Plant Transport

Importance of transport system

To move water and mineral ions- soil to roots and up leaves

Distribute sucrose around plant

Plants are large multicellular organisms, with a large diffusion distance and a small SA:V ratio- need transport systems

Slower metabolic rate than animals

Vascular tissue

Xylem

Phloem

Xylem

Transports water and minerals upwards

Phloem

Transports assimilates upwards or downwards

Vascular bundle in stem

Phloem outside

Cambium middle

Xylem inside

Ovals

Vascular bundle in root

Xylem cross shaped

Phloem quarters

Surrounded by endodermis

Vascular bundle in leaf

Xylem above

Phloem below

Xylem vessel structure

Long hollow tubes- dead cells

Walls impregnated with lignin for support, flexibility and waterproofing

Bordered pits- allow lateral movement of water between vessels

Phloem structure

Sieve plate- allow solute to pass through

Sieve tube elements- very little organelles

Companion cells carry out functions for sieve tube elements with more organelles

Linked by plasmodesmata with gaps to allow flow of minerals

Transpiration

Water loss due to evaporation from stomata of leaves

Transpiration rate factors

Number of leaves ( SA )

Number/ size of stomata

Light intensity ( Stomata open )

Temperature

Humidity

Wind ( keeps steeper water potential gradient )

Water availability

Potometer

Estimates transpiration rate

Record distance moved by an air bubble- indicates rate of uptake

Reducing potometer error

Ensure no unwanted air bubbles

Cut shoot underwater to prevent air entering cut end of xylem

Cut shoot at a slant to increase SA for water uptake

How does water enter roots?

By osmosis into the root hair cells in epidermis

Adaptations of root hair cells

Large SA

Many root hair cells

Thin cell wall

Low water potential

No chloroplasts

How does water move from root hair cells to xylem?

Symplast pathway

Apoplast pathway

Symplast pathway

water travels via osmosis through the cell surface membrane into the cytoplasm of the cells

Water moves from cell to cell through plasmodesmata

Apoplast pathway

Water travels by osmosis in the cellulose cell walls of the plant cells by osmosis

Water moves from the cell wall of one cell to cell wall of next until the endodermis

Casparian strip ( Apoplast pathway )

Blocks apoplast pathway at endodermis

Coating of Suberin on cell walls

Causes water to move across cell surface membrane of the cells

Movement of water up xylem

Up the stem

Mass flow

Cohesion- water molecules sticking together using H bonding

Pressure gradient- higher pressure in roots, lower in leaves

Adhesion- water molecules attracted to lignin in walls

Xerophytes

Plants adapted to living in dry conditions

Marram grass

Stomata sunk inside pits- sheltered from wind to reduce transpiration

Rolled leaves- air trapped, increasing humidity, decreasing water loss

Thick waxy cuticle to reduce evaporation

Cacti

Spines to reduce SA- reduces water loss

Close stomata when hottest

Wide spread roots to absorb maximum water

Hydrophytes

Plants adapted to wet conditions

Waterlilies

Air spaces- buoyancy

Stomata on upper epidermis- exposed to air for gas exchange

Many stomata- increase gas exchange

Large leaves- floating and SA

Thinner waxy cuticle

Translocation

Transport of assimilates throughout plant, in phloem tissue, from source to sink

Source

Site where assimilates are loaded into phloem

Sink

Site where assimilates are removed from phloem

Assimilates

Sucrose, amino acids

Active loading at source

Active transport of hydrogen ions out of companion cells

Creates hydrogen ion conc. gradient

Facilitated diffusion of H ions back into companion cells

Assimilates move into companion cells with the hydrogen ions via cotransport

Assimilates diffuse through the plasmodesmata from the companion cells into the sieve tube elements

Mass flow of assimilate through sieve tubes

Assimilates arriving at the sieve tube element at the source causes lower water potential

Water moves in to sieve tube element from xylem, via osmosis creating higher hydrostatic pressure at source

Assimilates ære unloaded at sink and water follows via osmosis- lowers sink pressure

Assimilates move from high to low pressure ( hydrostatic at source and sink)- mass flow

Unloading of assimilates into phloem

Assimilates move out of phloem by facilitated diffusion

Occurs due to assimilates in phloem at higher conc. than surrounding sink cells

Assimilates move down conc. gradient out of phloem into cells