Phloem pdf.

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1

Xylem

Transports water and minerals
from roots to shoots

2

Phloem

Transports photosynthetic
products from where they are made or stored (sources) to where they are required (sinks)

3

Two types of water-conducting
cells

Tracheids & Sieve tubes

4

Tracheids

All vascular plants
Vessel elements
▪Most angiosperms, few gymnosperms and seedless vascular
Cells are dead at maturity
Lignin

5

Sieve tubes

Angiosperms

Sieve-tube elements

▪Living cells, Lack nucleus and ribosomes, Sieve plates
Companion cells- Connect via plasmodesmata

6

The role of Bulk FLow

 Bulk flow- Movement of a fluid driven by pressure.Allows efficient long-distance transport

 Water and solutes move together

Xylem - tracheids and vessel elements

Phloem - sieve-tube elements of phloem

 Efficient movement is possible because…Mature tracheids and vessel elements have no cytoplasm & Sieve-tube elements have few organelles in cytoplasm

7

Translocation

The process by which the products of
photosynthesis are transported through phloem

Movement of sugars from sources to sinks

8

In angiosperms, sieve-tube elements are

the conduits for translocation.

9

Phloem sap

An aqueous solution that is high in sucrose. Travels from a sugar source to a sugar sink
▪Source - organ that is a net producer of sugar (mature leaves)
▪Sink - organ that is a net consumer or storer of sugar (tuber or bulb)

10

Storage organ

Sugar sink in summer and sugar source in winter

11

Movement from sugar source to sugar sink

Sugar must be loaded into sieve-tube
elements before being exported to sinks
Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways
Companion cells enhance solute movement between the apoplast and symplast

12
card image

The symplast pathway is where water moves between cytoplasm/vacuoles of adjacent cells. However, the apoplast pathway can only take water a certain way; near the xylem, the Casparian strip forms an impenetrable barrier to water in the cell walls, and water must move into the cytoplasm to continue

13

Phloem loading
requires

active transport
Proton pumping and cotransport of sucrose and H+ enable cells to accumulate sucrose

At sink, concentration of free sugar is lower than in the sieve tube because unloaded sugar is being consumed for growth and metabolism
Thus, sugar molecules diffuse from the
phloem to sink tissues and are followed by water (by osmosis)

14

Phloem sap moves through a sieve tube by......

bulk flow driven by positive pressure called
pressure flow

15

Sometimes there are more sinks than can be supported by sources....what is the result?

sugar sinks such as flowers, seeds, or
fruits are dropped