diffuse down their concentration gradient. c. Eventually, the particles reach dynamic equilibrium, where there is no net concentration on ei- ther side of the membrane. 2. Diffusion across a cell membrane does not require energy, so it is called passive transport. 3. Diffusion down the concentration gradients is the sole means by which oxygen enters your cells and carbon dioxide passes out of cells. D. 5.4 Osmosis is the diffusion of water across a membrane 1. One of the most important substances that crosses membranes by passive transport is water. 2. The diffusion of water across a selectively permeable membrane is called osmosis. 3. If a membrane, permeable to water but not to a solute, separates two solutions with different con- centrations of solute, water will cross the membrane, moving down its own concentration gradi- ent, until the solute concentration on both sides is equal. E. 5.5 Water balance between cells and their surroundings is crucial to organisms 1. Tonicity is a term that describes the ability of a surrounding solution to cause a cell to gain or lose water. 2. The tonicity of a solution mainly depends on its concentration of solutes inside the cell. 3. How will animal cells be affected when placed into solutions of various tonicities? a. In an isotonic solution, the concentration of solute is the same on both sides of a membrane, and the cell volume will not change. b. In a hypotonic solution, the solute concentration is lower outside the cell, water molecules move into the cell, and the cell will expand and may burst. c. In a hypertonic solution, the solute concentration is higher outside the cell, water molecules move out of the cell, and the cell will shrink. 4. For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmo- regulation, the control of water balance. 5. The cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different. a. The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much wa- ter and bursting when placed in a hypotonic environment. b. But in a hypertonic environment, plant and animal cells both shrivel. F. 5.6 Transport proteins can facilitate diffusion across membranes 1. Hydrophobic substances easily diffuse across a cell membrane. 2. However, polar or charged substances do not easily cross cell membranes and, instead, move across membranes with the help of specific transport proteins in a process called facilitated dif- fusion, which a. does not require energy and b. relies on the concentration gradient. 56 INSTRUCTOR GUIDE FOR CAMPBELL BIOLOGY: CONCEPTS & CONNECTIONS Copyright © 2015 Pearson Education, Inc. 3. Some proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules. 4. Other proteins bind their passenger, change shape, and release their passenger on the other side. 5. In both cases, the transport protein helps a specific substance diffuse across the membrane down its concentration gradient and thus requires no input of energy. 6. Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow. 7. The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin. G. 5.7 SCIENTIFIC THINKING: Research on another membrane protein led to the discovery of aquaporins 1. Dr. Peter Agre received the 2003 Nobel Prize in Chemistry for his discovery of aquaporins. 2. His research on the Rh protein used in blood typing led to this discovery. H. 5.8 Cells expend energy in the active transport of a solute 1. In active transport, a cell must expend energy to move a solute against its concentration gradi- ent. 2. The energy molecule ATP supplies the energy for most active transport. I. 5.9 Exocytosis and endocytosis transport large molecules across membranes 1. A cell uses two mechanisms to move large molecules across membranes. a. Exocytosis is used to export bulky molecules, such as proteins or polysaccharides. b. Endocytosis is used to take in large molecules. 2. In both cases, material to be transported is packaged within a vesicle that fuses with the mem- brane. 3. There are two kinds of endocytosis. a. Phagocytosis is the engulfment of a particle by the cell wrapping cell membrane around it, forming a vacuole. b. Receptor-mediated endocytosis uses membrane receptors for specific solutes. The region of the membrane with receptors pinches inward to form a vesicle. i. Receptor-mediated endocytosis is used to take in cholesterol from the blood. III. Energy and the Cell A. 5.10 Cells transform energy as they perform work 1. Cells are miniature chemical factories, housing thousands of chemical reactions. 2. Some of these chemical reactions release energy, and others require energy. 3. Energy is the capacity to cause change or to perform work. 4. There are two basic forms of energy. a. Kinetic energy is the energy of motion. b. Potential energy is energy that matter possesses as a result of its location or structure. 5. Thermal energy is a type of kinetic energy associated with the random movement of atoms or molecules. 6. Thermal energy in transfer from one object to another is called heat. 7. Light is also a type of kinetic energy; it can be harnessed to power photosynthesis. 8. Chemical energy is the a. potential energy available for release in a chemical reaction and b. the most important type of energy for living organisms to power the work of the cell. 9. Thermodynamics is the study of energy transformations that occur in a collection of matter. a. The word system is used for the matter under study. Copyright © 2015 Pearson Education, Inc. CHAPTER 5 The Working Cell 57 b. The word surroundings is used for everything outside the system; the rest of the universe. 10. Two laws govern energy transformations in organisms. a. Per the first law of thermodynamics (also known as the law of energy conservation), energy in the universe is constant. b. Per the second law of thermodynamics, energy conversions increase the disorder of the uni- verse. 11. Entropy is the measure of disorder or randomness. 12. Automobile engines and cells use the same basic process to make the chemical energy of their fuel available for work. 13. In the car and cells, the waste products are carbon dioxide and water. 14. Cells use oxygen in reactions that release energy from fuel molecules. 15. In cellular respiration, the chemical energy stored in organic molecules is used to produce ATP, which the cell can use to perform work. B. 5.11 Chemical reactions either release or store energy 1. Chemical reactions either a. release energy (exergonic reactions) or b. require an input of energy and store energy (endergonic reactions). 2. Exergonic reactions release energy. a. These reactions release the energy in covalent bonds of the reactants. b. Burning wood releases the energy in glucose as heat and light. c. Cellular respiration i. involves many steps, ii. releases energy slowly, and iii. uses some of the released energy to produce ATP. 3. An endergonic reaction a. requires an input of energy and b. yields products rich in potential energy. 4. Endergonic reactions a. start with reactant molecules that contain relatively little potential energy but b. end with products that contain more chemical energy. 5. Photosynthesis is a type of endergonic process. 6. In photosynthesis, a. energy-poor reactants (carbon dioxide and water) are used, b. energy is absorbed from sunlight, and c. energy-rich sugar molecules are produced. 7. A living organism carries out thousands of endergonic and exergonic chemical reactions. 8. The total of an organism’s chemical reactions is called metabolism. 9. A metabolic pathway is a series of chemical reactions that either a. builds a complex molecule or b.