CIE AS/A Level Biology -7.2 Transport mechanisms- Study Notes- New Syllabus

Ace A level Biology Exam with CIE AS/A Level Biology -7.2 Transport mechanisms- Study Notes- New Syllabus

Key Concepts:

state that some mineral ions and organic compounds can be transported within plants dissolved in water
describe the transport of water from the soil to the xylem through the:
• apoplast pathway, including reference to lignin and cellulose
• symplast pathway, including reference to the endodermis, Casparian strip and suberin
explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
state that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes
explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

CIE AS/A Level Biology 9700-Study Notes- All Topics

Transport of Substances in Plants

🌱 Key Point

Plants transport mineral ions and organic compounds dissolved in water through their vascular tissues.
This transport occurs in xylem and phloem depending on the substance.

🔬 Details of Transport

Substance	Transport Tissue	Direction & Form	Function
Mineral ions (e.g., nitrates, potassium, magnesium)	Xylem	Dissolved in water, upward from roots to leaves	Supply essential nutrients for growth and metabolism
Water	Xylem	Unidirectional, from roots → leaves	Maintains turgor, photosynthesis, cooling
Organic compounds (e.g., sucrose, amino acids)	Phloem	Bidirectional, from sources (leaves) → sinks (roots, fruits, growing tissues)	Provides energy and building blocks for growth and storage

📌 Key Points

Water acts as a solvent, allowing solutes to be transported efficiently.
Xylem mostly transports inorganic solutes, while phloem transports organic nutrients.
The movement in phloem is called translocation, which depends on pressure gradients created by active loading/unloading of solutes.

Summary:
– Mineral ions and organic compounds are transported dissolved in water.
– Xylem: transports water and dissolved minerals upward.
– Phloem: transports organic compounds to various plant parts.

Transport of Water from Soil to Xylem

🌱 Overview

Water absorbed by roots from the soil reaches the xylem through two main pathways: apoplast and symplast.
Both pathways ensure efficient water movement toward the vascular tissue.

1️⃣ Apoplast Pathway

Route: Water moves between cell walls and intercellular spaces without crossing the plasma membrane; travels passively along the extracellular matrix.
Structural Notes: Cellulose in cell walls allows water to diffuse; lignin (in xylem vessels and endodermis) provides rigidity and partially blocks apoplastic flow in certain regions.
Key Point: Fast pathway, does not involve cytoplasm, but water is blocked at the endodermis due to the Casparian strip.

2️⃣ Symplast Pathway

Route: Water moves through the cytoplasm of cells via plasmodesmata (cytoplasmic channels). Water enters root hair cells and passes cell-to-cell until it reaches the xylem.
Structural Notes: Endodermis (innermost cortex layer surrounding the stele) acts as a checkpoint. Casparian strip (band of suberin in endodermal cell walls) blocks apoplastic flow, forcing water into symplast.
Key Point: Symplastic movement is slower than apoplastic but allows controlled uptake of water and dissolved minerals.

📊 Comparison of Pathways

Feature	Apoplast Pathway	Symplast Pathway
Route	Along cell walls & intercellular spaces	Through cytoplasm via plasmodesmata
Membrane Crossing	No	Yes
Speed	Fast	Slower
Role of Endodermis	Blocked by Casparian strip	Water passes through cytoplasm to reach xylem
Selectivity	Low	High (selective ion uptake)

Summary:
– Water moves from soil to xylem via apoplast (fast, through cell walls) and symplast (controlled, through cytoplasm).
– Casparian strip in the endodermis blocks apoplastic flow, forcing water into symplast for selective uptake.
– Cellulose aids apoplast flow; suberin in Casparian strip ensures regulation.

Transpiration in Plants

🌱 Definition

Transpiration is the loss of water from plants in the form of water vapour.
It primarily occurs through leaves but can also occur via stems and flowers.

🔬 Mechanism of Transpiration

Evaporation: Water from mesophyll cells evaporates into the leaf’s intercellular spaces, increasing water vapour concentration inside.
Diffusion to Atmosphere: Water vapour exits the leaf through stomata along the concentration gradient (high inside → low outside). This is a passive process that does not require energy.

📌 Key Points

Transpiration generates transpiration pull, helping water move upward through xylem vessels.
It aids cooling of the plant by evaporation.
Assists in transport of mineral ions dissolved in water.
The rate of transpiration is influenced by light intensity, temperature, humidity, wind, and stomatal opening.

Summary:
– Transpiration = water evaporates from leaf mesophyll → diffuses out through stomata.
– Creates a water potential gradient, driving upward movement of water and dissolved minerals.
– Helps in cooling the plant and maintaining nutrient transport.

Movement of Water in Xylem – Cohesion, Tension, and Adhesion

🌱 Key Concepts

Water movement in xylem during transpiration is largely driven by physical properties of water, especially hydrogen bonding.

1️⃣ Cohesion-Tension Theory (Transpiration Pull)

Cohesion: Water molecules stick to each other via hydrogen bonds. Forms a continuous water column from roots to leaves in xylem vessels.
Tension (Transpiration Pull): Water evaporates from leaf mesophyll → creates negative pressure. Cohesive water molecules are pulled upward as a continuous column to replace lost water. This pull moves water and dissolved minerals from roots to leaves.
Key Point: Hydrogen bonding maintains the integrity of the water column, preventing it from breaking under tension.

2️⃣ Adhesion to Cell Walls

Water molecules stick to cellulose in xylem vessel walls via hydrogen bonds.
Adhesion prevents water column collapse, especially in narrow vessels.
Helps counter gravity, aiding upward movement.

📊 Summary Table – Role of Hydrogen Bonding

Property	Mechanism	Function in Xylem
Cohesion	H₂O molecules stick to each other	Maintains continuous water column; allows transpiration pull
Tension	Negative pressure from leaf evaporation	Pulls water upward through xylem
Adhesion	H₂O molecules stick to cellulose walls	Supports water column; prevents vessel collapse

Summary:
– Hydrogen bonding enables cohesion between water molecules and adhesion to xylem walls.
– Transpiration pull (tension) moves water upward efficiently.
– Cohesion and adhesion ensure continuous water transport from roots to leaves.

Adaptations of Xerophytic Leaves to Reduce Water Loss

🌱 Overview

Xerophytes are plants adapted to dry environments.
Their leaves have structural modifications that minimise water loss via transpiration.

🌵Example- Oleander (Nerium oleander)

Oleander is a xerophytic plant adapted to arid environments.
Its leaf structure minimizes water loss through multiple physical adaptations.

🔬 Key Structural Adaptations

Feature	Description	Function / Adaptation
Thick cuticle	Waxy layer covering upper epidermis	Reduces evaporation from the leaf surface
Upper epidermal tissue (multi-layered)	Several layers of epidermal cells	Provides additional barrier to water loss
Trichomes (hairs)	Hair-like projections on leaf surface	Break airflow, trap humid air around stomata → reduces transpiration
Stomatal crypts	Recessed cavities containing stomata	Protects stomata from direct wind and sun, maintains higher humidity in the crypt
Lower epidermal tissue	Contains stomata within crypts	Regulates water loss while allowing gas exchange

🌿 Transverse Section Note:

Water loss is minimized by the combination of thick cuticle, multi-layered epidermis, crypts, and trichomes.
Crypts create a microenvironment of higher humidity around stomata → slows diffusion of water vapour to the atmosphere.

🖊 Annotations for TS Drawing

Thick cuticle – outermost layer above upper epidermis.
Upper epidermal tissue – multi-layered beneath cuticle.
Trichomes – extend from upper or lower epidermis.
Stomatal crypt – cavity recessed into lower epidermis.
Stoma – within crypt.
Lower epidermal tissue – surrounds crypt, supports stomata.

Summary:
– Oleander leaves are adapted to dry climates with thick cuticle, multi-layered epidermis, trichomes, and stomatal crypts.
– These features reduce transpiration while maintaining gas exchange for photosynthesis.
– Annotated transverse sections highlight the structural basis of water conservation in xerophytes.

Transport of Assimilates in Phloem

🌱 Key Point

Assimilates (substances produced by photosynthesis), such as sucrose and amino acids, are dissolved in water.
These substances move through phloem sieve tubes from sources to sinks.

🔬 Details

Source: Plant organ where assimilates are produced or released.
Examples: Mature leaves (photosynthesis), storage organs during mobilization.
Sink: Plant organ where assimilates are used or stored.
Examples: Growing roots, shoots, fruits, seeds.
Movement: Occurs via mass flow or pressure-flow mechanism.
Driven by pressure gradients created by active loading of sucrose into phloem at sources and unloading at sinks.

📌 Key Points

Transport is bidirectional – from any source to any sink.
Water acts as a solvent, allowing smooth movement of sugars and amino acids.
Companion cells support sieve tube elements in active loading/unloading of assimilates.

Summary:
– Assimilates (sugars, amino acids) are dissolved in water and transported in phloem sieve tubes.
– Movement is from sources (production) to sinks (utilization or storage).
– Process relies on pressure gradients and companion cell activity.

Role of Companion Cells in Phloem Transport

🌱 Overview

Phloem sieve tube elements lack a nucleus and many organelles, so they rely on companion cells for metabolic support.
Companion cells actively load assimilates (e.g., sucrose) into sieve tubes using energy-dependent mechanisms.

🔬 Mechanism of Assimilate Loading

Proton Pump (H⁺-ATPase)
Located in the plasma membrane of companion cells.
Uses ATP to pump H⁺ ions out of the cell → creates a proton gradient (high H⁺ outside, low H⁺ inside).
Cotransporter Proteins / Symporters
Transport sucrose into companion cells against its concentration gradient.
Sucrose is co-transported with H⁺ ions moving back into the cell down the proton gradient.
This is called active loading because it uses energy indirectly (from ATP-driven proton pumps).
Transfer to Sieve Tube Elements
Sucrose moves from companion cells → sieve tube elements through plasmodesmata (cytoplasmic connections).
Increases solute concentration in sieve tubes → water enters by osmosis → generates hydrostatic pressure → drives mass flow to sinks.

📌 Key Points

Active loading allows efficient transport of sugars even against concentration gradients.
Companion cells provide energy and metabolic control for sieve tubes.
Proton pumps + cotransporters are essential for creating the pressure gradient needed for phloem translocation.

📊 Summary Table

Component	Role
Proton pump (H⁺-ATPase)	Uses ATP to pump H⁺ out, creating a proton gradient
Cotransporter / Symporter	Moves sucrose into companion cell along H⁺ gradient (active transport)
Plasmodesmata	Transfer loaded sucrose from companion cells to sieve tube elements
Result	High sucrose in sieve tubes → osmotic water entry → mass flow to sinks

Summary:
– Companion cells actively load sucrose into sieve tubes using proton pumps and cotransport proteins.
– This increases solute concentration, drawing water in → drives phloem sap from sources to sinks.
– Active loading is essential for efficient long-distance transport of assimilates.

Mass Flow in Phloem Sieve Tubes

🌱 Overview

Mass flow is the movement of phloem sap (assimilates + water) from sources (e.g., leaves) to sinks (e.g., roots, fruits) down a hydrostatic pressure gradient.
This is the main mechanism of long-distance transport in phloem.

🔬 Mechanism of Mass Flow

Loading at the Source
Sucrose and other assimilates are actively loaded into sieve tube elements from companion cells.
High solute concentration in sieve tubes → water enters by osmosis from adjacent xylem.
Result: high hydrostatic pressure at the source.
Movement Along the Tube
Phloem sap moves from high pressure (source) → low pressure (sink).
Movement occurs through sieve plates connecting sieve tube elements.
Bulk flow of solution carries sucrose, amino acids, and signaling molecules.
Unloading at the Sink
Sucrose is actively or passively removed from sieve tubes at the sink.
Water potential increases → water exits sieve tubes, reducing hydrostatic pressure.
Maintains pressure gradient, allowing continuous flow.

📌 Key Points

Mass flow is pressure-driven, not dependent on the weight of the sap.
Requires active loading at sources and unloading at sinks.
Companion cells are crucial for maintaining solute concentration and energy supply.

📊 Summary Table

Step	Process	Result
Source loading	Active loading of sucrose → water enters by osmosis	High hydrostatic pressure
Movement	Bulk flow along sieve tubes	Mass flow of sap toward sinks
Sink unloading	Sucrose removed, water exits	Low hydrostatic pressure, maintains pressure gradient

Summary:
– Mass flow in phloem is driven by hydrostatic pressure differences from sources to sinks.
– Water enters sieve tubes at sources and exits at sinks, creating a continuous flow.
– Companion cells maintain solute concentration and energy for active loading/unloading.

CIE AS/A Level Biology -7.2 Transport mechanisms- Study Notes

CIE AS/A Level Biology -7.2 Transport mechanisms- Study Notes- New Syllabus