Sugar Transport In Plants

Question 1

Translocation

Answer 1

Transport of mineral ions from 1 part to another in phloem

Question 2

Phloem structure

Answer 2

Made up of sieve tube elements, arranged end to end
Sieve tube elements , perforated to form sieve plates
No nucleus in elements
Share resources with companion cell
Sucrose, amino acids, K+, Cl-, PO4-, Mg2+ carried

Question 3

Why sucrose and not glucose?

Answer 3

Glucose is very reactive, sucrose is safer

Question 4

Sucrose

Answer 4

Fructose + glucose

Question 5

Sources and sinks

Answer 5

Sugars produced in photosynthesis at source
Sugars transported to sinks (used or stored)
Sinks can be above, below source
Translocation occurs up and down plant

Question 6

Mass flow theory

Answer 6

Sucrose manufactured
Diffuse down conc grad (fac diff) from photosynthesising cells into companion cells
H+ AT from companion cells into spaces in cell walls with ATP
H+ diff down conc grad though carrier protein into co transport protein with sucrose

Sucrose causes sieve tube elements to have -ve WP
Xylem, increase WP, water moves into sieve tubes from xylem osmosis
High hydrostatic pressure
Sucrose used at sink
Sucrose diff/AT into sink, lower WP
Water follows sucrose by osmosis, WPG
Hydrostatic pressure in region falls
Mass flow occurs down hydrostatic grad in sieve tube

Question 7

Evidence for Mass flow theory

Answer 7

Pressure in sieve tubes, sap released when cut
Conc of sucrose higher in source than sink
Downward phloem flow in day, not at night
Increased sucrose in leaf followed by increased sucrose in phloem after
Metabolic poisons inhibits translocation
Companion cell possess many mitochondria

Question 8

Evidence against Mass flow theory

Answer 8

Functions of sieve plates unclear
All solute moves at different speeds
Sucrose delivered in same ratio to all regions, conc not involved

Question 9

Investigating transport in plants

Ringing experiments

Answer 9

Woody plant, remove section of protective layer and phloem around circumference of stem
Area above swells, under withers and dies
Samples taken from swellings are rich in sucrose
Suggests that phloem than xylem responsible for transporting sugars

Question 10

Investigating transport in plants

Tracer experiments

Answer 10

use C14 5o make radioactive CO2 14
C14 isotope incorporated into sugars made during photosynthesis
Autoradiography traces sugar movement in plant
Take cross sections of stem, placing them on X-ray film
Film blackened where it is exposed to radiation
Black corresponds to phloem tissue in stem