Plant Nutrient and Water Transport

• Plants transport of water, minerals, and organic nutrients throughout their structure
• Plants need transport systems because of their size and immobility

Nutrient Transport

• Plants get essential nutrients from the soil through roots
• Plants create organic nutrients (sugars) in leaves through photosynthesis
• Transport systems distribute nutrients to all parts of the plant for growth, development, and reproduction

Water Uptake Mechanisms

• Water absorption happens mainly through root hairs, boosting the absorption surface area
• Water goes into roots via osmosis linked to the water potential gradient
• Solute concentration and pressure impacts water potential
• Water flows from the soil with higher water potential to root cells with lower water potential
• Once inside the root, water moves through the cortex via two main pathways: apoplast and symplast
• Apoplast pathway involves water moving through cell walls and spaces without crossing membranes
• Symplast pathway involves water entering a cell's cytoplasm then moving to other cells through plasmodesmata
• The endodermis, which has a Casparian strip, forces water and minerals entering the xylem to pass through a cell membrane
• The Casparian strip, made of suberin, stops water and ions from passing through
• This ensures no harmful substances enter the vascular tissue
• Water is transported upwards once it reaches the xylem

Transpiration Process

• Transpiration occurs when water evaporates from plant aerial parts, especially through leaf stomata
• This evaporation causes negative pressure, or tension, in the xylem, which then draws water upwards from the roots
• The transpiration pull helps water move in plants
• Several factors influence the rate of transpiration:
• Light: Stomata open allowing CO2 uptake for photosynthesis, also allowing water to escape
• Humidity: Higher humidity reduces transpiration rate because of decline in water potential
• Temperature: Higher temperatures increase evaporation, also increasing transpiration
• Wind: Wind increases transpiration by removing humid air
• Stomata are controlled by guard cells that causes them to open and close
• Stomata opens when guard cells are turgid (full of water)
• Stomata closes when guard cells are flaccid (lacking water)
• Light, CO2 concentration, and water availability affects stomata opening and closing
• Abscisic acid (ABA), a plant hormone, closes stomata in response to water stress

Xylem Function

• The xylem transports water and minerals from roots to the rest of the plant
• The xylem includes tracheids and vessel elements, which are dead when matured
• Tracheids are elongated cells with tapered ends found in all vascular plants
• Vessel elements are shorter and wider cells with perforated end walls called perforation plates, found in angiosperms
• These traits allow efficient water flow
• The transpiration-cohesion-tension mechanism drives water movement through the xylem
• Transpiration causes tension (negative pressure) in leaves
• Cohesion is the attraction between water molecules due to hydrogen bonding, allowing water to be pulled up
• Adhesion is the attraction between water molecules and the walls of the xylem which helps counteract gravity
• Minerals are transported along with water in the xylem, entering via active transport in the roots

Phloem Function

• Phloem is responsible for transporting organic nutrients, mainly sugars, from leaves (source) to other parts of the plant (sink)
• Phloem contains sieve tube elements and companion cells
• Sieve tube elements are living cells connected to form continuous sieve tubes
• Sieve plates at the ends of sieve tube elements assist phloem sap flow between cells
• Sieve tube elements lack nuclei and other organelles
• Companion cells linked to sieve tube elements provide them metabolic support
• Companion cells attach to sieve tube elements via plasmodesmata and help in sugar loading and unloading
• Translocation is the movement of sugars through the phloem, driven by pressure flow
• Sugars are actively transported into sieve tube elements at the source, raising solute concentration and lowering water potential
• Water goes into sieve tube elements from the xylem via osmosis, raising the pressure potential
• This pushes phloem sap towards the sink where sugars are unloaded
• At the sink, sugars are actively or passively transported out of sieve tube elements, increasing water potential
• Water leaves the sieve tube elements and returns to the xylem reducing pressure potential
• The pressure difference between source and sink drives phloem sap bulk flow
• The pressure flow model explains how sugars move from areas of high concentration (source) to areas of low concentration (sink)