CAIE A-Level Biology AS 7.2.8 Mass Flow in Phloem Questions
Explain pressure-driven mass flow in phloem using evidence from each question and mark scheme.
- Syllabus
- 2028–2030
- Course
- Biology 9700
- Level
- AS
Exam points
- Explain mass flow in phloem and the pressure gradients that drive assimilate movement.
CAIE A-Level Biology AS 7.2.8 Mass Flow in Phloem Questions question 1
[Maximum number: 4]
In most plants, sucrose is the main sugar that is transported from sources to sinks.
In the source, sucrose is transferred from a mesophyll cell to a phloem sieve tube through a companion cell.
Describe and explain how the transfer of sucrose into a phloem sieve tube from a companion cell can lead to the transport of the sugar to a sink.
presence of sucrose (in sieve tube) lowers water potential (of phloem sap) ;
A makes water potential more negative water enters (sieve tube), by osmosis / down water potential gradient ;
increases volume (in sieve tube) ;
increases hydrostatic pressure (in sieve tube at source) ;
A turgor pressure ref. to lower hydrostatic pressure (in sieve tube at sink) ;
Marking guidance:
allow ecf for no ref. to, hydrostatic/turgor detail ;
e.g. sucrose removed at sink
water follows sucrose that exits sink
movement of, (phloem) sap / sucrose, down pressure gradient / from high to low hydrostatic pressure ;
mass flow ;
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