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This text explains the importance of soil composition, the absorption of essential nutrients by roots, the process of water and mineral transport, and the conservation of water through cuticles and stomata. It also covers the role of phloem sap in distributing organic compounds and discusses parasitic plants.
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Ch 22 Plant nutrition and transport Subtitle
22.1 soil and air provide water and nutrients • A. Plants require 16 essential elements • Essential nutrients are chemicals that are vital for metabolism, growth, and reproduction. • 9 are macronutrients- C, O, H, N, K, Ca, Mg, P, and S • 7 are micronutrients- Cl, Fe, B, Zn, Mn, Cu, Mo • Pg. 467- chart
B. Soils have distinct layers • Soil is a complex mixture of rock particles, organic matter, air and water. • Also contains bacteria, archaea, fungi, protists and animals. Soil is critical to ecosystem function. • Soil is classified by texture according to its composition of sand, silt, and clay. • Over time, soil layers develop. Organic materials lie on the surface. • Humus, a chemically complex spongy organic substance occupies the upper layer of the topsoil or A horizon.
Plants stabilize the soil to prevent erosion. • Different climates have different soil types. • Human activities can increase erosion and ecosystem recovery is slow in heavily eroded areas.
C. Leaves and roots absorb essential elements • Plants get C, H, and O from water and the atmosphere. • Roots absorb water, but also the minerals that are dissolved in the soil’s water. • Organisms help plants get the nutrients they need. • Phosphorous can be a limiting factor in soil because it is poorly soluble. Mycorrhizae help plants boost phosphorous absorption. • Nitrogen needs to be fixed from the atmosphere into a form plants can use. Bacteria in the soil or in root nodules help fix N2 into NH4+ so plants can incorporate it into their tissues.
22. 2 Water and Dissolved minerals are pulled up to leaves • A. water vapor is lost from leaves through transpiration • Plants use a lot of water. A typical tree might consume 265 liters of water in a single day. • Water is essential for plants because it is a medium for metabolic processes. It’s needed for photosynthesis, hydrolysis, and to maintain turgor pressure. • Plants lose most of the water that they absorb. When plants lose water through leaves it is called transpiration. • Low humidity, wind and heat cause transpiration rates to soar. • When plants close stomata to combat water loss, photosynthesis also slows.
22.2 B. Xylem transport • Xylem- vascular tissue that transports water and minerals (xylem sap) • Functional cells of xylem(tracheids and vessel elements) are dead at maturity. Metabolic activity cannot drive water transport through plants. • The cohesion-tension theory explains how xylem sap moves in a plant. • Water is cohesive (the molecules stick together). When water evaporates from the leaf, additional water diffuses out into the mesophyll. Water molecules leaving the vein attract adjacent molecules, effectively pulling molecules up the stem. Water also enters through the roots. • Root hairs and mycorrhizal fungi increase surface area for water and mineral absorption.
Water and mineral can travel through roots in 2 ways. • 1. extracellular pathway- solution moves in the spaces between cells or along the cell walls. • 2. intracellular pathway- water can move from cell to cell via plasmodesmata • The solution flows through the outer portion of the root until it gets to the endodermis where the Casparian strip forces the materials to enter the cells of the endodermis. • Materials that cross the endodermis continue into the xylem, making their way up the plant.
This mechanism uses the properties of water to distribute fluids so that the plant does not burn energy. • If there is not enough water in the soil, plants can wilt.
c. The cuticle and stomata help conserve water • The cuticle is an important water-saving adaptation in land plants. • Other water-saving features: • Stomata- pores that can close to shut down transpiration. • Stomata are made up of guard cells. The guard cells use a potassium gradient to open and close. Most plants close guard cells at night so that photosynthesis is not affected.
22.3 Organic compounds are pushed to nonphotosynthetic cells • A. phloem sap contains sugars and other organic compounds • The compounds carried in phloem are carried in phloem sap. (includes water, minerals as well as carbohydrates.) • Phloem sap also includes amino acids, hormones, enzymes, and mRNA molecules. • B. The pressure flow theory explains phloem function • This theory suggests that phloem sap flows from sources to sink. • Sources- any plant part that produces or releases sugars • Sink- any plant part that does not photosynthesize.
Plants expend energy to load sugars into sieve tube elements. Water moves by osmosis into the phloem sap. This increases turgor pressure that drives phloem sap through the sieve tubes. • When the sap gets to the “sink” it is moved out of the tubes by active transport. Then water will move out of the sap causing a decrease in turgor pressure. • A given organ may act as a source or a sink.
22.4 Parasitic plants tap into another plant’s vascular tissue • Most plants are self-sufficient. However, some are parasites. • Parasitic plants acquire water, minerals, and food by tapping into the vascular tissues of the host plant. • When a parasitic seedling sticks to a host, it will grow a root through the host’s epidermis and form a structure called a haustorium, which connects the parasite’s vascular tissues to those of the host. • Dodder is an example of a parasitic plant. The most common parasitic plants are the many species of mistletoe. • Most plants infected with mistletoe are weakened, but do not die.
