PLANT TRANSPORT
Xylem: Responsible for transporting water and nutrients from the roots to the rest of the plant.
Phloem: Transports sugars and other metabolic products downward from the leaves.
Sieve tubes: Specialized structures within the phloem that facilitate the movement of sugars.
Companion cells: Assist sieve tubes by providing energy and support, ensuring efficient transport of nutrients throughout the plant.
Tracheids: Water-conducting cells in the xylem that help in the transport of water and minerals, providing structural support to the plant. Xylem vessels: Composed of vessel elements that allow for rapid water transport, these structures play a crucial role in moving water and dissolved minerals from the roots to the rest of the plant.
Xylem parenchyma: Living cells that store nutrients and assist in the lateral transport of water and minerals, contributing to the overall efficiency of the xylem system.
Phloem parenchyma: These cells assist in the storage and lateral transport of sugars and other organic nutrients, playing a supportive role in the phloem's function.
Sieve tube elements: Specialized cells in the phloem that facilitate the long-distance transport of carbohydrates, primarily sugars, from the leaves to other parts of the plant, ensuring that energy is distributed where needed.
Companion cells: These cells are closely associated with sieve tube elements and are crucial for the maintenance and functionality of the sieve tubes, providing metabolic support and aiding in the transport of nutrients.
Tracheids: Long, narrow cells in the xylem that aid in water conduction and provide structural support, allowing for effective water movement across various distances within the plant.
Plant Transport: Transpiration and Water Movement
Transpiration
Definition:
Loss of water vapour from the plant's aerial parts, particularly through the stomata, cuticle, and lenticels in the bark.
Occurs Through:
Stomata: Primarily located on the undersides of leaves; pores that facilitate gas exchange and transpiration.
Cuticle: A waxy layer covering the leaf surface, which reduces water loss but allows some evaporation.
Lenticels: Small openings in the bark of woody plants that enable gas exchange while minimizing water loss.
Water Concentration Gradient:
The water concentration is highest in the xylem and lowest in spongy mesophyll cells, creating a driving force for water movement within the plant.
Passage of Water Across the Leaf
As water evaporates from the cell walls of spongy mesophyll cells into the air spaces, a decrease in water concentration occurs in those cells. This leads to:
Osmosis: Water is drawn from adjacent mesophyll cells, creating a continual gradient that facilitates water movement from the xylem into the mesophyll cells below stomata to replace the lost water.
Factors Affecting Transpiration Rate
Temperature: Higher temperatures increase the rate of transpiration due to steeper concentration gradients and reduced humidity, promoting faster evaporation of water.
Humidity: Lower humidity levels enhance transpiration by increasing the gradient between the moisture inside the leaf and the dryer air outside.
Air Movement: Wind or air currents remove moisture near leaf surfaces, thereby increasing the concentration gradient and stimulating higher rates of transpiration.
Light: Stomata require light for photosynthesis and generally open during the day, allowing water vapor to diffuse out of the leaf into the atmosphere.
Structure of the Stoma
Definition:
Pores formed by pairs of guard cells that regulate gas exchange (carbon dioxide in, oxygen out) and control water loss during transpiration.
Guard Cell Features:
Unevenly Thickened Walls: This allows for the stomatal opening and closing through curvature when the guard cells become turgid.
Inelastic Cellulose Microfibrils: These prevent the guard cells from expanding uniformly, thus controlling the diameter of the stomatal pore during turgidity.
Chloroplast Presence: Only the guard cells contain chloroplasts among epidermal cells; they utilize light for photosynthesis, creating carbohydrates that influence osmotic pressure and stomatal function.
Mechanism of Stomatal Opening and Closing
During daylight: Potassium ions actively accumulate in guard cells, increasing osmotic pressure, resulting in water influx and cell turgidity, which opens the stomata.
In darkness: Potassium ions exit the guard cells, leading to reduced turgidity as water exits, causing the stomata to close.
Xerophytes
Adaptations for Dry Conditions:
Reduced Leaf Surface Area: Such as spines instead of broad leaves to minimize water loss.
Thick Cuticles: A thicker waxy cuticle on leaves helps reduce evaporation.
Stomata Positioning: Stomata located primarily on the lower side of the leaf reduces exposure to direct sunlight.
Leaf Curling: Some species can curl their leaves to trap moisture and reduce surface area exposed to air.
Leaf Hairs: Hairs on leaves can trap humid air, creating a microclimate that decreases the evaporation rate.
Functions of Transpiration:
Cooling Mechanism: Transpiration cools the plant through the evaporation of water from the leaf surface.
Nutrient Distribution: Facilitates the distribution of dissolved mineral salts throughout the plant optimizing growth and metabolism.
Structure and Function of the Xylem
Functions:
Water and Mineral Transport: Essential for maintaining hydration and nutrient supply to various plant parts.
Structural Support: Provides rigidity and structural integrity, helping plants stand upright.
Structure:
Composed of various cell types including parenchyma, fibers, and vessel elements, which are hollow and lignified.
Lumen Structure: The hollow lumen of vessel elements aids in capillary action, allowing for efficient water transport; dead cells within facilitate the free flow of water.
Ascent of Water Up the Stem
Water loss from leaves creates tension in the xylem, pulling water from roots through the transpiration stream.
Cohesion-Tension Theory: Cohesion forces (water molecules sticking together) help maintain a continuous column of water, while adhesion forces (water adhering to vessel walls) support upward movement against gravity.
Root Pressure: Additionally contributes to pushing water up the xylem to a limited extent, especially during nighttime or in hydrated soils.
Movement of Water into the Roots
Water moves from the surrounding soil into root hair cells via osmosis, following a concentration gradient from areas of high water potential in soil to low water potential in root cells.
Uptake of Ions by the Root
Mechanisms:
Active Transport: Required for the uptake of essential ions against their concentration gradient energy.
Passive Diffusion: Utilized for the uptake of ions that are present in high concentrations in the soil solution.
Translocation in the Phloem
Definition:
Movement of sucrose and amino acids from sources (where they are produced, e.g., leaves) to sinks (where they are stored or used, e.g., roots or fruits).
Phloem Structure:
Composed of sieve tube elements (aligned end-to-end to form a continuous tube) and companion cells that support sieve elements.
Sieve Plates: Facilitate mass flow of nutrients due to pressure gradients created by active and passive transport processes.
Mechanism of Translocation:
Bidirectional Movement: Nutrient transport can occur in both directions depending on plant needs; primarily involves sugars as sucrose along with minerals and hormones.
Pressure Gradient: Drives the mass flow from source to sink, where high pressure at the source promotes movement towards areas of lower pressure at the sink.
Measuring the Rate of Transpiration
Potometers: Tools used to measure water uptake in plant shoots, with types varying from bubble potometers (tracking air bubbles) to weight potometers (measuring changes in weight).
Importance:
Ensuring proper sealing of junctions and maintaining constant environmental conditions during experiments is crucial for accuracy in measuring transpiration rates.
Things i don’t quite understand
what is lignin
what are sieve plates