L6_3.3.7 Translocation BCS

Translocation

LO: To describe and explain the mechanism of
translocation in plants

Q2: Why do plants
need to translocate

assimilates

throughout the

plant?

What is Osmosis?

Draw

Draw a xylem

Draw

Draw a phloem

Contents of Sieve Tubes

• Aphids can be used to

‘sample’ phloem sap

• Such experiments also

demonstrate mass flow of
phloem sap (around 10000 x
faster than diffusion)

Conclusion: The rate of movement
is too quick to be caused just by
diffusion.

Translocation

• Assimilates are organic molecules the plant

has made e.g. products of photosynthesis
such as sucrose.

• Assimilates are transported around the plant

via sieve tube elements

• Production sites known as SOURCES
• Storage or usage sites known as SINKS

Translocation is the transport of

assimilates through a plant

Explain why leaves and green fruits act as sources
while ripe fruits, roots, old leaves and stems are

sinks

Translocation

Phloem sap moves by
translocation, which
involves:
1) Active loading at

sources tissues

2) Mass flow through

sieve tubes

3) Active unloading at

sink tissues

Hotspot

Where is the highest concentration of glucose?

Hotspot

Where is the highest water potential?

Hotspot

Where are the H⁺ ions actively pumped?

Hotspot

Where is the sucrose/H⁺ protein pump?

Hotspot

which cell has the most mitochondria?

1. Active loading at the source

• Companion cell uses ATP

to pump H+out into cell
wall

• H+ diffuses back (down

gradient) into companion
cell via cotransporter
proteins accompanied by
sucrose.

• Concentration of sucrose

is now high in companion
cell → diffuses into sieve
tube elements via
plasmodesmata

Companion cell Source cell

High [H+]

Low [H+]

Active loading - Also called
secondary active transport

SOURCE
2. Mass flow

1. Increasing sucrose concentration in sieve

tubes near source makes Ψ more –ve.

2. Water moves into phloem from xylem

3. At sink sucrose used up or converted to

starch so Ψ less –ve than source.

4. Water moves out of sink into xylem

5. This creates a flow due to differences in

hydrostatic pressure →known as mass
flow

More
positive
hydrostatic
pressure

More
negative
hydrostatic
pressure

SINK

This process is

completely

passive

3. Transfer of sucrose from sieve tube

elements

• Sucrose diffuses or is

actively transported
out of sieve tube
elements by
companion cells into
sink.

Card Sort

Try completing the card sort for this

process

Reorder

Put the following in order

1

2

3

4

• 1
• 2
• 3
• 4

Reorder

Put the following in order

1

2

3

4

5

• 1
• 2
• 3
• 4
• 5

1. Triose sugars are produced in photosynthesising leaves, some of

which are converted to sucrose.Sucrose moves from mesophyll cells
across leaf to phloem tissue, using either symplast or apoplast
pathway.

2. At the phloem tissue, a large excess of H+ions is created in the

apoplast outside the companion cells as H+ions are pumped out of
companion cells into the cell wall using ATP.

3. H+ions move back into the cell down their concentration gradient,

through a protein which acts as a cotransporter for H+ions and
sucrose.

4. Sucrose molecules then move from the companion cell into the sieve

tube, through the plasmodesmata which connect them (the symplast
pathway).

5. Loading a high concentration of sucrose into a sieve element greatly

decreases the water potential of the sap inside it.

6. Water moves down the water potential gradient by osmosis,

entering the sieve element. This causes a corresponding increase
in hydrostatic pressure.

7. A hydrostatic pressure gradient is created between source and sink

and causes a mass flow of water and dissolved solutes through the
through the sieve tubes, from high to low pressure.

8. At the sink, sucrose moves out of the phloem into surrounding

tissue by either the symplast or apoplast routes. Similar methods to
those used for loading are used for this unloading process.

9. Once in the tissue, sucrose is converted into something else by

enzymes, e.g invertase enzyme hydrolyses sucrose into glucose and
fructose. This decreases sucrose concentration and maintains
concentration gradient.

10.Water follows sucrose by osmosis, maintaining the hydrostatic

pressure gradient and allowing mass flow to continue.

Translocation Qs

Open Ended

Explain, with a suitable example, how some parts of the plant can act as both a 'source' and a 'sink' (2)

Open Ended

With reference to the diagram explain the process of the loading of sucorse into the phloem and its movement in the phloem (6)

Translocation Qs