Plant Transport - Translocation

Transport in
Plants 
Kaz-Hoshay Blackwood -
Harrison 
This Photo by Unknown author is licensed underCC BY-ND.

Transport In Plants – Vascular Bundle  
• The xylem and phloem are usually next to each other in plants.  
• They and the vascular cambium form the vascular bundle.  
 
• Vascular Bundle in a
young dicotyledon stem 
• The vascular cambium produces

new xylem and phloem as the
plant grows 
• In the stem, the xylem is toward

the inside and the phloem
toward the outside 
 
Transport In Plants –
Vascular Bundle  
Mature Dicot stem with annual rings. Each ring
shows how many years the plant has existed. Each
ring a set of xylem produced by the cambium.  
• Vascular Bundle in a dicotyledon

root 
• The all the xylem cells group up

to forms a 't' ,'x' or star shape in
the centre 
• the phloem is outside of it. 
 
• Vascular Bundle in a dicotyledon

leaf 
• The phloem is at the bottom and

the xylem at the top 
 
Transport In Plants - Phloem 
Transport In Plants – Phloem 
• They are made of sieve tube element and companions cells. 
• Each sieve tube element is controlled by a companion cell 
• Just like xylem, they stack on top of each other 

Transport In Plants – Phloem 
• They are alive when they fully mature 
• Their end walls are NOT completely gone, they are perforated,
like sieves 
• That’s why they are called sieve tubes 

• The sieve tubes have little cytoplasm but no nucleus 
•This is why it is controlled by the companions cell. 
• The flow of sugars and amino acids is two way in the sieve tubes. 
 
• Companion cells are connected to sieve tubes by holes in the walls.  
• they have many mitochondria to provide energy for themselves and the sieve tubes  
• Companion cell nuclei control the companion cell and the sieve tube elements  
• The movement substances through the Phloem is called Translocation 
 
Translocation moves sugars (mostly sucrose, some glucose) and amino acids from sources to sinks 
Sources – organs that carry out photosynthesis. E.g. Leaves and some stems, or parts that release
sugars. E.g storage organs when food is scarce, and leaf cotyledons when seeds are growing.  
Sinks – organs and tissues that use and/or store sugars and nutrients. Eg.Stems, roots, leaves,
storage organs (bulbs, corms, tubers, etc.) and fruits. The sugars are use for respiration, stored or
converted to another form, like starch or proteins.  
 
CROSS SECTION OF PHLOEM LONGITUDINAL SECTION OF PHLOEM

Pressure Flow Hypothesis/Mass Flow Theory 
Pressure Flow hypothesis is the theory that explains how sugars move through the phloem in plants.  
Theory states that dissolved sugars move from sources to sinks.  
The process happens due to a pressure building up inside the phloem sieve tubes due to the sugars
being moved into them.  
The pressure inside the sieve tubes cause the sap to be pushed either up or down 
Pressure Flow Hypothesis 
• The sugars are moved from the source cells

into the phloem by active transport.  
• The high concentration sugars in phloem

causes some water from xylem to move by
osmosis into phloem 
• The sugars and water filling phloem and pushing

against the walls causes pressure to be
created 
 
• The pressure inside the phloem forces the sap

(organic food and water) to move either up or
down.  
• The sap moves through the stem passively, no

energy required to move it.  
• Along the way, some of the water returns to

the xylem beacuse of the pull created by
transpiration 
• When sap gets to target (leaf, stem, fruit,

etc), organic food is unloaded by active
transport and water moves back into xylem 
• The pressure inside the phloem decreases

as water exits toward the xylem.  
INVESTIGATING SUGAR
MOVEMENT THROUGH
PLANTS 
RINGING EXPERIMENT
RINGING EXPERIMENT
APHID EXPERIMENT
Aphids feed on sap by pushing their stylets into the phloem. Aphid is anesthetized and body removed from plant,
leaving mouthpart behind. Sap will ooze from the stylet for a couple days. The analysed sap will show organic
materials.  
RADIOISOTOPES EXPERIMENT
Isotopes are alternate forms of elements. E.g. Carbon exists as Carbon 12, carbon 13 and carbon 14.  
If a plant is supplied with Carbon dioxide that has a radioactive form of carbon in it, it will make glucose
containing the radioactive carbon. After a while, when the radioactive carbon dioxide is switched with normal
carbon dioxide and the plant is checked, only the phloem will have radioactive substances, because that is
where the food made by photosynthesis is carried.  
HOW IS THE STRUCTURE OF
PHLOEM IS SUITED FOR ITS
FUNCTION? 
Phloem structure and function 
Phloem cells have: 
1. perforated end walls - they are not not completely gone because cell walls and cell
membranes are essential to staying alive 
2. small amounts of cytoplasm - need to have cytoplasm so it can be alive and direct
the flow of sugars 
3. companion cells - they have dense cytoplasm, nucleus, extra mitochondria and other
organelles, enough for itself and sieve tube element, so it can control itself and sieve
tubes. Companion cells also use energy to move sugars from source cells into the
sieve tubes. Phloem cells have to alive in order to use energy move sugars 
References- Links to sources 
https://intl.siyavula.com/read/science/grade-10-lifesciences/support-and-transport-sy
stems-in-plants/05-support-and-transport-systems-in-plants-02 
https://diffzi.com/wp-content/uploads/2018/11/Active-Water-Absorption-vs.jpg 

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Transport in

This Photo by Unknown author is licensed underCC BY-ND.

• They and the vascular cambium form the vascular bundle.

• The vascular cambium produces

• In the stem, the xylem is toward

• Vascular Bundle in a dicotyledon

• The all the xylem cells group up

• the phloem is outside of it.

• Vascular Bundle in a dicotyledon

• The phloem is at the bottom and

• They are made of sieve tube element and companions cells.

• Each sieve tube element is controlled by a companion cell

• Just like xylem, they stack on top of each other

• They are alive when they fully mature

• Their end walls are NOT completely gone, they are perforated,

• That’s why they are called sieve tubes

• The sieve tubes have little cytoplasm but no nucleus

•This is why it is controlled by the companions cell.

• Companion cells are connected to sieve tubes by holes in the walls.

• The movement substances through the Phloem is called Translocation

CROSS SECTION OF PHLOEM LONGITUDINAL SECTION OF PHLOEM

Theory states that dissolved sugars move from sources to sinks.

• The sugars are moved from the source cells

• The high concentration sugars in phloem

• The sugars and water filling phloem and pushing

• The pressure inside the phloem forces the sap

• The sap moves through the stem passively, no

• Along the way, some of the water returns to

• When sap gets to target (leaf, stem, fruit,

• The pressure inside the phloem decreases

Phloem cells have:

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• They and the vascular cambium form the vascular bundle.  

• the phloem is outside of it. 

• They are made of sieve tube element and companions cells. 

• Each sieve tube element is controlled by a companion cell 

• Just like xylem, they stack on top of each other 

• They are alive when they fully mature 

• That’s why they are called sieve tubes 

• The sieve tubes have little cytoplasm but no nucleus 

•This is why it is controlled by the companions cell. 

• Companion cells are connected to sieve tubes by holes in the walls.  

• The movement substances through the Phloem is called Translocation 

Theory states that dissolved sugars move from sources to sinks.  

Phloem cells have: 