Translocation

Question 1

What is glucose converted to for transport

Answer 1

Glucose, made in the leaves, is converted to sucrose for transport- converted back to glucose when it reaches cells that need it

Question 2

Source to sink

Answer 2

Translocation- organic compound from source to sink(tissues that need them) in phloem. Requires energy- up and down plant.

Question 3

What are assimilates

Answer 3

Products of photosynthesis that are transported, although glucose is made in photosynthesis, main assimilate is sucrose. Sucrose content of most cell sap is only around 0.5% but it can be 20-30% or the phloem sap content

Question 4

Main sources of assimilates in a plant:

Answer 4

-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.

Question 5

Main sinks in a plant are:

Answer 5

-roots that are growing and/or actively absorbing mineral ions.
-meristems that are actively dividing
-any parts of plant that are laying down food stores, such as developing seeds, fruits or storage organs.

Question 6

Fluid moving in phloem

Answer 6

Phloem sap

Question 7

Two types of tissue in phloem

Answer 7

Sieve tube element-long line of cells arranged end to end- almost all organelles have been lost like nucleus and vacuole-free to transport phloem sap.End walls contain large pores- sieve plates- sieve plates allow phloem sap to move between cells.

Companion cells-contain nucleus as well as large amounts of mitochondria, microscopic channels link the companion cells to the sieve tube element cells.-plasmodesmata- molecules such as atp and proteins can move through the plasmodesmata.
Role of companion cells is to provide essential molecules like protein and atp to the sieve tube element cells.

No lignin in cell walls.

Question 8

What supports the phloem

Answer 8

Fibres and sclereids- both have thickened cell walls containing lignin.

Fibres are long and narrow whereas sclereids have a variety of shapes

Question 9

Process of translocation

Answer 9

At source- sucrose is loaded into phloem by an active process

Cell membrane phloem companion cell, a protein on the membrane uses atp to pump hydrogen ions out of the cytoplasm and into the spaces of the cell wall-active transport- creates a concentration gradient for hydrogen ions, with not hydrogen ions on the outside of the cell membrane, hydrogen ions now flow through a cotransporter protein down the concentration gradient back into the cell. This inward flow of hydrogen ions is coupled with an inward flow of sucrose into the companion cell.

Companion cells have large number of mitochondria, provide the atp needed for the active transport of hydrogen ions, foldings on membrane increase surface area.

Now high conc of sucrose, sucrose can now diffuse through the plasmodesmata from companion to sieve tube element cells. Now high conc of sucrose in sieve tube element cells, this lowers water potential in sieve tube element. Water moves into sieve tube element by osmosis from surrounding xylem- increases hydrostatic pressure in sieve tube element, as a result phloem cell sap moves up or down sieve tube element towards sink- bulk movement of phloem sap is called mass flow. Sucrose moves of at sink- converted to glucose or starch. Sucrose leaves, increases water potential in sieve tube element, water moves out of sieve tube element by osmosis