Mass transport is the movement of materials over large distances due to pressure differences

Phloem = transport of organic substances, sugars, other soluble products of photosynthesis from the leaves to the rest of the plant

Xylem = transport of mineral and water through the roots and the stem to the leaves

Transport of Water in the Xylem

• Water absorbed by the roots through root hairs by osmosis and transported by xylem vessels - evaporation of water from leaves pulls water into the stem of the plant - transpiration - passive process

Structure of Xylem Vessels

Transport water and dissolve mineral ions

• Cell walls contain lignin = strengthens xylem walls against tension = waterproof

• Lignified walls = cell contents die = hollow lumen with no cytoplasm = little resistance to mass flow of water and minerals

• Walls of xylem contains tiny holes = vessel gets blocked/damaged and diverted so the upwards movement of water continues in adjacent vessel

• The Vessel lose their end walls = reduces resistance to flow - forms a continuous vessel for a column of water to move from roots → leaves

Transpiration

Process of water movement through a plant and its evaporation from the leaves

The Cohesion Tension Theory

• Water vapour diffuses out of air spaces down a WP gradient - transpiration - size of each stomata pore controls the rate of transpiration

• Water evaporates from mesophyll cells → air spaces = water vapour build up in air spaces

• WP in mesophyll cells lower than xylem vessels = water moving into mesophyll cells by osmosis

• This creates a low hydrostatic pressure (top of the xylem) and lowers the vol

• Water in the xylem is under tension → water pulled to the leaves

• Water forms H bonds between each other and stick together = cohesion - continuous columns of water are maintained due to cohesion and pulled up xylem due to transpiration

• Water then moves by osmosis from soil → roots

Adhesion of water molecules - inward pull on the vessel walls as water is pulled up = negative pressure in the xylem = decrease in diameter

Cohesion tension theory is a passive process so no atp is used as no energy is required as energy is provided by the sun in the form of heat energy

Evidence for cohesion tension theory

• Change in diameter = due to rate of transpiration = more tension during the day and less tension during the night due to adhesion of water molecules which causes the diameter to shrink

• If a xylem vessel is broken and air enters - tree can no longer draw up water due to air bubbles - continuous column of water is broken so cohesion of water molecules is broken

• When a xylem is broken - water does not leak out so under pressure = air drawn in under tension

• Respiratory inhibitors do not inhibit this process

Factors Affecting Transpiration Rate

Light = higher light intensity means faster rate of transpiration so stomata open to allow carbon dioxide in for photosynthesis so stomata close when its dark so there is little transpiration

Temperature = higher temperature means transpiration happens at a faster rate as more energy so evaporate from cells inside cells which increases WP gradient in and out of the leaf so water diffuses out faster

Wind Speed = faster wind speed means that transpiration occurs at a faster rate due to lots of air movement which blows water molecules away from the stomata which increases the WP gradient between the inside and outside of the leaf so water diffuses out faster

Humidity = Lower humidity means transpiration happens at a faster rate - if air around the plant is dry than the WP gradient between the leaf and air increases so water diffuses out faster

Transport in the Phloem

Mineral ions and organic soluble molecules are transported around the plant as sap through translocation

Structure of phloem vessels

• Phloem tissue is made up of sieve tube elements = long, thin living cells arranged end to end - unlike the xylem the sieve tube elements don’t break down but contain small pores (perforations/sieve plates)

• Sieve tube elements contain little cytoplasm, no nucleus and very few organelles for more space for transport of materials

• Sieve tube elements - associated with companion cells to help with living functions which are biochemically active which contain a large nucleus, dense cytoplasm and many mitochondria - the cells are connected to the sieve tube elements by cytoplasmic connections = plasmodesmata

Translocation = bidirectional unlike transpiration = unidirectional

Transfer of Sucrose into sieve elements

• sucrose is synthesised from products of photosynthesis in cells with chloroplasts

• the sucrose diffuses down a conc gradient by facilitated diffusion from photosynthesising cells into companion cells

• H ions actively transported from companion cells into spaces within cell walls using ATP

• H ions the diffuse down the conc gradient through carrier proteins into sieve tube elements

• Sucrose molecules are transported with H ions by co transport - protein carriers = co transport proteins

Mass Flow Theory

Mass flow of sucrose through sieve tube elements - bulk movement of a substance through a channel/area in a specified time

• The sucrose if produced during photosynthesis and is actively transported into sieve tubes

• This causes sieve tubes to have a lower WP

• The xylem have a much higher WP - water moves from xylem → sieve tubes by osmosis = high hydrostatic pressure

• At respiring cells - sucrose is actively transported from sieve tubes elements through companion cells into sink cells - used in respiration or converted into starch for storage

• This reduces WP in part of the sieve tube elements and water leaves the tubes by osmosis → xylem

• Hydrostatic pressure of sieve tubes are then lowered

• Water entering the sieve tube elements at the source and leave at the sink = high hydrostatic pressure at the source and low at the sink

• Therefore mass flow of sucrose solution down the hydrostatic pressure gradient in sieve tubes

Evidence for/against mass flow theory

For :

• Hydrostatic pressure in the phloem - release of sap when it is cut

• Conc of sucrose is higher in leaves/source than in the roots/sink

• Flow in the phloem during day and reduces during night/shaded

• Increase in sucrose → increases in phloem

• Metabolic inhibitors/ lack of oxygen inhibits translocation of sucrose in phloem

• Companion cells have many mitochondria and produce ATP readily

Against :

• Theory makes function of sieve plates unclear - would hinder mass flow of sucrose → suggested that they have a structural function to prevent bursting of sieve tubes under pressure

• Not all solutes move at the same speed → they should all move the same speed during mass flow

• Sucrose is moved at around the same rate to all sinks → rather than faster at sinks with a lower sucrose conc that mass flow suggests

Investigating Transport in Plants

1. Ringing Experiments

• Complete ring of bark is removed - contains phloem whilst leaving the xylem intact = downward movement of sugars is prevented → region of the stem above missing phloem swells with liquid with high sucrose/ dissolved organic substances

• Some non-photosynthetic tissues below the area wither and die but the area above continues to grow

• Conclusion = phloem is the tissue responsible for translocating sugars downwards in plants - flow would continue if the xylem transported sugars

2. Tracer Experiments

• Isotope 14C → forms radioactive carbon dioxide → atmosphere will then contain 14C then the 14C will be used in forming sugars during photosynthesis

• Radioactive sugars then traced as they move using autoradiography - film goes black where the plant have been exposed to the radiation in the 14C sugars

• Blackened regions are found to show where the phloem is located in the stem as other tissues don’t blacken the film which shows that the phloem is responsible for translocation of sugars

3. Puncture Experiments

• If the phloem is punctured with a hollow tube the sap comes out → shows high pressure inside the phloem

• If the xylem is punctured the air is sucked in → shows low pressure inside the xylem

• Aphids - greenfly have specialised mouthparts which use the penetrate the phloem tubes - separates phloem sap from carbon dioxide