Transport in plants

How Plants Get Water and Nutrients

Plants absorb nutrients and water through their roots, but photosynthesis — the process by which plants create their fuel — occurs in the leaves. Therefore, plants need to get fluids and nutrients from the ground up through their stems to their parts that are above ground level.

The veins of a maple leaf. 

The leaves of plants also contain veins, through which nutrients and hormones travel to reach the
cells throughout the leaf. Veins are easy to see some leaves (a maple tree, for instance). In some plants the veins are hard to see, but they're in there.

Sap is the mix of water and minerals that move through the xylem. Carbohydrates move through the phloem. There are several different "modes of transportation" through the xylem and phloem; their main function is to keep all cells of the plant hydrated and nourished.

On this page we will be following the flow of water from the ground up, from the soil, into the roots and to the leaves.

Tissue systems.

The dermal tissue system covers the exterior of a plant.

The vascular tissue system transports materials between roots and shoots.

The ground tissue system is located between the dermal tissue and the vascular tissue, and is responsible for most of the plant's metabolic functions.

Vascular tissues in plants

Just as animals, plants also contain vascular tissues (xylem), which transports water and minerals up from the roots to the leaves, and phloem, which transports sugar molecules, amino acids, and hormones both up and down through the plant.

Xylem & phloem tissues run close to each other. Together they form two different structures, depending on where they are in the plant.  In the stem and leaves they form the vascular bundle, and in the roots they form the Stele.

The Stele: Vascular tissue in the root. 

Vascular tissue in the stem. 

Transpiration - How plants win against gravity. 

How does water enter the plant?

Water enters the roots by osmosis, is pulled up the xylem and the water is pulled out of the stomata in the leaf via evaporation. 

Its starts at the roots. 

Water enters the plants through the root hairs (thin, permeable, large surface area), moving down a water potential gradient (by osmosis).

Why?

Because the vacuoles of root cells contain a strong solution of dissolved substances, hence a lower water potential than the soil. So naturally the water travels via osmosis into the root hair. 

Water (and dissolved minerals) need to get from the root hair in the epidermal layer, through the cortex and into the xylem to be carried up to the leaves. 

There are three possible pathways for the water to travel through. 

Apoplast : between the root cells along the permeable cell walls. This is the fastest as it travels along the network of fully permeable cell walls. 

Symplast: through the cell membranes, cytoplasm or plasmodesmata (gaps in the cell walls).

Vacuolar: from vacuole to vacuole.

As the water approaches the xylem, the apoplast pathway is blocked by a layer of cells (endodermis) surrounding the pericycle. 

Endodermis cells have thickened walls with suberin (impermeable). 

Suberin forms a band around these cells called Casparian strip. 

The water travelling along the apoplastic pathway can't move past this strip in the walls of the endodermis, so it is forced into the symplast pathway and through it’s pits, it enters the xylem vessel. This provides the plant with control as to which minerals can enter the xylem, and thus the plant. It could transport minerals such as nitrate ions and ammonium ions via active transport. 

The water moves from the endodermal cells into the xylem. 

It is thought that the endodermal cells actively secrete salts into the xylem, giving the xylem a more negative water potential. 
This causes the water to flow into the xylem from the cortex. 










This flow of water into the xylem causes the pressure to increase inside the xylem, this is called hydrostatic pressure and it pushes the water in the xylem up. The pressure pushing the water up from the roots is called root pressure. This is ONE of the theories that has been proposed to explain how water travels up the plant. 


You don't need to know this, but it is interesting. 

Nitrogen transport into the plant. 

Nitrogen usually enters the plant as nitrate ions/ammonium ions which diffuse along the concentration gradient into the apoplast stream. 

The nitrate and ammonium ions enter the symplast by active transport against the concentration gradient. 

The nitrates and ammonium then flows via plasmodesmata in the cytoplasmic stream.

AGAIN This lowers the water potential in the xylem, causing water to be drawn through the endodermis.

The Xylem: The water channels of the plant.

The xylem is a long continuous tube that runs the full length of the plant from the roots to the leaves, and right up to the plant apex. Water and solutes travel up through this straw like structure very easily.

Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the plant. For this reason, the transport systems are arranged differently in root and stem. 

In the root it has to resist forces that could pull the plant out of the ground. In the stem it has to resist compression and bending forces caused by the weight of the plant and the wind.

The structure of xylem tissue

4 different types of cells make up the xylem. These are xylem vessel elements, tracheid, fibres and xylem parenchyma. Of all four, only the xylem parenchyma are living cells. 

Parenchyma - cells with thin walls generally used as a storage.

Fibres – long cells with thickened cell walls (these help with the support of the plant).

Tracheids: Dead cells that have lignin and form a system, with vessel elements, to transport water.

Xylem vessel elements -Dead cells (walls made of lignin, impermeable to water). 

The vessels and tracheids form a system of tubes through which the water can travel. 

• As they develop, the vessels and tracheids incorporate lignin into their cellulose walls. 
• The lignin is hard, strong and waterproof. This build up and causes the cells to die. 
• There is an empty space left inside as the cytoplasm leaves and the end walls break down. This leaves a continuous tube for the water to travel through. 

The lignin strengthens the xylem and causes the xylem to be water proof. 

Gaps in the walls called pits to allow movements between vessels and/or living tissues nearby.

Transpiration - Evaporation from the leaves - pulls the water up the xylem. 

In order to move water against the pull of gravity, sometimes many meters up, the plants need to use some pretty hefty forces.

The forces involved are

• Transpirational Pull
• Cohesion and Adhesion
• Capillarity
• Root Pressure

Watch this video for an overview.

                                                

1. Transpirational Pull (Transpiration)

The evaporation and therefore loss of water through the stomata of the leaves, leaves a ‘gap’ inside the leaf tissue. 

This gap has a lower water potential as it is always losing water. Water travels down its water potential gradient into the air space, from the xylem. 

The water travels through the leaf tissue from the xylem into the air space the same way in which water travels from the root hairs to the xylem, that is through the symplastic, apoplastic and vacuolar pathways.

 

2.       Cohesion and Adhesion forces.

As the water is pulled out of the xylem by transpiration, a whole column of water is pulled along with it. A column that extends all the way back down the xylem to the roots. This column, of course is made by water molecules, but what causes these water molecules to stick together to form a column that can be pulled all at once? It is due to the special properties of water.

Cohesion

The oxygen atoms in the water molecule have small negative charges. The hydrogen atoms have small positive charges. These opposite charges attract in a very important biological molecular attraction called a hydrogen bond.

As one water molecule gets pulled up by the transpiration pull, it pulls all the water molecules it has hydrogen bonds with along with it, creating one long unbroken column of water.

            Adhesion

The same small positive and negative charges on the water molecule allow it to adhere (stick) to the opposing charges on the fibrous walls of the xylem vessels. These vessels are called hydrophilic because they are attracted to water molecules.

 The theory that water moves up the xylem using transpirational pull, adhesion and cohesion is called the Cohesion-tension theory.

There are two other theories to explain how water moves up plants.

3. Root pressure

In the roots, endodermis cells (around the xylem vessels) actively transport mineral ions into the xylem, reducing its water potential. As water is drawn in, the hydrostatic pressure in the root increases and water is pushed upwards.

4. Capillarity

Water molecules ‘climb up’ in narrow tubes (µm) because they are attracted (adhesion) to molecules with opposite charges (polar) of the tube. The narrower the tube the higher the water goes. It is thought this mainly occurs in small plants.

Environmental factors that affect the rate of transpiration.

Click here to do a virtual lab investigating the environmental factors. 

About 99% of water absorbed by the plant can be lost by evaporation. If a plant loses more water than it absorbs, it wilts. If a plant loses an excessive amount of water, it reaches a point where it cannot regain its turgor, and it dies. But how does it keep water in? During the day the stomata in the leaf need to be open in order for the exchange of gases between the leaf and the atmosphere. However the presence of open pores in the leaf mean it is vulnerable to water loss.

Leaves lose water at different rates, depending on what environmental conditions they are in. The different conditions that will affect the rate of transpiration (the rate at which water is lost from the leaves) are:

-       Temperature

-       Humidity

-       Air movement

Temperature

            When the temperature is hot, the kinetic energy of the water molecules increases (they get faster). This means the acceleration of evaporation of water from the walls of the mesophyll cells and if the stomata are open, the loss of water from the leaf. As the water moves away from the leaf at a faster rate, the water potential outside the leaf decreases, so it can hold more moisture. More water is lost from the leaf to compensate.

Humidity

When the atmosphere is humid, it contains a lot of water molecules. This means that the difference in water potential between the inside of the leaf and the atmosphere is not as great. The rate of transpiration decreases because the water molecules are not moving down the water potential gradient as fast as they have previously.

Air movement.

When the air is still around a leaf, the water vapour moving out of the leaf increases the humidity of the atmosphere directly next to the stomata. This protective layer of water vapour, slows the rate of transpiration down. However if the air next to the leaf is moving, such as in windy conditions, the water vapour is moved away from the stomata and the rate of transpiration will increase.

Light also influences the rate of transpiration due to the fact that light influences the opening and closing of stomata, so as a results impacts upon the rate of transpiration.

Click on the screen shot below to get an interactive overview of Water Transport. 

Adaptations of plants to survive different water supplies.

Plants can be classified due to the adaptations they have to survive with regards to water supply.

Hydrophytes – plants that survive in an excess of water.

Xerophytes   - adapted to survive in an environment that lacks water.

Mesophytes - terrestrial plants which are adapted to neither a particularly dry nor particularly wet environment. 

Hydrophytes

e.g. water lily, live with their roots submerged in the mud

 at the bottom of a pond and have floating leaves on the surface 

Hydrophytes are plants that have adapted to living in aquatic environments (saltwater or freshwater). These plants require special adaptations for living submerged in water, or at the water's surface. Aquatic plants can only grow in water or in soil that is permanently saturated with water. 

As hydrophytes do not have a problem retaining water, due to the abundance of water in their environment, they have less need to regulate transpiration, which would require more energy and be of little benefit to the plant. They also have a number of adaptations to keep them afloat so the leaves are able to photosynthesise efficiently.

Characteristics of aquatic plants

• Thin cuticle on outside of leaf. Thick cuticles reduce water loss from the epidermis; thus most hydrophytes have no need for thick cuticles.
• Stomata that are open most of the time, because water is abundant and there is no need for it to be retained in the plant. This means that guard cells on the stomata are generally inactive.
• Stomata on the upper side of the leaves.
• A less rigid support structure as water pressure supports them.
• Flat leaves on surface plants for flotation.
• Large air spaces in the stem and leaf tissue for flotation.
• Smaller roots: water can travel directly into leaves through osmosis; thus large root systems are not required for water uptake.
• Feathery roots: no need to support the plant.

Xerophytes

In dry environments, a typical plant would evaporate water faster than the rate at which water was replaced in the soil, leading to wilting. To reduce this effect, xerophytic plants exhibit a variety of specialized adaptations to survive in such conditions.

• ·      They may absorb water from their own storage, allocate water specifically to sites of new tissue growth lose less water to the atmosphere and so convert a greater proportion of water in the soil to growth or have other adaptations to manage water supply and enable them to survive.

Eg Marram Grass 

Like other Xerophytes, Marram Grass is well adapted to its surroundings in order to thrive in an otherwise harsh environment.

Marram grass lives in very dry conditions that increase loss of water, sandy conditions drain water quickly, and very windy conditions will further increase rates of transpiration. To survive, Marram grass has a number of adaptations to help it survive.

·       a rolled leaf. This creates a localized environment of water vapour potential within the leaf, and helps to prevent losses of this precious water.

·       The stomata sit in small pits within the curls of the structure, which make them less likely to open and to lose water.

·       The folded leaves have hairs on the inside to slow or stop air movement. This slowing of air movement once again reduces the amount of water vapour being lost.

·       A waxy Plant cuticle on the leaf surface also prevents evaporation from the leaf surface.

Mesophytes

Mesophytes are plants of temperate regions and flourish in habitats with 

adequate water supply. They need to survive unfavourable times of the year

by shedding their leaves, surviving underground or as dormant seeds.

Because they live in temperate zones, they need to withstand hot conditions, dry conditions and dry conditions, depending on the time of year/season. These plants are found in average conditions of temperature and moisture and grow in soil that has no water logging.

General characteristics. Mesophytes generally require a more or less continuous water supply. They usually have larger, thinner leaves compared to xerophytes, sometimes with a greater number of stomata on the undersides of leaves.

Dry conditions: Because of their lack of particular xeromorphic adaptations, when they are exposed to extreme conditions they lose water rapidly, and are not tolerant of drought. They avoid excessive moisture loss by closing their stomata and tend to make up for any water lost during the day by replenishing the supply at night.

Cold conditions where ground is frozen:

• ·      shed their leaves – no longer needed as the sunlight over summer has produced enough starch for the plants energy needs over winter. Leaves use up energy needlessly as there is not enough sunlight in winter to make them useful.
• ·      Some plants only survive underground as a bulb or a corm. The parts of the plant that are above ground die as a result of the cold wind or frost. An underground organ is protected from the cold wind and can survive.
• ·      Some plants (called annuals) grow, flower and die in the same year. They survive the winter as a dormant seed in the ground, to flourish once the summer arrives again.

Translocation - the movement of sugars from source to sink. 

The purpose of translocation

The products of photosynthesis are made in the leaf, but needed in other parts of the plant. 

The leaves are called the source as they are the source of the useful products (sucrose/glucose). These products are transported to parts of the plant that need them to grow or a place of storage called the sink. 

The transport of these products is called translocation, and the vessel that they are transported along is called the phloem.

The structure of the phloem

Like the xylem, the phloem is made of several types of cells.

Sieve tubes: adapted for the transport of the sucrose and glucose

Made of cells called sieve elements, that stand end on end.

·      

     Sieve elements still have their end walls, but these walls are perforated with pores. 

     The ends of sieve elements are called sieve plates.

      Long filaments made of phloem protein, extend from the cytoplasm of one tube element to the next, through the pores in the sieve plate.  Sieve elements have no nucleus and most of their organelles disintegrate during their development, but they are still alive. For this reason each sieve element needs a companion cell to help it survive.

Companion cells

·      Dense cytoplasm.

·      Large central nucleus.

·      Many mitochondria.

·      Connected to the sieve tube element by plasmodesmata.

Experimental evidence that the phloem is involved in translocation.

Bark ringing experiments: 

The role of bark (phloem) in sugar movement in plants. Mason and Maskell (1928) demonstrated that removing a complete ring of bark (a) while leaving the wood (xylem) intact prevented downward movement of sugars. When a strip of bark was retained between upper and lower stem parts (b), sugars flowed downwards in direct proportion to the width of the remaining bark.

Radiotracing and detection using aphid mouth parts:

1.    The plant is given CO₂ that is radioactively labeled and put in a bright place.

2.    The plant incorporates that CO₂ into any glucose it is producing via photosynthesis. This glucose is now radioactive and able to be followed though the plant.

Detection of radioactive sugars using photographic paper.

The products can be detected at the source and different sinks by placing these on photographic paper and then developing the paper. Any radioactivity is detected by fogging on the negatives.

Results: when a cross section of the stem is pressed against the photographic material, fogging only occurs where the phloem tissue was in contact with the film.

Detection of radioactive sugars using aphids

Aphids place their stylets directly into the phloem. In order to collect the contents of the phloem, scientists allow the aphids to do this, then under humane conditions, remove the mouthparts from the aphid, leaving the mouthparts in the phloem. Any sugars traveling down the phloem, are pushed out of the phloem through the stylet so the scientists can collect them very easily.

Scientists have used this to see how long it takes radioactive carbon dioxide to be integrated into sugars and then translocated. This has been observed to be more rapid than could be accounted for by diffusion.

The presence of radioactive sugars in the phloem by this method (as this is the only place that stylet has been inserted) has shown that it is indeed the phloem that transports the photosynthetic products.