9.2 Transport in the phloem of plants Flashcards
Diagram showing transport and exchange in plants
What portion of plant cells perform photosynthesis?
A small portion
How do the plant cells that perform photosynthesis get energy?
The cells that do, usually the mesophyll cells of the leaf, produce large enough quantities to supply their own needs and the needs of the rest of the plant.
What is phloem?
A vascular tissue that has evolved to efficiently conduct sugar molecules from a source where they are present in abundance to a sink where they are needed because they are being used or stored.
Phloem consists of columns of living cells called ___
Sieve tubes
Each cell in a sieve tube is called a ___
Sieve element (or sieve tube element).
What do sieve elements contain and how does this affect their needs?
-Limited cytoplasm with a few mitochondria and other organelles, but the nucleus, vacuole, cytoskeleton, and many other structures are broken down.
-Thus, sieve elements require ‘life support’ from companion cells that run alongside each element.
How are sieve elements and companion cells connected?
By pores called plasmodesmata
Diagram showing a sieve tube cell and companion cell connected by plasmodesma
What are sieve elements largely filled with?
-Fluid called phloem sap, which is a combination of water, dissolved sucrose and other carbohydrates, amino acids, proteins, some types of minerals, and plant hormones.
-The sap is contained in a single plasma membrane that extends throughout the sieve tube, from one cell to the next.
Describe the structure of sieve elements (other than organelles)
-Sieve elements are largely filled with a fluid called phloem sap
-They have perforated end walls called sieve plates that allow sap to flow like a river from cell to cell.
Diagram of phloem in a longitudinal section
.
Sieve elements, sieve tubes, and sieve plates
-Sieve elements or sieve tube elements are elongated living cells.
-Many sieve elements are connected end to end to form a sieve tube.
-The walls connecting sieve elements become perforated during development to form sieve plates.
What does the movement of sugars and organic molecules in phloem require?
-Unlike the movement of water in the xylem, the movement of sugars and other organic molecules in the phloem requires active transport.
-This requires phloem cells to be living, unlike xylem vessels.
-Like the xylem, the structure of the phloem is closely related to its function.
What are the structures in a phloem?
-Reduced organelles in sieve elements
-Companion cells
-Plasmodesmata
-Sieve plate
-Cell membrane
What is the function of reduced organelles in sieve elements?
Absence of cell structures (including nucleus, cytoskeleton, Golgi, ribosomes and vacuole) frees the lumen to conduct a large volume of sap
What is the function of companion cells?
Metabolic support cells (containing all the standard organelles) provide biomolecules (e.g. enzymes) necessary to maintain life functions in the sieve elements
What is the function of plasmodesmata?
Openings between the sieve elements and companion cells allow communication and support from companion cells
What is the function of sieve plates?
Pores through the horizontal cells that join sieve elements allow sap to flow freely
What is the function of the cell membrane of the phloem?
Presence of a fully functional cell membrane in sieve elements that contains specialised protein pumps provides the structures needed to control the composition of sap
What is phloem well-adapted to do?
To move sugar-rich sap through its sieve tubes
What does sugar-rich sap contain?
The sap is actively loaded with carbohydrates, primarily sucrose, at a source (a tissue with a high concentration of dissolved sugars).
Give an overview of what happens to sap during translocation
The sap flows toward a sink (a tissue that requires sugars for storage or use in respiration) in a process called translocation.
Describe the speed of translocation
-Translocation in angiosperms may move sugars at a rate of 10 to 200 centimeters per hour.
-Though translocation rate is influenced by a variety of factors, it can generally supply sugars to a sink hundreds of times faster than diffusion would.
Table showing sinks and sources in translocation (make into separate flashcards?)
What are sources?
Photosynthesising tissues and storage organs that are exporting sugars to other parts of the plant.
What are sinks?
Organs that cannot produce (sufficient) sugars and need them for respiration or storage.
The same organ may be a ___ at some stages and a ___ at others
Sink
Source
Give an example of how the same organ may be a sink at some stages and a source at others
-For example, while an onion is developing and growing it is a sink because it is receiving sugars for storage.
-Later, when the onion begins to sprout, it will release its stored carbohydrates to nourish the embryonic leaves, and thus will be a source.
-As a result, phloem movement can be bidirectional, meaning the sap can flow in different directions.
-This is in contrast to the xylem, where water flow is unidirectional, always moving from roots to shoots.
Diagram showing translocation in the phloem
Steps 1-4 of translocation
1) Sucrose produced by a source (in photosynthesis or released from storage organs) is actively loaded using ATP into phloem tubes. (Technically, companion cells are actively loaded and sucrose then diffuses through plasmodesmata into the sap of the sieve tube elements.)
2) The active loading of sucrose increases the solute concentration in the sieve tubes.
3) Water then moves from xylem vessels to the sieve tubes by osmosis, moving from an area of lower solute concentration to higher solute concentration.
4) Water is essentially incompressible, meaning its volume has almost no change at different pressures. As water enters sieve elements and pushes on the phloem cell walls, it causes increased internal pressure. The pressure exerted by water is called hydrostatic pressure.
Steps 5-9 of translocation
5) The high hydrostatic pressure at the source squeezes the sap through the pores of the sieve plates, away from the source, and towards the sink. This movement is called mass flow, or bulk flow because the water and solutes are moving together.
6) At the sink, companion cells unload sugars from the sieve tube. This may be active or passive transport, depending on the relative sugar concentrations. As sugars leave the sieve tube, the concentration of solutes decreases.
7) Decreasing solute concentration allows water to return to xylem vessels.
8) The decrease in water in the sieve tube causes lower hydrostatic pressure near the sink . This allows sap to flow from the areas of high hydrostatic pressure by the source to areas of low hydrostatic pressure by the sinks.
9) As phloem sap flows from source to sink, it flows down hydrostatic pressure gradients. This process is also referred to as the pressure-flow mechanism.
Diagram of mass flow from source to sink in phloem
Diagram of an aphid penetrating a plant to suck phloem sap
Diagram showing the aphid’s stylet is being severed but phloem sap continues to flow from the stylet
Diagram of phloem sampling from aphid stylets at different distances can determine the rate of translocation
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If the aphid shown in the figure below had penetrated the xylem rather than the phloem with its stylet, what would you be most likely to find at the stylet after the aphid is removed?
No droplets emerging
The suction of transpiration pull could make water evaporate from the surface of the stylet but it is unlikely that there would be enough pressure to squeeze droplets out of the stylet.
Diagram of a transverse section of a young dicot stem (shoot)
Light micrograph of a transverse section of the root of a buttercup
The vascular tissue is at the center with the xylem forming an ‘X’ shape and the phloem filling the spaces around it.
Electron micrograph of a transverse section of monocot root
Which letter indicates phloem on this micrograph?
D
What does this diagram show?
A monocot stem
The vascular bundles are found in a scattered arrangement throughout the stem.
