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Which cells in the phloem tissue are primarily responsible for the translocation of substances?
Which cells in the phloem tissue are primarily responsible for the translocation of substances?
What experimental technique did M. Malpighi use to study phloem transport?
What experimental technique did M. Malpighi use to study phloem transport?
What is the primary component that Sieve tube elements are rich in?
What is the primary component that Sieve tube elements are rich in?
Which of these options describes a consequence of girdling on a plant?
Which of these options describes a consequence of girdling on a plant?
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Which type of organisms lack P-protein in their sieve tube elements?
Which type of organisms lack P-protein in their sieve tube elements?
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In what forms can P-protein appear in sieve tube elements?
In what forms can P-protein appear in sieve tube elements?
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What role does P-protein serve in sieve elements?
What role does P-protein serve in sieve elements?
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What type of cells primarily store and release food molecules in phloem tissue?
What type of cells primarily store and release food molecules in phloem tissue?
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What role do xylem parenchyma cells serve when modified as transfer cells?
What role do xylem parenchyma cells serve when modified as transfer cells?
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What is a distinctive feature of intermediary cells?
What is a distinctive feature of intermediary cells?
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What defines the direction of sap translocation in phloem?
What defines the direction of sap translocation in phloem?
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What types of organs typically serve as sources in plants?
What types of organs typically serve as sources in plants?
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What type of cells are primarily responsible for accumulating sugars during the first year of the wild beet's lifecycle?
What type of cells are primarily responsible for accumulating sugars during the first year of the wild beet's lifecycle?
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What is the primary function of plasmodesmata in xylem parenchyma and intermediary cells?
What is the primary function of plasmodesmata in xylem parenchyma and intermediary cells?
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Which statement is true regarding sinks in plants?
Which statement is true regarding sinks in plants?
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What can be inferred about the cost of maintaining plasmodesmata in cells?
What can be inferred about the cost of maintaining plasmodesmata in cells?
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What is the primary function of phloem translocation in plants?
What is the primary function of phloem translocation in plants?
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Which term describes the conversion of photoassimilates to storage forms like starch or sucrose?
Which term describes the conversion of photoassimilates to storage forms like starch or sucrose?
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Which of the following tissues surrounds the small veins in the vascular tissue of leaves?
Which of the following tissues surrounds the small veins in the vascular tissue of leaves?
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What are the cells called that conduct sugars and other organic materials in the phloem?
What are the cells called that conduct sugars and other organic materials in the phloem?
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Which type of plants primarily exhibit highly differentiated sieve tube elements?
Which type of plants primarily exhibit highly differentiated sieve tube elements?
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Phloem translocation is significant from an agricultural perspective mainly because it affects which of the following?
Phloem translocation is significant from an agricultural perspective mainly because it affects which of the following?
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What role does carbon partitioning serve in plants?
What role does carbon partitioning serve in plants?
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What is the location of phloem in relation to other vascular tissues in plants with secondary growth?
What is the location of phloem in relation to other vascular tissues in plants with secondary growth?
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What is the primary sugar transported in the phloem?
What is the primary sugar transported in the phloem?
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Which of the following substances is contained in the phloem sap?
Which of the following substances is contained in the phloem sap?
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In which species is photosynthate readily transferred from a partly defruited branch to a partly defoliated branch?
In which species is photosynthate readily transferred from a partly defruited branch to a partly defoliated branch?
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What is the most abundant substance transported in the phloem?
What is the most abundant substance transported in the phloem?
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Which of the following hormones has been found in sieve elements?
Which of the following hormones has been found in sieve elements?
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Which inorganic solute is relatively immobile in the phloem?
Which inorganic solute is relatively immobile in the phloem?
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What types of proteins found in the phloem are involved in the sealing of wounded sieve elements?
What types of proteins found in the phloem are involved in the sealing of wounded sieve elements?
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Which of the following is NOT typically considered part of the phloem transport system?
Which of the following is NOT typically considered part of the phloem transport system?
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What is the process by which sugars are transported from mesophyll cells to sieve elements called?
What is the process by which sugars are transported from mesophyll cells to sieve elements called?
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Which structures are often regarded as a functional unit during the loading of sugars?
Which structures are often regarded as a functional unit during the loading of sugars?
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What is the first step in the import of sugars into sink cells?
What is the first step in the import of sugars into sink cells?
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What term describes the transport of photosynthates from the sieve elements to sink cells?
What term describes the transport of photosynthates from the sieve elements to sink cells?
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Which process describes the movement of sugars from the source to the sink in plants?
Which process describes the movement of sugars from the source to the sink in plants?
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What term is used to describe the regulation of fixed carbon into various metabolic pathways?
What term is used to describe the regulation of fixed carbon into various metabolic pathways?
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After sieve element unloading, how are sugars transported to sink cells?
After sieve element unloading, how are sugars transported to sink cells?
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What happens to sugars in sink cells after they are imported?
What happens to sugars in sink cells after they are imported?
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What is the term used to describe the differential distribution of photosynthates within a plant?
What is the term used to describe the differential distribution of photosynthates within a plant?
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Which two factors determine sink strength?
Which two factors determine sink strength?
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What effect does cooling a sink tissue have on photosynthate transport?
What effect does cooling a sink tissue have on photosynthate transport?
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Which of these activities can potentially limit the rate of uptake by a sink?
Which of these activities can potentially limit the rate of uptake by a sink?
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What would likely happen if the size of a sink tissue is increased?
What would likely happen if the size of a sink tissue is increased?
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What is the role of acid invertase and sucrose synthase in photosynthate transport?
What is the role of acid invertase and sucrose synthase in photosynthate transport?
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A mutant corn plant with a defective enzyme for starch synthesis would likely display what characteristic?
A mutant corn plant with a defective enzyme for starch synthesis would likely display what characteristic?
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Which of the following statements about sink activity is true?
Which of the following statements about sink activity is true?
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Study Notes
Introduction to Translocation in Phloem
- Translocation is the movement of assimilates (products of photosynthesis) from source leaves to sink tissues where they are stored or metabolized
- Phloem is the vascular tissue responsible for translocation
- Phloem bundles are often surrounded by a bundle sheath, which isolates the veins from the intercellular spaces in the leaf
- Carbon partitioning is the efficient distribution of photosynthetic energy throughout the plant
- Carbon allocation is the conversion of photoassimilates to sucrose or starch
- Phloem translocation affects crop yield, herbicide effectiveness, and plant productivity
Pathways of Translocation
- Phloem is generally found on the outer side of primary and secondary vascular tissues
- Sieve elements: conduct sugars throughout the plant
- Sieve tube elements (angiosperms)
- Sieve cells (gymnosperms)
- Companion cells: involved in metabolic functions
- Fibers and sclereids provide support and protection
- Phloem removal (girdling) experiments show photosynthates move through phloem
- Water and other inorganic molecules are also transported through the phloem
Companion Cells and Sieve Elements
- Each sieve tube element is associated with one or more companion cells
- Companion cells are connected to sieve elements by plasmodesmata
- Companion cells take over some of the metabolic functions of sieve elements
- Companion cells contain mitochondria, which provide energy (ATP) to sieve elements
- Three distinct companion cell types exist in mature, exporting leaves:
- Ordinary companion cells
- Transfer cells
- Intermediary cells
Materials Translocated in Phloem
- Sucrose is the primary transported sugar
- Amino acids, hormones, and some inorganic ions are also transported
- Concentrations of sucrose can range from 0.3 to 0.9 M
Rates of Movement
- Translocation rates can be measured as velocity or mass transfer rate
- Mass transfer rates (grams per hour per cm²) are preferred to measure velocity through sieve elements
Pressure-Flow Model of Translocation
- Phloem translocation is generally explained by the pressure-flow model
- The model proposes a pressure gradient between source and sink driven by osmotic pressure
- Phloem loading leads to a lower solute potential and higher turgor pressure at the source
- Phloem unloading results in higher solute potential and lower turgor pressure at the sink
- Water moves from high water potential (source) to low water potential (sink)
Phloem Loading and Unloading
- Phloem loading occurs in mature source leaves
- Triose phosphate, formed by photosynthesis, is converted to sucrose
- Sucrose moves from mesophyll cells to sieve elements
- Sieve element loading concentrates sucrose and other solutes in sieve elements
- Sieve element unloading occurs in sink tissue
- Unloading to sink cells is the reverse of the processes in source tissues
Factors Affecting Sink Strength
- Sink strength is a function of sink size and sink activity
- Sink size is the total weight of sink tissue and sink activity is the rate of photosynthate uptake per unit sink weight
- Sink strength affects translocation patterns
- Cooling a sink tissue inhibits activities that require metabolic energy and slows down the speed of transport.
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Description
This quiz explores the process of translocation in phloem, detailing the movement of assimilates from source leaves to sink tissues. It covers the structure and function of phloem, including sieve elements and companion cells, as well as the impact of translocation on crop yield and plant productivity.
