Translocation Flashcards
(12 cards)
State the form in which carbohydrates are transported in plants.
Sucrose
What is translocation?
The movement of organic solutes around a plant in the phloem
What is a source? And what is a sink?
- Source => regions of a plant that produce assimilayes (e.g. glucose) by photosyntheiss or from storage materials e.g. leaves, storage organs
- Sink => regions of a plant that require assimilates to supply their metabolic needs e.g. roots, fruits
What are assimilates?
The products of photosynthesis that are transported around a plant eg. sucrose
What are 3 examples of sources?
- Green leaves and green stems
- Storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period
- Food stores in seeds when they germinate
What are 3 examples of sinks?
- Roots that are growing and/or actively absorbing mineral ions
- Meristems that are actively dividing
- Any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs
What is phloem loading?
- The way in which plants load assimilates (e.g. sucrose) into the phloem to be transported
- It can occur actively or passively
Describe the symplast route for phloem loading and explain how it occurs.
Sucrose production in mesophyll cells:
- Sucrose is produced in mesophyll cells by photosynthesis.
Movement through plasmodesmata:
- Sucrose moves passively through the cytoplasm of adjacent cells via plasmodesmata (small cytoplasmic channels between plant cells)
- It passes through bundle sheath cells and companion cells into the sieve tube elements of the phloem.
Concentration gradient maintained:
- A concentration gradient is maintained by the conversion of sucrose into larger sugars (like raffinose or stachyose) in intermediary cells or companion cells
- This prevents sucrose from diffusing back and keeps the gradient going
Mass flow begins:
- Once loaded into sieve tubes, the sucrose reduces water potential, drawing in water by osmosis from the xylem
- This creates hydrostatic pressure that drives the mass flow of phloem sap toward sinks (e.g. roots, growing tissues) where there is a low turgor pressure
Describe the apoplast route for phloem loading and explain how it occurs
Sucrose production in mesophyll cells:
- Sucrose is made in photosynthesising mesophyll cells via photosynthesis.
Movement through the apoplast:
- Sucrose diffuses out of the mesophyll cells into the cell walls (apoplast), moving through the cell wall spaces towards the companion cells and sieve tube elements of the phloem
Active transport into companion cells:
- Hydrogen ions (H⁺) are actively pumped out of companion cells into the apoplast using ATP-powered proton pumps
- This creates a proton gradient (more H⁺ outside the companion cell).
Sucrose-H⁺ co-transport:
- Sucrose is transported into the companion cells through co-transporter proteins that allow H⁺ ions to re-enter along the gradient, bringing sucrose with them (secondary active transport).
Sucrose enters sieve tube elements:
- Sucrose moves from the companion cells into the sieve tube elements via plasmodesmata
Mass flow begins:
- The high concentration of sucrose lowers the water potential in the phloem.
- Water enters by osmosis from the xylem, increasing turgor (hydrostatic) pressure
- This pressure drives the mass flow of phloem sap toward sinks (e.g., roots, fruits, growing tissues) where there is a low turgor pressure
How are companion cells adapted to their function?
- Many mitochondria to produce ATP for active transport
- Infoldings in cell membranes to increase surface area for transport
Describe the process of phloem unloading.
- The sucrose is unloaded from the phloem at any point into the cells that need it
- The main mechanism of phloem unloading seems to be by diffusion of the sucrose from the phloem into the surrounding cells
- The sucrose rapidly moves on into other cells by diffusion or is converted into
another substance (for example glucose for respiration, starch for storage) so that a concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells - The loss of the solutes from the phloem leads to a rise in the water potential of the phloem. Water moves out into the surrounding cells by osmosis
- Some of the water that carried the solute to the sink is drawn into the transpiration stream in the xylem
Describe and explain 4 sources of evidence for the processes involved in translocation.
- Advances in microscopy allows us to see adaptations of the companion cells for active transport
- If the mitochondria of the companion cells are poisoned, translocation stops
- The flow of sugars in phloes is about 10,000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow
- Aphids can be used to demonstrate the translocation of organic solutes in the phloem. Using evidence from aphid studies, it has been shown that there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore the flow rate in the phloem is lower closer to the sink that it is near the source. The concentration of sucrose in the phloem sap is also higher near to the source than near the sink
