IB Biology Plant Science TRANSLOCATION IB Biology Plant Science

9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots). Phloem tissue transports sugars and amino acids from sources which include photosynthetic tissue (leaves and stems) and storage organs, to sinks which include the fruits, seeds and roots of the plant. This transport is known as active translocation and requires energy.

Learning objective Explain how organic compounds are transported within plants.

Organic substances such as sucrose and amino acids are transported in the phloem

The phloem forms sieve tubes

from sources to sinks Source Sink

The source can be the leaves (due to photosynthesis) and the sink be the roots or fruit….

… but can also be from the roots or seed to the growing areas of the plants during germination or new spring growth TRANSLOCATION

Therefore flow can be in either direction (unlike the xylem).

Mass flow hypothesis 1 1. Loading (of sugars/organic compounds) into the phloem by the companion cells using active transport

2. Causes high concentration (hypertonic) of solutes in phloem Mass flow hypothesis 2 2. Causes high concentration (hypertonic) of solutes in phloem

3. Water moves from the xylem to the phloem by osmosis Mass flow hypothesis 3 3. Water moves from the xylem to the phloem by osmosis

4. Rise in hydrostatic pressure at source (in the phloem) Mass flow hypothesis 4 4. Rise in hydrostatic pressure at source (in the phloem)

Mass flow hypothesis 5. This creates a hydrostatic pressure gradient with the higher pressure being at the source. 5

Mass flow hypothesis 6. Water flows downs the hydrostatic pressure gradient from source to sink 6

Mass flow hypothesis 7. Active transport of sucrose and other organic compounds out of the phloem at the sink further helps in producing the hydrostatic pressure gradient. 7

Mass flow hypothesis 8. Mass flow can be reversed at certain times such as during spring growth when sugars/organic compounds sored in the roots are translocated to the growing regions. 8

May 2018 Examination Explain how organic compounds are transported within plants. a. transported in/translocated in/loaded into phloem✔ b. in sieve tubes ✔ c. by mass flow ✔ d. from sources to sinks ✔ e. from leaves/other example of source to roots/other example of sink ✔ f. loading (of sugars/organic compounds) by active transport ✔ g. cause high concentration of solutes (in phloem/sieve tubes) ✔ h. water uptake (in phloem/sieve tubes) by osmosis/water diffuses into phloem ✔ i. rise in (hydrostatic) pressure at source (in phloem) ✔ j. creates a (hydrostatic) pressure gradient/higher pressure in source than sink ✔ k. flow can be in either direction/bidirectional ✔