Chapter 9: Transport In Plants Flashcards
The need for a transport system
- Size : larger plants need effective transport systems to move substances both up and down from the tip of the roots to the topmost leaves and stems
- Metabolic rate : Many internal and underground parts of the plant do not photosynthesise so they require oxygen and glucose transported to them and waste products of cell metabolism removed
Mineral ions need to be transported to all cells to produce proteins and enzymes required by the cell - SA:V : Relatively small SA:V overall for exchange of gases with the air so they cannot rely on diffusion alone to meet their metabolic demands
Vascular System
Roots: Vascular bundles in the centre to help the plant withstand tugging strains from the force of the wind on the stem and leaves
- Root hair surrounds the outside
Stem: Vascular bundles surround the edge to provide strength and support
- Parenchyma between the vascular bundles is packing and supporting tissue
Leaves: Vascular bundles carried in the main vein through the midrib of the organ to help support the structure of the leaf + branching veins spreading through the leaf help in transport and support
- palisade mesophyll on the outer surface is the main photosynthetic tissue
Xylem
Non-living tissue made of mainly xylem vessels surrounded by packed xylem parenchyma that transports water + mineral ions and provides support
- unidirectional flow of materials from shoots to leaves
Xylem vessels are a long hollow tubes with no cytoplasm or end cell walls so water is able to travel against gravity
- cohesion/adhesion (capillary action)
Thick cell walls are stiffened with lignin to make them waterproof and prevent from collapsing under the transpiration pull + provides mechanical strength
- lignin is formed in patterns: spiral, annular or reticulate to allow xylem to stretch as the plant grows + enable stem to bend
- incomplete lignification leaves gaps (bordered pits) to allow lateral movement of water between vessels
Xylem parenchyma stores food and contains tannin deposits
- bitter chemical that protects plant tissue from herbivore attack
Phloem
Living tissue made of sieve tube elements and companion cells that transports assimilates + amino acids in a sap around the plant
- bidirectional flow of materials from source to sink
Sieve tube elements are elongated cells that line end to end to form a long, hollow sieve tube
- ends have perforated walls called sieve plates to allow sap to flow through sieve tube elements + act as structural support (no lingin) to keep lumen open / block if sieve tube elements are injured
- they have no nucleus + very little cytoplasm to allow space for sap to flow
Companion cells linked to sieve tube elements by plasmodesmata (cytoplasmic strands link pores through cellulose walls to sieve tube elements)
- large nucleus + lots of mitochondria for the active loading of assimilates into sieve tubes
Transport of water into the plant
First water moves into plant through root hair cells (contain root hairs that are thin extensions from cell with large SA:V and thin layers)
Symplast Pathway
- movement of water though symplast (continuous cytoplasm linked by plasmodesmata)
- movement due to water potential gradient between cells until reaches xylem cells
Apoplast pathway
- movement though apoplast (cell walls and intercellular spaces)
- movement due to cohesion which creates a tension leading to a continuous flow of water through apoplast
Vacuolar pathway
- movement through vacuoles of neighbouring cells
Transport of water though the plant
Water moves through apoplast and symplast pathways until reaching the epidermis
Casparian strip (band of Suberin) causes water in apoplast to join symplast
Active transport of minerals into xylem decreases water potential (causes root pressure-independent of transpiration but not the major factor for water movement) so water moves into xylem through osmosis
When entering xylem, water moves though apoplast pathway
Water moves up the xylem vessel through adhesion(to carbohydrates in vessels) and cohesion
Process of transpiration
loss of water from the leaves and stem of a plant
Consequence of gas exchange (lower water potential in air leads to evaporation of water vapour)
Transpiration stream:
- passive process
- water evaporates from the surface of mesophyll cells into air spaces which then diffuses out of leaf through stomata
- this maintains a concentration gradient
Mechanism of movement in transpiration
Capillary action
- due to adhesion + cohesion
- allows movement up a narrow column against gravity
Transpiration pull
- water pulled up in a continuous stream to replace water lost by evaporation in the leaves
- results in tension in the xylem
Cohesion tension theory
- a model explaining the movement of water through a plant due to transpiration
- evidence (changing diameter of trees throughout the day, when xylem vessel is broken - air drawn in instead of the leakage of water)
Measuring transpiration
Potometer: measures water uptake in plants
- secure all joints to ensure the water loss measure is only due to transpiration
Rate of water uptake = distance moved by air bubble/time taken to move that distance
Limitations:
- only measure water uptake, not loss
- damage to plant when cutting it
- no resistance to water being pulled up as there are no roots
Factors effecting transpiration
Light intensity
- increase in intensity increases rate
- more stomata open for water loss
Relative Humidity
- measure of amount of water vapour in air in comparison to total concentration of water that the air can hold
- increase in relative humidity decreases rate
- lower water potential gradient when humidity increases
Temperature
- increase in temperature increases rate
- increased KE of particles
- higher concentration of water that can be held by the external air before becoming saturated
Air movement
- increase in wind speed increases rate
- increase in water potential gradient as water around stomata is moved
