Transport in plants Flashcards
(23 cards)
What are the main transport tissues in plants
Xylem and phloem
What is the cambium
A layer of unspecialised cells that divide creating more specialised cells in the xylem and phloem
Xylem
End walls breakdown to form a hollow tube
Impermeable to water
Water can leave via pits in walls
Transports water in one direction (up)
Lignified so strengthens the plant
Starts off as living tissue but cells die as it becomes lignified
Phloem
Transports in both directions
Sieve tubes made up of many cells joined together to create a long tube
No lignification
Walls between cells become perforated to form the sieve plates
Mature cells have no nucleus but survive due to companion cells (linked via plasmodesmata)
Living tissue that transports food around the plant e.g. sucrose
Types of xylem
Begins as unlignified living tissue known as the protoxylem
Becomes lignified and dead tissue which is known as the metaxylem
Evidence for the movement of water through the xylem
If the cut end of a shoot is placed in eosin dye, dye can be seen being carried into the transport system and through to the vascular tissue of leaves
Removing a ring of bark killing phloem but not xylem, eosin dye shows upward movement of plant is unaffected (ringing experiment)
Provide plant with water containing radioactive isotopes, these can be tracked by autoradiography
Which cells absorb water into roots
Root hair cells (large surface area)
Movement across root
Water absorbed by root hair cell
Creates a conc. gradient in roots
Water moves into cell by osmosis
Causes water to continually move up by transpiration
2 pathways of water
Symplast pathway
Apoplast pathway
Symplast pathway
Water moves by diffusion down conc. gradient from root hair cells to xylem through interconnected cytoplasm (symplast) of the cells
Apoplast pathway
Water is pulled by attraction between water molecules across adjacent cell walls
This occurs until the water reaches the endodermis containing a waterproof layer (the casparian strip)
Translocation
The movement of substances around plants
Transpiration
The loss of water vapour from the surface of the plant
Cohesion tension theory
The theory of transpiration based on the idea that the stream of water molecules stick together by cohesive forces, so that when a molecule is lost by evaporation it puts a tension on the column and another molecule of water is pulled up to replace it
The transpiration stream
The movement of water up from the soil through the root hair cells and across the roots to the xylem, then up the xylem, across the leaf until it is lost by evaporation from the spongy mesophyll cells and diffuses out of the stomata down the concentration gradient
Modelling transpiration
Josef Bohm showed adhesive forces between water and therefore transpiration upwards
Potometer
Demonstrates the uptake of water
Factors affecting transpiration
Light - stomata open in the light
Temp - higher temp increases evaporation
Wind - increases transpiration as reduces still air around the stomata
Humidity - high humidity decreases transpiration as there is a reduced concentration gradient
Guttation
Drops of water are forced out of the leaves as a result of root pressure
Root pressure
The pressure that results when salts are secreted from root hair cells in xylem sap, increasing conc. in the root and moving more water into the xylem by osmosis
Assimilates
The substances transported in the phloem - main one is sucrose
Transport of assimilates in the phloem
Sources (where substance is made) to sinks (tissues that need the substance)
Mass flow hypothesis
Developed by Munch
Proposes that translocation occurs due to pressure gradients
