phloem 2

Does phloem sap contain more sugar at source than sink?: 

Does phloem sap contain more sugar at source than sink? Bukht Awer

Source and Sink: 

Source and Sink The leaves are considered the source because they make the sugars The sink or the roots are where the sugars are stored Sugar and other nutrients a plant needs are produced in the mesophyll cells of the leaves and then moved to the sieve elements. Sieve elements are phloem conducting cells (2)

Slide 3: 

Phloem sap is the liquid that enters the sieve tubes and travels down to the sink and other place it is needed. The sap contains sugars, amino acids and nutrients for the plant. The sap is actively loaded in to the sieve tube elements by a process called phloem loading. Phloem loading is critical for plant growth (1) The main route for sugar transport is apoplastic Apoplastic pathway- A pathway that goes through the cell wall

Expierment : 

Expierment In the experiment the researchers used spinach leaves to determine the amount of sucrose and amino acids in the plant leaves.

Slide 5: 

For the experiment the researchers had spinach that been grown in a humid chamber with tungsten and mercury lighting. The 49 days old leaves were frozen in Nitrogen dioxide To examine the phloem sap they cut through the styles of the plant with a laser beam revealing all the sap.

results: 

results The researchers found that the amino acids are located mostly three regions of the leaf (1) the vacuole Chloroplast region Cytoplasm

Slide 7: 

GAPDH is the chloroplast material that is most in the middle region. The cytosolic is represented by the PEPCX, which is mostly in the lower fractions of the plant. The material from vacuole is shown by the Mann and it is mainly found in fraction with the highest density

Slide 8: 

Figure 2 shows the amount of amino acids and sucrose in each fraction of the plant. The amino acids are in all three compartments of the leaf (1)

Slide 9: 

2/3 of the amino acids in the phloem sap were found in the acids glutamate, glutamine and aspartate. The average concentration of sucrose amounted to 0.8 M (1)

Slide 10: 

Most of the sucrose is not found in the three regions observed above Instead it is found in the sieve tube members of the plant the sucrose concentration in the sieve tubes is one order of magnitude higher than in the cytosol of the mesophyll cells (1) the sucrose is actively loaded in to the sieve tube member and then transported The sugar is usually accumulated in the source of cells and this causes the water to move up in to the phloem sap by osmosis (3)

Slide 11: 

The researchers evalulated the maximum value of sucrose content in the cytosol by distributing all of the sucrose found in the fractions of the lowest density to the cytosol (1) The result they found was < 103mM and this says that most of the sucrose is found in the cytoplasm but in reality a good amount is found in the vacuole and the sieve tubes. The maximum value should be considerably lower than the one above.

conclusion: 

conclusion The research indicates the best place to extract sugar is from the source of the cells Most of the sucrose is near the source of the plant where it is created instead of the sink

Reference: 

Reference 1. Riens B, Lohaus G, Heineke D, Heldt HW (1991) Amino acid and sucrose content determined in the cytosolic, chloroplastic, and vacuolar compartments and in the phloem sap of spinach leaves. Plant Physiol 97:227–233 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1080988/ 2. Gottwald J.R., Krysan P.J., Young J.C., Evert R.F. & Sussman M.R. (2000) Genetic evidence for the in planta role of phloem specific plasma membrane sucrose transporters. Proceedings of the National Academy of Sciences USA 97, 13979–13984. http://www.pnas.org/content/97/25/13979.short 3. Raven, Peter H., and Peter H. Raven. Biology . Boston: McGraw-Hill Higher Education, 2005, pp 767-781. http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html 4.  E.A. Rennie and R. Turgeon, A comprehensive picture of phloem loading strategies. Proc Natl Acad Sci U S A , 106 (2009), pp. 14162–14167. http://jxb.oxfordjournals.org/content/58/15-16/4061