4.3 Pressure Flow Model for Phloem Transport
4_Phloem-Translocation
Phloem Translocation
Overview
xylem (blue) carries water from the roots upwards phloem (orange) carries products of photosynthesis from the place of their origin (source) to organs where they are needed (roots, storage organs, flowers, fruits – sink); note that e.g. the storage organs may be source and leaves may be sink at the beginning of the growing season
Nefronus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
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Introduction
Learning Objectives
- Differentiate between source and sinks in a plant.
- Explain the pressure-flow model for sugar translocation in phloem tissue.
- Describe the roles of proton pumps, co-transporters, and facilitated diffusion in the pressure-flow model.
- Recognize how different sugar concentrations at sources and different types of sinks affect the transport pathway used for loading or unloading sugars.
- Compare and contrast the mechanisms of fluid transport in the xylem and phloem.
Key Terms
co-transporter - proteins that move two molecules across a membrane at the same time, one with the concentration gradient other against the concentration gradient
facilitated diffusion - the movement of molecules across a membrane with the concentration gradient via a specific protein
girdling - to cut through and remove the bark of a tree trunk all the way around
phloem - one of the vascular tissues in plants, transport photosynthates
photosynthates - carbohydrates produced during photosynthesis
pressure-flow model - the hypothesis that explains the movement of sugars in phloem
proton pump - use energy from ATP to create an electrochemical gradient across the plasma membrane.
sink - tissue or site in a plant where photosynthates are utilized or stored
source - tissue or site in a plant where photosynthates are produced usually green leaves
translocation - the movement of sugars from source to sink
Plants need an energy source to grow. In growing plants, photosynthates (sugars produced by photosynthesis) are produced in leaves by photosynthesis and are then transported to sites of active growth where sugars are needed to support new tissue growth. During the growing season, the mature leaves produce excess sugars that are transported to storage locations including ground tissue in the roots or bulbs (a type of modified stem). Many plants lose leaves and stop photosynthesizing over the winter. At the start of the growing season, they rely on stored sugars to grow new leaves to begin photosynthesis again.
Sugars move from “source” to “sink”
Locations that produce or release sugars for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation, or movement of sugar. The points of sugar delivery—such as roots, young shoots, and developing seeds—are called sinks. Sinks include areas of active growth (apical and lateral meristems, developing leaves, flowers, seeds, and fruits) or areas of sugar storage (roots, tubers, and bulbs). Storage locations can be either a source or a sink, depending on the plant’s stage of development and the season.
The photosynthates from the source are usually translocated to the nearest sink through the phloem sieve tube elements. For example, the highest leaves will send sugars upward to the growing shoot tip, whereas, lower leaves will direct sugars downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). Note that the fluid in a single sieve tube element can only flow in a single direction at a time, but fluid in adjacent sieve tube elements can move in different directions. The direction flow also changes as the plant grows and develops, such as in the following situations:
- In the middle of the growing season, actively photosynthesizing mature leaves and stems serve as source, producing excess sugars which are transported to sinks where sugar use is high. Sinks during the growing season include areas of active growth meristems, new leaves, and reproductive structures. Sinks also include sugar storage locations, such as roots, tubers, or bulbs. At the end of the growing season, the plant will drop leaves and no longer have actively photosynthesizing tissues.
- Early at the start of the next growing season, a plant must resume growth after dormancy (winter or dry season). Because the plant has no existing leaves, its only source of sugar for growth is the sugar stored in roots, tubers, or bulbs from the last growing season. These storage sites now serve as sources, while actively developing leaves are sinks. Once the leaves mature, they will become sources of sugar during the growing season.
Translocation: Transport from Source to Sink
The most accepted hypothesis to explain the movement of sugars in phloem is the pressure-flow model for phloem transport. This hypothesis accounts for several observations:
- Phloem is under pressure.
- Translocation stops if the phloem tissue is killed.
- Translocation proceeds in both directions simultaneously (but not within the same tube).
- Translocation is inhibited by compounds that stop the production of ATP in the sugar source.
In very general terms, the pressure-flow model works like this: a high concentration of sugar at the source creates a low solute potential (Ψs), which draws water into the phloem from the adjacent xylem. This creates a high-pressure potential (Ψp), or high turgor pressure, in the phloem. The high turgor pressure drives the movement of phloem sap by “bulk flow” from source to sink, where the sugars are rapidly removed from the phloem at the sink. Removal of the sugar increases the Ψs, which causes water to leave the phloem and return to the xylem, decreasing Ψp.
This video provides a concise overview of sugar sources, sinks, and the pressure-flow hypothesis.
Transport pathways in sugar translocation
Before we get into the details of how the pressure-flow model works, let’s first revisit some of the transport pathways:
- Diffusion occurs when molecules move from an area of high concentration to an area of low concentration. Diffusion does not require energy because the molecules move down their concentration gradient (from areas of high to low concentration).
- Proton pumps use energy from ATP to create electrochemical gradients, with a high concentration of protons on one side of a plasma membrane. This electrochemical gradient can then be used as a source of energy to move other molecules against their concentration gradients via co-transporters.
- Co-transporters are channels that perform a type of secondary active (energy-requiring) transport. Co-transporters move two molecules at the same time: one molecule is transported along (“down”) its concentration gradient, which releases energy that is used to transport the other molecule against its concentration gradient.
- Symporters are a type of co-transporter that transports two molecules in the same direction; both into the cell, or both out of the cell.
- Antiporters are a type of co-transporter that transports two molecules in opposite directions; one into the cell, and the other out of the cell.
Each of these transport pathways plays a role in the pressure-flow model for phloem transport.
Pressure Flow Model for Phloem Transport
Photosynthates, such as sucrose, are produced in the mesophyll cells (a type of parenchyma cell) of photosynthesizing leaves. Sugars are actively transported from source cells into the sieve-tube companion cells, which are associated with the sieve-tube elements in the vascular bundles. This active transport of sugar into the companion cells occurs via a proton-sucrose symporter: the companion cells use an ATP-powered proton pump to create an electrochemical gradient outside of the cell. The cotransport of a proton with sucrose allows movement of sucrose against its concentration gradient into the companion cells.
From the companion cells, the sugar diffuses into the phloem sieve-tube elements through the plasmodesmata that link the companion cell to the sieve tube elements. Phloem sieve-tube elements have reduced cytoplasmic contents and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap (Figure 2.4.2.).
The presence of high concentrations of sugar in the sieve tube elements drastically reduces Ψs, which causes water to move by osmosis from the xylem into the phloem cells. This movement of water into the sieve tube cells causes Ψp to increase, increasing both the turgor pressure in the phloem and the total water potential in the phloem at the source. This increase in water potential drives the bulk flow of phloem from source to sink (Figure 2.4.3.).
Sugar loading and unloading into and from phloem sieve tube members can be apoplastic or symplastic. Unloading at the sink end of the phloem tube can also occur either by diffusion if the concentration of sugar is lower at the sink than in the phloem, or by active transport, if the concentration of sucrose is higher at the sink than in the phloem.
If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the sucrose concentration in the sink cells is usually lower than in the phloem sieve-tube elements because the sucrose at the sink is rapidly metabolized for growth.
If the sink is an area of storage where sugar is converted to starch, such as a root or bulb, then the sugar concentration in the sink is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly converted to starch for storage.
If the sink is an area of storage where the sugar is stored as sucrose, such as sugar beet or sugar cane, then the sink may have a higher concentration of sugar than the phloem sieve-tube cells. In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells.
Once sugar is unloaded at the sink cells, the Ψs increase, causing water to diffuse by osmosis from the phloem back into the xylem. This movement of water out of the phloem causes Ψp to decrease, reducing the turgor pressure in the phloem at the sink and maintaining the direction of bulk flow from source to sink.
This video has a detailed discussion of the pressure-flow hypothesis.
Movement of Fluid in Xylem vs Phloem
| Xylem | Phloem |
Driving force for fluid movement | transpiration (evaporation) from leaves, combined with cohesion and tension of water in the vessel elements and tracheids (passive; no energy required) | Active transport of sucrose from source cells into phloem sieve tube elements (energy required) |
Cells facilitating fluid movement | Non-living vessel elements and tracheids | Living sieve tube elements (supported by companion cells) |
Pressure potential | Negative due to the pull from the top (transpiration, tension) | Positive due to push from source (Ψp increases due to influx of water which increases turgor pressure at source) |
Attributions
Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Introduction to Organismal Biology at https://sites.gatech.edu/organismalbio/ is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Botany (Ha, Morrow, and Algiers) is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers.