ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Thirty-Seven Outline INTRODUCTION Transport Process Must Occur for Plants to Function fig 37.1 Phloem carries a solution of carbohydrates from leaves to other parts Xylem carries water from roots to other plant parts Passive Forces Drive the Movement of Plant Liquids Not driven actively by a pump like in animals Passive forces rely on narrow transport tubes THE SOIL Produced from Weathered Rocks Composition related to structure of parent rocks Includes 92 naturally occurring elements tbl 2.1 Elements combined into inorganic compounds called minerals Composition of Soil Organic materials result from biological activities fig 37.2 Effects are obvious in the topsoil as well as deeper layers Soil fertility related to decomposition, recycling organic debris Topsoil = mineral particles + living organisms + humus Varying size of soil particles Coarse sand has largest particle size Clay has smallest particle size Half of soil volume is occupied by empty space Filled with air or water Not all water in soil is available to plants Water in Soil Nature of water affects interaction with soil Water forms hydrogen bonds with itself and other materials Water stays tightly bound to negatively charged clay particles Water adheres to small particles with greater surface area, like clay Best soil for plant growth Contains balanced mixture of fine and coarse particles Called loam Water drains through soil by force of gravity Water held in soil pores and available to plants is called capillary water Amount of water remaining after gravity drainage is called field capacity Water unavailable to plants associated with permanent wilting point WATER MOVEMENT THROUGH PLANTS Effects of Gravity on Water Tube with closed upper end placed in a bucket full of water Weight of water forces it down the tube Weight of air on water in bucket forces water up the tube Forces counter each other, water rises to 10.4 feet at sea level Weight of water pulls itself down in tube taller than 10.4 meters Tube has vapor-filled vacuum at upper end Water in tube cavitates fig 37.3 Movement of Water in Plants Not caused by capillary action (cohesion in small diameter tubes) Caused by tension resulting from evaporation at leaves Water molecules removed from leaves Water replenished by new molecules drawn in at roots Absence of cavitation of water in plants Reduced by tensile strength resulting from cohesion of water Tensile strength increased in vessels with small diameters Cavitation occasionally occurs in localized regions of xylem Cavitation bubbles block ends of vessel elements and tracheids Bubbles unable to pass through end pores, flow is blocked Overall flow upward continues through parallel elements Potential energy of water Pressure potential exerted by atmosphere Solute potential driven by diffusion forces Water potential = pressure potential + solute potential Forces that result in upward movement of water in plants Positive pressure of atmosphere Negative pressure caused by evaporation Osmotic absorption at roots Transpiration Majority of water taken up by roots is lost to atmosphere Exit leaves through stomata in form of water vapor Water first passes into intercellular spaces between spongy mesophyll fig 35.27 Water in spaces renewed by flow from leaf veinlets Plant must have continual source of water to survive Must minimize water lost to atmosphere Must admit CO2 Plant features evolved to balance these two situations The Absorption of Water by Roots Most water enters through root hairs fig 35.5 Root hairs always turgid due to solute potential Transport of minerals requires expenditure of ATP energy Water and minerals pass into conducting elements of xylem fig 37.4 Non-selectively follow cell walls and spaces between cells Selectively go through plasma membrane and through protoplasm of adjacent cells Water and mineral passage stopped at Casparian strip Endodermal cells selectively control mineral movement Transpiration may cease at night due to high relative humidity of air Active transport of ions still occurs in roots Water passes inward via osmosis, called root pressure Pressure strong enough to force water from cut ends of plant Strong pressure causes guttation, water is forced out of veins fig 37.5 Occurs only in very short plants Water Movement in Plants Tension created in xylem during active transpiration Walls of vessels pulled close together Diameter of tree trunk may be less during the day than at night Negative water potential also occurs in columns of dead xylem cells Warming of leaves and small branches increases rate of evaporation Creates pull at upper end of water column Pulls water in through roots Sun is ultimate source of potential energy The Regulation of Transpiration Rate Control short term loss of water by closing stomata Must counter balance water loss with need for CO2 Intercellular spaces must be moist for CO2 to enter cells Changing water pressure in guard cells regulates stomata fig 37.6 Guard cells are only epidermal cells with chloroplasts Distinctive curved shape, cell wall thicker next to stomatal opening Turgid cells have bowed shape, open stomata Turgidity results from active ion pumping Requires expenditure of energy Chloroplasts provide ATP When active transport stops ions move out by diffusion With lower solute potential water leaves guard cell Guard cells become flaccid, close stomata Ions important to stomatal regulation Primarily potassium, K+ Changes in gradient results in rapid stomata opening or closing Accompanied by passage of Cl- inward or H+ outward When water is scarce guard cells become flaccid, stomata close Abscissic acid effects passage of K+ Ions pass out of guard cells Stomata close Hormone binds to receptors on guard cell plasma membrane Other Factors Regulating Transpiration Concentration of CO2 Guard cells become flaccid with high concentrations Stomata close because there is no need for more CO2 Temperature: stomata close when temperature is above 30% to 34% C In dark stoma will open if CO2 is low CAM photosynthesis allows plants to conserve water by taking in CO2 at night and fixing it during the day Seasonal dormancy regulates water loss Deciduous plants lose leaves during dry seasons, including winter Annual plants exist only in the form of seeds Leaf morphology regulates water loss Thick, hard leaves with few stomata are more resistant to drying Wooly trichomes trap humid layer of air near the leaf surface Stomata may be present in pits in the leaf surface NUTRIENT MOVEMENT Movement of Ions Occurs Through Protoplasts of Cells, Not Through Walls Passage is active and carrier-mediated Specific ions maintained at levels different from those of the soil Lose capacity to absorb nutrients when deficient in oxygen Ions Ultimately Transported Throughout Plant Seasonal abundance of phosphorus, potassium, nitrogen, iron in xylem Essential nutrients may be translocated from parts to be shed Calcium cannot be translocated once it is deposited CARBOHYDRATE MOVEMENT Carbohydrates Translocated from Leaves to Other Plant Parts May be concentrated in storage organs like tubers Generally stored in the form of starch May be converted to transportable forms like sucrose Translocation studied using aphids fig 37.7 Sucrose comprises most of dry matter of phloem liquid Movement may be as rapid as 50 to 100 centimeters per hour Flow of Phloem Liquid Mass flow results from hydrostatic pressure of osmosis fig 37.8 Sucrose actively loaded into phloem of leaf veinlets Source is location of sucrose production Sink is region where sucrose is actively unloaded Change in solute potential allows water to pass in as well Sucrose unloaded from sieve tubes at the sink Solute potential decreases as sucrose is removed Water moves from source to sink, sucrose moves passively with it Transport through phloem does not require energy Loading and unloading of materials requires energy Energy supplied in form of ATP by companion or parenchyma cells ATP passes through plasmodesmata into sieve tubes PLANT NUTRIENTS Two Kinds of Inorganic Nutrients Are Required by Plants Macronutrients are required in relatively large amounts Include carbon, hydrogen, oxygen, nitrogen, potassium, calcium, phosphorus, magnesium and sulfur Each nutrient approaches or exceeds 1% of its dry weight Importance has been known for the last century Micronutrients are required in trace amounts Include iron, chlorine, copper, manganese, zinc, molybdenum, boron Constitute several hundred to less than one part per million Importance only recently recognized Important nutrients determined by hydroponic culture Known or suspected nutrients left out of culture medium Plants grown and studied for abnormal symptoms fig 37.9 The Roles of Plant Nutrients tbl 37.1 Some functions are common among plants and other organisms Role of potassium in the regulation of guard cell turgor pressure Calcium is an essential component of middle lamellae, membranes Phosphorus important in molecules like nucleic acids and ATP Nitrogen is essential in nucleic acids, amino acids and chlorophyll Some plants have special requirements Grasses require silica to retard consumption by herbivores fig 37.10 Nitrogen-fixing bacteria in legumes require cobalt Soybeans require nickel Desert and salt-marsh plants require additional sodium Some plants concentrate nutrients they do not use themselves Fertilizer Natural communities recycle and reuse nutrients Cultivated crops require input of additional nutrients Exposed soils lose nutrients to erosion Crops carry nutrients away from source Tropical soils particularly devoid of nutrients Additional minerals include nitrogen, phosphorus, potassium May be source of pollution in certain circumstances Grading of commercial fertilizers Three numbers reflect percentage of N, P and K Proportions needed depend on natural fertility and type of crop Organic fertilizers Include manure and dead animal remains Are not nutritionally superior to inorganic fertilizers Provide additional source of humus Enhance capacity to hold water and nutrients