7.2.57 · Transport in Plants

Movement in the Phloem — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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7.2.5 Movement in the Phloem — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define translocation, identify sources and sinks, and describe the evidence for the movement of assimilates in the phloem.

Phloem sap
Difference from xylem
Why sucrose
Changing roles
Radioactive tracers
Sap that clots

Part 1 of 7Translocation

Translocation

Translocation the transport of assimilates (chiefly sucrose, with amino acids, hormones and some mineral ions) through phloem sieve tubes from source to sink, requiring metabolic energy (ATP).

Phloem sap

Phloem sap it is mostly water with sucrose and other dissolved substances.

Difference from xylem

Difference from xylem xylem carries water upward passively in dead vessels,
whereas phloem carries a concentrated sugar solution in living cells, in either direction depending on sources and sinks.

Why sucrose

Why sucrose it is soluble, chemically unreactive and carries much energy per molecule, with less osmotic effect than glucose.
- (a non-reducing sugar)

Part 2 of 7Sources and sinks

Organ	Source	Sink
Mature green leaf	Yes, makes sucrose by photosynthesis	No
Young leaf	No	Yes, needs sugars for growth
Storage organ (potato tuber, tap root)	In spring, when starch is broken down to sucrose	In summer, when sucrose is converted to starch
Growing root tip, meristem, fruit, seed, flower	No (germinating seed stores are a source)	Yes

Changing roles

Changing roles the role of an organ can change during the life of the plant.

Direction

Direction sucrose in one phloem tube may flow upward to a bud while another flows downward to a root, though each sieve tube carries sap in one direction at a time.

Part 3 of 7Evidence for translocation in the phloem

1Ringing

Ringing removing a ring of bark including phloem makes the stem swell above the ring, where sugar accumulates, and the tissue below eventually dies,
while the xylem still carries water.

2Radioactive tracers

Radioactive tracers a leaf supplied with ¹⁴CO₂ later shows radioactivity in the phloem in autoradiographs of stem sections, showing sugars made in the leaf are transported there.

3Aphids

Aphids an aphid inserts its stylet into a single sieve tube,
- and if the aphid is removed leaving the stylet, sap exudes under pressure and can be analysed for sucrose.

4Sap that clots

Sap that clots in plants such as castor oil, sap can still be collected and studied.

5Inhibitors and low temperature

Inhibitors and low temperature they reduce translocation, showing living cells are needed and that some steps, such as loading, need energy.

6Microscopy

Microscopy advances in microscopes show the adaptations of companion cells, including the importance of mitochondria.

Part 4 of 7Rate of translocation

1Measured rates

Measured rates tracers give about 0.3 to 1.5 metres per hour, much faster than diffusion could achieve over such distances.

2Mass flow

Mass flow this suggests pressure drives bulk movement of the solution.

3Calculation

Calculation rate = distance travelled by the tracer ÷ time.

4Other substances

Other substances hormones such as auxin and gibberellins and amino acids also travel in phloem to growing points.

Part 5 of 7Interpreting phloem data

Sucrose with distance

Sucrose with distance concentration in sap decreases with distance from a source, because sucrose is progressively unloaded into sinks along the path.

Ringing data

Ringing data compare the mass of sugar above and below a ring using percentage difference, and explain with reference to blocked phloem.

Phloem versus xylem

Phloem versus xylem phloem is living tissue carrying organic solutes either way with a pressure gradient,
- and xylem is dead tissue carrying water and ions upward with tension.

Part 6 of 7Why the plant needs translocation

Why the plant needs translocation

Linking parts

Linking parts it connects the parts that make food with the parts that use it, and a leaf exports the surplus sucrose it does not need.

Changing sinks

Changing sinks young leaves and root tips during early growth, flowers and developing fruits during reproduction,
- and storage organs such as tubers and bulbs near the end of the season.

Crop yield

Crop yield breeders select plants that direct more assimilates to the edible sink,
- such as wheat grain or potato tuber, without increasing total photosynthesis.

Conversion to starch

Conversion to starch at the sink, sucrose may be made into insoluble starch, which lowers its osmotic effect and steepens the unloading gradient.

Part 7 of 7More from the specification

Apoplast Pathway

Water and dissolved minerals move through non-living interconnected cellulose cell walls and extracellular spaces via mass flow, continuing until the suberised Casparian strip in the endodermis diverts flow into the symplast.

Symplast Pathway and Osmosis

Water moves between living cells via interconnected cytoplasm and plasmodesmata down a water potential gradient from epidermal root hairs into the vascular cylinder, crossing selectively permeable cell membranes by osmosis.

Transpiration and the Transpiration Stream

Transpiration is the loss of water vapour from aerial parts of the plant through open stomata,
which generates negative hydrostatic pressure (tension) in leaf xylem that pulls the continuous transpiration stream upward from the roots.

Cohesion-Tension Mechanism in Xylem

Hydrogen bonds between polar water molecules maintain strong cohesion throughout the continuous water column,
while adhesive forces to hydrophilic cellulose cell walls prevent column collapse under negative pressure.

Transpiration stream

Continuous movement of water up the xylem from roots to leaves.

Hydrostatic pressure

The pressure a fluid exerts on the walls of its container.

Transpiration

Loss of water vapour from leaves by evaporation and diffusion through stomata.

Stomata

Pores in the leaf through which water vapour leaves.

Apoplast

Pathway through cell walls.

Symplast

Pathway through cytoplasm and plasmodesmata.

Osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Mesophyll

The tissue in the middle of a leaf where photosynthesis takes place.

Water potential

A measure of the tendency of water to move; pure water has the highest.

Apoplast pathway

Movement of water through cell walls and spaces without crossing a membrane.

Key terms

Term	Meaning
Translocation	Movement of assimilates in the phloem from sources to sinks.
Assimilate	An organic substance made by the plant and transported, such as sucrose.
Source	A region that loads assimilates into the phloem.
Sink	A region that removes assimilates from the phloem.
Sucrose	A disaccharide, the main sugar transported in phloem.
Autoradiography	Technique for locating radioactive substances by their effect on photographic film.
Stylet	The mouthpart of an aphid that penetrates sieve tubes.

Avoid these mistakes

Saying phloem transports only downward.

Saying leaves are always sources — young leaves are sinks.

Saying translocation is the movement of water in the xylem.

Saying the phloem transports glucose — the main sugar transported is sucrose.

Forgetting that sinks can change over the year, as in tubers.

Last updated: 15 October 2026