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Mass Transport in Plants

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Mass Transport in Plants

Mass transport in plants is the movement of substances in a single direction and speed. This is seen in the xylem and phloem, transport vessels in plants. The xylem is responsible for transporting water and minerals up the plant via the transpiration stream. The phloem transports amino acids and sugars in both directions: up and down the plant.

The transpiration stream refers to the process of water evaporation from the leaves.

Substances can move in four main ways:

  • Diffusion: the movement of substances down their concentration gradient.

  • Facilitated diffusion: the movement of substances down their concentration gradient through membrane proteins.

  • Osmosis: water movement down a water potential gradient (from high to low water concentration) through a partially permeable membrane.

  • Active transport: the movement of substances against their concentration gradient, using energy in the form of ATP.

If you're interested in learning more about the different transport methods, refer to our explanation on Transport Across Cell Membranes.

Mass transport in xylem

Xylem is the vascular plant tissue that facilitates the movement of water up the plant into the leaves. To understand water transport in plants, you need to study key concepts such as water potential and the different water movement types.

The role of water, solute, and pressure potentials

Water potential is the concentration of water molecules in a solution. Water potential is affected by solute concentration and pressure. Water will move towards low water potential (high concentration of solutes). Water potential is the key factor in driving the transpiration stream up the plant.

Solutes are substances dissolved in a solvent.

Water potential is higher in the soil than the plant, which allows it to diffuse into the root cells. The plant will manipulate the concentration of solutes (solute potential) to aid osmosis from the soil.

Pressure (turgor) potential can be either positive (compression) or negative (vacuum) in the plant cells. When the cell is at a maximum pressure potential, it becomes turgid. Plants can manipulate the pressure by opening and closing stomata and altering solute concentrations.

Water potential in the plant cells can be calculated using the following equation:

Ψ = ΨS + ΨP

  • Ψ is the total water potential in megapascals
  • ΨS is the solute potential
  • ΨP is the pressure potential

Diffusion and water movement in cells

Before entering the xylem, water in the roots will move via apoplast and symplast pathways through diffusion. This type of diffusion is osmosis; it involves water moving down a water potential gradient through a partially permeable membrane. There are two main pathways for water movement: the apoplast pathway and the symplast pathway.

Apoplast pathway

  • Water moves through the spaces of the cell walls, dead cells (xylem) and xylem tubes.

  • Due to cohesive forces, more water is pulled up into the xylem.

  • The water will eventually reach a Casparian strip made of waxy suberin and is impermeable to water. Casparian strips will direct water in this pathway to enter the cytoplasm, where it becomes a part of the symplast pathway.

Symplast pathway

  • Water will travel via the cytoplasm, vacuoles and plasmodesmata of the cells.

  • Water moves by osmosis; the neighbouring cell has a lower water potential, so water moves into it.

Cohesion-tension theory

Cohesion (water molecules clinging to each other) and tension (water molecules clinging to the walls of the xylem) are the main drivers of the transpiration stream. Evaporation through the leaf stomata creates a negative water potential which forces water to move upwards towards the leaves.

Mass transport in phloem

Amino acids and sugars, such as sucrose, are transported in another type of vascular tissue called the phloem. They are transported in a bi-directional movement from the leaves (source) to the growing parts of the plant (e.g., shoots and roots), roots (sinks), flowers and fruits.

A source refers to the region of the plant where food is made, such as leaves.

A sink is where food is stored or used, such as the root.

Translocation is the movement of sugars and amino acids from the source to the sinks.

Mass flow hypothesis

Mass flow describes the movement of fluids down a pressure or temperature gradient, and it explains the transportation of food from sources to sinks. The mass flow hypothesis states that:

  1. Sucrose is co-transported into sieve tube elements from the companion cells, reducing the sieve tube's water potential.

  2. Water moves from the xylem (high water potential) into the phloem (low water potential), which increases the hydrostatic pressure in the phloem.

  3. Sources have a higher hydrostatic pressure, while the sinks have a lower hydrostatic pressure. This allows solutes to move to the sinks down the concentration gradient.

  4. Sinks will use or store the solutes, increasing the water potential in the phloem. Water will move out of the phloem, down the hydrostatic pressure gradient.

To find out more about mass flow, check out our article Phloem.

For and against the mass flow hypothesis

This hypothesis is still under investigation. Below, you will find supporting and conflicting evidence for mass flow.

Supporting

Against

When the stem is cut, the sap oozes out. This suggests there is high hydrostatic pressure in the sieve tube.

The presence of sieve plates (end walls) seems to interfere with the mass flow.

Sources such as leaves have a higher concentration of sugars than the sinks, such as roots.

Mass flow refers to substances moving at the same speed; however, solutes of different sizes should move at different speeds.

Higher sucrose concentration in the leaves will experience a similar increase in sucrose in the phloem.

Sucrose concentrations in different sinks will vary. However, sugars are delivered at similar rates. The mass theory would suggest that sugars would move towards it quicker if the concentration is lower in a particular sink.

Translocation of substances will be inhibited in low oxygen conditions or if metabolic poisons are present.

Table 1. Statements for and against the mass flow hypothesis.

Active loading

Active loading can also be referred to as apoplastic loading. Sucrose is actively loaded (requires ATP) into the sieve tube elements from companion cells.

  1. Hydrogen ions (protons) are pumped from companion cells to surrounding cells via proton pumps.

  2. The proton concentration becomes much higher in the surrounding cells than in the companion cells.

  3. Hydrogen ions will diffuse back into the companion cells (down the gradient) through co-transporter proteins.

  4. When hydrogen ions move down the gradient, they take a sucrose molecule (against the concentration gradient). This also happens from the companion cell to the sieve tube element.

Co-transport describes the simultaneous movement of a substance down its concentration gradient and another substance up its concentration gradient. In companion cells, hydrogen ions diffuse down their concentration gradient while sucrose travels up.

Tracer and ringing experiments

Tracer and ringing experiments investigate the translocation of sugars in the plant.

The ringing experiment

  • A ring containing phloem bark and cortex is removed to leave the xylem in the centre.

  • Since the xylem is intact, water transport will not be affected, but sugar transport will stop at the ring as the phloem has been removed. This causes a swelling in the tissue, thus supporting the concept of phloem translocation.

The tracer experiment

  • Plants are grown in a laboratory containing radioactively labelled carbon dioxide (C14).

  • Through photosynthesis, the radioactive carbon is incorporated into the sugars.

  • Autoradiography (a technique used to detect radioactive material in the sample) is used to observe the movement of sugars in the plant.

  • The results show that the radioactive carbon is only present in the phloem and absent in the xylem, thus supporting the concept of phloem translocation.

Mass transport in non-vascular plants

Non-vascular plants (bryophytes), including mosses, liverworts and hornworts, lack a vascular bundle and do not have xylem or phloem to transport water, minerals and food. Instead, non-vascular plants contain much simpler tissue.

  • Bryophytes will obtain water by osmosis, and nutrients will diffuse into the plant. Only the plant parts that are close to water and nutrient sources will take them up. They do not have a transport system (vascular tissue) to distribute it within the plant.

  • Bryophytes do not have roots like vascular plants do; instead, they have rhizoids. Rhizoids resemble seeds; however, they function differently. They will anchor the plant but are not able to take up water. They will absorb water by osmosis, such as the other parts of bryophytes.

Mass Transport in Plants - Key takeaways

  • Mass transport describes the movement of substances in a single direction and speed. Mass transport in plants takes place in the xylem, which carries water and inorganic ions, and phloem, which carries sugars and amino acids.

  • Water in the xylem moves in one direction and is driven by the transpiration stream. Water potential and the cohesive properties of water are key in maintaining the transpiration stream.

  • Mass transport in phloem is bi-directional from the leaves (sources) to the growing shoots and storage organs (sinks).

  • Non-vascular plants do not have xylem or phloem. They will solely depend on the diffusion of substances into the plant.

Frequently Asked Questions about Mass Transport in Plants

To transport water, inorganic nutrients and food in bulk where it is needed.

The xylem moves water and minerals through transpiration.


The phloem moves sugars and amino acids through translocation.

Mass flow refers to the movement of fluids down a pressure or temperature gradient.

  • Facilitated diffusion 
  • Osmosis
  • Active transport

Osmosis is used to transport water into root hair cells and up the plant to the leaves.

Final Mass Transport in Plants Quiz

Question

Water and minerals in xylem  bi-directionally. True or False?

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Answer

False

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Question

What is meant by water potential and why is it important to the transpiration stream?

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Answer

Water potential is the concentration of water molecules in a solution. Water potential is higher at the roots, where the water moves from the soil. The water will move towards the lower water potential - up the plant which maintains the transpiration stream.

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Question

What are the two water properties that maintain the transpiration stream?

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Answer

Cohesion and adhesion.

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Question

What is solute potential?


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Answer

The concentration of solutes.

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Question

The higher the solute potential, the higher the water potential. True or False?


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Answer

False

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Question

Complete the equation: Water potential = Solute potential + ____________.


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Answer

Water potential = Solute potential + Pressure potential

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Question

In the roots, before entering xylem, water will move via two pathways. What are they?


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Answer

Apoplast and symplast pathways.

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Question

What is the Casperian strip?


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Answer

 Waxy, thickened central root endodermis.

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Question

What is cohesion?


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Answer

Attraction between two molecules of the same kind.

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Question

What is a source and the sink? Give an example for each.


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Answer

Source is the place where the food is made, the leaves; sinks are the locations where the food is used or stored, e.g. the root.

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Question

What is translocation?


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Answer

Movement of sugars and amino acids from the source to the sinks.

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Question

 Give one supporting and one disagreement statement for the mass flow hypothesis.


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Answer

 Statements (any statements supporting/against are correct):

  • Supporting: Sources such as leaves have a higher concentration of sugars than the sinks, such as roots.

  • Against - Sucrose concentrations in different sinks will vary, however, sugars are delivered at similar rates. Mass theory would suggest that if the concentration is lower in a particular sink, sugars would move towards it quicker and to the other.

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Question

What is active loading in phloem?


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Answer

 Sucrose is actively loaded (requires ATP) into the sieve tube elements from companion cells.

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Question

What are the tracer and ringer experiments used to investigate?


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Answer

Tracer and ringer experiment is used to investigate the mass transport of substances  in plants.

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Question

Why would you put radioactive carbon into the plant (by using carbon dioxide in the air) to investigate the mass flow of sugars (tracer experiment)?


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Answer

Using autoradiography, radioactive carbon, in the form of sugars can be observed moving in the plant.

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Question

Plants have two transport systems. Xylem transports water and inorganic ions. What is the name of the other system and what does it transport?

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Answer

Phloem. It transports amino acids and sugars.

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Question

Xylem and phloem transport substances in a bi-directional flow. True or False?


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Answer

Partially true. Xylem transports water in one direction and phloem transports materials bi-directionally.

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Question

What are the two main properties of water molecules that help the mass flow of water?


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Answer

Adhesion and cohesion.

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Question

How does cohesion occur between the water molecules?


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Answer

The cohesive forces between water molecules occur due to hydrogen bonds. Water molecules are charged due to an unequal sharing of electrons. This allows for hydrogen bonds between water molecules.

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Question

Fill in the blanks about adhesion. 


Adhesion is the _________ between the two __________ substances. In xylem, water molecules are _______ to the ________ of xylem. Water molecules are charged and xylem ________ are too. Water will move via ________ action up the plant.

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Answer

Adhesion is the attraction between the two different substances. In xylem, water molecules are attracted to the walls of xylem. Water molecules are charged and xylem walls are too. Water will move via capillary action up the plant.

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Question

What will create surface tension in xylem’s sap?

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Answer

Transpiration stream.

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Question

What are the four types of cells present in xylem?


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Answer

Tracheids, xylem vessel elements, parenchyma and sclerenchyma.

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Question

What would happen if xylem had end walls?


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Answer

Mass water flow would be disrupted.

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Question

How are the pits in the lignified xylem walls useful?


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Answer

Pits allow xylem to withstand the water pressure as it fluctuates throughout the plant.

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Question

How is the xylem and phloem complex called?


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Answer

A vascular bundle.

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Question

What is cambium?


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Answer

An actively dividing layer of cells that promotes growth.

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Question

What do the primary and secondary growths do?


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Answer

The primary growth increases the length in shoots and the root. The secondary growth causes an expansion in plant’s size.

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Question

How will the vascular bundles be arranged in the dicot plant’s stem?


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Answer

In a ring-like manner.

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Question

How are xylem and phloem arranged in a dicot root?

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Answer

Xylem appears in an x-shaped pattern and phloem is arranged around it.

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Question

Name two differences between the structure of xylem and phloem.


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Answer

The two differences (any differences are correct):


  1. No end walls in the xylem but present in the phloem.
  2. One way system in xylem but bi-lateral movement in phloem.

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Question

What does phloem transport?

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Answer

Sugars and amino acids.

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Question

Xylem and phloem are the two specialised transport structure in the plant. True or False?

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Answer

True.

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Question

What is the direction of transport of substances in the phloem?

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Answer

Substances are transported bi-directionally.

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Question

What is the name of the tissue where xylem, phloem and cambium reside?

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Answer

The vascular bundle.

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Question

What is cambium?

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Answer

An actively dividing bundle of cells between xylem and phloem.

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Question

What is the name of the seed leaf that gives the name to monots and dicots?

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Answer

Cotyledon.

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Question

How are vascular bundles arranged in the stem of a dicot plant?

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Answer

The vascular bundles are arranged in the ring-like structure. Xylem is present in the inner part of the cambium ring and phloem is present at the exterior. 

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Question

What are phloem fibres?

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Answer

Non-living schlerenchyma cells that provides support to the plant.

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Question

What are parenchyma cells?

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Answer

Permanent ground tissue that will form the bulk of a plant.

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Question

What are the three main structural features of sieve tube elements?

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Answer

  1. Connected by sieve plates transversely which allows organic matter flow between sieve element cells.
  2. Sieve tubes have abandoned their nucleus and reduced the amount of organelles they have to focus on translocation (movement) of organic material.
  3. Thick and rigid cell walls which allows the phloem to withstand the hydrostatic pressure.

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Question

Why do companion cells have many mitochondira?

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Answer

In order to produce ATP for active transport of materials between the sources and sinks and sieve tube elements.

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Question

How do companion cells increase the surface area for substance absorption?

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Answer

Plasma membrane folds inwards to increase surface area to volume ratio for substance absorption.

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Question

What is meant by a cytoplasmic junction?

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Answer

Cell to cell or cell to extracellular matrix connection through the cytoplasm.

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Question

Fill in the blanks about transport of materials in phloem.


______ ______ and sugars such as sucrose are transported in phloem. They will be transported in _________ movement from the leaves to the _______ parts of the plant (e.g. shoots and roots), ______ organs (e.g. roots), flowers and fruits. Therefore, materials are transported from ______ to ______.


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Answer

Amino acids and sugars such as sucrose are transported in phloem. They will be transported in bi-directional movement from the leaves to the growing parts of the plant (e.g. shoots and roots), storage organs (e.g. roots), flowers and fruits. Therefore, materials are transported from sources to sinks.


Show question

Question

Give any two differences between xylem and phloem.

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Answer

For example: 


  1. Phloem transports sugars and amino acids while xylem transports water and minerals.
  2. Phloem has sieve plates between cells but xylem has no end walls (continuous flow of material).

Show question

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