Mass Transport in Plants

what is transpiration?

loss of water vapour by evaporation via the stomata

how does light intensity affect transpiration?

more light means more evaporation of water as more of the stomatas are open

how does temperature affect transpiration?

faster rate of transpiration as the water molecules have more kinetic energy so move faster

how does humidity affect transpiration?

more humidity around the plant means more moisture in the air so less transpiration as water potential gradient is reduced

how does wind affect transpiration?

more wind means faster rate of transpiration as the wind carries away the water vapour

benefit of cohesion between water molecules

water is polar so H bonds form between the water molecules, so water mols can stick together and water can travel up the xylem in a continuous column

benefit of adhesion

water mols stick to the walls of the xylem vessels so water doesn’t fall back down due to gravity

how does root pressure force movement of water?

as water moved into the roots, the volume of liquid increases so the root pressure increases which forces the water upwards

explain the cohesion-tension theory

• water vapour evaporates out of stomata, a loss of water means there is a lower pressure

• due to the negative pressure created, more water is pulled up the xylem to replace the lost water

• due to the H bonds, H20 molecules are cohesive so they stick together which creates the continuous column of water

• the water mols are adhesive so stick to the walls of the xylem so they can be pulled up against gravity

• whilst the column of water is moving upwards, tension at the root is created, which makes the xylem narrower so more water is contact with the xylem walls and therefore more is moved up

explain 3 ways in which xylem vessels are adapted

• The end walls are removed from each end, forming an uninterrupted tube, so water can pass through easily

• Their walls are thickened with a woody substance called lignin, which helps to support the xylem vessels and stops them collapsing inwards

• There are small pits in the cell wall where is no lignin so water and ions can move into and out of the vessels

how is sucrose transported from the source to the sieve tube element?

• photosynthesis occurring in the chloroplasts of leaves creates the organic substances

• this creates a high concentration of sucrose at the site of production, so the sucrose diffuses down its conc gradient into the companion cell via facilitated diffusion

• active transport of hydrogen ions occurs from the companion cells into the spaces within the cell walls using energy

• this creates a conversation gradient, so the hydrogen ions move down their conc gradient via carrier proteins into the sieve tube elements

• co transport of sucrose with the hydrogen ions occurs vis protein co-transporters to transport the sucrose into the sieve tube element

how does sucrose move within the sieve tube element?

• this increase of sucrose in the sieve tube element lowers the water potential

• water enters the sieve tube element from the surrounding xylem vessels via osmosis

• this increase in water volume in the sieve tube element increases the hydrostatic pressure, forcing liquid towards the sink

how does sucrose transport to the sink (respiring cells)?

• sucrose is used in respiration at the sink or stored as starch

• more sucrose is actively transported into the sink cell, causing the water potential to decrease

• so water moves from the sieve tube element to the sink cell via osmosis

• the volume of the sieve tube element has decreased so the hydrostatic pressure will decrease

• therefore the movement of these organic substances is due to the differences in hydrostatic pressures

how to ensure phloem is how these substances are transported?

tracers

• tracks the location of radioactively labelled carbon

• plants are provided with only radioactively labelled CO2 and this absorbs into the plant overtime, and is used in photosynthesis to create the sugars

• thin slices from the stems are then cut and placed on an x-ray film that turns black when exposed to radioactive materal

• this highlights where the sugars have ben transported in the phloem, and so where the phloem is

ringing experiment

• a ring of bark and phloem is removed off a tree trunk

• if the part of the tree above the ring starts to swell, as the phloem cant move down

• the solution in the swollen part can be taken and tested for sugars, to show that when the phloem is removed, the sugars cant be transported and proves phloem transports sugars