Transport - Plants

Transpiration stream

• water is drawn from xylem vessels to replace evaporated water lost during transpiration

  • occurs due to capillary action

  • loss of water — generates tension(negative pressure)

Xylem Vessels

• formed from long lines of cells that are connected at each end

• as xylem cels develop, cell walls between the connected cells degrade and the cell contents are broken down

  • allows for unimpeded flow

Parenchyma cells

• forms the cortex and pith of the stem

• act as storage structures for starch

  • cortex is region located directly beneath epidermis while pith is the central region of the stem

Vascular tissue

• arranged in a ring of vascular bundles

  • Xylem transports water and dissolved mineral ions from roots to leaves

  • Phloem transports organic solutes from leaves to other plants

Root Hairs

• specialized epidermal cells present in roots to absorb water and mineral ions from the soil

Endodermis

forms the boundary between the vascular tissue and cortex in a root

Root Pressure Generation

1. minerals are actively transported from soil(lowers the water potential)

2. creates water potential gradient

3. minerals are actively transported to the xylem

4. water potential is reduced

5. water enters the xylem and generates a positive pressure potential — pushes column of water upwards

Translocation

• transport of organic solutes in phloem tissue

  • liquid transported — phloem sap

• transports sugars from source to sink

Sources

• regions of plants where organic solutes originate

  • eg. mature green leaves, green stems

  • produces glucose — turned into sucrose

  • storage organs

    • eg. tubers & tap roots

Sinks

• regions of plants where organic compounds are required for growth

  • e.g. meristems

  • roots that are growing or actively taking up minerals

  • young leaves in bud

• sources can become sinks

Phloem adaptations - Sieve Tube Cells

• composed of sieve tube cells & companions cells

  • sieve tubes — a continuous tube through which phloem sap flows

  • Perforated sieve plates separate cells, allow the passage of sugars

  • Reduced cytoplasm and few organelles to allow for free flow of phloem sap

Phloem adaptations — Companion cells

• closely associated with sieve tube

  • aid with the loading and unloading of dissolved substances

• contain many mitochondria — generate ATP for active loading of sucrose into sieve tube

Sieve tube structure

• sieve plates — continuous movement of sugars

• cellulose cell wall — strengthens wall to withstand hydrostatic pressures

• no nucleus, vacuole or ribosomes — maximises space for assimilates

• thin cytoplasm — reduces friction

Companion Cell

• nucleus & other organelles — provides metabolic support

• transport proteins — moves assimilates in & out of sieve tube

• lots of mitochondria — provide ATP for active transport of assimilates

• Plasmodesmata — link with sieve tube, allows movement of assimilates

Translocation

• transport of sucrose/assimilates from source to phloem to sink

  • active process — requires ATP

Translocation

1. active transport used to load organic compounds

2. high concentrations of solutes in phloem at

3. raised hydrostatic pressure — generates hydrostatic pressure gradient between source & sink

   1. contents flow to sink down pressure gradient

4. sucrose is unloaded from phloem at sink — lowers water potential