PLB 105 Lecture 5 Phloem

What is the function of phloem?

Functions in photosynthate (product of photosynthesis i.e. sugar) transport and support
Composed of three major cell types:
- Sieve Elements (sieve tube members, sieve cells)
- Parenchyma (companion, albuminous cells)
- Sclerenchyma (fibers/lignin)
Callose found in the cell walls

Sieve Tube Members (STMs)

sieve plates and associated with companion cells found in angiosperms

Sieve Cells

longer cells, more primitive, found in gymnosperms/ferns

function in transport/conduction of sucrose axially and laterally throughout the plant

Conducting Elements

thick PRIMARY cell wall
enucleate at maturity but considered living
plasma membrane, plastids, mitochondria (limited numbers) and proteins intact

sieve areas

thin-walled areas like pits in xylem

sieve plates are found in _______ and are like _________.

sieve tube members, perforation plates

What is the difference between xylem vessel elements and phloem sieve tube elements?

Phloem sieve plate has companion cells and are PRIMARY walled
- they both transport fluids, and xylem has secondary lignin wall

Development of Sieve Tube Members

• sieve element precursor divides

• wall thickens, aggregation of p-protein

• sieve plate formation starts

• tonoplast (vacuole), nucleus, ER, ribosomes degenerate

• dispersion of p-protein

Sieve Tube Element & CC

1. The phloem cell divides longitudinally

2. Companion Cell develops alongside young STM

3. STM degenerates nucleus and disperses P-protein bodies while companion cell keeps nucleus

4. STE opens pores → sieve plates. Companion cell has lots of plasmodesmata to connect with sieve cell

apl

altered phloem development

mutants develop xylem where phloem should occur

Sieve Plate Development

1. ER + Golgi near end walls associated with plasmodesmata

2. Callouses deposition in plates between wall & ER around PD

3. pore digestion

4. p-protein aggregates person P1 (loose strands) to P2 (crystalline form)

Callose plug

in case of fungal or microbial invasion, phloem can be shut off by making callose plugs

• when callose deposition increases, it is NOT digested at this time.

P-protein bodies

phloem-protein

fibular, not aggregated

sensitive to pressure changes

plug up sieve plate

P protein function

• actin-like contractile

• function in disease prevention by forming ‘slime plugs’

• callose plugs also function in same way

Transfer Cells

Type of Parenchyma in phloem

• Membrane is invaginated to increase SA

• Transfer products of chloroplasts in parenchyma

Phloem Fibers Sclerenchyma

• Fiber Cap lignin in secondary wall of phloem

Fibers and Sclereids

• function in support only

• secondary cell wall

• dead at maturity

• adds support/structure to phloem and periderm (tree bark)

Phloem

major cell types are sieve tubes & companion cells

• both are alive but SIEVE TUBES LACK NUCELI AT MATURITY

• both cell types are thin walled

• sieve tubes and companion cells are sister cells and have many plasmodesmata connections

• parenchyma cells can function as transfer cells

• fibers provide structural support to the phloem

Where is ATP used for transport in the phloem?

at source cell and at sink cell

• need ATP to pump sucrose into sieve cell and remove sucrose into the cell

Is sucrose transport in plant active or passive process?

Both

Active when unloading and loading sucrose with companion cells and parenchyma cells but Pressure flow is passive due to water moving osmotically