transport in plants

What is a dicotyledonous plant

Produces two seed leaves

Has broad surfaces leaves

Has a network of veins running throughout

Visualise the composition of the stem

Xylem inside phloem outside

Why do the stems have xylems on the inside ?

To help support the plant and for flexibility, due to the lignin in its cell walls

Why is there parenchyma in the stem and where

In the centre for storage and strengthening fibres

Visualise the root

What does the endodermis do

Supply the xylem with water

Why is the xylem on the inside of the root

To help the plant withstand pulling forces like of water and growth aka mechanical forces

Name all the facts you know about the xylem

• they have bordered pits

• A long continuous column with no end plates

• Cell has dead lignin in its walls which thickens it and has spiral patterns for flexibility

• Transports water up the plant

Name all the facts you know about phloems

• transports assimilates (organic compounds) from the source leaf to sink root up and down the plant

• Consist of sieve tube cells and companion cells that are linked in cytoplasm by their plasmodesmata allowing comms and flow of substances eg minerals

• Companion cells help in active loading of sucrose

• Sieve tube elements are what actually make the tube up

Where does water move to in a plant

A place of lower water potential, more negative

Describe the apoplast pathway

Water moves thru the cellulose fluid filled cell walls

the walls are thick and open so it’s easy for water to move thru as no cell membranes

• little control

  stops at the endodermis due to waterproof casparian strip made from Suberin- forced water into the cellulose fluid filled by passing thru membrane to go into the simplest pathway - membrane selectively permeable so can control xylem water uptake

Desc the symplast pathway

Water travels through cytoplasm connected by plasmodesmata in gaps in cell walls

Desc movement of water through soil into root

Cells in epidermis of root actively transport minerals in so lowers WP so water moves into root hair cell cytoplasm by osmosis down the concentration gradient

What three ways does water move up stem ?

Capillary action

Transpirational pull

Root pressure

Desc capillary action

Cohesion = attraction to other water molecules

Adhesion = attraction to xylem walls

Desc root pressure

Water and minerals move into xylem from the root which pushes water in xylem up a few cm

Desc transpirational pull

Cohesive forces of water form a long column, if water gets lost at the top by transpiration more must be drawn up the stem, that pulling creates tension and lignin is what’s helping the plant to stay intact and not break because it strengthens and toughened the wall

If the column gets broken water can flow into the adjacent xylem vessels by the bordered pits

How do the following affect transpiration rate :

• light

• Temperature

• Wind

• Humidity

• Water availability

Light = increase as stomata open

Temp= increase as more KE and WP gradient higher

Humidity = decrease as WP gradient lower

Air = increase as less humidity blowing away water vapour and maintains gradient by blowing water Vapor away

Water = decrease as stomata will close up

Xerophyte adaptations

• small leaves like pines or needles to reduce the surface area for transpiration

• Sunken stomata and hairs to trap moist air, effectively increasing the humidity so the diffusion gradient and reducing transpiration

• Thick waxy cuticle

• Fewer stomata

Hydorphyte adaptations

Large flat leaves that are buoyant so they can be on the surface close to the sun and have a large SA

Thinly lignified walls

Thin waxy cuticle

Stomata on upper surface of leaf to exchange with air not water

Xylem is reduced as less need to transport water

Cells adapted to not absorb too much water via cell wall

Plant adaptations

What actyally is translocation

Movement of assimilates from source eg where it’s made (leaf) or where ist stored eg tuber, seeds etc to where it is used eg meristem, growing flowers/ developing seeds etc

Describe active loading

1. H+ ions pumped out of companion cell by proton pumps via active transport, increasing conc outside of the cell and decreasing inside

2. H+ move back in but via cotransporter proteins with sucrose attached

3. Sucrose enters cell and reduces WP

4. Water moves into cell via osmosis forming cell sap with sucrose

5. Sucrose diffuses down a conc gradient into the sieve tube

6. Water follows and its addition causes hydrostatic pressure so the contents move to the area of lower pressure, sink. They move by mass flow

What is unloading ?

At the sink the sucrose moves out and diffuses into that tissues to be used up

What is the mass flow hypothesis

Due to presence of a hydrostatic pressure gradient- Starts from the active loading of sucrose into sieve tube at source , causing water to move in but at the same time sucrose is being unloaded at sink, water ofc follows and this lowers the hydrostatic pressure at the sink