To describe the structure and function of phloem

21 April 2023WALT

WILFDescribe and recognise the components of phloem tissueDistinguish between phloem and xylem tissueDescribe the term TRANSLOCATIONExplain the evidence for translocation

The role of phloemPhloem transports the products of photosynthesis from the site of production (SOURCE) to the place where they are used (SINK). This is known as TRANSLOCATION

A Typical source is…?

Leaf

A sink could be…..?

A storage organ or growing tip

(Fruit / Tap root/ root & shoot meristems)

Phloem tissue Is living

Is made of a variety of cells:

Sieve tube elements

Companion cells

Phloem fibres

Phloem parenchyma

Sieve tubesMade of sieve elements laid end to end.

Sieve plates are the end walls that are perforated by pores.

Cytoplasmic filaments containing phloem protein extend through the pores.

element

Mature sieve tube elements have no nucleus and the cytoplasm is pushed to the sides.

They are closely associated with companion cells

Sieve tube elements are kept alive and supported by companion cells. These have very thin cell walls, dense cytoplasm, a centrally placed nucleus and lots of mitochondria. They connect to the sieve tube elements with plasmodesmata.

Evidence for translocationDirect samplingAphids insert a stylet into the sieve tubes to feed on the sap. This can be collected and analysed

Evidence for translocationRINGING EXPERIMENTS

Evidence for translocationRADIOACTIVE LABELLING

An isotope of Carbon dioxide is applied to a leaf.The CO2 is fixedin the sugar produced during photosynthesis. Its’ progress through the plant is traced using AUTORADIOGRAPHY

Evidence for translocationAUTORADIOGRAPHY

This involves placing the leaf exposed to the radioactive isotope, on a photographic film in the dark for 24 hours.

When the film is developed, the film goes ‘foggy’ where there is radioactivity

What the evidence leads toTranslocation is the transport of the products of photosynthesis. These are soluble organic materials, sucrose & amino acids

Translocation occurs in both directions (up and down).Fogging occurs in the leaves above the leaf exposed to the radioactivity as well as the roots

Theories of translocation

MASS FLOW HYPOTHESIS (1937)

A passive mass flow of sugars from the highest concentration in the phloem in the leaf (source) to the lowest concentration in the growing or storage areas (sink)

Theories of translocationARGUMENTS (evidence against Mass flow)•Translocation occurs much faster than by diffusion(25-100cm h-1 compared to 0.2 day h-1)•No explanation of the presence of sieve plates•Sucrose and amino acids appear to flow in different directions and at different rates in the same tissue.

•Phloem tissue has a high O2 consumption. Translocation stops when a respiratory poison is added.•No role for the companion cells

Recent theories suggest that

Active transport is involved

Recent theories suggest that Observation of protein filaments passing through the sieve pores suggest that different solutes are transported along different filaments

Bi directional movements along individual sieve tubes could be due to streaming in the cytoplasm.Although there has to be some mechanism to transport solutes across the sieve plates

Scientists haven’t yet agreed!

Key words

SOURCE

PHLOEM

TRANSLOCATION

SINK

SIEVE TUBES

SIEVE PLATES

COMPANION CELLS

PHLOEM FIBRES

PHLOEM PARENCHYMA

MASS FLOW

APHID

STYLET

FIXATION

AUTORADIOGRAPHY

RADIOACTIVE LABELLING

MERISTEM

Define the word on a post-it. Then go and test understanding on each other.

To describe the structure and function of phloem

21 April 2023WALT

WILFDescribe and recognise the components of phloem tissueDistinguish between phloem and xylem tissueDescribe the term TRANSLOCATIONExplain the evidence for translocation

HOW MANY CAN YOU DO WITH CONFIDENCE? WHAT WOULD HELP YOU IMPROVE?