Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles4. Plant Physiology © The McGraw−Hill Companies, 2008494Plant Physiology CHAPTER OUTLINE Plant Transport Systems 50Transpiration 51 Absorption of Water from the Soil 51 A CLOSER LOOK 4.1 MineralNutrition and the Green Clean 52Water Movement in Plants 53 Translocation of Sugar 53 Metabolism 54A CLOSER LOOK 4.2 Sugar and Slavery 55Energy 57 Redox Reactions 57 Phosphorylation 57 Enzymes 58 Photosynthesis 58Energy from the Sun 58 Light-Absorbing Pigments 58 Overview 58 The Light Reactions 59 The Calvin Cycle 62 Variation to Carbon Fixation 63 Cellular Respiration 64Glycolysis 65 The Krebs Cycle 65 The Electron Transport System 68 Aerobic vs. Anaerobic Respiration 69 Chapter Summary 70 Review Questions 70 Further Reading 70 KEY CONCEPTS 1. The movement of water in xylem is a passive phenomenon dependent on the pull of transpiration and the cohesion of water molecules whereas the translocation of sugars in the phloem is best described by the Pressure Flow Hypothesis.2. Plants are dynamic metabolic systems with hundreds of biochemical reactions occurring each second, which enable plants to live, grow, and respond to their environment. 3. Life on Earth is dependent on the flow of energy from the sun, and photosynthesis is the process during which plants convert carbon dioxide and water into sugars using this solar energy with oxygen as a by-product. 4. In cellular respiration, the chemical-bond energy in sugars is converted into an energy-rich compound, ATP, which can then be used for other metabolic reactions.CHAPTER Products of photosynthesis are translocated in the phloem and stored in various plant organs. These pumpkins are excellent examples of how energy from the sun is transformed into food by these processes.Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles4. Plant Physiology © The McGraw−Hill Companies, 200850UNIT IIIntroduction to Plant Life: Botanical Principles Although plants lack mobility and appear static to the casual observer, they are nonetheless active organ-isms with many dynamic processes occurring within each part of the plant. Materials are transported through specialized conducting systems; energy is harnessed from the sun; storage products are manufactured; stored foods are broken down to yield chemical energy; and a multitude of products are synthesized. Put simply, plants are bustling with activity. This chapter will consider some of the major trans-port and metabolic pathways in higher plants. PLANT TRANSPORT SYSTEMS As described in Chapter 3, there are two conducting, or vascular, tissues in higher plants, the xylem and the phloem, each with component cell types. Water and mineral transport in the xylem will be described first. Tracheids and vessel elements, which consist of only cell walls after the cyto-plasm degenerates, are the actual conducting components in xylem.The source of water for land plants is the soil. Even when the soil appears dry, there is often abundant soil mois-ture below the surface. Roots of plants have ready access to this soil water; leaves, however, are far removed from this water source and are normally surrounded by the relatively drier air. The basic challenge is moving water from the soil up to the leaves across tremendous distances, sometimes up to 100 meters (300 feet). This challenge is, in fact, met when water moves through the xylem. There are three components to this movement: transpiration from the leaves, the uptake of water from the soil, and the conduction in the xylem ( fig. 4.1 ). Figure 4.1Transpiration-Cohesion Theory of xylem transport. (a) As transpiration occurs in the leaf, it creates a cohesive pull on the whole water column downward to the roots, where water is absorbed from the soil. (b) Vessel elements join to form a long vessel thatmay reach from the roots to the stem tip.XylemPhloemThe tension createdby transpiration pullswater up into leaves.XylemH2OPhloemXylem(a) Xylem transportWater is absorbedby the roots.Water is cohesive andforms a continuouscolumn in xylem.H2OVesselelement(b)Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles4. Plant Physiology © The McGraw−Hill Companies, 2008CHAPTER 4Plant Physiology 51TranspirationTranspiration, the loss of water vapor from leaves, is the force behind the movement of water in xylem. This evapora-tive water loss occurs mainly through the stomata (90%) and to a lesser extent through the cuticle (10%). When stomata are open, gas exchange occurs freely between the leaf and the atmosphere. Water vapor and oxygen (from photosynthesis) diffuse out of the leaf while carbon dioxide diffuses into the leaf ( fig. 4.2a ). The amount of water vapor that is transpired is astounding, with estimates of 2 liters (0.5 gallons) of water per day for a single corn plant, 5 liters (1.3 gallons) for a sun-flower, 200 liters (52 gallons) for a large maple tree, and 450 liters (117 gallons) for a date palm. Imagine the quantities of water lost each day from the acres of corn and wheat planted in the farm belt of the United States! Clearly, transpiration by plants is a major force in the global cycling of water. It is the action of the guard cells that regulates the rate of water lost through transpiration and, at the same time, regulates the rate of photosynthesis by controlling the CO 2uptake. Each stoma is surrounded by a pair of guard cells, which have unevenly thickened walls. The walls of the guard cells that border the stoma are thicker than the outer walls. When guard cells become turgid they can only expand outward owing to the radial orientation of cellulose fibrils; this outward expansion of the guard cells opens the stomata. Stomata are generally open during daylight and closed at night. As long as the stomata are open, both transpiration and photosynthesis occur, but when water loss exceeds uptake, the guard cells lose turgor and close the stomata ( fig. 4.2b ). (See A Closer Look 6.1—The Influence of Hormones on Plant Reproductive Cycles.) On hot, dry, windy days the high rate of transpiration frequently causes the stomata to close early, resulting in a near shutdown of photosynthesis as well as transpiration. A fine balance must be struck in this photosynthesis-transpiration dilemma to allow enough CO 2for