Learning Outcomes 5-6-7

Question 1

active transport

Answer 1

Requires energy (usually from ATP) to move solutes against their concentration gradient.

Question 2

example of a process using active transport

Answer 2

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

Question 3

passive transport

Answer 3

no energy input required. ex: diffusion

Question 4

chemical potential gradient

Answer 4

the difference in chemical potential of solutes across a membrane, drives the movement of solutes from high potential to low potential

Question 5

how are proton gradients established

Answer 5

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).

Question 6

the proton gradient is used by which enzyme, and for what purpose

Answer 6

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

Question 7

channels, carriers, pumps

Answer 7

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

Question 8

Symport

Answer 8

Moves two molecules in the same direction across a membrane (e.g., sucrose-H⁺ symport).

Question 9

Antiport:

Answer 9

Moves two molecules in opposite directions (e.g., Na⁺/K⁺ pump in animals).

Question 10

Phloem:

Answer 10

Located toward the outer part of the vascular bundles, responsible for transporting sugars and nutrients.

Question 11

Xylem:

Answer 11

Located toward the inner part, responsible for water and mineral transport.

Question 12

4 major cell types in phloem (phloem elements)

Answer 12

Sieve Tube Elements
Companion Cells
Phloem Parenchyma
Phloem Fibers

Question 13

sieve tubes

Answer 13

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)

Question 14

companion cells

Answer 14

connected by plasmodesmata, every sieve tube has at least one to support in metabolic processes

Question 15

callose deposition

Answer 15

plants deposit callose at the sieve plates to seal damaged sieve tubes and prevent sap loss.

Question 16

Phloem Parenchyma

Answer 16

Involved in storage and lateral transport

Question 17

Phloem Fibers

Answer 17

Provide mechanical support.

Question 18

Plasmodesmata

Answer 18

channels connecting plant cells, allowing the transport of solutes, water, and signaling molecules between cells.

Question 19

companion cells and sieve tube elements are linked through

Answer 19

plasmodesmata

Question 20

Companion cells help with

Answer 20

loading and unloading of sugars

Question 21

intermediary cells

Answer 21

specialized companion cells involved in symplastic (plasmodesmata) loading

Question 22

intermediary cells have an extensive network of [blank] to connect them to [blank]

Answer 22

plasmodesmata, mesophyll cells

Question 23

companion cells are involved mainly with apoplastic or symplastic

Answer 23

apoplastic

Question 24

Have extensive cell wall ingrowths to increase surface area for solute transfer.

Answer 24

transfer cells

Question 25

explain the function of intermediary cells

Answer 25

facilitate passive movement of passive transport of sugars from mesophyll cells into intermediary cells, then actively into sieve tube elements

Question 26

polymer trapping mechanism

Answer 26

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

Question 27

source definition, example

Answer 27

produce sugars, leaves

Question 28

sink, define and exampls

Answer 28

store or consume sugars,

Question 29

Role of Sucrose Symporter

Answer 29

the active loading of sucrose into the phloem by coupling sucrose transport with proton movement (H⁺ gradient).

Question 30

Allocation

Answer 30

distribution of photosynthates (the products of photosynthesis, such as sugars) to different metabolic pathways (growth, maintenance of storage)

Question 31

partitioning

Answer 31

distribution of photosynthates and other resources to different plant organs or tissues.

Question 32

explain pressure flow model

Answer 32

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.

Question 33

how is sucrose loaded and unloaded

Answer 33

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