7.12 Chapter Summary
BioCast
1. Drawing Plan Diagrams of Transverse Sections
Key Structures to Include
- Stems: Vascular bundles arranged in a ring.
- Roots: Vascular cylinder (stele) with xylem and phloem arranged in specific patterns.
- Leaves: Vascular bundles (veins) with xylem and phloem organized accordingly.
Tips for Drawing
- Use Clear Labels: Ensure each part is accurately labeled (e.g., xylem, phloem, cambium, cortex).
- Accurate Proportions: Reflect the relative sizes and arrangements as seen under a microscope.
- Consistent Symbols: Develop a set of symbols or shorthand for repeated structures to save time.
- Practice with Photomicrographs: Regularly practice drawing from various microscope slides and images to improve accuracy and speed.
Example: Stem Transverse Section
- Outer Layer: Epidermis.
- Cortex: Between epidermis and vascular bundles.
- Vascular Bundles: Arranged in a ring; each bundle contains:
- Xylem (towards the center)
- Phloem (towards the exterior)
- Vascular Cambium: Located between xylem and phloem, responsible for secondary growth.
- Pith: Central region surrounded by vascular bundles.
2. Distribution of Xylem and Phloem in Transverse Sections
Stems
- Arrangement: Vascular bundles in a ring.
- Xylem: Located towards the center of each bundle.
- Phloem: Positioned towards the outer side of each bundle.
- Cambium: Between xylem and phloem, allowing for secondary growth.
Roots
- Vascular Cylinder (Stele):
- Xylem: Typically forms a star shape with multiple arms.
- Phloem: Located between the arms of xylem.
- Endodermis: Surrounds the stele, acting as a selective barrier.
- Cortex: Between the endodermis and epidermis.
Leaves
- Midrib: Contains major vascular bundles.
- Xylem: Generally located towards the upper side of the vascular bundle (closest to the epidermis).
- Phloem: Located towards the lower side (closest to the mesophyll).
- Bundle Sheath Cells: Surround the vascular bundles, especially prominent in monocots but present in dicots as well.
Visual Patterns
- Stems vs. Roots vs. Leaves: Note the differing arrangements and densities of vascular tissues depending on the organ type.
3. Drawing and Labeling Transport Tissue Elements
Xylem Vessel Elements
- Structure:
- Long, Tubular Cells: Joined end-to-end to form vessels.
- Thick, Walled Lignified Cell Walls: Provide strength for water transport.
- Pits: Regions of the wall that are thinner to facilitate water movement between cells.
- No Nucleus at Maturity: Allows unobstructed water flow.
- Drawing Tips:
- Draw elongated, cylindrical cells connected end-to-end.
- Include visible thickened walls and occasional pits.
Phloem Sieve Tube Elements
- Structure:
- Long, Narrow Cells: Form sieve tubes for nutrient transport.
- Sieve Plates: Perforated end walls between adjacent sieve tube elements.
- Companion Cells: Closely associated cells that aid in the function of sieve tubes.
- Drawing Tips:
- Illustrate elongated cells with large sieve plates.
- Show interconnected sieve tube elements with gaps (sieve pores).
Companion Cells
- Structure:
- Nuclear and Cytoplasmic Connection: Adjacent to sieve tube elements, facilitating transport functions.
- Highly Active Metabolism: Supports the maintenance and function of sieve tubes.
- Drawing Tips:
- Depict smaller, closely associated cells next to sieve tube elements.
- Highlight the dense cytoplasm and nucleus within companion cells.
Electron Micrographs Insights
- Xylem: Show detailed lignin deposition and pit structures.
- Phloem: Reveal the fine sieve plates and the intimate association with companion cells.
4. Relating Structure to Function
Xylem Vessel Elements
- Structure Features:
- Lignified Walls: Provide structural support to withstand negative pressure during water transport.
- Pits: Allow water and mineral ions to move laterally between vessels and other xylem cells.
- Continuous Tubes: Facilitate efficient, unimpeded upward water flow.
- Function:
- Water and Mineral Transport: Moves water from roots to shoots, essential for photosynthesis and nutrient distribution.
Phloem Sieve Tube Elements
- Structure Features:
- Perforated Sieve Plates: Enable the free flow of sap between sieve tube elements.
- Long, Tubular Shape: Maximizes the transport capacity.
- Lack of Organelles: Reduces obstruction, allowing for efficient nutrient flow.
- Function:
- Nutrient Transport: Distributes sugars and other organic compounds from sources (e.g., leaves) to sinks (e.g., roots, fruits).
Companion Cells
Structure Features:
- Dense Cytoplasm: Supports active transport mechanisms.
- Nucleus Present: Maintains metabolic activities necessary for phloem function.
- Plasmodesmata Connections: Facilitate communication and transport between companion cells and sieve tubes.
Function:
- Support Sieve Tubes: Manage the loading and unloading of sugars and other nutrients into sieve tubes.
- Energy Provision: Provide ATP and other resources needed for active transport processes.
5. Transport of Mineral Ions and Organic Compounds
Dissolved in Water:
- Mineral Ions (e.g., nitrate, phosphate) and Organic Compounds (e.g., sugars, amino acids) are transported within plants.
- Solution: These substances are dissolved in the xylem and phloem sap, facilitating their movement throughout the plant.
6. Transport of Water from Soil to Xylem
Apoplast Pathway
- Definition: Movement of water through the cell walls and intercellular spaces, bypassing the cell membranes.
- Components:
- Cellulose: Provides structural support; forms the cell wall matrix.
- Lignin: Reinforces cell walls, especially in xylem vessels, preventing collapse under tension.
- Pathway: Soil → Root Hair Cells → Cell Walls (Apoplast) → Xylem Vessels
Symplast Pathway
- Definition: Movement of water through the cytoplasm of cells, connected via plasmodesmata (cytoplasmic channels).
Key Structures:
- Endodermis: The innermost layer of the cortex in roots.
- Casparian Strip: A band of suberin (a waxy substance) in the endodermis cell walls that blocks the apoplast pathway.
- Suberin: Prevents passive flow of materials, forcing water to enter the symplast.
- Pathway: Soil → Root Cells → Symplast → Endodermis → Xylem Vessels
7. Transpiration Process
Role: Drives the continuous movement of water from roots to leaves.
Evaporation: Water evaporates from the stomata (pores) on the leaf’s internal surfaces.
Diffusion: Water vapor diffuses from inside the leaf to the atmosphere, creating a negative pressure that pulls more water upward.
8. Hydrogen Bonding in Water Movement
Cohesion-Tension Theory
- Cohesion: Hydrogen bonds between water molecules create a continuous water column in the xylem.
- Tension: Evaporation at the leaf surface generates negative pressure (tension) that pulls water upward.
Adhesion
- Definition: Hydrogen bonds between water molecules and cellulose in the xylem walls.
- Function: Prevents the water column from breaking, aiding continuous flow.
9. Adaptations of Xerophytic Leaves to Reduce Water Loss
Annotated Drawing Description
- Thick Cuticle: A waxy layer covering the leaf surface to minimize water loss.
- Reduced Stomata: Fewer stomata, often sunken, to decrease transpiration rates.
- Leaf Hairs (Trichomes): Provide a barrier to reduce airflow over the leaf surface, limiting water loss.
- Compact Leaf Structure: Minimizes surface area exposed to the environment.
Example: Succulent Leaves
- Storage Tissues: Store water within leaf tissues.
- CAM Photosynthesis: Stomata open at night to reduce water loss.
10. Movement of Assimilates in Phloem Sieve Tubes
Assimilates: Organic molecules like sucrose and amino acids produced in source tissues (e.g., leaves).
Transport Direction: From source (where assimilates are produced) to sink (where assimilates are used or stored, e.g., roots, fruits).
Phloem Sieve Tubes: Specialized vessels for the transport of assimilates, interconnected by sieve plates.
11. Transfer of Assimilates by Companion Cells
Companion Cells: Specialized cells adjacent to sieve tube elements; crucial for loading and unloading assimilates.
Mechanism:
- Proton Pumps: Actively transport H⁺ ions out of companion cells, creating a proton gradient.
- Cotransporter Proteins: Use the proton gradient to symport sucrose into the companion cells from the surrounding cells.
- Function: Facilitates the movement of sucrose into the phloem sieve tubes for transport to sinks.
12. Mass Flow in Phloem Sieve Tubes
Hydrostatic Pressure Gradient:
- Loading at Source: Active transport of sucrose into phloem increases osmotic pressure, causing water to enter by osmosis, generating high pressure.
- Unloading at Sink: Sucrose is removed from phloem, decreasing osmotic pressure, causing water to exit by osmosis, generating low pressure.
- Mass Flow: The pressure difference drives the bulk movement of phloem sap from source to sink.
- Direction: Always flows from areas of higher pressure (source) to lower pressure (sink).
Questions
1. Outline the transport needs of plants and the fact that some mineral ions and organic compounds can be transported within plants dissolved in water.
Transport Needs:
- Water and Mineral Transport: Essential for photosynthesis and metabolic activities. Water is absorbed by roots and transported upward to all parts of the plant.
- Organic Compound Transport: Sugars (produced in leaves via photosynthesis) and amino acids are transported to areas where they’re needed for growth and storage.
Role of Water as a Solvent:
- Minerals and Organic Compounds: Dissolve in water, allowing xylem to transport mineral ions and phloem to carry sugars and amino acids.
- Efficiency: Dissolved substances can move easily through vascular tissues, supporting metabolic functions across the plant.
2. Draw, label, and describe the overall structure of herbaceous dicotyledonous stems, roots, and leaves using a light microscope.
Herbaceous Dicot Stem:
- Vascular Bundles: Arranged in a ring near the periphery, with xylem towards the inside and phloem towards the outside.
- Pith: Central region filled with parenchyma cells, mainly for storage and support.
Herbaceous Dicot Root:
- Central Xylem: Star-shaped arrangement in the root center.
- Phloem: Located between the arms of the xylem.
- Endodermis: Surrounds vascular tissue, regulates water and mineral entry into xylem.
Herbaceous Dicot Leaf:
- Vascular Bundle in Midrib: Contains xylem (upper side) and phloem (lower side).
- Mesophyll Layers: Palisade (photosynthesis) and spongy mesophyll (gas exchange).
- Epidermis with Stomata: Facilitates gas exchange and transpiration.
3. Draw the structure of the transport tissues xylem and phloem using the high-power lens of a light microscope.
Xylem:
- Vessel Elements: Long tubes with thick lignified walls, no end walls, and large lumens for unimpeded water flow.
- Tracheids: Long, tapered cells with thick lignified walls and pits to allow lateral water movement.
- Pits: In cell walls, allow water to move laterally.
Phloem:
- Sieve Tube Elements: Long cells with perforated sieve plates, facilitating sap flow between cells.
- Companion Cells: Support sieve tube elements with ATP, essential for active transport.
- Parenchyma: Provide support and store nutrients.
4. Explain the process of transpiration.
Definition: Loss of water vapour from the leaf to the atmosphere, primarily through stomata.
Process:
- Evaporation: Water evaporates from mesophyll cell walls into leaf air spaces.
- Diffusion: Water vapour exits through open stomata, down a water potential gradient.
- Driving Force: The Sun’s energy causes evaporation, creating a water potential gradient from soil to leaf.
Importance: Helps with nutrient transport, leaf cooling, and water uptake from roots.
5. Describe the adaptations of the leaves of xerophytic plants with the aid of annotated drawings.
Adaptations for Water Conservation:
- Thick Cuticle: Prevents water loss by covering the leaf surface.
- Sunken Stomata: Located in pits to reduce water loss and maintain humid air around stomata.
- Rolled Leaves: Marram grass can roll its leaves, trapping moist air and reducing transpiration.
- Hairy Epidermis: Traps a layer of moisture, reducing the steepness of the water potential gradient.
6. Explain how water moves across a leaf through the apoplast and symplast pathways.
Apoplast Pathway:
- Water moves along cell walls without entering the cells, bypassing cell membranes.
- Fast pathway since water does not encounter resistance from cell membranes.
Symplast Pathway:
- Water enters the cell cytoplasm via osmosis and moves between cells through plasmodesmata.
- Slower movement as it relies on osmosis and moves through the cell’s living part.
7. Relate the structure of xylem to its functions.
Structural Adaptations:
- Lignified Walls: Provide strength to resist tension and prevent collapse.
- Hollow Lumen: No internal cell contents to obstruct water flow.
- Pits: Allow lateral movement of water between vessels or to adjacent cells.
- Continuous Tubes: End walls break down to form long tubes for efficient water transport.
8. Explain the movement of water up the xylem from root to leaf, including the roles of cohesion-tension and adhesion.
Cohesion-Tension Theory:
- Cohesion: Water molecules stick together via hydrogen bonds, creating a continuous column.
- Tension: Water evaporation from leaves pulls water up due to tension in the xylem.
Adhesion: Water adheres to hydrophilic xylem walls, assisting upward movement and preventing backflow.
Transpirational Pull: Water moves up the xylem as a result of water loss in leaves creating tension.
9. Describe the transport of water from the soil to the root xylem through the apoplast and symplast pathways.
- Soil to Root Hairs: Water enters root hair cells by osmosis due to a water potential gradient.
- Pathways in Root Cortex:
- Apoplast Pathway: Water moves through cell walls until blocked by the Casparian strip in the endodermis.
- Symplast Pathway: Water moves through cytoplasm and plasmodesmata into the xylem.
- Endodermis Control: Casparian strip forces water into symplast pathway, allowing selective absorption of ions.
10. Relate the structure of phloem to its functions.
Sieve Tube Elements:
- Sieve Plates: Large pores allow rapid sap flow.
- Thin Cytoplasm: Enables easier sap movement without major obstacles.
Companion Cells:
- Mitochondria-Rich: Provide ATP for active transport of assimilates.
- Close Association with Sieve Tubes: Plasmodesmata allow efficient transfer of substances to sieve tubes.
11. Explain that assimilates dissolved in water, such as sucrose and amino acids, move through phloem sieve tubes from sources to sinks.
Assimilates in Phloem:
- Source: Leaf cells produce sucrose from photosynthesis, loaded into phloem.
- Sink: Growing regions or storage organs remove sucrose for energy or storage.
Phloem Sap: Contains sucrose, amino acids, and hormones dissolved in water, flowing under pressure.
12. Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink.
Process of Mass Flow:
- Source Loading: Sucrose enters sieve tubes, lowering water potential and causing water to flow in by osmosis.
- High Hydrostatic Pressure: Builds up at source end, pushing phloem sap towards the sink.
- Sink Unloading: Sucrose is removed, water exits phloem by osmosis, lowering pressure.
- Result: Pressure gradient drives sap flow from high-pressure source to low-pressure sink.
13. Explain how companion cells transfer assimilates to phloem sieve tubes.
Mechanism:
- ATP-Dependent: High metabolic activity and many mitochondria in companion cells provide energy for active transport.
- Active Transport of Sucrose: Companion cells pump H+ ions out, creating a gradient.
- Co-Transport of Sucrose: Sucrose moves with H+ ions back into companion cells, then into sieve tubes via plasmodesmata.