Plant Transport Mechanisms

• Land plants have adapted to increased competition through evolution and increased numbers.
• Adaptations include a vascular system for support and long-distance transport, and increased surface area for sunlight absorption through phyllotaxy and shoot branching.

Vascular Tissues Anatomy

• Tracheids and vessel elements are part of the xylem with pits and perforations for water transport.
• Sieve tube elements and companion cells are part of the pholem for sugar transport

Xylem

• Xylem is made of dead cells with thick cell walls.
• Xylem is located in the wood of plants.
• It transports water and nutrients using capillary action, supporting stem.

Phloem

• Phloem is made of living cells that lack thick cell walls, located in the bark of the plant.
• Transports food (sugar-sap) up and down the stem, like an elevator and does not support the stem

Plant Systems

• Shoots perform photosynthesis, reproduction, food and water transport, and storage.
• Roots provide anchorage, water and mineral absorption, and reproduction.

Shoot System

• Leaves are for photosynthesis
• Stems transport water and nutrients

Photosynthesis

• Leaves take in water, carbon dioxide, and light to produce oxygen and sugar.

Root System

• Roots absorb water and minerals from the soil.

• Leaves take in CO2 and release O2 through stomata.

• Transpiration creates a force in leaves that pulls xylem sap upward.

• Phloem sap flows both ways between shoots and roots, moving from sugar production or storage sites to usage sites.

• Roots exchange gases with soil, taking in O2 and discharging CO2.

• Plants use a variety of transport processes due to the diversity of substances and large distances and barriers. Mechanisms transport substances over both short and long distances.

• Plants employ different mechanisms to transport substances over short or long distances

Solutes transport

• Selective plasma membrane permeability helps short-distance movement of substances.
• Active and passive transport are used.
• Membrane pumps and transport proteins are helpful

Proton Pumps

• Proton pumps transport H+ ions and play a primary role in basic transport
• Ion channels also aid

Water Transport

• Water transport occurs via osmosis of free water.
• Aquaporins, which are transport proteins in cell membranes, facilitate water molecule transport.

Water Potential

• Water potential predicts the direction of water flow, moving from high to low water potential.
• Water potential is the sum of solute potential and pressure potential, measured in megapascals (MPa).
• Solute potential is directly proportional to molarity.
• Pressure potential is the physical pressure on a solution and can be positive or negative.

Solute Potential

• Solute potential is based on the molarity/concentration of solute, water moves from regions of lower solute potential to higher solute potential.

Pressure Potential

• Positive pressure results in cell turgidity.

• Negative pressure results from transpiration

• Water moves from one end of the cell to another in seconds via diffusion.

• Transporting water from roots to the top of a tree can take centuries.

Bulk Flow

• Bulk flow addresses long-distance transport needs.
• A gradient between the roots and leaves is the driving force for water movement in the xylem.
• Dead xylem cells decrease blockage.
• Phloem lacks organelles to reduce blockage.

Mechanisms of transport in plants

• Diffusion
• Active transport
• Bulk flow

Transport Pathways

• Apoplast: Includes everything external to plasma membranes, like cell walls, extracellular spaces, and dead cells.
• Symplast: Includes the entire cytosol mass of living cells and plasmodesmata.

Vascular Tissue Functions

• Vascular tissues transport food (products of photosynthesis) and water,
• Xylem transports water and minerals up, while phloem transports products of photosynthesis down.

Soil to Xylem Pathway

• 1. Absorption of water and nutrient minerals by the root cells
• 2. Transport of water and minerals into the xylem
• 3. Bulk flow and transpiration in xylem-shoot-leaves

Absorption

• Root hairs increase surface area for this purpose.

• The root cortex controls mineral selectivity.

• Endodermis acts as a checkpoint for mineral passage into the vascular cylinder.

• The Casparian strip, made of suberin, prevents solute leakage and keeps unneeded substances out

Xylem sap

• Xylem sap (water + minerals) moves long distances via bulk flow to leaf veins
• Factors move xylem sap in the plant that includes root pressure and transpiration

Root pressure

• Pushing xylem sap is minor because positive pressure is insufficient to overcome gravity.
• Guttation occurs with greater root pressure than transpiration.

Transpiration

• Transpiration accounts for xylem sap movement.

• Cohesion-Tension Hypothesis states transpiration provides the pull, and cohesion transmits it.

• Tension indicates xylem sap is normally under negative pressure.

• Cohesion is the force between water molecules.

• Water exits the xylem, becomes vapor, and exits the leaf via stomata.

Water and Tension

• Outside air and the atmosphere influence water tension

• Adhesion helps pulls against gravity

• Stomata regulate transpiration rate, with 95% of water loss occurring through them.

• Guard cells control stomatal shape to regulate water and gas exchange.

Effects of Transpiration

• Without enough water in the leaves shoots may wilt
• It can lower the plant temperature.
• Evaporitive cooling helps prevent the denaturation of enzymes for photosynthesis.

Translocation

• It is important to move the product of photosynethesis with:
• Sieve tube elements
• Sieve plates

Pholem

• Contains Hormones, Minerals, Sugar (most prevalent), Amino acids
• Sugar source are plant organs that produce suga
• Sugar sink is are net consimers or depositories like growing roots or fruit.

Explore plant transport mechanisms focusing on water potential, apoplast vs. symplast pathways, and the Casparian strip. Understand how water and minerals are absorbed by roots and transported throughout the plant. Learn about the roles of perforation plates and aquaporins.