Translocation

What tissue is involved in translocation?

Phloem

What is translocation?

The transport of assimilates such as sucrose in plants

What are phloem vessels made of?

Living cells

What is the function of phloem vessels?

Transport nutrients to growing and storage tissues

What are sieve tube elements?

Specialised cells that form tubes to transport sucrose

What substance is transported in the phloem?

Sucrose dissolved in solution

What are companion cells?

Cells that provide ATP for active processes in phloem

Why do companion cells need many mitochondria?

To produce ATP for active transport

How are sieve tube elements and companion cells connected?

By plasmodesmata

What are plasmodesmata?

Gaps between cell walls allowing movement of substances

What are assimilates?

Products of photosynthesis such as sucrose

What is a source?

A region that releases sucrose into the phloem

Give an example of a source

Leaves

What is a sink?

A region that removes sucrose from the phloem

Give examples of sinks

Roots and meristems

Is translocation an energy requiring process?

Yes

What is active loading?

The process of loading sucrose into the phloem using ATP

What happens first in active loading?

Companion cells use ATP to pump H+ ions out

Why are hydrogen ions pumped out of companion cells?

To create a diffusion gradient

How do hydrogen ions re-enter companion cells?

By diffusion

How does sucrose enter companion cells?

By co-transport with H+ ions via facilitated diffusion

What happens to sucrose concentration in companion cells?

It increases

How does sucrose enter sieve tube elements?

By diffusion through plasmodesmata

What effect does sucrose entry have on water potential in sieve tubes?

It lowers water potential

Where does water enter the sieve tube from?

The xylem

How does water enter the sieve tube?

By osmosis

What pressure is created in sieve tubes at the source?

High hydrostatic pressure

How does water move along the sieve tube?

From high to low hydrostatic pressure

How is sucrose removed at the sink?

By diffusion or active transport

What happens to water potential in sieve tubes at the sink?

It increases

What happens to water at the sink?

It leaves the phloem and returns to the xylem

What happens to pressure in the phloem at the sink?

It decreases

What is mass flow?

The movement of substances down a hydrostatic pressure gradient

What theory explains translocation in plants?

The mass flow hypothesis

What does mass flow transport?

Assimilates such as sucrose from source to sink

What evidence shows pressure exists in sieve tubes?

Sap flows out when a stem is cut

How does sucrose concentration differ between source and sink?

Higher in sources than sinks

What happens after sucrose levels increase in leaves?

Sucrose concentration increases in the phloem

How do metabolic poisons affect translocation?

They inhibit sucrose transport

Why does lack of oxygen inhibit translocation?

ATP production stops

Why do sieve plates challenge mass flow theory?

They appear to slow down flow

What is one suggested function of sieve plates?

Prevent bursting under pressure

Why is unequal solute speed evidence against mass flow?

Mass flow predicts equal speed

Why is uniform delivery rate evidence against mass flow?

Flow should favour lowest sucrose concentration

What is a ringing experiment?

Removal of bark and phloem from a stem

What tissue remains after ringing?

Xylem

What happens above the ring in a ringing experiment?

Sucrose accumulates and tissue swells

What happens below the ring?

The tissue dies

What does the ringing experiment show?

Sucrose is transported in the phloem

What is a tracer experiment?

Use of radioactive substances to track transport

What radioactive gas is used in tracer experiments?

Carbon dioxide labelled with carbon-14

How does radioactive carbon enter sugars?

Through photosynthesis

How are labelled sugars tracked in plants?

Using autoradiography

What does a blackened region on autoradiography indicate?

The presence of transported sugars

What conclusion do tracer experiments support?

Sugars are transported in the phloem