Mass Transport (Plants)

what are dissolved mineral ions transported through (1)

xylem

what are dissolved organic compounds transported through (1)

phloem

what are plant roots responsible for ()

• uptake of water

• uptake of mineral ions

what type of process is the uptake of water (1)

osmosis

what type of processes are involved in the uptake of minerals (2)

• diffusion

• active transport

why must plants take in a constant supply of water (3)

• to compensate for the continuous loss of water via the transpiration stream

• so that they can photosynthesise

• to produce proteins

define transpiration (1)

the loss of water vapour via the stomata by diffusion

state the ways in which transpiration is important for plants (3)

• it provides means of cooling the plant

• helps the uptake of mineral ions

• provides turgor pressure of the cells which provides support to leaves

state the 4 main stages of transpiration (4)

1. roots take up water from soil

2. water is drawn up the stem to the leaves

3. veins carry the water into the leaves

4. water evaporates from the leaves

define transpiration pull (1)

how a column of water is pulled up the xylem as a result of transpiration

define apoplastic pathway (1)

movement of water through the cell walls of a plant

define symplastic pathway (1)

movement of water from the cytoplasm of a cell

state and explain the roles of the stomata (2)

• controls transpiration via the pairs of guard cells that surround it

• when open, allows for a greater rate of transpiration and of gaseous exchange

explain the transpiration stream (5)

1. water vapour diffuses from air spaces through stoma

2. water evaporates from a mesophyll cell wall into the air spaces , creating a transpiration pull

3. water moves through the mesophyll cell wall or out through the mesophyll cytoplasm

4. water leaves a xylem vessel through a non-lignified area

5. water moves up the xylem vessels to replace the water lost from the leaf

define translocation within phloem tissue (1)

the transport of assimilates from source to sink, requiring ATP

what is the name of the liquid which is being transported through the phloem (1)

phloem sap

what does the phloem sap consist of (5)

• mainly sucrose

• water

• amino acids

• hormones

• minerals

what are the different sources of assimilates (3)

• green leaves and green stem

• storage organs

• food stores in seeds

what type of process is the loading and unloading of the sucrose from the source to the phloem (1)

an active process

why are carbohydrates generally transported in the form of sucrose within plants (2)

• it allows for efficient energy transfer and increased energy storage

• it is less reactive than glucose as it is a non-reducing sugar

explain what happens when sucrose molecules are taking the apoplastic pathway (8)

• modified companion cells pump hydrogen ions out of the cytoplasm and into cell wall

• via a proton pump using ATP

• this results in large conc of hydrogen ions in the cell wall of companion cell

• resulting in the hydrogen ions moving down the conc gradient back to the cytoplasm of the cell

• through a cotransporter protein

• which also carries sucrose molecules into the companion cell

• against the conc gradient for sucrose

• which then move into sieve tubes

state and explain how the walls of sieve cells adapted for mass transport (5)

• they have intercellular spaces

• with small volume of cytoplasm and large vacuole in them

• so phloem sap can flow easily through corridor with low resistance

• have thick cell walls

• that help them withstand the pressure exerted by the mass flow of sugars

what are the sinks where assimilates are required (4)

• meristems

• roots

• developing seeds

• storage organs

what did the mass flow hypothesis consist of

• two partially permeable membranes

• containing solutions with different concs of ions

• these membranes were placed into two chambers containing water

• and were connected via a passageway

• two membranes are joined via a tube

explain how mass flow of organic solutes occurs in plants

• pressure difference is generated by actively loading sucrose into the sieve elements of the source

• which lowers the water potential in the sap

• this results in water moving into the sieve elements via osmosis

• this increases the hydrostatic pressure at the source

• allowing water to flow into the sink via osmosis as solutes are unloaded

• creating a low hydrostatic pressure

• creating a hydrostatic pressure gradient

state and explain the supporting evidence for the mass flow hypothesis (8)

• when phloem sieve tubes are punctured, phloem sap oozes out

• suggesting that it is under pressure

• phloem sap taken from near a source has higher sucrose conc than sap from near a sink

• suggesting the different water potentials in the different locations

• when a plant virus is applied to leaves under light, the virus can be seen moving down the plant towards the roots

• demonstrating the bulk flow of substances in one direction

• when plant virus is applied to leaves under dark, it is not transported

• suggesting that photosynthesis is required for translocation to occur

state and explain the contradicting evidence for the mass flow hypothesis (6)

• rate of translocation of amino acids was measured, and found that they travel slower than sucrose

• mass flow hypothesis states the flow should be at the same rate

• some experiments show substances moving in opposite direction

• mass flow hypothesis starts that flow should be in one direction

• suggested that some sieve tubes translocate at different times

• mass flow hypothesis states that all sieve tubes are involved at the same time

state and explain the ringing experiment (4)

• remove a ring of surface tissue from the stem of a plant

• plant is then exposed to a radioactive tracer

• radioactive CO2 is used to photosynthesise the plant

• amount of radioactive carbon present in different parts of the plant is detected

explain the results from the ringing experiments (2)

• phloem is involved in the transport of sucrose

• in the phloem, transport of sucrose occurs both upwards and downwards

use the mass flow hypothesis to explain how pressure id generated inside a phloem tube

• sucrose actively transported into phloem

• lowering the water potential in it

• water moves into phloem by osmosis from xylem

why is phloem reduced during the hottest part of the day

• higher rate of transpiration

• water lost through stomata

• causes less water movement from xylem to phloem

what measurements can be used to calculate the rate of water movement

• time taken

• initial and final mass

• number of xylem vessels

explain why water moves up a plant stalk

• water transpires from leaves

• water potential gradient pulls up water

• hydrogen cohesion maintains the column

describe the cohesion-tension theory of water transport in the xylem (5)

• water loss from leaf due to transpiration

• lowers water potential of leaf cells

• water pulled up via xylem

• water molecules cohere by hydrogen bonds

• maintaining a water column

• adhesion of water molecules to walls of xylem

describe the mass flow hypothesis for the mechanism of translocation in plants (5)

• sugars are actively transported from source to phloem

• by companion cells

• lowering water potential of sieve cells and water enters by osmosis

• increase in pressure causes mass movement towards sink

• sugars used in root for respiration for storage

Explain the factors that affect transpiration

• Light

• More light causes more stomata to open therefore more evaporation

• Temperature

• More heat means more kinetic energy therefore faster molecules moving therefore more evaporation

• Humidity

• More water vapour therefore water potential outside is greater than inside therefore reduces water evaporation

• Wind

• More wind will blow away humid air therefore maintaining a water potential gradient therefore more evaporation