CH9 translocation

Question 1

What is translocation

Answer 1

• process that moves organic compound from sources to sinks
• active process
• substances can be transported up and down the plant

Question 2

What are assimilates

Answer 2

• products of photosynthesis
• mainly sucrose

Question 3

Main sources of assimilates

Answer 3

• 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 4

Main sinks in a plant

Answer 4

• roots that are growing or actively absorbing mineral ions
• meristems that a re actively dividing
• any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs

Question 5

Phloem loading

Answer 5

• soluble products of photosynthesis moving into phloem from sinks
• two main ways: symplast route and apoplast route

Question 6

Symplast route

Answer 6

• sucrose from source moves through cytoplasm of mesophyll cells and into sieve tubes by diffusion through the plasmodesmata
• sucrose ends up in sieve elements and water follows by osmosis
• creates a pressure of water that moves the sucrose through the phloem by mass flow

Question 7

Apoplast route, from source to companion cells

Answer 7

• sucrose from source travels through cell walls and inter-cell spaces to companion cells and sieve elements
• by diffusion down a concentration gradient, which is maintained by removal of sucrose into phloem vessels

Question 8

Movement of sucrose into cytoplasm of companion cells

Answer 8

• active process
• hydrogen ions are actively pumped out of companion cell into surrounding tissue using ATP
• hydrogen ions return to companion cells down a concentration gradient via a co-transport protein
• sucrose is molecule that is transported, increasing concentration in companion cells and into sieve tubes sieve tube elements

Question 9

Adaptations of companion cells for active transport

Answer 9

• many infoldings in cell membranes for an increased surface area
• have many mitochondria to supply ATP

Question 10

Apoplast route, from companion cells to phloem

Answer 10

• build up of sucrose in companion cells and sieve tubes elements causes water to move in by osmosis, leading to a build up of turgor pressure due to rigid cell walls
• water carrying assimilates moves into tubes of sieve elements, reducing the pressure in the companion cells and moves up or down plant by mass flow to areas of lower pressure
• solute accumulation in source phloem leads to an increase in turgor pressure that forces sap to regions of lower pressure in sinks

Question 11

Phloem unloading

Answer 11

• sucrose is unloaded from phloem at any point that cells need it
• main mechanism is diffusion, and sucrose is then converted into another substance
• maintains steep concentration gradient

Question 12

Evidence for details of translocation

Answer 12

• advances in microscopy allow us to see adaptations of companion cells for active transport
• if mitochondria of companion cells are poisoned, translocation stops
• flow of sugars in phloem would be much slower if it was diffusion alone