3.2.2: Transport in plants Flashcards
Why do plants require a transport system?
1) Metabolic demands
2) Size: diffusion alone would take too long/distance substances would need to diffuse is too great
3) Low SA:V ratio: cannot rely on diffusion alone
What does the phloem transport?
Sucrose dissolved in water (forms sap)
What does the xylem transport?
Water and dissolved mineral ions
Describe the structure of the xylem vessel
- Long, hollow columns of dead cells fused together end to end
- There are no cell contents to obstruct flow
- Walls of the vessel are thick and lignified
- Lignin and fibres prevent collapse under transpiration pull
- Bordered pits allow movement between vessels
Why do the xylem vessels have bordered pits?
To allow the sideways movement of water between xylem vessels or into other plant cells.
What is translocation?
The movement of assimilates in the phloem.
Assimilates
Products of photosynthesis
Source
Part of the plant that releases sucrose into the phloem.
How does sucrose move into the phloem?
1) Companion cells use ATP to transport H+ ions into surrounding tissues (LOADING)
2) H+ ions diffuse back into companion cell down conc. gradient through co-transporter protein
3) Sucrose travels with H+ in co-transporter proteins into companion cells
4) Conc of sucrose in companion cells increases –> sucrose diffuses into phloem through plasmodesmata
Transpiration
The loss of water vapour from a plant’s leaves and stem
Transpiration stream
Movement of water through a plant until it is lost by evaporation from the leaves
Describe the structure of the phloem
- Sieve tube elements –> lined up end to end, walls perforated by many pores so as not to obstruct sucrose mass flow
- Companion cells perform metabolic processes to keep sieve tube elements alive
Why do plants need water?
- Photosynthesis
- Maintain cell turgor
- Medium to transport dissolved minerals
- Solvent for reactions
- Hydrolysis rections
- Coolant (evaporation)
Root pressure theory
Active pumping of minerals into the xylem causes water to move in. This results in root pressure which “pushes” water part way up the xylem.
Evidence for root pressure theory
- Cyanide –> no ATP –> no root pressure
- Increased temp. –> increased root pressure –> increased kinetic energy, ROR, ATP production
- When stem cut (and cohesion stopped) sap can exude from cut end
Sucrose moves by
mass flow
Sympoplast
Water moves through cytoplasm, passes into next cell via plasmodesmata. Changing water potential of cells; highest water potential = outside root cells
Apoplast
Passing through cellulose cell wall
Reaches endodermis –> must enter cytoplasm to avoid being stopped by Casparian strip
Factors affecting transpiration
- Light –> photosynthesis, stomata open
- Relative humidity –> relate to water vapour potential gradient
- Temperature –> molecules have more kinetic energy = higher rate evaporation//air can hold more water before saturation
- Air movement (water vapour potential gradient)
- Soil water availability: more water = higher rate of transpiration
How are stomata opened
1) K+ ions pumped into guard cell (active transport)
2) Water moves in via osmosis
3) Turgor increases
4) Inner wall less flexible than outer –> becomes bean shaped, stomata open
Adaptations of xerophytes
- Sunken stomata
- Needles/spines
- Hair on leaves
Hydrophytes adaptations
- Stomata on upper surface of leave
- Air spaces in stem (low resistance internal pathway for moving substances)
- Small roots (decrease SA, reduce water uptake)
- Wide leaves w/ air spaces (capture light, easier for gas exchange)
