Plants have their own transport system, just like humans! Mass transport in plants is when substances are moved in one direction at a certain speed. This happens in the xylem and phloem, which are like highways for plants. The xylem carries water and minerals from the roots to the rest of the plant through a process called transpiration. The phloem carries amino acids and sugars up and down the plant. There are four ways substances can move around in plants: diffusion, facilitated diffusion, osmosis, and active transport. If you want to know more about how substances move in and out of cells, check out our explanation on Transport Across Cell Membranes.

Mass transport in xylem

Xylem is a special tissue in plants that helps water move up from the roots to the leaves. To really understand how water moves in plants, you'll need to learn about some important ideas like water potential and different types of water movement.

The role of water, solute, and pressure potentials

To understand how water moves in plants, we need to talk about water potential. Water potential is basically the concentration of water molecules in a solution. It's affected by the concentration of solutes (like minerals) and pressure. Water likes to move from areas of high water potential to areas of low water potential (where there are more solutes). This is what drives the transpiration stream up the plant.

In the soil, water potential is higher than in the plant, so water diffuses into the root cells. The plant can manipulate the concentration of solutes (solute potential) to help with osmosis from the soil.

Pressure potential (also known as turgor) can be positive (compression) or negative (vacuum) in plant cells. When the pressure potential is at its maximum, the cell becomes turgid. Plants can control the pressure by opening and closing stomata and changing the concentration of solutes.

You can calculate the total water potential in plant cells using this equation:

Ψ = ΨS + ΨP

Ψ is the total water potential in megapascals, ΨS is the solute potential, and ΨP is the pressure potential.

Diffusion and water movement in cells

When water enters the roots, it moves through two different pathways: the apoplast pathway and the symplast pathway. In the apoplast pathway, water moves through the spaces between the cell walls and dead cells like xylem. This movement is driven by cohesive forces, which pull more water up into the xylem. However, eventually, the water will reach a Casparian strip made of waxy suberin which is impermeable to water. This Casparian strip will direct water to enter the cytoplasm and become part of the symplast pathway.

In the symplast pathway, water moves through the cytoplasm, vacuoles, and plasmodesmata (channels connecting plant cells). Water moves in this pathway by osmosis, with water moving from areas of high water potential to areas of low water potential. This means that when water moves from one cell to another, it's because the neighboring cell has a lower water potential, so the water moves into it.

Overall, the combination of these two pathways allows water to be transported efficiently up the plant, from the roots to the leaves, where it is used in photosynthesis and other essential plant processes.

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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

In addition to water, plants also need to transport nutrients like amino acids and sugars (such as sucrose) from the leaves where they are produced to other parts of the plant, like the roots and flowers, where they are needed. This transport happens in another type of vascular tissue called the phloem.

Unlike the xylem, which transports water and minerals in a unidirectional movement from roots to leaves, the phloem transports sugars and amino acids in a bi-directional movement. The movement is from the leaves (source) to the growing parts of the plant (shoots and roots), roots (sinks), flowers, and fruits.

In the phloem, a source refers to the region of the plant where food is made, such as the leaves, while a sink is where food is stored or used, such as the root. Translocation is the process of moving sugars and amino acids from the source to the sink.

Phloem transport requires energy and is an active process. The concentration of sugars in the source is higher than in the sink, so they move down a concentration gradient. This movement requires energy, which is provided by ATP (adenosine triphosphate), the energy currency of the cell.

Overall, the phloem plays a crucial role in the distribution of nutrients throughout the plant, ensuring that all parts of the plant have access to the nutrients they need to grow and function properly.

Mass flow hypothesis

The mass flow hypothesis proposes that the movement of nutrients in the phloem occurs through a pressure gradient, with the high hydrostatic pressure in the source regions and lower pressure in the sink regions. This hypothesis suggests that sucrose is co-transported into sieve tube elements from the companion cells, reducing the sieve tube's water potential. As a result, water moves from the xylem (high water potential) into the phloem (low water potential), which increases the hydrostatic pressure in the phloem. The pressure gradient allows solutes to move to the sinks down the concentration gradient, where they are used or stored.

There is evidence to support the mass flow hypothesis, such as the observation that sap oozes out of the cut stem, indicating high hydrostatic pressure in the sieve tube. However, there are also conflicting pieces of evidence that challenge the hypothesis. For example, sieve plates (end walls) may interfere with the mass flow, and sources like leaves have a higher concentration of sugars than the sinks, such as roots. Additionally, the theory would suggest that sugars should move towards a sink quicker if the concentration is lower in that sink, but sugars are delivered at similar rates regardless of the sugar concentration in different sinks. Finally, the translocation of substances is inhibited in low oxygen conditions or if metabolic poisons are present, indicating that the mass flow hypothesis may not fully explain nutrient transport in these conditions.

Overall, while the mass flow hypothesis is still under investigation, there is evidence both supporting and conflicting with this model of nutrient transport in plants.

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Active loading

Active loading of sucrose, also known as apoplastic loading, is an alternative model to explain the movement of nutrients from sources to sinks in plants. In this model, sucrose is actively loaded into the sieve tube elements from companion cells, which require energy in the form of ATP.

The process of active loading involves the use of proton pumps that transport hydrogen ions (protons) from companion cells to surrounding cells. This leads to a higher proton concentration in the surrounding cells compared to the companion cells. Hydrogen ions then diffuse back into the companion cells through co-transporter proteins, which are capable of moving a sucrose molecule against its concentration gradient. This mechanism enables the simultaneous movement of hydrogen ions down their concentration gradient and sucrose up its concentration gradient.

Co-transport is a vital process in active loading, where two substances move in opposite directions across a membrane. In companion cells, hydrogen ions diffuse down their concentration gradient while sucrose travels up, which enables the movement of sucrose from the source to the sink.

Active loading has several advantages over the mass flow hypothesis, as it can explain the movement of nutrients against their concentration gradient, which would not be possible with mass flow alone. Additionally, the active loading model can account for the differences in sugar concentration between sources and sinks. However, active loading requires more energy than mass flow, and its exact mechanism is still under investigation.

Overall, the active loading model provides an alternative explanation for the transport of nutrients in plants, emphasizing the importance of active processes that require energy in the form of ATP.

Tracer and ringing experiments

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

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. 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, also known as bryophytes, lack vascular tissue and do not have xylem or phloem to transport water, minerals, and food. Instead, these plants have much simpler tissue, and they obtain water through osmosis, with nutrients diffusing into the plant. However, only the plant parts that are close to the water and nutrient sources will take them up, as there is no transport system to distribute them within the plant.

Bryophytes do not have roots like vascular plants do; instead, they have rhizoids. These structures anchor the plant but are not able to take up water. Instead, they absorb water through osmosis, much like the other parts of bryophytes.

In contrast, vascular plants have specialized tissues, including the xylem and phloem, that enable the transport of water, minerals, and food throughout the plant. The xylem carries water and inorganic ions, while the phloem carries sugars and amino acids. Water in the xylem moves in one direction and is driven by the transpiration stream, which is maintained by water potential and the cohesive properties of water.

The mass transport of substances in plants is a crucial process that occurs in the xylem and phloem. In the phloem, mass transport is bi-directional and occurs from the leaves (sources) to the growing shoots and storage organs (sinks). However, non-vascular plants solely depend on the diffusion of substances into the plant, as they do not have xylem or phloem.

In summary, the lack of vascular tissue in non-vascular plants, such as bryophytes, means that they cannot perform mass transport of substances like vascular plants. Instead, they rely on the diffusion of substances into the plant, with only the parts close to water and nutrient sources taking them up.

Mass Transport in Plants

Why do plants need mass transport?    

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

What are the two methods of mass transport in plants?

The xylem moves water and minerals through transpiration.The phloem moves sugars and amino acids through translocation.

What is mass flow in plants?

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

What are the three types of transport in plants?

Facilitated diffusion OsmosisActive transport

How is osmosis used in transport by a plant?

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

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