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Plant Nutrition and Transport Chapter 29 Impacts, Issues Leafy Cleanup Crews The EPA is using hybrid plants to remove some dangerous toxins from highly contaminated sites – a process known as phytoremediation 29.1 Plant Nutrients and Availability in Soil Nutrients • Elements or molecules essential for an organism’s growth and survival Plants require sixteen elemental nutrients available from soil, water, and air • Nine macronutrients, required in large amounts • Seven micronutrients, required in trace amounts Plant Nutrients and Deficiency Symptoms Properties of Soil Soil consists of mineral particles mixed with decomposing organic material (humus) • Water and air in spaces between particles Mineral particles in soil differ in size (sand, silt, and clay) which affects compaction • Clay particles are negatively charged, and can hold positively charged ions dissolved in water Soils and Plant Growth Different soil types affect growth of different plants • Most plants grow best in soils containing 10 to 20 percent humus • Soils with equal proportions of sand, silt, and humus (loams) have the best oxygen and water penetration • Swamps and bogs have too much organic matter How Soils Develop Soils develop over thousands of years Most form in layers (horizons) with distinct properties (soil profiles) Topsoil (the A horizon) contains the most organic material Soil Horizons O HORIZON Fallen leaves and other organic material littering the surface of mineral soil A HORIZON Topsoil, with decomposed organic material; variably deep [only a few centimeters in deserts, elsewhere extending as far as 30 centimeters (1 foot) below the soil surface] B HORIZON Compared with A horizon, larger soil particles, not much organic material, more minerals; extends 30 to 60 centimeters (1 to 2 feet) below soil surface C HORIZON No organic material, but partially weathered fragments and grains of rock from which soil forms; extends to underlying bedrock BEDROCK Fig. 29-2, p. 495 Leaching and Erosion Leaching • Process by which water removes soil nutrients and carries them away • Fastest in sandy soils Soil erosion • A loss of soil under the force of wind and water • Increases with sparse vegetation and poor farming practices Erosion Due to Poor Farming Practices 29.1 Key Concepts Plant Nutrients and Soil Many plant structures are adaptations to limited amounts of water and essential nutrients The amount of water and nutrients available for plants to take up depends on the composition of soil Soil is vulnerable to leaching and erosion 29.2 How Do Roots Absorb Water and Nutrients? Root specializations such as root hairs, mycorrhizae, and nodules help the plant absorb water and nutrients Root Hairs Root hairs • Thin extensions of root epidermal cells that enormously increase surface area available for absorbing water and dissolved mineral ions • New root hairs constantly form just behind the root tip Mycorrhizae Mycorrhizae • Forms of mutualism between root and fungi in which both species benefit • Fungal hyphae share minerals absorbed from soil • Root cells provide fungus with food Root Nodules Root nodules • Masses of root cells infected with bacteria that fix atmospheric nitrogen into a form usable by plants (nitrogen fixation) • A mutualism between certain types of soil bacteria and legumes Root Specializations How Roots Control Water Uptake Osmosis drives water from soil into the walls of parenchyma cells of the root cortex Water enters cell cytoplasm by diffusion or through aquaporins; active transporters pump dissolved mineral ions into cells Water and ions move from cell to cell through plasmodesmata The Casparian Strip Endodermis between the cortex and vascular cylinder secretes a waxy substance which forms a waterproof band (Casparian strip) between plasma membranes of endodermal cells The Casparian strip forces water and ions to enter the vascular cylinder through plasmodesmata or through endodermal cell membranes (controlled by transport proteins) Exodermis Exodermis • A layer of cells just below the root surface that can deposit a Casparian strip that functions like the one next to the vascular cylinder Control of Water and Ion Uptake by Transport Proteins Fig. 29-5a, p. 497 vascular cylinder epidermis endodermis primary phloem primary xylem cortex A In roots, the vascular cylinder’s outer layer is a sheet of endodermis, one cell thick. Fig. 29-5a, p. 497 Fig. 29-5b, p. 497 vascular cylinder B Parenchyma cells that make up the layer secrete a waxy substance into their walls wherever they touch. The secretions form a Casparian strip, which prevents water from seeping around the cells into the vascular cylinder. tracheids and vessels in xylem sieve tubes in phloem endodermal cell Casparian strip Fig. 29-5b, p. 497 Fig. 29-5c, p. 497 C Water and ions can only enter the vascular cylinder by moving through cells of the endodermis. They enter the cells via plasmodesmata or via transport proteins in the cells’ plasma membranes. Casparian strip Vascular cylinder water and nutrients Cortex Fig. 29-5c, p. 497 Animation: Root functioning 29.3 How Does Water Move Through Plants? The upward movement of water through xylem, from roots to leaves, is driven by two properties of water: evaporation and cohesion Tracheids and vessel members • Water conducting tubes of xylem • Cells are dead at maturity • Lignin-impregnated walls remain Tracheids and Vessel Members Fig. 29-6a, p. 498 perforation in the side wall of tracheid a Tracheids have tapered, unperforated end walls. Perforations in the side walls of adjoining tracheids match up. Fig. 29-6a, p. 498 Fig. 29-6b, p. 498 vessel member b Three adjoining vessel members. The thick, finely perforated end walls of dead cells connect to make long tubes that conduct water through xylem. Fig. 29-6b, p. 498 Fig. 29-6c, p. 498 perforation plate c Perforation plate at the end wall of one type of vessel member. The perforated ends allow water to flow freely through the tube. Fig. 29-6c, p. 498 Cohesion-Tension Theory Continuous negative pressure (tension) created by evaporation of water from leaves and stems (transpiration) pulls water upward through xylem Hydrogen bonds among water molecules (cohesion) in continuous columns inside xylem tubes keep water from breaking into droplets Cohesion-Tension Theory Fig. 29-7a, p. 499 mesophyll (photosynthetic cells) vein upper epidermis A The driving force of transpiration stoma Evaporation of water molecules from above ground plant parts puts water in xylem into a state of tension that extends from roots to leaves. For clarity, tissues inside the vein are not shown. Fig. 29-7a, p. 499 Fig. 29-7b, p. 499 xylem vascular cambium phloem B Cohesion of water inside xylem tubes Even though long columns of water that fill narrow xylem tubes are under continuous tension, they resist breaking apart. The collective strength of many hydrogen bonds keeps individual water molecules together. Fig. 29-7b, p. 499 Fig. 29-7c, p. 499 vascular cylinder endodermis water cortex molecule root hair cell C Ongoing water uptake at roots Water molecules lost from the plant are being continually replaced by water molecules taken up from soil. Tissues in the vein not shown. Fig. 29-7c, p. 499 Animation: Transpiration 29.2-29.3 Key Concepts: Water Uptake and Movement Through Plants Certain specializations help roots of vascular plants take up water and nutrients Xylem distributes absorbed water and solutes from roots to leaves 29.4 How Do Stems and Leaves Conserve Water? Water is an essential resource for all land plants Water-conserving structures (cuticle and stomata) and processes are key to the survival of land plants The Water-Conserving Cuticle Cuticle • A translucent, water-impermeable layer coating the walls of all plant cells exposed to air • Consists of epidermal cell secretions: waxes, pectin, and cellulose fibers embedded in cutin Controlling Water Loss at Stomata Stomata • Openings through the plant epidermis that regulate water vapor loss and gas exchange • Formed by two guard cells Guard cells open or close the stoma depending on the amount of water in their cytoplasm • Swollen cells open stoma • Collapsed cells close stoma Controlling Water Loss at Stomata Environmental cues open or close stomata • Water availability (abscisic acid released by root cells) • Carbon dioxide levels in leaf (aerobic respiration) • Light intensity (triggers potassium pumps) • Air pollution (prevents photosynthesis) Stomata and Industrial Smog 29.4 Key Concepts Water Loss Versus Gas Exchange A cuticle and stomata help plants conserve water, a limited resource in most land habitats Closed stomata stop water loss but also stop gas exchange Some plant adaptations are trade-offs between water conservation and gas exchange 29.5 How Do Organic Compounds Move Through Plants? Phloem distributes the organic products of photosynthesis through plants Concentration and pressure gradients in the sieve-tube system of phloem force organic compounds to flow to different parts of the plant Phloem: Sieve-Tube Members and Sieve Plates one of a series of living cells that abut, end to end, and form a sieve tube companion cell (in the background, pressed tightly against sieve tube) perforated end plate of sieve-tube cell, of the sort shown in (b) Fig. 29-10a, p. 502 Organic Products of Photosynthesis Plants store carbohydrates as starch, and distribute them as sucrose and other small, water-soluble molecules Pressure-Flow Theory Translocation • Gradients set up by companion cells move organic molecules into sieve tubes at sources, and unload them at sinks Pressure-flow theory • Internal pressure (turgor) builds up in sieve tubes at a source, pushing solute-rich fluid to a sink, where sucrose is removed from the phloem Translocation of Organic Compounds: Sources and Sinks Fig. 29-12a, p. 503 Translocation SOURCE (e.g.,mature leaf cells) interconnected sieve tubes A Solutes move into a sieve tube against their concentration gradients by active transport. WATER B As a result of increased solute concentration, the fluid in the sieve tube C The pressure flow becomes difference hypertonic. pushes the fluid from the source D Both pressure to the sink. Water and solute moves into and concentrations out of the sieve gradually tube along the decrease as the way. fluid moves from source to sink. E Solutes are SINK (e.g., unloaded into developing sink cells, which root cells) then become hypertonic with respect to the sieve tube. Water moves from the sieve tube into sink cells. Fig. 29-12a, p. 503 Fig. 29-12b, p. 503 upper leaf epidermis photosynthetic cell sieve tube in leaf vein companion cell next to sieve tube lower leaf epidermis Typical source region Photosynthetic tissue in a leaf Fig. 29-12b, p. 503 Fig. 29-12c, p. 503 sieve tube Typical sink region Actively growing cells in a young root Fig. 29-12c, p. 503 29.5 Key Concepts Sugar Distribution Through Plants Phloem distributes sucrose and other organic compounds from photosynthetic cells in leaves to living cells throughout the plant Organic compounds are actively loaded into conducting cells, then unloaded in growing tissues or storage tissues Summary: Processes that Sustain Plant Growth ATP formation by roots respiration of sucrose by roots absorption of minerals and water by roots transport of sucrose to roots transport of minerals and water to leaves photosynthesis Fig. 29-13, p. 504 Animation: Cohesion-tension theory (or Water transport) Animation: Interdependent processes Animation: Soil profile Animation: Stomata Animation: Translocation in phloem Animation: Uptake of nutrients by plants Animation: Water absorption Video: Leafy clean-up crews Video: Sequoias