Translocation

what is translocation?

the transport of assimilates throughout the plant in the phloem by mass flow (source to sink)

what are assimilates?

• mainly sucrose

• amino acids

where are sugars made (and what type of sugar and what is it turned into)?

• in the leaves

• glucose is made

• every other glucose is turned into sucrose

Why can mass flow in the phloem occur both up and down a plant?

Different parts of the plant require assimilates at different times, so mass flow can move up or down depending on where sugars are needed.

Can mass flow occur in both directions in the same sieve tube?

Yes, because different regions of the plant act as sinks at different times, causing assimilates to move either up or down.

what is a source of sucrose?

where sucrose is produced and then loaded into the phloem

what are the main sources in plants?

• Green leaves and green stems (when they are fully grown)

• Storage organs (e.g. seeds, bulbs) ~ during the spring (during Autumn, when they are developing they are sinks)

• Food stores un germinating seeds

what is a sink of sucrose?

where sucrose is used up and therefore unloaded from the phloem

what are the main sinks in plants?

• Meristematic and growing tissues, such as small leaves - not big enough to produce all the glucose they need for respiration, shoot tips and root tips

• Roots

• Storage tissues such as seeds, bulbs or seeds during development

what does a pressure difference in the phloem cause?

• sap to move either up or down in the phloem (from high pressure to low pressure)

what is the first step at the source of a plant?

loading

what are 2 ways sucrose is loaded into the phloem at the source?

• the symplast pathway

• the apoplast pathway

what is the symplast route when loading the phloem?

• the sucrose diffuses through the cytoplasm in neighbouring mesophyll cells, moving through plasmodesmata

• this does not require ATP so it is passive

• the sucrose lowers the water potential of the cytoplasm in the neighbouring cell

• water follows by osmosis
   

is the symplast route for loading at the source passive or active?

passive

describe the apoplast route for loading sucrose into the phloem

• Sucrose diffuses through the cell walls to the companion cells down a concentration gradient

• This gradient is maintained by the constant movement of sucrose into phloem vessels (from where sucrose moves to a sink)

• At the companion cells, sucrose is moved into the cytoplasm of the companion cell across the cell surface membrane by an active process.

• Companion cells actively transport H+ ions out into surrounding tissue using ATP

• H+ ions return by diffusion down their concentration gradient via co-transporter proteins

• They bring sucrose along with them

• As sucrose builds up in the companion cells it diffuses into the sieve tube through the plasmodesmata down its concentration gradient.

• The build-up of sucrose lowers the water potential of the sieve tube element and so water follows by osmosis (from the xylem and surrounding cells) - both symplast and apoplast

• This leads to high hydrostatic pressure in the sieve tube element at the source

where does sucrose diffuse to and from at the sink?

• sucrose diffuses out of the sieve tube elements and into the surrounding cells down its concentration gradient

what happens when sucrose diffuses into the surrounding cells?

• the water potential of the cell lowers

• water follows in by osmosis down its water potential gradient

• This leads to low hydrostatic pressure at the sink, thereby creating a hydrostatic pressure gradient between the source and the sink

what causes the sap to be transported through the plant?

the hydrostatic pressure gradient between the source and the sink

what is sap made up of?

• sucrose

• amino acids

does the sap move up or down the plant via transcloaction?

• it can be moved up or down depending on the relative positions of the source and the sink

fill in the gaps

how does sucrose more from high hydrostatic pressure (source) to low hydrostatic pressure (sink)?

mass flow

how is sucrose unloaded out of the phloem?

• At the sink, sucrose diffuses out of the sieve tube elements and into the surrounding cells down its concentration gradient

• As sucrose diffuses into a cell, it lowers the water potential of that cell, and so water therefore follows by osmosis down its water potential gradient

• This leads to low hydrostatic pressure at the sink, thereby creating a hydrostatic pressure gradient between the source and the sink

• This pressure difference (between the source and the sink) means that sap (sucrose and amino acids) can be transported over many metres

• Depending on the relative positions of the source and sink, sucrose can be moved either up or down the plant by translocation