4- transport in plants Flashcards
xylem tissue structure
- Composed of vessel elements and tracheids. Both are long, tube-like structures that are dead at maturity, providing unimpeded pathways for water movement.
- Vessel elements are shorter and wider, with perforations in their end walls.
- Tracheids are narrower with tapering ends and have pits but no perforations.
xylem tissue role in transport
- Transports water and dissolved minerals from roots to the rest of the plant in one direction (upwards).
- Provides structural support to the plant due to thick, lignified walls.
phloem tissue structure
• Consists mainly of sieve tube elements and companion cells.
• Sieve tube elements are long, cylindrical cells lacking a nucleus at maturity, with end walls (sieve plates) that have pores to allow for fluid movement.
• Each sieve tube element is associated with one or more companion cells, which are nucleated cells that help in the functioning of the sieve tube elements.
• Smaller than xylem.
phloem tissue role in transport
• Responsible for the transport of photosynthates (sugars and products of photosynthesis from the leaves (source) to other parts of the plant (sinks) such as roots and stems.
• This transport process, known as translocation, can occur in any direction (upwards or downwards) depending on the plant’s needs.
apoplastic pathway
• Non-living parts: Water moves along cell walls and extracellular spaces.
• Passive process: Water enters the plant through the root hair cell walls, travels cell to cell via their adjacent walls, and continues this path until it reaches the endodermis layer of the roots.
• Barrier: The Casparian strip in the endodermis blocks the apoplastic pathway, preventing water from bypassing the plant’s control system in the endodermal cells.
symplastic pathway
• Living parts: Water moves from cells through their cytoplasm, connected by plasmodesmata.
• Active process: Water enters the plant through the root hair cell membrane, moves into the cytoplasm, and can then pass directly from cell to cell through the plasmodesmata.
• Continuity: The symplastic pathway provides a continuous route from the root hair cells to the xylem in the centre of the root.
comparison between apoplastic and symplastic pathway
• Speed: Apoplastic pathway is generally quicker due to the lack of resistance in cell walls.
• Control: Symplastic pathway offers more control over water movement.
importance of water movement
• Turgor pressure maintenance: Helps maintain plant rigidity.
• Nutrient transport: Assists in the transport of nutrients from soil to plant.
• Photosynthesis: Provides the necessary water for the photosynthesis process.
cohesion- tension theory
• Explains the mechanism of water transport from roots to shoots in plants.
• Based on the physical properties of cohesion, tension, and adhesion.
cohesion
• Attraction between water molecules.
• Hydrogen bonding: Due to the polar nature of water, molecules stick together.
• Forms a continuous water column:
From the roots to the leaves.
tension
• Negative pressure or suction created in the xylem vessels.
• Transpiration: Evaporation of water from the leaves creates a suction force.
• Pulls water upwards: This tension pulls water up from the roots.
adhesion
• Attraction between water molecules and xylem walls.
• Capillary action: Adhesion and cohesion allow water to rise against gravity in xylem tubes.
transpiration-cohesion- tension mechanism
• Starts with transpiration: Water evaporation from leaf surfaces creates a water deficit.
• Creates tension: This deficit pulls water up due to cohesive forces between water molecules.
• Continuous water column: This process continues as long as there’s a water supply and transpiration occurs.
role of stomata
• Opening and closing: Controls the rate of transpiration and hence the tension.
• Influenced by: Environmental factors affect stomatal behaviour and thus water transport rate.
temperature affect on rate of transpiration
• Rate change: The rate of transpiration increases with rising temperature.
• Reason: Higher temperatures enhance water evaporation from the stomata, speeding up the transpiration process.
light affect on rate of transpiration
• Rate change: The rate of transpiration increases with more light.
• Reason: Light leads to the opening of stomata (photosynthesis activity), allowing more water to escape.
humidity affect on rate of transpiration
• Rate change: The rate of transpiration decreases with increasing humidity.
• Reason: High humidity reduces the water vapour pressure gradient between the inside of the leaf and the outside environment, slowing down water diffusion out of the leaf.
air movement affect on rate of transpiration
• Rate change: The rate of transpiration increases with greater air movement.
• Reason: Increased air movement (wind) removes the humid air layer near the stomata, maintaining a high water vapour pressure gradient and thus enhancing transpiration.
mass flow hypothesis
A model explaining how sugars are
transported in the phloem of plants.
mass flow hypothesis key components and processes
• Source and sink: Sugars are produced at the source (e.g. leaves) and used or stored at the sink (e.g. roots, fruits).
• Active loading: Sugars are actively loaded into phloem at the source, which requires energy.
• Water influx: This increases the osmotic pressure, causing water to move into the phloem from the xylem, creating a pressure gradient.
• Down the gradient: Sugars (sap) move from the high-pressure area (source) to the low-pressure area (sink) through sieve tubes in the phloem.
• Unloading at the sink: Sugars are actively unloaded at the sink, again requiring energy.
• Water returns to xylem: The decrease in osmotic pressure at the sink allows water to move back into the xylem.
mass flow hypothesis strengths
• Experimental support: Some experimental data, such as aphid stylet method and radioactively labelled carbon tracing, provide evidence for this hypothesis.
• Simplicity and generality: The hypothesis offers a simple and general explanation for the bulk movement of sugars from ‘source’ to ‘sink’.
• Pressure gradient: The concept of a pressure gradient created by active loading and unloading of sugars explains the directional flow of sap.
mass flow hypothesis weaknesses
• Lack of definitive proof: Despite supportive evidence, there’s still debate and ongoing research in this area.
• Not all solutes move at the same speed: If mass flow were the only mechanism, all solutes should move at the same rate, which is not observed.
• Functional sieve plates: The role and function of sieve plates are not explained by the mass-flow hypothesis.
• Directionality issue: The hypothesis struggles to explain the observed multidirectional movement of solutes.
