Mass transport plants

what is the xylem

the tissue that transports water in the stem and leaves of plants.

why are xylem vessels x shaped

withstand mechanical forces

what is the outer part of the xylem called

endodermis

xylem vessels are

dead tissues with long open ends

what is transpiration

loss of water vapour via the stomata by diffusion

what enters and exits through the stomata

carbon dioxide enters

water exits

what is the transpiration stream

movement of water up the stem

why does water leave the leaves during transpiration

humidity in the air is less then the humidity in the air spaces in the stomata, water potential gradient. water molecules diffuse through the air spaces in the stomata into the surrounding air

how does water enter the root

there soil has a higher water potential than the root hair cell (due to dissolved cell sap ) so the water moves along the water potential gradient into the root hair cell via osmosis

what are the 2 pathways water can take across the cortex of the root to the xylem

symplast pathway

apoplast pathway

what is the symplast pathway

where water enters the cytoplasm through the plasma membrane and passes from one cell to the next through plasmodesmata

what is the plasmodesmata

the channels which connect the cytoplasm of one cell to the next.

what is the apoplast pathway

where the water moves (diffusion) through the water filled spaces between cellulose molecules in the cell walls. When the water reaches the endodermis, it encounters a layer of suberin which is known as the Casparian strip, which cannot be penetrated by water. the water that has been moving through the cell walls must now enter the symplast pathway.

what is the casparian strip

thick, waterproof, waxy band of **suberin** within the cell wall that blocks the apoplast pathway and is impassable for water

how does water move up the stem (cohesion tension theory )

Water is lost from the leaf because of transpiration which lowers the water potential of mesophyll/leaf cells. Water moves from xylem into mesophyll lowering water potential and pressure at the top of the xylem. Water is pulled up the xylem along the pressure gradient, which creates tension, to replace this lost water as water coheres and ‘sticks’ together forming a continuous water column. Additionally water adheres to the walls of the xylem.

why is water cohesive

stick together with hydrogen bonds

how does high air flow affect rate of transpiration

Increases

good airflow removes water evaporated from the stomata and surrounding leaf setting up a high concentration gradient, increasing water loss

how does high humidity affect rate of transpiration

decrease

Lower concentration gradient of water in the air compared with gaps in the stomata so water is less likely to leave

how does high light intensity affect transpiration

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increases when light intensity is high guard cells are turgid so stomata is open therefore more water is lost

how does high temperature affect transpiration

increases

at high temp particles have more kinetic energy so transpiration occurs faster as water evaporate from mesophyll and diffuse away at a faster rate

what is a potometer used to investigate

the effect of environmental factors on the rate of transpiration

method for potometer practical

Cut a shoot underwater ( prevent air from entering the xylem)

Place the shoot in the tube and set up apparatus

make sure it is airtight

allow single air bubble to form in capillary tube

set up environment and allow plant to adjust for 5 minutes

record start of bubble and leave for set time and record end value

reset bubble by opening tap in resvoir

repeat experiment with changing variable

how would you change the environmental conditions:

air flow

humidity

light intensity

temperature

air flow : hairdryer/ fan

humidity: spray with water and place in plastic bag

light intensity : lamp

temperature: hot or cold room

what is the phloem

the tissue that transports organic substances in plants.

what are sources

the sites of photosynthesis

what are sinks

where sugars are either directly used or stored

translocation can move sugars in either directions because

sinks can be above or below sources

what organic molecules does the phloem transport

sucrose or amino acids

what inorganic ions are transported using the phloem

potassium

sodium

phosphate

magnesium

chloride

properties of phloem cells

vascular tissues with sieve elements (arranged end to end)

surrounded by companion cells

what are the sieve elements of phloem cells

long thin structures

between each sieve tube element is a perforated end wall called a sieve plate

why do sievetubes only have cell wall and cell membrane

more capacity to carry sucrose

companion cells are involved in

ATP production for active processes - lots of mitochondria for respiration

why is sucrose used in mass flow instead of glucose

less reactive

disaccharide vs monosaccharide

what are the 3 phases of mass flow theory

1) transfer of sucrose into sieve elements from photosynthesizing tissues

2) mass flow of sucrose from the sieve tube elements

3) transfer of sucrose from the sieve tube elements into storage or other sink cells

why was mass flow theory developed

rate of movement of substances within plants is too fast to be explained by simple diffusion

process of mass flow

1) sucrose diffuses from source to companion cells (facilitated diffusion)

2) sucrose is actively transported from companion cells into phloem using ATP, this causes the water potential to decrease

3) water moves from xylem to phloem along water potential gradient by osmosis which increases hydrostatic pressure

4) At the same time sucrose leaves phloem (diffusion) to be used in respiration or stored as starch at sink cells which increases water potential

5) water moves out of phloem into xylem, via osmosis which decreases hydrostatic pressure

6) sucrose at the ‘top’ of the phloem will move down hydrostatic pressure gradient from source to sink

why is sucrose actively transported into sink cells

at sink cells, sucrose is either used for respiration or stored as starch

as water enters at the sources and leaving at the sinks

high hydrostatic pressure at the source

low hydrostatic pressure at the sinks

general movement of sucrose in the phloem

down the hydrostatic pressure gradient in the sieve tubes

what do ringing experiments involve

removal of a ring of surface tissues from the stem of the plant while leaving the stem core intact - removes only the phloem leaving the xylem intact

after the ringing experiments

the plant is exposed to a radioactive tracer - 14CO2 (readily absorbed by the leaves and used in photosynthesis)

as a result of the plant using the radioactive tracer during photosynthesis

the sucrose will be radioactive so its movement, via translocation can be traced, amount of sucrose in different areas can be detected

if mass flow hypothesis is correct

the bulk flow of phloem sap should be in one direction (from source to sink) and occur at the same rate in any sieve tube at the same time

what happens in the xylem is damaged during the ringing process

the plant will not have an adequate supply of water and will wilt

what do results of tracer and ringer experiments show

the **phloem is involved** in the transport of sucrose (There is no radioactive sucrose detected past the ringing point on the stems )

In the phloem the transport of sucrose occurs both upwards and downwards (Sucrose is translocated from the source (leaves) to the sink tissues above and below)

what is the evidence supporting mass flow theory

When the phloem sieve tube is punctured phloem sap oozes out -This suggests that it is under pressure

Phloem sap taken from near a source has a higher sucrose concentration than sap taken from near a sink - different water potentials would result in osmosis into/out of the sieve tubes at those two locations

in dark - no translocation (for translocation to occur photosynthesis and the production of sucrose is required in the source tissue)

what is the evidence against mass flow theory

rate of translocation of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose ( should be flowing at the same rate)

detected different substances moving in opposite directions ( should be flowing in the same)

sieve tube elements purpose?