OCR (A) Biology A-Level: Topic 3.3 Transport in Plants

Introduction

• Plants require a transport system to distribute nutrients to all cells.
• Xylem Tissue: Facilitates water and mineral transport upwards through transpiration (passive).
• Phloem Tissue: Transports sugars to all plant parts via translocation (active).

Vascular Bundle

In Roots

• Components: Xylem and phloem, vital for transport and structural support.
• Xylem Arrangement: X shape in the center, withstands mechanical forces.
• Surrounding Structures: Endodermis (water supply to xylem) and pericycle (meristem cells).

In Stem

• Xylem Location: Inside for support and flexibility.
• Phloem Location: Outside of the vascular bundle.
• Cambium: Meristem layer for new xylem and phloem production.

In Leaf

• Structures: Midrib and veins, forming a network for transport and support.
• Dicotyledonous Leaves: Veins spread out from midrib.

Xylem and Phloem

Xylem Features

• Transport water/minerals, provide structural support.
• Composed of dead tissue, forming continuous columns.
• Contains pits for lateral water movement, thickened with lignin.
• Water movement is unidirectional (upward).

Phloem Features

• Composed of living cells, involved in nutrient translocation.
• Structure: Sieve tube elements and companion cells.
• Sieve Tube Elements: Transport sugars (e.g., sucrose) as sap, bidirectional.
• Companion Cells: Produce ATP for active processes.
• Plasmodesmata: Cytoplasmic links for substance flow.

Transpiration

• Water absorption through roots, upward movement, released as vapor through leaves.
• Stomata: Entry for CO2, exit for water and oxygen.
• Transpiration Stream: Water movement up the stem, supports photosynthesis, growth, and temperature regulation.
• Osmosis & Evaporation: Water moves from xylem to mesophyll, evaporates, diffuses out.
• Potometer: Measures transpiration rate.
• Factors affecting rate: leaf characteristics, light, temperature, humidity, air movement, water availability.

Xerophytes Adaptations

• Adapted to dry conditions, minimize water loss.
• Features: Smaller leaves, dense mesophyll, thick cuticles, stomata closure, hairs/pits for moisture, leaf rolling.

Hydrophytes Adaptations

• Adapted to aquatic environments.
• Features: Thin cuticle, many open stomata on upper surfaces, wide leaves, air sacs for buoyancy.

Water Movement in Roots

• Water enters via root hair cells, moves to xylem due to water potential gradient.
• Root Hair Cells: Increase surface area for water/mineral uptake.
• Mineral Uptake: Active transport against concentration gradient.

Water Pathways

• Symplast Pathway: Water moves through cytoplasm via plasmodesmata.
• Apoplast Pathway: Water moves through cell wall spaces, bypasses plasma membranes.

Endodermis and Casparian Strip

• Casparian strip blocks apoplast pathway; water enters symplast pathway.

Water Transport Up Stem

• Water moves from xylem to mesophyll down gradient.
• Root Pressure: Active transport of minerals into xylem, osmosis-driven water push.
• Tension-Cohesion Theory: Water cohesion and surface tension support upward movement.
• Capillary Action: Cohesion pulls water along xylem walls.

Translocation

• Energy-requiring transport of assimilates in phloem.
• Active Loading: Companion cells use ATP for hydrogen ion transport, creating diffusion gradient.
• Sucrose Movement: Enters sieve tubes, reduces water potential, draws in water, increases pressure.
• Pressure Flow: Mass flow from source to sink driven by hydrostatic pressure gradient, supplies nutrients where needed.