9.2 Transport in the phloem of plants Flashcards
What is translocation?
Translocation is the movement of organic compounds from sources to sinks
What is a source?
Where the organic compounds are synthesised
What is a sink?
Where the compounds are delivered to for use or storage
What are the sources in plants?
Photosynthetic tissue aka leaves
What are the sinks in plants?
Roots
Fruits
Seeds
How are organic compounds transported from sources to sinks?
Via the phloem
What is the phloem?
A vascular tube system
It is responsible for transporting organic compounds from sources to sink
How are sugars transported and why?
They are transported as sucrose because it is soluble but metabolically inert
What is nutrient- rich viscous fluid of the phloem?
Plant sap
What are the two types of cells phloem sieve tubes composed of?
Sieve element cells and companion cells
In addition to sieve element cells and companion cells, what cells does the phloem also have and what do they do?
Sclerenchymal and parenchymal cells which fil additional spaces and provide support
What are sieve element cells?
They are long and narrow cells that are connected together to form the sieve tube
How are sieve elements connected?
By sieve plates at their transverse ends which are porous to enable flow between cells
What do sieve elements lack and how is it helpful?
They lack a nuclei and have reduced organelles to increase flow rate
What helps sieve elements withstand hydrostatic pressure?
Thick and rigid cell walls
What do companion cells do?
Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
Why do companion cells have an infolding plasma membrane?
It increases surface area to volume ratio to allow more material exchange
Why do companion cells have many mitochondria?
To fuel the active transport of materials between the source and sink and the sieve tube
What do companion cells have to move materials in and out of the sieve tube?
Transport proteins
If the sieve element did not have the support of a companion cell what would it not be able to do and why?
Sustain independent metabolic activity because sieve elements have no nuclei and fewer organelles
What exists between sieve elements and companion cells in relatively large numbers?
Plasmodesmata
What does plasmodesmata do?
Connect the cytoplasm of companion and sieve element cells and mediate the symplastic exchange of metabolites
What does the phloem transport?
Transports food and water
How many ways is the movement in phloem?
Two ways
What do connecting sieve cells share?
A highly perforated sieve plate
What mediates the movement of sap?
Hydrostatic pressure from the xylem
How can you usually differentiate xylem and phloem cells in a microscopic image?
By the diameter of their cavity
Does the xylem or the phloem have a larger cavity?
The xylem
What are xylem and phloem grouped into?
Bundles that extend from the roots to the shoots in vascular plants
Where can you find differences in between plant types?
Distribution and arrangement
What are two different plant types?
Monocotyledones and Dicotyledons
In monocotyledons roots how does the stele present itself?
The stele is large and vessels will form a radiating circle around the central pith
When talking about the vessels surrounding the stele in roots, where are xylem vessels located?
More internally
When talking about the vessels surrounding the stele in roots, where are phloem vessels located?
More externally
In dicotyledons roots how does the stele present itself?
The stele is very small and the xylem is located centrally with the phloem surrounding it
In dicotyledons roots how do xylem vessels form?
They form a cross shape
In dicotyledons roots where are phloem situated?
In the surrounding gaps
In monocotyledons stems, where are the vascular bundles found?
In a scattered arrangement throughout the stem
In monocotyledons stems, where are the phloem vessels positioned?
Positioned externally towards the outside of the stem
In dicotyledons stems, how are vascular bundles arranged?
In a circle around the centre of the stem
In dicotyledon stems what separates phloem and xylem vessels and in which order are they separated?
The cambium
Xylem on the inside and phloem on the outside
How does the organic compounds produced at the source loaded into phloem sieve tubes?
Actively by companion cells
How can materials pass into the sieve tube?
Via interconnecting plasmodesmata
How can materials travel across the intervening cell wall?
They can be pumped by membrane proteins
What describes the transport of apoplastic loading of sucrose into the phloem sieve tubes?
Active transport that requires ATP expenditure
What are actively transported out of phloem cells and how?
Hydrogen ions are actively transported out of phloem cells by proton pumps
How is a proton gradient created?
The concentration of hydrogen ions consequently builds up outside of the cell
How do hydrogen ions diffuse back into the phloem cell?
Via a co transport protein which requires sucrose movement
What does the diffusion of hydrogen ions back into the cell result in?
A build up of sucrose within the phloem sieve tube for subsequent transport from the source
What makes the sap solution at the source to become hypertonic?
The active transport of solutes into the phloem by companion cells
What does hypertonic sap solution at the source result in?
Water to be drawn from the xylem via osmosis
Why does hydrostatic pressure at the source increase?
The incompressibility of the water that has built up in the phloem
What does the increase in hydrostatic pressure force?
It forces the phloem sap to move towards areas of lower pressure
What does the phloem sap moving towards areas of lower pressure cause?
Phloem transports solutes away from the source and to the sink
What happens to the solutes that are unloaded from the phloem?
They are transported into sinks
What do companion cells unload from the phloem?
Solutes
What is the sap solution at the sink?
Hypotonic
What does the sap solution being hypotonic then cause?
Water being drawn out of the phloem and back into the xylem via osmosis
What does water being drawn out of the phloem and back into the xylem via osmosis ensure?
That the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
What does a low hydrostatic pressure at the sink mean?
Phloem sap will always move from the source towards the sink
What are organic molecules when they are transported into the sink?
Either metabolised or stored with the tonoplast of vacuoles
What are aphids?
A group of insects which feed mainly on sap from phloem
What is a stylet?
A mouthpiece of aphids which pierces the plants sieve tube to allow sap to be extracted
What aids the penetration of the stylet into the sieve tube?
Digestive enzymes that soften the intervening tissue layers
What happens if the stylet is severed and why?
Sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube
Why are aphids good for providing a measure of phloem transport rates?
Because aphids collect sap at various sites along a plants length
When measuring phloem transport using aphids, where is the plant grown?
Within a lab with the leaves sealed within a glass chamber containing radioactively labelled CO2
When measuring phloem transport using aphids, what will the leaves convert the CO2 into?
Into radioactively labelled sugars which are transported by the phloem
When measuring phloem transport using aphids, where are the aphids placed and then what happens?
Along the plants length and are then encouraged to feed on the phloem sap
When measuring phloem transport using aphids, what happens once the feeding has commenced?
The aphid stylet is severed and sap continues to flow from the plant at the selected positions
When measuring phloem transport using aphids, why is the sap analysed?
For the presence of radioactively labelled sugars
When measuring phloem transport using aphids, how can it be calculated?
Based on the time taken for the radioisotope to be detected at different positions along the plants length
What is phloem transport also known as?
Translocation rate
What determines the translocation rate?
The concentration of dissolved sugars in the phloem
What affects the concentration of dissolved sugars in the phloem sap?
The rate of photosynthesis
The rate of cellular respiration
The rate of transpiration
The diameter of the sieve tubes
What affects the rate of photosynthesis?
Light intensity
CO2 concentration
Temperature
What affects the rate of cellular respiration?
Any factor which physically stresses the plant
What does the rate of transpiration determine?
How much water enters the phloem
What does the diameter of sieve tubes affect?
Hydrostatic pressure but it may differ between plant species
