9.2 (Plant Biology) Flashcards
Define translocation, phloem sap, source and sink.
Translocation: transport of organic solutes in a plant
Phloem sap: the substance in which organic compounds are transported from sources to sinks
Source: a site where organic compounds are loaded into the phloem
Sink: a site where compounds are unloaded from the phloem
List example source and sink tissues.
Sources:
Photosynthetic tissues - mature green leaves - green stems.
Storage organs that are unloading their stores - storage tissues in germinating seeds - tap roots or tubers at the start of the growth session.
Sinks:
Roots that are growing or absorbing mineral ions using energy from cell respiration.
Parts of the plant that are growing or developing food stores -developing fruits -developing seeds -growing leaves -developing tap roots or tubers
State that phloem transport is bidirectional.
Phloem transport is bidirectional because sucrose gets accumulated in the phloem tissue and absorbs water creating a high turgor pressure. Other areas have low turgor pressure due to loss of water.
Outline why pressure in the phloem increases due to the movement of water into the phloem.
The build up of sucrose and other carbohydrates draws water into the companion cells through osmosis. The rigid cell walls combined with the incompressibility of water result in a build-up of pressure. Water will flow from this area of high pressure to an area of low pressure. At the sink end, sucrose is withdrawn from the phloem and either utilized as an energy source for such processes as growth or converted to starch. In either case, the loss of solute causes a reduction in osmotic pressure and the water that carried the solute to the sink is then drawn back in to the transpiration stream in the xylem.
State that sucrose is the most prevalent solute in phloem sap.
sucrose is the most prevalent solute in phloem sap.
Outline why sucrose is used for phloem transport, as opposed to glucose.
Sucrose is not as readily available for plant tissues to metabolize directly in respiration and therefore makes a good transport form of carbohydrate as it will not be metabolized during transport.
Describe the active transport of sucrose into the phloem via a co-transport protein.
Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source
State that the phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem.
The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
State that water moves into the phloem by osmosis.
This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
State that water moves from area of higher pressure to area of lower pressure and that the movement of water also moves the solutes dissolved in it.
the solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
Consequently, water is drawn out of the phloem and back into the xylem by osmosis
This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
Hence, phloem sap will always move from the source towards the sink
When organic molecules are transported into the sink, they are either metabolized or stored within the tonoplast of vacuoles
Outline the structure and function of sieve tube cells, with specific mention of the rigid cell wall and sieve plates.
Sieve elements are long and narrow cells that are connected together to form the sieve tube
Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
Sieve elements have no nuclei and reduced numbers of organelles to maximize space for the translocation of materials
The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
Outline the structure and function of companion cells, with specific mention of mitochondria and cell membrane infolding.
Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
State two ways xylem cells can be identified in cross sections of stem and root.
Xylem cells are generally larger than phloem cells. Within one vascular bundle, phloem cells tend to be closer to the outside of the plant in stems and roots.
State that aphids consume phloem sap as the main component of their diet.
Phloem sap is nutrient rich compared with many other plant products and the nutrients in it are small soluble molecules that do not need to be digested. Despite this, the only animals to consume it as the main part of their diet are insects belonging to a group called the Hemiptera, which includes aphids, whitefly, mealybugs and psyllids.
Outline how aphids have been used to measure the rate of flow and composition of phloem sap.
A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
The sap is then analysed for the presence of radioactively-labelled sugars
The rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length
