mass transport in plants

MASS TRANSPORT IN PLANTS

Xylem and phloem are mass transport systems: 

• Xylem - water and mineral ions in solution from roots to the leaves by transpiration. 

• Phloem - organic substances e.g. sugars in solution up and down the plant by translocation. 

XYLEM STRUCTURE

Xylem vessels are very long, tube‑like structures formed from dead cells and are lined with lignin. 

XYLEM ADAPTATIONS

Cells joined with no end walls forming a long continuous tube → water flows as a continuous column

● Cells contain no cytoplasm / nucleus → easier water flow / no obstructions

● Thick cell walls with lignin → provides support / withstand tension / prevents water loss

● Pits in side walls → allow lateral water movements

MOVEMENT OF WATER 

ROOT PRESSURE 

• Water in roots pushes water up the xylem 

  • But not all the way up, so only in small plants. 

CAPILLARY ACTION 

• Water has a tendency to move up into small tubes 

  • But this is too slow so plants require another way of transporting water. 

COHESION-TENSION AND EVAPORATION 

This is the method which plants use to transport water from the roots and out of the leaves by transpiration.

FLOW OF WATER FROM SOIL TO ROOTS 

OSMOSIS 

1. Water potential is higher within the soil than the root hair cell. 

2. This is because of the dissolved substances in the cell sap.

3. Water is taken up by the roots of a plant and through the endodermis, before being moved into xylem tissue, which is in the centre of the root. 

Root hair cells function to increase the surface area in order for water to absorbed and for nutrients to be pumped across against the concentration gradient by active transport.

Apoplastic

Water and dissolved mineral ions and salts move from the root hair cells to the xylem by travelling from cell to cell directly through cell walls and through intercellular spaces of neighbouring cells. Water moves through the cell walls through spaces between cellulose molecules, down the water potential gradient. When the water and mineral ions are near the xylem, the Casparian strip forms an impenetrable barrier to water in the cell walls. This means that it must move into the cytoplasm to get to the xylem cortex, now moving my symplastic flow. This gives the plant control over the ions that enter its xylem vessels, since water must cross a plasma membrane to get there. 

Sympastic

Water moves from the root hair cells to the xylem by travelling from cell to cell through cytoplasm. Water and minerals are immediately filtered because they cross the  cell surface membrane of the root hair well which is partially permeable. Water moves through plasmodesmata (small channels in cell walls that connect neighbouring cells). As the water and minerals have already been filtered, they bypass the Casparian strip and move straight into the xylem. The water moves down the water potential gradient between cells. As soon as the water reaches the xylem it is rapidly transported away, meaning the water potential gradient is maintained. 

TRANSPIRATION AND MOVEMENT ACROSS LEAF 

Transpiration is the evaporation of water from a plant’s surface, especially the leaves. 

• Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf. 

• The humidity of the atmosphere is usually less than that of the air spaces next to the stomata. 

• As a result there is a water potential gradient from the air spaces through the stomata to the air. 

• Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air. 

TRANSPIRATION AND ENERGY

The transportation pull is a passive process, and therefore does not require metabolic energy to take place. 

1. The xylem vessels through which the water passes are dead and so cannot actively move the water. 

2. Xylem vessels have no end walls which means that xylem forms a series of continuous, unbroken tubes from root to leaves, which is essential to the cohesion-tension theory of water flow up the stem. 

3. Energy is nevertheless needed to drive the process of transpiration. 

4. This energy is in the form of heat that evaporates water from the leaves and it ultimately comes from the sun.

COHESION-TENSION THEORY 

1. Water evaporates from the leaves via the (open) stomata due to transpiration.  

2. Reducing water potential in the cell,  increasing water potential gradient

3. Water is drawn out of xylem to the mesophyll cells. 

4. Tension in the xylem is created due to the transpiration pull. 

5. Water is cohesive due to hydrogen bonding (polar molecules) so they have a tendency to stick together. 

6. The cohesive forces between water molecules pull water up as a column. 

7. As water moves up, there is a lower water potential in the roots so more enters the roots via osmosis, down water potential gradient. 

8. Water is therefore moving up the xylem, against gravity. 

FACTORS AFFECTING TRANSPIRATION RATE

light intensity

heat

wind

humidity

light intensity

the lighter it is the faster the transpiration rate - positive correlation. 

• This is because the stomata open when it gets light to let in CO₂ for photosynthesis. 

• When it’s dark the stomata are usually closed, so there’s little transpiration.

TEMPERATURE

the higher the temperature the faster the transpiration rate - positive correlation.  

• Warmer water molecules have more kinetic energy so they evaporate by diffusion from the cells inside the leaf faster. 

• This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

HUMIDITY

the lower the humidity, the faster the transpiration rate - negative correlation 

• If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate.

• If the air is moist, there is a lower water potential gradient so diffusion is slower out of the leaf. 

WIND

the windier it is, the faster the transpiration rate - positive correlation. 

• Lots of air movement blows away water molecules from around the stomata. 

• This increases the water potential gradient, which increases the rate of transpiration.

STRUCTURE OF PHLOEM

• Living cells joined end to end forming tubes.  

• Living cells are sieve tube elements.

  • But have no nucleus and few organelles. 

• There is a companion cell for each sieve tube element. 

  • Carry out the living functions and provide energy for active transport of solutes. 

TRANSLOCATION

Translocation is the movement of solutes/organic substances through the plant bidirectionally from the source to the sink. 

MASS FLOW MECHANISM OF TRANSLOCATION

1. At the source, sucrose moved into phloem by active transport. 

2. This increases sucrose concentration and decreases water potential in phloem. 

3. Water moves into the phloem from xylem and surrounding tissues by osmosis (down water potential gradient). 

4. This results in an increase in hydrostatic/turgor pressure in sieve tube elements. 

5. Solutes and water move from high to low pressure so move away from the source. 

6. Sucrose is absorbed (assimilated) at the sink. 

7. So sucrose concentration in phloem decreases and water potential increases. 

8. Water moves back to the xylem by osmosis, down water potential gradient.