9.2 - Transport in the Phloem of Plants Flashcards
Plants transport organic compounds from sources to sinks
1)Define translocation, phloem sap, source and sink
2) list example source and sink tissues
3) state the phloem transport is bidirectional
Translocation is the movement of organic compounds from sources to sinks. Organic molecules such as sucrose and amino acids move vis phloem tubes in plants
Phloem is composed of living tissue called sieve tube members that are joined end to end to form a tube that conducts food materials through the plant - bordered by companion cells - can carry sugars in variety of directions (bidirectional)
Sources produce sugars by photosynthesis (leaves or green stems or roots) and deliver these via the phloem to sinks (roots, buds, stem, seed, fruits).
At the source, sugar and other organic molecules loaded into the sieve tube creating higher solute conc in sieve tube - water from surrounding tissue ensters vis osmosis creating hydrostatic pressure at source that pulls sap to sink
Incompressibility of water allows transport along hydrostatic pressure gradients
1) Outline why pressure in the phloem increases due to the movement of water into the phloem
At normal temperature the compressibility of water is so low - any changes in pressure at one end of the transport vessels will result in equal change in pressure at other end of the transport vessel
Build up of sucrose and solutes causes water to enter the companion cells through osmosis - this causes a build up os hydrostatic pressure. Water flows from high to low pressure. Sucrose is used as an energy source at the sink for growth or converted to starch for storage - water is carries the solute is drawn back into the xylem
Active transport is used to load organic compounds into phloem sieve tubes at the source
1) State that sucrose is the most prevalent solute in phloem sap
2) Outline why sucrose is used for phloem transport as opposed to glucose
3) Describe the active transport of sucrose into the phloem via a co-transporter protein
Organic compounds produced at the source actively loaded into phloem sieve tubes by companion cells. At the source sugars are loaded through active transport in process called phloem loading
In some plants - sugars travel through cells to cell walls of companion cells and some sieve cells. Then actively transported into the phloem by sucrose transport protein (Apoplast) Protons pumped out of the companion cells from tissues by active transport - ATP as energy source - creates a proton gradient - H binds with sucrose and flows back into the companion cell tube complex through co-transporter protein
Sucrose can also travel through plasmodesmata (symplast route)
High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
1) State the phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem
2) State that water moves into the phloem by osmosis
Active transport of solutes into phloem makes sap hypertonic - causes water to enter by osmosis
Raised hydrostatic pressure cause the contents of the phloem to flow towards sinks
1) State that water moves from area of higher pressure to area of lower pressure an that the movement of water also moves the solutes dissolved in it
at the source, sugar and other organic molecules are loaded into the sieve tube members thus increasing solute conc within the sieve tube cells. Water from surrounding tissues, enters the sieve tube members by osmosis following the conc gradient
Structure - function relationship of phloem sieve tubes
1)State the function of phloem including loading of carbs at a source, transport and carbs through plat and unloading at sink
2) Outline the structure of sieve tube cells with specific mention of the rigid cell wall and sieve plates
3) Outline the structure and function of companion cells with specific mention of mitochondria and cell membrane infolding
Sieve tube cell are living - membranes help maintain sucrose and organic molecule conc by active transport
Companion cells - perform many of the genetic and metabolic functions of the sieve tube cell to maintain its viability
Mitochondria - energy supply
Infolding of plasma membrane - Increases phloem loading capacity
Plasmodesmata - connect the cytoplasm of companion cells with sieve tube elements
active transport - allows for the accumulation of sucrose in the sieve tube cell
Rigid cell walls - allows the creation of enough pressure to achieve the flow of phloem in the sieve tube cell
Identification of xylem and phloem in microscope images of stem and root
xylem and phloem / Dicot and Monocot ( vascular bundle / epidermis
Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide
1) State that aphids consume phloem sap as the main component of their diet
2) Outline how aphids have been used to measure the rate of flow and composition of phloem sap
Radioactively labeled carbon 14 contained within c02 can be fixed by plants by photosynthesis - the carbon in these sugars created by photosynthesis is metabolized be the plant and will be found in different molecules within the plant - can be detected using film
Aphids uses stylets to tap into phloem - aphid body cut from the stylet after stylet inserted into the phloem - phloem sap continues through the stylet - radioactive carbon can be detemined in the cap