Learning Outcomes 5-6-7 Flashcards
active transport
Requires energy (usually from ATP) to move solutes against their concentration gradient.
example of a process using active transport
Loading sucrose into the phloem via the sucrose-proton symporter. Using the gradient from pushing H+ out of the cell (high [ ] outside), by H ATPase, this gradient drives the symporter to transport sucrose in
passive transport
no energy input required. ex: diffusion
chemical potential gradient
the difference in chemical potential of solutes across a membrane, drives the movement of solutes from high potential to low potential
how are proton gradients established
established by H ATPase, pumping protons out of the cell, via ATP hydrolysis which breaks down ATP into ADP + inorganic phosphate, against gradient (low to high).
the proton gradient is used by which enzyme, and for what purpose
ATP synthase to generate ATP during photosynthesis/respiration. ATP synthase uses the proton gradient and its proton motive force to synthesize ATP. Protons move down the gradient and this drives energy for synthesis from ADP + pi to ATP
channels, carriers, pumps
channels: down gradient, allows ions or molecules to move
carriers: bind specific solutes, undergo conformational change to transport them
pumps: against gradient, uses energy from gradient to transort solutes
Symport
Moves two molecules in the same direction across a membrane (e.g., sucrose-H⁺ symport).
Antiport:
Moves two molecules in opposite directions (e.g., Na⁺/K⁺ pump in animals).
Phloem:
Located toward the outer part of the vascular bundles, responsible for transporting sugars and nutrients.
Xylem:
Located toward the inner part, responsible for water and mineral transport.
4 major cell types in phloem (phloem elements)
Sieve Tube Elements
Companion Cells
Phloem Parenchyma
Phloem Fibers
sieve tubes
wide, stacked end to end, lacks nuclei, ribosomes, MT to maximize space to transport sap
perforated structures, sieve plates are,
sieve plates allow flow of phloem sap (perforated structures)
companion cells
connected by plasmodesmata, every sieve tube has at least one to support in metabolic processes
callose deposition
plants deposit callose at the sieve plates to seal damaged sieve tubes and prevent sap loss.
Phloem Parenchyma
Involved in storage and lateral transport
Phloem Fibers
Provide mechanical support.
Plasmodesmata
channels connecting plant cells, allowing the transport of solutes, water, and signaling molecules between cells.
companion cells and sieve tube elements are linked through
plasmodesmata
Companion cells help with
loading and unloading of sugars
intermediary cells
specialized companion cells involved in symplastic (plasmodesmata) loading
intermediary cells have an extensive network of [blank] to connect them to [blank]
plasmodesmata, mesophyll cells
companion cells are involved mainly with apoplastic or symplastic
apoplastic
Have extensive cell wall ingrowths to increase surface area for solute transfer.
transfer cells
explain the function of intermediary cells
facilitate passive movement of passive transport of sugars from mesophyll cells into intermediary cells, then actively into sieve tube elements
polymer trapping mechanism
symplastic phloem loading via intermediary cells
involves conversion of sucrose into larger sugars like raffinose and stachyose within the intermediary cells.
they cannot diffuse back through plasmodesmata and are too big, this ensures one way movement into sieve tube and traps them there
source definition, example
produce sugars, leaves
sink, define and exampls
store or consume sugars,
Role of Sucrose Symporter
the active loading of sucrose into the phloem by coupling sucrose transport with proton movement (H⁺ gradient).
Allocation
distribution of photosynthates (the products of photosynthesis, such as sugars) to different metabolic pathways (growth, maintenance of storage)
partitioning
distribution of photosynthates and other resources to different plant organs or tissues.
explain pressure flow model
Loading of sucrose at the source reduces water potential, causing water to enter from the xylem.
The increased pressure pushes the sap toward the sink.
At the sink, sucrose is unloaded, increasing water potential, and water returns to the xylem.
how is sucrose loaded and unloaded
loading at source
apoplastically - sucrose is loaded actively from mesophyl cells via cell wall into companion cells, using the sucrose symporter (using photon gradient) to push sucrose into companion cells into sieve tubes.
symplastically - sucrose is loaded from mesophyll to companion cells to sieve tubes via plasmodesmata passively. Then the polymer trapping mechanism applies, where sucrose is converted to larger sugars like raffinose in intermediary cells and cannot diffuse back through the plasmodesmata, ensuring one way movement.
loading sucrose at the source reduces the water potential, which causes a positive pressure that pushes water into sieve tubes from xylem, then this cause bulk flow to move from source to sink
unloading at sink
apoplastically - movemtn via cell wall where sucrose is exported into cell wall from sieve tubes and exported into sink via active transport, where it may be used for metabolism, storage, or cell wall biosynthesis
symplastically - movemnt via plasmodesmata where sucrose is exported into sink passively via plasmodesmata (common in growing tissues where sucrose is quickly metabolized)
unloading sucrose at the sink results in higher water potential in sieve tubes, since there’s less sucrose now, so water will leave the phloem
