9.1 Transport in the Xylem of Plants

Plant Leaves Absorb radiant energy Optimize the rate of photosynthesis Cross Section of a leaf

Leaf Part Leaf Function Waxy Cuticle Water conservation Stomata Opening in leaf/gas exchange Guard cells Control stomata opening Vascular Tissue Xylem: transports water and minerals from the roots to the shoot by passive transport Phloem: transports nutrients throughout the plant by active transport Collenchyma cells Support vascular bundles (xylem and phloem) and give leaf structure Chloroplasts (mainly in the mesophyll, spongy and pallisade layers) Carry out photosynthesis Spongy layer: filled with air spaces to help facilitate gas exchange Pallisade layer: packed with chloroplasts

Transpiration The passive movement and evaporation of water from the stems and leaves of a plant Made of two types of cells Tracheid cells Vessel elements Pits in the sides of the cells allow water to flow between the cells of the xylem

Cohesive & Adhesive Properties Cohesive properties: (hydrogen bonding) hold water molecules together as they travel up the xylem Evaporation creates a “pull” causing them to move up xylem Water is also entering the plant causing a “push” of water Adhesive properties: water is attracted to the sides of they xylem cells Water molecules adhere to sides of vessels and resist gravitation pull downward

Stomata and Guard Cells opening and closing of stomata controlled by guard cells 1. stomata open and close due to changes in turgor pressure 2. gain and loss of water in the guard cells is largely due to the transport of potassium ions 3. A hormone abscicis acid causes potassium ions to rapidly diffuse across guard cells, causing them to close (due to lack of available water) 4. other factors like carbon dioxide levels and circadian rhythms affect opening and closing

Stomata & Guard cells

Abiotic Factors and Transpiration Rates Environmental Factor Effect Light Speeds up transpiration by warming leaf and opening stomata Humidity Decreasing humidity increases transpiration because of the greater difference in water concentration Wind Increases the rate of transpiration because humid air near the stomata is carried away Temperature Increasing temperature causes greater transpiration because more water evaporates Soil water If the intake of water at the roots doesn’t keep up with transpiration , turgor loss occurs causing the stomata to close decreasing transpiration Carbon Dioxide High CO2 levels in the air around the plant usually causes guard cells to lose turgor and stomata close

Root System and Water Uptake Water and mineral uptake Occurs in the roots which have adapted in several ways: roots are covered in tiny hairs that increase the surface area for greater water absorption 2. Branching allows roots to cover more area and increase uptake 3. Cortex cell walls are permeable for osmosis

Uptake of Mineral Ions 1. Diffusion of mineral ions a. Requires concentration gradient (not common because ions are usually in low conc. In the soil) 2. Mutualistic relationships with fungal hyphae a. Fungi have long multinucleated cells called hyphae b. Form dense networks called mycelium that grow around roots c. Plant root provides fungi with nitrates in exchange for needed minerals 3. Mass flow of water in the soil carrying ions

Active Transport of Minerals Mineral ions are taken into the plant by active transport a. root hairs secrete hydrogen ions into soil particles, exchanging them for mineral ions (calcium, magnesium, iron etc.) b. ATP is hydrolyzed in order to provide the energy for this process

Adaptions of Xerophytes Xerophytes live in arid (dry) environments They have developed adaptations to help with transpiration Reduced leaves sometimes spines Rolled leaves: block the wind from reaching stomata Deep roots for more surface area Thickened waxy cuticle Reduced number of stomata Hairs to help trap water Water storage organ Low growth CAM and C4 plants: fix CO2 at very low concentrations, stomata open less frequently