The document discusses the transport of organic nutrients through the phloem tissue in plants. It describes how photosynthesis produces sugars in source tissues like leaves, which are then actively loaded into the sieve tube elements and companion cells of the phloem. This loading creates pressure that pushes the phloem sap from sources to sinks, where the sugars are unloaded to support growth and storage. Key components of the phloem discussed include sieve plates, sieve-tube members, companion cells and the pressure-flow hypothesis to explain long-distance translocation of nutrients from sources to sinks throughout the plant.

1. TRANSPORT IN
PHLOEM

2. PRESNETED BY:
M.SOHAIL RIAZ
PRESENTED TO:
MISS IQRA MUNIR

3. PHLOEM
• The phloem is the tissue that translocates the products of
photosynthesis from mature leaves to areas of growth and
storage, including the roots
TRANSLOCATION:
Is the transport of organic nutrients in the plant

4. CONT………
• Translocation stops if the phloem tissue is killed
• Translocation proceeds in both directions simultaneously
(but not within the same tube)
• Translocation is inhibited by compounds that stop
production of ATP in the sugar source

6. CONT…….
Things Phloem
Made of Living cells
Cell wall thin
Cell wall material Cellulose
Permeability of cell wall permeable
Cross walls Perforated cross walls called
sieve plates

8. PATHWAYS OF TRANSLOCATION
• The two long-distance transport pathways—the phloem and
the xylem—extend throughout the plant body
• The phloem is generally found on the outer side of both
primary and secondary vascular tissues
• In plants with secondary growth the phloem constitutes the
inner bark

9. CONT………
• The cells of the phloem that conduct sugars and other
organic materials throughout the plant are called sieve
elements.
• Sieve element is a comprehensive term that includes both
the highly differentiated sieve tube elements typical of the
angiosperms and the sieve cells of gymnosperms.
• In addition to sieve elements, the phloem tissue contains
companion cells and parenchyma cells

10. CONT………
• In some cases the phloem tissue also includes fibers and
sclereids (for protection and strengthening of the tissue) and
laticifers (latex-containing cells)
• However, only the sieve elements are directly involved in
translocation.

11. PHLOEM
• Food and minerals move through tubes formed by chains of
cells, sieve-tube members:
• Sieve plates
• Companion cell

12. SIEVE PLATES
• An area of relatively large pores present in the common end
walls of sieve tube elements
• Sieve plates are mostly located on the overlapping adjacent
end walls
• They are slender, tube-like structures composed of
elongated thin-walled cells, placed end to end
• The main function of sieve tubes is to transport sugars and
nutrients up and down the plant

13. SIEVE-TUBE MEMBERS
• Characteristics:
• Living cells arranged end-to-end to form food conducting
cells of the phloem
• Lack lignin in their cell walls
• Mature cells lack nuclei and other cellular organelles
• Alive at functional maturity
• Its function is to transport products of photosynthesis

14. COMPANION CELLS
• Characteristics:
• Connected to sieve-tube members via plasmodesmata
• Transport of sugar from source to sink occur through
companion cells
FUNCTIONS:
• Support sieve-tube members
• May assist in sugar loading into sieve-tube members

16. PHLOEM SAP
• Is an aqueous solution that is mostly sucrose
• Travels from a sugar source to a sugar sink

17. CITRUS SINENSIS L. OSBECK (SWEET ORANGE)

18. SOURCE:
• Any exporting organ
• Storage organ during export phase
• mature leaves
SINK:
• Any non-photosynthetic organ that does not produce enough
photosynthetic products to support their own growth and
development

19. SOURCE-SINK RELATIONSHIPS
• Source-sink relationships can explain the direction of
phloem translocation within the plant
• Source - produces more carbohydrates than required for its
own needs
• Sink - produces less carbohydrates than it requires

21. SOURCE-SINK PATHWAYS FOLLOW PATTERNS
• Although the overall pattern of transport can be stated as
source to sink
• Not all sources supply all sinks in a plant
• Certain sources supply specific sink
• Proximity of source to sink is a significant factor.
• Importance of various sinks may shift during plant
development Roots and shoots major sinks during
vegetative growth

22. CONT………
• Fruits become dominant sinks during reproductive
development
• Young leaf is completely dependent on carbohydrates from
other sources.
• So it is a strong sink
• As the leaf grows it increasingly provides for its own
carbohydrate needs
• Mature leaf is largely a carbohydrate exporter (source)

23. MATERIALS TRANSPORTED
• The phloem is the vascular system for moving
(translocating) sugars produced in photosynthesis and other
substances throughout the plant.

24. COMPOSITION OF PHLOEM SAP IN CASTOR
BEAN(RICINUS COMMUNIS)
components concentration
Sugar 80.0-106.0
Amino acids 5.2
Organic acids 2.0-3.2
Protein 1.45-2.20
Potassium 2.3-4.4
Chloride 0.355-0.675
Phosphate 0.350-0.550
Magnesium 0.109-0.122

25. THE MECHANISM OF PHLOEM TRANSPORT
• Phloem transports sugars from a “source” to a “sink”
• In source tissue:
• Sugars are moved from photosynthetic cells and actively
loaded (uses ATP energy) into companion cells and sieve
tube elements.

26. THE PRESSURE -FLOW MODEL
• Phloem loading leads to a buildup of sugars (the phloem
cells become hypertonic)
• Due to which:
1. Solute potential decreases
2. Water potential decreases
• In response, water enters sieve elements from xylem via
osmosis
• Thus phloem turgor pressure increase

27. IN SINK TISSUE:
• Phloem unloading leads to lower sugar concentration (the
phloem cells become hypotonic)
• Due to which:
• 1. Solute potential increases
• 2. Water potential increases
• So, Water leaves the phloem and enters sink sieve elements
and xylem (via osmosis)
• Thus phloem turgor pressure decreases

28. THE PRESSURE-FLOW HYPOTHESIS
• The best-supported theory to explain the movement of food
through the phloem is called the pressure-flow hypothesis
• The pressure-flow theory explains how sap moves in a plant
from source to sink

30. PROCESS OF TRANSLOCATION OF FOOD
• Sugars begin at a source and are pumped into phloem tube
cells
• Osmosis moves water into the cells and raises pressure
• Pressure moves the sap
• The leaf is a source of sugar, since it makes sugar by
photosynthesis
• Glucose and fructose made by photosynthesis are linked to
make sucrose

31. CONT……
• Active transport is used to load sucrose into phloem tubes
against a diffusion gradient
• The high concentration of sucrose in the sieve tube cells of
the phloem causes water to move in by osmosis, which
raises pressure and causes the sap to move
• A developing fruit is one example of a sink
• Sucrose may be actively transported out of phloem into the
fruit cells

32. CONT……..
• In a root, sucrose is converted into starch, which keeps
sugar moving in by diffusion
• As the sugar concentration drops in the sieve tube cells,
osmosis moves water out of the tube
• As water moves out by osmosis, the pressure in the sieve
tube cells drops
• The pressure difference along the column of sieve tube cells
keeps the sap flowing

34. MECHANISM OF TRANSLOCATION OF FOOD
THROUGH THE PHLOEM EFFECT BY:
• Translocation through the phloem is dependent on
metabolic activity of the phloem cells
• Chilling its petiole slows the rate at which food is
translocated out of the leaf
• Oxygen lack also depresses it
• Killing the phloem cells puts end to it

35. SUMMARY
• Locations that produce or release sugars for the growing
plant are referred to as sources.
• Sugars produced in sources, such as leaves, need to be
delivered to growing parts of the plant via the phloem in a
process called translocation, or movement of sugar
• The points of sugar delivery, such as roots, young shoots,
and developing seeds, are called sinks.

37. THANK YOU