Transport in Plants

Transport in Plants

• Plants transport water and minerals from the soil to the leaves for photosynthesis and other metabolic processes.
• Plants use xylem and phloem tissues to transport water and nutrients.
• Xylem is a complex tissue made up of different cell types (vessels, fibres, parenchyma).
• Xylem vessels are continuous tubes through which water is transported.
• Lignin strengthens the xylem vessels making them waterproof and resistant to collapsing.
• Narrow vessels increase adhesion helping move water up the plants through cohesion.
• Xylem vessel elements are dead cells with no cytoplasm allowing a continuous column of water to move.
• Phloem is another complex tissue containing living cells like sieve tubes and companion cells, as well as supporting cells such as fibres and parenchyma.
• Phloem transports sugars produced via photosynthesis from leaves to other parts of the plant.
• Sieve tube elements are elongated cells that form the main transporting component in phloem.
• Sieve plates connect sieve tubes with pores that transport substances along.
• Companion cells provide metabolic support for sieve tube cells.
• Companion cells have dense cytoplasm, a large nucleus, many mitochondria, as well as a well-developed endoplasmic reticulum (RER) to synthesize proteins.

Transpiration in Plants

• Transpiration is the loss of water from a plant's leaves via stomata as water vapour
• Stomata are small pores mainly on the underside of leaves that allow gas exchange to occur, water can escape through openings as well.
• Most water is lost from stomata but some is lost from the cuticle layer of the leaf.
• Some of water loss via cuticle is from diffusion of water.
• Waxy cuticle layer on leaves reduces loss of water.
• Less water lost through thickened cuticle layers.
• Woody plants have lenticels where gas and water exchange occurs.
• Spongy mesophyll is for gas exchange.
• Transpiration creates a water potential gradient from roots to shoots and influences water flow.

External Factors Affecting Transpiration

• Air movements (wind): Windy days have a quicker removal of water vapour that reduces humidity around the leaves, which increases transpiration.
• Humidity: Low humidity increases transpiration rates.
• Temperature: Higher temperatures increase water evaporation and transpiration rates.
• Water supply: Droughts or dry soil reduces water intake through roots, thus reducing transpiration
• Light: Plant stomata open during the day and water vapour is released, thus transpiration increases in the light.

Internal Factors Affecting Transpiration

• Leaf surface area: Larger leaves have greater surface area and more stomata that increases transpiration rates.
• Number and distribution of stomata: Stomata are often found more sparsely or on the lower surface of the leaf to reduce water loss.
• Thickness of cuticle: Thinner layers mean higher transpiration rates.

Functions of Transpiration

• Cooling the plant: Transpiration helps cool the plant, through evaporation of water causing a cooling effect.
• Transporting water to shoots and leaves: Transpiration from the leaves creates a pulling force which helps water flow to shoots and leaves.
• Absorption and translocation of minerals: The process helps push more water up to the leaves thus minerals are transported as well.

Xerophytes

• Xerophytes are plants adapted to live in dry conditions.
• Specialized leaves reduce water loss (sunken stomata and curled leaves.)
• Extensive root systems increase water uptake.
• Swollen stems store water.
• Thick waxy cuticle reduces water loss.

Features of Xerophytes

• Smaller leaves: Smaller surface area for water loss
• Densely packed spongy mesophyll: Reduces exposed surface area for water loss
• Thicker waxy cuticle: Impermeable to water, controls evaporation
• Trichomes: Trap humid air; helps keep water vapour potential lower
• Rolled leaves: Maintain humid air around stomata
• Extensive roots: Maximises uptake of water

Transport Mechanisms

• Transport systems describe how water and minerals are transported from the soil to the xylem and from roots to leaves.
• Includes the symplastic and apoplastic pathways, and the Casparian strip.
• Sucrose is loaded into phloem tubes by companion cells using proton pumping and co-transporter mechanisms.
• Mass flow of phloem sap is driven by hydrostatic pressure gradients from source to sink.

Water Uptake by Roots

• Plants need to replace lost water.
• Water moves into root hair cells due to differences in water potential.
• Water moves across parenchyma cells to the xylem.
• Water is transported throughout the plant via xylem vessels.
• As water moves through the root system, water is drawn in.

Pathways of Water Uptake

• Water can be transported through the root via apoplast, symplast, or vacuolar pathways.
• Apoplast: Through cell walls
• Symplast: Through cytoplasm
• Vacuolar: Through vacuoles.

Cohesion-Tension Theory

• States the mechanism for water transport in xylem through cohesion and tension..
• Water cohesion helps move water up the xylem.
• Adhesion (attraction between water and xylem walls)
• Tension (pulling force) from transpiration in leaves which pulls water up the xylem.

Explore the mechanisms of water and nutrient transport in plants, focusing on xylem and phloem. Understand the structure and function of these tissues, including key components like vessels and sieve tubes, and their roles in photosynthesis and metabolic processes.