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Transport in Plants #2

Transport in Plants

Plants need a transport system to:

  • Move water and minerals from the roots up to the leaves
  • Move sugars from the leaves to the rest of the plant

The Vascular Tissues

Water and soluble mineral ions travel upwards in the xylem tissue

Assimilates such as sugars travel in both directions in the phloem tissues

In a dicotyledonous plant (one that has two seed leaves) the xylem and phloem are present in the vascular bundle along with other types of tissue such as collenchyma and sclerenchyma.

Meristem – a layer of dividing cells, also called the pericycle

Cambium – the layer of meristem that divide to produce new xylem and phloem cells

Xylem

  • Long cell with thick lignified walls
  • The lignin waterproofs the walls of the xylem and causes the cell to die
  • The end walls of the xylem cells then decay and the contents leaves the cells
  • It is them a tube with no end walls called a xylem vessel
  • Lignin strengthens the vessel walls and prevents collapse when water levels are low
  • The spiralling patterns of lignin allow flexibility to the stems and branches of the plant
  • In some place the lignin is not complete and gaps called bordered pits form which connect the xylem vessel to other xylem vessels and to the parenchyma

Phloem

  • Made of sieve tube elements and companion cells
  • Sieve tube elements transport sugars and assimilates.
  • When mature the sieve tubes lack a nucleus and contain very few organelles
  • Companion cells subsidise their metabolic needs
  • The sieve tube does contain a smooth endoplasmic reticulum which can be found at the plasma membrane and near the plasmodesmata
  • All sieve tube cells have groups of pores at their ends called sieve plates
  • The pores are reinforced by platelets of a polysaccharide called callose
  • Companion cells are a specialised form of parenchyma cell

 

 

Transpiration

Transpiration is the loss of water vapour from the upper parts of the plant, particularly the leaves as some water may evaporate through the upper leaf surface but this loss is limited by the waxy cuticle. Most water vapour evaporates from the underside where the stomata open during the day for gaseous exchange.

  1. Water enters the leave through the xylem and moves by osmosis into the cells of the spongy mesophyll.
  2. Water evaporates from the cell wall of the spongy mesophyll
  3. Water vapour moves by diffusion out of the leaf through the open stomata. This relies of a difference between the water vapour potential of the leaf and the outside environment.

The importance of transpiration system:

  • Transports useful mineral ions up the plant
  • Maintains cell turgidity
  • Supplies water for growth, cell elongation and photosynthesis
  • Supplies water that, as it evaporates, keeps the plant cool on hot days

Factors that affect the rate of transpiration:

Light intensity – in light the stomata open more for gaseous exchange as the light is needed for photosynthesis

Temperature – high temperatures increase the rate of evaporation, increase the rate of diffusion through the stomata because the water molecules have more energy, decrease the relative water vapour potential in the air, allowing for more rapid diffusion of the water molecules out into the air from the leaf

Relative humidity – higher humidity decreases the rate of transpiration as there is a smaller water vapour potential gradient between the leaf and the environment

Air movement – increases the rate of transpiration as this carries away water vapour from the plant, increasing the water vapour potential gradient

Water availability – if water is less available the plant will preserve the water by closing the stomata

Measuring Transpiration rate:

The main route of water loss from a plant is through the stomata of the leaf.

It is not easy to measure the rate at which water vapour is lost from the leaves.

However, it is relatively easy to measure the rate at which a plant stem takes up water, and can give a good estimate of the rate of transpiration

The amount of water lost by the plant may be investigated experimentally using a potometer – a device that measures water movement through the plant  – assumption: Water loss = Water uptake

The Transpiration Stream

This is the movement of water from the soil through the plant to the air surrounding the leaves. The main driving force is the water potential gradient between the soil and the air in the leaf air spaces.

The root hair cells absorb water and minerals from the soil.

The water then moves across the root cortex down a water potential gradient to the endodermis of the vascular bundle. Water may also travel through the apoplast pathway as far as the endodermis but must enter the symplast pathway due to a casparian strip blocking the way.

The movement of water across the root is driven by an active process that occurs in the endodermis. The endodermis is a layer of cells surrounding the medulla and xylem. This layer of cells is also known as the starch sheath as it contains granules of starch.

The plasma membranes contain transporter proteins which actively pump mineral ions from the cytoplasm of the cortex cells into the medulla and xylem.

This makes the medulla and xylem more negative in terms of WP so the water moves in by osmosis.

Root pressure

This is the pressure when the endodermis actively moves water and minerals into the medulla with no way to go back. This pressure forces the water and minerals up into the xylem. It can push water up a few metres but cannot account solely for water getting to the top of tall trees.

Transpiration pull

The loss of water by evaporation from the leaves must be replaces by water from the xylem. Water molecules are attracted to each other by cohesion and so pull each other up into the leaf.

Because this mechanism involves cohesion between the water molecules and tension occurring to pull the chain up the xylem and into the leaf, it is therefore called the cohesion-tension theory.

Capillary Action

The same forces that hold water molecules together in cohesion also cause the water to adhere to the sides of the xylem vessels. This helps the water molecules crawl up the walls of the vessel towards the leaves again.

Adaptions of plants to the availability of water

Hydrophyte – a plant adapted to living in water or where the ground is very wet

Xerophyte – a plant adapted to living in dry conditions

Terrestrial Plants

Most terrestrial plants reduce water loss with:

  • A waxy cuticle on the leaf will reduce the water loss due to evaporation through the epidermis
  • The stomata are closed at night, when there is no light for photosynthesis
  • The stomata are often found on the under surface of the leaves not on the top surface, which reduces the evaporation due to direct heating from the sun
  • Deciduous plants lose their leaves in winter when the ground may be frozen and water less available and also when temperatures are too low for photosynthesis

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Other xerophytic features

  • Closing the stomata when water availability is low
  • Low water potential inside their leaf cells made by maintaining a high salt concentration in the cells
  • A very long tap root that can reach the water deep underground (the water table)

Many plants that live in humid areas may contain specialised structures at the tips or margins of their leaves called hydathodes which release water droplets which may then evaporate from the leaf surface.

Translocation

Translocation occurs in the phloem and is the movement of assimilates throughout the plant.

Source – a part of the plant that loads materials into the transport system e.g. leaves photosynthesise and the sugars made are moved to other parts of the plant

Sink – a part of the plant where materials are removed from the transport system e.g. the roots receive sugars and store them as starch. Other times of the year this starch can be converted back into sugars and used to grow the plant so the roots may also be a source.

Active loading

  1. ATP is used to pump H+ ions out of the companion cells.
  2. The concentration of H+ ions outside the cell increases and a gradient is formed.
  3. The hydrogen ions diffuse back into the cell with sucrose through facilitated diffusion. This is known as cotransport.
  4. The cotransport required ATP to move the H + ions out of the cell and to let the sucrose enter the cell against its concentration gradient.
  5. As the concentration of sucrose in the companion cell increases, it can diffuse out of the plasmodesmata and into the sieve tube elements of the phloem.

 

Sucrose is moved along the phloem by mass flow. It is moved with other assimilates such as amino acids in sap. The sap can be made to travel up or down the plant depending on the difference in hydrostatic pressure between the two ends of the tube. The sucrose and water enters the phloem at the source causing a concentration gradient that will push it towards the sink. The assimilates and water leaving at the sink lowers the hydrostatic pressure. Although the sap may be flowing in different directions in different sieve tube elements, the fact that it is always flowing the same way in one group of cells makes it mass flow.