Transport in plants (3.1.3)

Dicotyledonous plants (dicots) produce seeds with two cotyledons (‘seed leaves’), which store food for the developing embryo and form the first leaves.

Dicots have a vascular system of xylem and phloem running through their stems, roots, and leaves. These transport tissues are arranged in vascular bundles.

The xylem and phloem are located in the centre of the dicot root. In a transverse cross-section, xylem forms a cross or star-shaped pattern, while the phloem is situated between the arms of the xylem cross.

This arrangement provides structural support. It helps the root push through soil and gives the plant strength to withstand strains from stems and leaves blowing in the wind.

In dicot leaves, the xylem and phloem form a network of veins that provide both mechanical support and efficient transport within the leaf structure.

The xylem is usually located on the upper side of each vein, while the phloem is found on the lower side.

In a transverse cross-section of dicot stems, the vascular bundles are located near the outside of the stem in a ring arrangement.

The xylem is on the inner side of each vascular bundle, while the phloem is on the outer side.
This arrangement provides a scaffolding effect, reducing bending while still allowing some flexibility, so the stem does not snap.

In a longitudinal section of a dicot stem, the xylem and phloem run as continuous strands along the length of the stem.

The xylem is on the inner side of each vascular bundle, while the phloem is on the outer side.

Meristematic tissue contains unspecialised stem cells that elongate and differentiate into xylem or phloem, allowing growth throughout the plant’s life.

This tissue can be found in the vascular cambium, located between xylem and phloem in the vascular bundles of stems.

To dissect and prepare a stem:

1. Use a sharp scalpel to cut a thin transverse or longitudinal section.
2. Place the sample on a microscope slide with a drop of water (wet mount).
3. Use toluidine blue (TBO) to stain the xylem blue and phloem pink.
4. Place a coverslip over the sample and gently squash it, allowing light to pass through for microscopy.

Xylem vessels can also be seen in plants like celery by placing cut stems in coloured dye for 24 hours. The dye moves up the stem with the transpiration stream.

Transverse and longitudinal sections show dye-filled xylem, demonstrating the pathway of water from roots to leaves.

Xylem is a tissue made of cells joined end to end with no end walls between them. This creates a long, hollow tube structure that allows continuous, uninterrupted water transport.

The xylem cells that make up the vessels are dead, meaning they contain no cytoplasm or organelles. This gives the xylem an empty lumen, which reduces resistance to the flow of water.

The vessels are narrow, helping water rise by capillary action.

Xylem vessel walls are impregnated with spirals of lignin around the lumen of the vessel. The spiral pattern provides flexibility, while lignin provides support and additional mechanical strength, preventing the vessels from collapsing inwards under the tension generated during water transport.

Lignin is also waterproof, which minimises water loss from the vessel.

Pits within the walls of the xylem are non-lignified, allowing lateral movement of water between other cells and xylem vessels.

Water is absorbed into the root by osmosis, down a water potential gradient.

This gradient is established when mineral ions are actively transported from the soil into the cytoplasm of root hair cells, making the water potential inside the root lower than in the soil.

To reach the xylem, water moves from the root hair cells through the cortex and endodermis through one of two routes: the symplast or apoplast pathway.

In the symplast pathway, water diffuses through the plasma membrane of root cells and enters the cytoplasm. It then moves from cell to cell through specialised cell junctions called plasmodesmata.

Water moves from a region of higher water potential to lower water potential by osmosis, down a water potential gradient.

This pathway is slow because entry into the pathway is regulated by crossing the plasma membrane of the root hair cell, which creates greater resistance to movement.

In the apoplast pathway, water moves between cells through the cell walls and intercellular spaces.

Water fills the spaces within the loose network of cellulose fibres in plant cell walls.

Adhesion occurs between water molecules and the cell walls, helping water move through the walls by capillary action.

Cohesion between water molecules also helps pull additional water molecules along.

Water moves down a pressure gradient by mass flow. This pathway is faster because water does not initially cross cell membranes.

When water moving through the apoplast pathway reaches the endodermis, it is blocked by a waterproof barrier called the Casparian strip. This strip contains suberin, which prevents water from continuing through the cell walls.

As a result, water must enter the cytoplasm of the endodermal cells, forcing it into the symplast pathway before it can enter the xylem.

Crossing the plasma membrane at this point allows the plant to control which substances enter the xylem, preventing harmful or toxic substances from being transported.

Mineral ions are actively pumped into the xylem vessels from the endodermis of the roots against their concentration gradient using carrier proteins / ion pumps, which requires ATP.

By lowering the water potential in the xylem, water moves in from the endodermis through the symplastic pathway by osmosis.

The inflow of water generates pressure, giving the water a small push upward in the xylem.

Stomata are pores mainly on the underside of leaves that allow carbon dioxide to enter and oxygen to leave the plant.

Each stomatal pore is surrounded by kidney-shaped guard cells with a thicker inner wall and thinner outer wall.

When guard cells take in water and become turgid, they swell and bend, opening the stomata. This happens during the day when photosynthesis occurs in the presence of sunlight.

When guard cells lose water and become flaccid, the stomata close. This happens at night when photosynthesis cannot occur.

Transpiration is the loss of water vapour from a plant by evaporation from the leaf and diffusion into the atmosphere through the stomata.

It occurs as a consequence of gas exchange through the stomata.

Water evaporates from the surfaces of the spongy mesophyll cells into the air spaces within the leaf and then diffuses out of the leaf through the open stomata down a water vapour potential gradient.

In the cohesion–tension theory, transpiration reduces the water potential in the mesophyll cells, causing water to move by osmosis from the xylem into these cells. This creates tension (negative pressure) in the xylem, causing water to move down a pressure gradient by mass flow from roots to leaves.

In the cohesion–tension theory, the cohesion between water molecules, forms a continuous column of water in the xylem, which is pulled upwards as a result of the tension created by transpiration.

Adhesion occurs between water molecules and the walls of the xylem vessels, helping the water column move up the vessel by capillary action and resist the effects of gravity.

A potometer is an apparatus that measures a plant’s water uptake. It is assumed that water uptake is directly related to water loss by the leaves, and therefore, it can be used to measure the rate of transpiration.

To estimate the rate of transpiration per of leaf surface area:

1. Flatten the leaves onto graph paper and draw around the leaf.
2. Count the number of squares covered by the leaf, including only squares that are more than 50 covered.
3. Double the leaf area to account for both the upper and lower leaf surfaces.
4. Divide your calculated transpiration rate by the leaf area to get the rate per .

The rate of transpiration can be measured by recording the mass loss of a plant over time.

The plant is placed on a mass balance and weighed at regular intervals. As water evaporates from the leaves, the plant’s mass decreases, giving an estimate of the transpiration rate.

Xerophytes have several adaptations to reduce water loss.

Marram grass has sunken stomata, leaf hairs, and rolled leaves, which help reduce transpiration by:

• reducing air movement and trapping water vapour, increasing humidity around the stomata. This lowers the water potential gradient between the leaf and the surrounding air, reducing water loss.
• reducing the surface area exposed for transpiration.

Xerophytes have several adaptations to reduce water loss.

Cacti reduce water loss by:

• Having spines instead of leaves, reducing their surface area:volume ratio and therefore reducing transpiration.
• Closing their stomata during the hottest part of the day.
• Having a thick, waxy cuticle, which is waterproof. Its thickness increases the diffusion distance, while the wax layer reduces evaporation from the leaf surface.

Hydrophytes are plants which live in aquatic environments, such as water lilies.

Given that water is abundant in their environment, adaptations to reduce water loss are not needed.

Hydrophytes have large air spaces in their leaves and stems, which help them float on water and increase the amount of light they receive for photosynthesis. These air spaces can also store oxygen for respiration.

The stomata are mainly located on the upper surface of the leaf, allowing gas exchange to occur with the air, where the carbon dioxide concentration is higher.

Hydrophytes have flexible stems and leaves, which are supported by the surrounding water and can bend with water movement, preventing damage.

The waxy cuticle of the leaf is very thin or absent because water loss by transpiration is not an issue.

The leaves have a large surface area to maximise photosynthesis. They may also have a large number of stomata, which can remain open for longer periods to maximise gas exchange.

Sieve tube elements are cells stacked end to end to form long, hollow, continuous tubes, which allow mass flow of assimilates with minimal resistance. Although sieve tube elements are living cells, they contain very little cytoplasm, no nucleus, and few organelles, further reducing resistance and facilitating the flow of substances.

The end walls of sieve tube elements, called sieve plates, contain pores that allow assimilates to pass through and connect the cytoplasm of adjacent cells. The sieve plates also limit the size of molecules that can pass and slightly reduce the rate of flow.

The cell walls of sieve tube elements are thin, not lignified (cellulose only), and do not contain pits.

Companion cells act as the “life support” for sieve tube elements, carrying out the living functions. They contain dense cytoplasm, a large nucleus, and many mitochondria, and are connected to the sieve tube elements via plasmodesmata.

The mitochondria and other organelles provide energy for active transport, allowing the companion cells to load sucrose from the source (where assimilates originate) into the sieve tube elements through active loading.

Sucrose moves between leaf cells via the passive symplast pathway through plasmodesmata or the active apoplast pathway through cell walls and intercellular spaces.

For the active loading of sucrose into the companion cells from the surrounding tissue, protons
() are actively pumped out using ATP and a proton pump. This creates a proton gradient.

Protons then re-enter the cell via co-transport proteins, carrying sucrose molecules with them. This process actively loads sucrose into companion cells, from where it diffuses into sieve tube elements for translocation.

The entry of sucrose into sieve tube elements at the source decreases the water potential in the sieve tube elements. This causes water to enter the phloem by osmosis from the surrounding cells and xylem vessels. This results in higher hydrostatic pressure within the sieve tube elements.

When the sucrose arrives at the sink, it diffuses out of the sieve tube elements into the surrounding cells. This causes the water potential inside the sieve tube elements to increase so water leaves by osmosis down a water potential gradient. This lowers the hydrostatic pressure.