mass transport in plants

structure and function of xylem

- long cells with no end walls, to allow movement of a continuous column of water
- no organelles or cytoplasm, to allow easier water flow (no obstruction)
- supported by rings of lignin, to withstand the tension of water and provide strength
- contains pits in walls, to allow lateral movement (side to side) of water

transpiration (cohesion-tension theory)

- water vapour lost from mesophyll cells due to evaporation of water through stomata
- lowers WP of mesophyll cells
- hydrogen bonds stick water molecules together - cohesion
- forming a continuous water column
- adhesion of water molecules to walls of xylem
- water pulled up xylem creating tension
- water from xylem replaces water lost from mesophyll cells
- via osmosis from soil with higher WP, to root hair cells with lower WP

evidence for the cohesion-tension theory

1. diameter of trunks is narrower at midday because tension stretches trunks. this coincides with highest rates of transpiration and evaporation

2. when xylem breaks there is a lack of cohesion, as a result water no longer reaches top leaves

3. when xylem breaks, water does not leak out of a broken vessel, instead air is drawn in due to tension

measuring the rate of transpiration

1. leafy shoot cut (at an angle to increase SA) underwater to prevent air bubbles and maintain a continuous column of water
2. potometer and reservoir is completely filled with water, joints all sealed with waterproof jelly (/vaseline)
3. introduce air bubbles into capillary tube
4. distance moved by air bubble in a given time is measured
5. tap can be opened, and reservoir allows air bubble to return to start point to repeat
6. repeat and calculate mean
7. volume of water lost calculated - (pi)r²I (radius of capillary tube, I = distance moved by air bubble)

rate of transpiration

- about 99% of water taken up by a plant is lost via transpiration
- so, rate of uptake is almost equal to rate of transpiration
- rate of uptake can be measured by potometer, and transpiration estimated

rate of transpiration is increased when:
- environment is not humid
- environment has a high temp
- environment is windy

mass flow theory / translocation

1. in source (leaf) sucrose is actively transported into phloem by companion cells
2. this lowers the WP of sieve cells, and water enters by osmosis from the xylem
3. this produces high hydrostatic pressure
4. mass flow to respiring cells
5. sugars unloaded by active transport / facilitated diffusion (from phloem to sink cells)
6. sugars used in root for respiration for storage

sucrose

- glucose is made during photosynthesis
- converted into sucrose (fructose + glucose)

phloem

vessel composed of two types of specialised cells, connected by plasmodesmata: sieve tube elements and companion cells

sieve tube elements:
- sieve plates
- cells are alive when mature
- no nucleus and very few organelles
- thick walls to resist high pressure

companion cells:
- lots of mitochondria to produce ATP for active transport
- ribosomes for the synthesis of carrier proteins

why does water move from xylem to phloem via osmosis?

positive WP in xylem and negative WP in phloem → high hydrostatic pressure

sucrose is transported to sink cells

- to be used for respiration
- sucrose is hydrolysed into glucose and fructose
- glucose is required for respiration

evidence for mass flow theory

ringing experiment:
- cut the bark and phloem in a ring
- immediately above the cut, a build up of sugar sap is visible - sucrose is moving from source → sink
- tissue below cut dies - it hasn’t been able to respire due to a lack of sucrose

tracer isotopes:
- plant exposed to ¹⁴CO₂, and uses this to synthesise sucrose
- isotopes traced using autoradiograph
- by cutting thin sections of the plant stem over time, we can see the movement of sucrose
- movement occurs from sucrose to sink, via phloem

aphids:
- bright green bugs that feed on plants
- feed on sugar water, obtained by piercing plants and sucking it out
- only observed to piece into sieve tube elements
- sap is extracted as it’s under high pressure

diurnal changes:
- sucrose content in phloem mirrors content in leaves
- in leaves at midday: more sunlight, greater rate of photosynthesis, more sucrose
- so, more sucrose actively transported into phloem, decreasing phloem WP, more osmosis from xylem, higher hydrostatic pressure, greater rate of mass flow
- sucrose conc is highest during the day cuz that’s when photosynthesis occurs cuz of sunlight

mass transport in plants

- plants require a transport system to ensure all cells receive nutrients and water
- xylem - enables movement of water via transpiration (passive)
- phloem - enables movement of sugars/organic substances via translocation (active)
- the vessels are arranged in vascular bundles

pressure

push force

tension

pull force

A student measured the time taken for water movement. Give two other measurements she made to calculate the rate of water movement.

1. initial and final mass of beaker and all contents

2. number of groups of xylem vessels

Give the reason for adding a layer of oil to the water in the beaker.

prevent evaporation

The student used a sharp scalpel to cut the celery. Describe how she should ensure she handled the scalpel safely during this procedure.

cut away from body, against a non-slip surface

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.

Suggest why.

1. plant has roots

2. xylem cells very narrow

Name the process that produced the ¹⁴CO₂ released from the trunk.

respiration