Introduction to Mass Transport in Plants
In smaller organisms, substances can move around simply by diffusion. However, as you learned in the Surface Area to Volume Ratio chapter, large multicellular organisms like plants have a small surface area to volume ratio. This means they need specialized "highways" to move water, minerals, and sugars over long distances. This process is called mass transport.
In this chapter, we will look at the two main systems plants use: the xylem (for water and minerals) and the phloem (for sugars). Don't worry if this seems like a lot of steps at first; we will break it down into simple, logical stories!
1. Transport of Water: The Xylem
The xylem is responsible for moving water and dissolved mineral ions upwards from the roots to the leaves. To understand how water moves against gravity without a "pump" like a heart, we use the cohesion-tension theory.
The Cohesion-Tension Theory
Think of this like a long chain of people holding hands, being pulled up from the top of a building. Here is the step-by-step process:
- Transpiration: Water evaporates from the leaves through the stomata. This reduces the water potential of the leaf cells.
- Tension: Because water is lost, a "pull" (negative pressure or tension) is created, drawing water up from the xylem into the leaf.
- Cohesion: Water molecules are "sticky" because of hydrogen bonds. This is called cohesion. Because they stick together, they form a continuous, unbroken column of water all the way from the roots to the leaves.
- Adhesion: Water molecules also stick to the walls of the xylem vessels. This is called adhesion and helps the water column stay upright.
Key Takeaway: Water moves upwards because it is "pulled" by evaporation at the leaves, and the water molecules stick together in a long, continuous chain.
Quick Tip: If the water column breaks (e.g., an air bubble enters), the movement of water stops because the "chain" of cohesion is broken!
2. Transport of Sugars: The Phloem
While water goes up, the sugars made during photosynthesis (like sucrose) need to go wherever they are needed—this could be down to the roots for storage or up to a growing fruit. This process is called translocation, and it happens in the phloem.
The Mass Flow Hypothesis
The current best explanation for how this works is the mass flow hypothesis. It relies on the movement of substances from a source (where they are made) to a sink (where they are used).
Step 1: Transfer of sucrose into the phloem (The Source)
Sucrose is produced in the leaves (the source). It is actively transported into the phloem's sieve tube elements. This requires energy in the form of ATP.
Step 2: Movement of water into the phloem
As the concentration of sucrose in the phloem increases, the water potential inside the phloem decreases (becomes more negative). This causes water to move from the nearby xylem into the phloem by osmosis.
Step 3: Mass flow of sucrose
The entry of water creates a high hydrostatic pressure at the source. At the sink (e.g., a root), sucrose is being removed and used. This increases the water potential at the sink, so water leaves the phloem there, creating a low hydrostatic pressure. The sucrose solution then flows down the pressure gradient from source to sink.
Summary Box: Mass flow is a passive process resulting from active transport of sugars. It moves from high pressure (source) to low pressure (sink).
3. Evaluating the Evidence for Mass Flow
AQA examiners love to ask you to evaluate evidence. How do we know mass flow is actually happening? Scientists have used two famous experiments:
A. Ringing Experiments
In a ringing experiment, a section of the outer layers (including the phloem) is removed from a woody stem, while leaving the xylem intact in the center.
- Observation: The bark just above the missing ring begins to swell.
- Explanation: The sugars moving down from the leaves are blocked by the missing phloem. This shows that phloem (and not xylem) is responsible for transporting sugars.
B. Tracer Experiments
Plants are grown in an atmosphere containing a radioactive isotope of carbon, \(^{14}C\). This \(^{14}C\) is used by the plant to make radioactive sugars.
- Observation: We can track the movement of these sugars by placing the plant on X-ray film (a technique called autoradiography). The areas that turn black show where the radioactive sugar is.
- Explanation: The radioactivity is found strictly within the phloem tissue, confirming it is the pathway for translocation.
Arguments For and Against Mass Flow
Don't worry if this seems tricky; just remember one "pro" and one "con" to start with!
Evidence supporting the theory:
- There is a measurable pressure gradient in the phloem (higher pressure at the source).
- Concentration of sucrose is higher in the leaves (source) than in the roots (sink).
- Sieve tubes are required; if they are damaged, the process stops.
Evidence questioning the theory:
- The function of sieve plates is unclear; they seem like they would slow down mass flow rather than help it.
- Not all solutes move at the same speed, which you would expect if they were all "flowing" together in one mass.
- Sucrose is delivered to all sinks at roughly the same rate, rather than going faster to the ones with the lowest pressure.
Common Mistakes to Avoid
1. Confusing Xylem and Phloem: Remember Xylem for X-tra water (up only) and Phloem for Food/Photosynthate (up and down).
2. Forgetting ATP: While mass flow itself is a physical process, the loading of sucrose at the source is active and requires ATP from respiration.
3. Misusing "Water Potential": Always specify that water moves from a higher (less negative) water potential to a lower (more negative) water potential.
Quick Review: Key Vocabulary
Transpiration: Evaporation of water from leaves.
Cohesion: Water molecules sticking to each other.
Translocation: Transport of organic substances in the phloem.
Hydrostatic Pressure: The "pushing" pressure exerted by a fluid.
Source: Where sucrose is made (e.g., leaf mesophyll cells).
Sink: Where sucrose is used or stored (e.g., roots, growing shoots).