biology module 3 transport in plants

Adaptations of plants to increase SA: V ratio

• Plants have a branching body shape

• Leaves are flat and thin

• Roots have root hairs

function of xylem tissue

• Vascular tissue that carries dissolved minerals and water up the plant

• Structural support

• Food storage

where is xylem tissue found

in vascular bundles

location of vascular bundles in xylem

• In the roots the vascular bundle is found in the centre and the centre core of this is xylem tissue. This helps the roots withstand the pulling strains they are subjected to as the plant transports water upwards and grows

• In the stems the vascular bundles are located around the outside and the xylem tissue is found on the inside (closest to the centre of the stem) to help support the plant

• In the leaves the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The xylem tissue is found on the upper side of the bundles (closest to the upper epidermis)

function of phloem tissue

• Transport organic compounds, particularly sucrose, from the source (eg. leaf) to the sink (eg. roots). The transport of these compounds can occur up and down the plant

location of vascular bundles in phloem

In the roots the vascular bundle is found in the centre and on the edges of the centre core is the phloem tissue

In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis)

In the leaves, the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)

what are the four cell type that make up the xylem

1. Tracheids (long, narrow tapered cells with pits)

2. Vessel elements (large with thickened cell walls and no end plates when mature)

3. Xylem parenchyma

4. Sclerenchyma cells (fibres and sclereids)

lignified cell walls in xylem

adds strength to withstand the hydrostatic pressure so the vessels do not collapse, impermeable to water

no end plates in xylem

allows the mass flow of water and dissolved solutes as cohesive and adhesive forces are not impeded

no protoplasm in xylem

doesn’t impede the mass flow of water and dissolved solutes

pits in wall in xylem

lateral movement of water allows continual flow in case of air bubbles forming in the vessels

small diameter of vessels in xylem

helps prevent the water column from breaking and assists with capillary action

what are sieve tube elements in the phloem

main conducting cells, line up end to end to form a continuous tube

sieve plates with sieve pores in sieve tube elements

allows for continuous movement of the organic compounds

cellulose cell wall in sieve tube elements

strengthens the wall to withstand the hydrostatic pressures that move the assimilates

no nucleus, vacuole, or ribosomes in sieve tube elements

maximises space for the translocation of the assimilates

thin cytoplasm in sieve tube elements

reduces friction to facilitate the movement of assimilates

companion cells

Each sieve tube element has a companion cell associated with it as companion cells control the metabolism of their associated sieve tube member

They also play a role in loading and unloading of sugars into the phloem

nucleus and other organelles present in companion cells

provides metabolic support to sieve tube elements and helps with loading and unloading of assimilates

transport proteins in plasma membrane of companion cells

moves assimilates into and out the sieve tube elements

large numbers of mitochondria

to provide ATP for the active transport of assimilates into or out of the companion cells

plasmodesmata

the link to sieve tube elements which allows organic compounds to move from the companion cells into the sieve tube elements

dicotyledonous plants

Seeds that contain two cotyledons (seed leaves)

Network of veins

Leaves that typically have broad blades (leaf surface) and petioles (stalks)

Tap root with lateral branches

what is transpiration

the loss of water vapour from a plant to its environment by evaporation and diffusion, its a consequence of gaseous exchange at the stomata

advantage of transpiration

It provides a means of cooling the plant via evaporative cooling

The transpiration stream is helpful in the uptake of mineral ions

The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

what is a transpiration stream

The transpiration stream refers to the movement of water from the roots to the leaves

The evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules causes the movement of water through a plants xylem

It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells

air movement affecting rate of transpiration

• There is usually a lower concentration of water molecules in the air outside the leaf

• When the air is relatively still water molecules can accumulate near the leaf surface. This creates a local area of high humidity which lowers the concentration gradient and the rate of transpiration

• Air currents can sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration

temp affecting rate of transpiration

• There is usually a lower concentration of water molecules in the air outside the leaf

• When the air is relatively still water molecules can accumulate near the leaf surface. This creates a local area of high humidity which lowers the concentration gradient and the rate of transpiration

• Air currents can sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration

light intensity affecting rate of transpiration

• Stomata close in the dark, their closure greatly reduces the rate of transpiration

• When the light is sufficient for the stomata to open, the rate of transpiration increases

• Once the stomata are open any increase in light intensity has no effect on the rate of transpiration

• Stomata will remain open at relatively low light intensities

humidity affecting rate of transpiration

• If the humidity is high that means there is a large concentration of water molecules in the air surrounding the leaf surface

• This reduces the concentration gradient between inside the leaf and the outside air which causes the rate of transpiration to decrease

• At a certain level of humidity, an equilibrium is reached; there is no concentration gradient and so there is no net loss of water vapour from the leaves

apoplast pathway

• Water moving on the apoplast, or apoplastic, pathway travels within the cell walls and intercellular spaces of plant tissue

  • Note that this is not osmosis because the water does not cross any cell membranes

• Water is drawn across the root via the apoplast pathway due to cohesive forces between water molecules:

  • Water moves upwards in the xylem due to transpiration

    • Cohesion between water molecules means that more water is drawn along the apoplast pathway within the root to replace the water that has moved upwards

symplast pathway

• Water moving on the symplast, or symplastic, pathway travels via cell cytoplasm and vacuoles

  • Water enters the symplast pathway and moves between cells, and into cell vacuoles, by osmosis

  • Water can also move from cell to cell by diffusion via the plasmodesmata

• Water is drawn across the root via the symplast pathway as follows:

  • water moves into root hair cells from the soil by osmosis, increasing the water potential of the root hair cell

  • water moves down its water potential gradient into neighbouring root cells, increasing their water potential

  • water continues to move across the root from high to low water potential

water movement into the xylem

• When water reaches the centre of the root it must cross the endodermis to enter the xylem

• The cells of the endodermis are surrounded by a waxy band known as the Casparian strip, which forms an impassable barrier to water

  • The waxy material is known as suberin

• The Casparian strip blocks the cell walls of the endodermis cells, preventing water from entering the xylem via the apoplast pathway and instead forcing it into the symplast pathway

  • it is thought that this may help the plant control which mineral ions reach the xylem

water movement in the xylem

• Water is drawn upwards in the xylem due to transpiration as follows:

  • water evaporates from the surface of cells in the leaves, lowering the water potential of leaf cells

  • water is drawn out of the xylem and into leaf cells by osmosis down its water potential gradient

  • more water molecules are drawn upwards in the xylem in a continuous column due to forces of cohesion between water molecules

    • attractive forces of adhesion between water molecules and the sides of the xylem aid this process

  • The upward movement of water in the xylem is known as the transpiration stream

movement of water in the leaves

• Water moves through the leaves of plants due to transpiration as follows:

  • Water vapour diffuses out of leaf air spaces and into the surrounding environment down a water vapour potential gradient

  • The loss of water vapour from the air spaces creates a water potential gradient between leaf mesophyll cells and the leaf air spaces, so more water moves from the leaf mesophyll cells into the air spaces

    • Water first moves from the cell cytoplasm to the cell surface, before evaporating into the air space

  • Losing water lowers the water potential of the leaf mesophyll cells, so water moves into the cells by osmosis from neighbouring cells and the xylem

• Note that water movement through the leaf also occurs via the apoplast and symplast pathways

translocation in the phloem

the transport of assimilates from source to sink and requires the input of metabolic energy (ATP)

source of assimilates

• The source of the assimilates could be:

  • Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose)

  • Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)

  • Food stores in seeds (which are germinating)

the sinks

• The sinks (where the assimilates are required) could be:

  • Meristems (apical or lateral) that are actively dividing

  • Roots that are growing and / or actively absorbing mineral ions

  • Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)

why are carbohydrates are generally transported in plants in the form of sucrose

• It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)

• It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported

loading of assimilates eg. sucrose

• The pathway that sucrose molecules use to travel to the sieve tubes is not fully understood yet. The molecules may move by the:

  • symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion

  • apoplastic pathway (through the cell walls) which is an active process

• If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an active process and therefore requires ATP as an energy source

• The large concentration of hydrogen ions in the cell wall of the companion cell results in the hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell

• The hydrogen ions move through a cotransporter protein. While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose

• The sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells

• Companion cells have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pump

• This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the mesophyll

unloading of assimilates eg. sucrose

• The unloading of the assimilates (eg. sucrose) occurs at the sinks

• Scientists believe that the unloading of sucrose is similar to the loading of sucrose, with the sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways

• To maintain a concentration gradient in the sink tissue, sucrose is converted into other molecules. This is a metabolic reaction so requires enzymes (eg. invertase which hydrolyses sucrose into glucose and fructose)

why does mass flow occurs

due to the presence of a hydrostatic pressure gradient

• The pressure gradient is generated by actively loading sucrose into the sieve elements at the source; this lowers the water potential in the sieve tube

• Water moves into the sieve elements by osmosis; this increases the hydrostatic pressure at the source

• At the same time, solutes are unloaded from the sieve elements at the sink, causing water to follow by osmosis; this lowers the hydrostatic pressure at the sink

• The difference in hydrostatic pressure between the source and the sink creates a hydrostatic pressure gradient

what are xerophytes

• Xerophytes (from the Greek xero for ‘dry’) are plants that are adapted to dry and arid conditions

• Xerophytes have physiological and structural (xeromorphic) adaptations to maximise water conservation

xerophytic adaptations