8.4Transport in Plants
Translocation Core — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 15 parts Lesson 7 of 9 Unit quiz
Lesson summary
8.4 Translocation — IGCSE BIOLOGY notes: Describe translocation of sucrose and amino acids in phloem from sources to sinks (Supplement).Sugars — Mainly sucrose, made from glucose produced in photosynthesis.Amino acids — Made using glucose and nitrates absorbed from the soil.Trans…
- Sieve plates
- Changing direction
- Cells — The tubes are made of living cells, unlike xylem vessels which are dead
- Source — A part of the plant where substances are made or released
- Sink — A part of the plant where substances are stored or used
- Direction — Substances always move from a source to a sink
Section 1 of 15What is translocated
Sugars
- Mainly sucrose, made from glucose produced in photosynthesis.
Amino acids
- Made using glucose and nitrates absorbed from the soil.
Translocation
- The transport of sucrose and amino acids in the phloem from regions of production to regions of storage or use.
Section 2 of 15Phloem tubes
Cells
- The tubes are made of living cells, unlike xylem vessels which are dead.
Sieve plates
- Cells are joined end to end with holes in the end walls that let substances flow from cell to cell.
Section 3 of 15Sources and sinks
Source
- A part of the plant where substances are made or released.
Sink
- A part of the plant where substances are stored or used.
Direction
- Substances always move from a source to a sink.
Changing direction
- The direction depends on the stage of development or the time of year.
Section 4 of 15Changes through the year
| Time | Source | Sink |
|---|---|---|
| Winter | Storage organs | Other parts of the plant, for respiration and repair |
| Spring growth | Storage organs such as roots | Growing areas |
| Summer | Photosynthesising leaves | Roots, where sucrose becomes glucose and is stored as starch |
Section 5 of 15Xylem and phloem compared
| Tissue | What is moved | Process | Direction | Cells |
|---|---|---|---|---|
| Xylem | Water and mineral ions | Transpiration stream | One way, roots to leaves | Dead |
| Phloem | Sucrose and amino acids | Translocation | All directions | Living |
Section 6 of 15More from the specification
Mesophyll
- The tissue in the middle of a leaf where photosynthesis takes place.
Movement
- An action by an organism or part of it causing a change of position or place.
Stomata
- Pores in the leaf through which water vapour leaves.
Humidity
- The amount of water vapour in the air.
Section 7 of 15What translocation is
Translocation
Translocation the movement of organic solutes, mainly sucrose and amino acids, through the phloem from regions of production to regions of use or storage.
Energy
- Energy unlike the transpiration stream it uses energy.
Direction
- Direction it moves in both directions, always from a source to a sink.
Sucrose
- Sucrose glucose from photosynthesis is converted to sucrose, which is soluble, transportable and does not react on the way.
Amino acids
- Amino acids made using glucose and nitrate absorbed from the soil.
Section 8 of 15Sources and sinks
| Source | Sink | |
|---|---|---|
| Meaning | Releases sugars into the phloem | Removes sugars from the phloem |
| Examples | Green leaves; storage organs releasing stores in spring | Roots, growing shoots, flowers, fruits, tubers and bulbs |
Section 9 of 15Seasonal pattern of translocation
Winter
- Winter storage organs supply sucrose for respiration and amino acids for cell repair.
Spring
- Spring roots are the source and growing areas are the sinks.
Summer
- Summer leaves photosynthesise fast and are the source;
- roots are the sink, where sucrose is converted to glucose and stored as starch.
Storage
- Storage insoluble starch does not affect water potential, so cells do not swell or burst.
Section 10 of 15Movement of sap (pressure-flow)
1
Sucrose is actively loaded into the sieve tubes at the source, using energy from companion cells.
2
Water enters the sieve tubes by osmosis, raising the pressure at the source.
3
Sap flows along the tube towards the sink, where pressure is lower.
4
Sucrose is unloaded at the sink and used for respiration, growth or stored as starch.
Section 11 of 15Loading and unloading in more detail
1Loading
- Loading sucrose moves from mesophyll cells into companion cells and then into the sieve tube.
- The companion cell uses ATP to pump hydrogen ions out;
- they return through a co-transport protein carrying sucrose in.
2Source end
- Source end the concentrated sap has a low water potential, so water enters from nearby xylem by osmosis and raises the pressure.
3Sink end
- Sink end sucrose is removed and converted to starch or used in respiration,
- so the sap has a higher water potential and water leaves by osmosis,
- giving low pressure.
4Mass flow
- Mass flow the pressure difference drives sap along the tube, and removal of sucrose at the sink maintains the gradient.
Section 12 of 15Experimental evidence
| Evidence | What it shows |
|---|---|
| Ringing experiment (ring of bark including phloem removed) | Sugars accumulate above the ring and the bark swells, so phloem carries sugars |
| Radioactive tracer (leaf given 14CO2) | Labelled sucrose is made and later found in other parts, mainly in the phloem |
| Aphid stylets | Sap collected from the phloem is rich in sugar, with higher concentration nearer the leaf |
| Metabolic poisons | Flow slows when respiration is blocked, so energy is needed |
Section 13 of 15Source and sink are functions
Source and sink are functions
Summer storage
- Summer storage plants make more sugar than they need and store the excess as starch in roots, tubers, bulbs and seeds.
Spring
- Spring enzymes break the starch back to soluble sugars, which are translocated to buds, so the storage organ becomes a source.
Meaning
- Meaning source and sink describe the function at a given time, not a fixed place.
Section 14 of 15Key terms
| Term | Meaning |
|---|---|
| Translocation | Movement of sucrose and amino acids in phloem from sources to sinks. |
| Source | A part of a plant that releases sucrose or amino acids. |
| Sink | A part of a plant that uses or stores sucrose or amino acids. |
| Sucrose | The sugar mainly transported in phloem. |
| Sieve plate | Perforated end wall of a phloem cell. |
| Phloem | Living tissue that transports sucrose and amino acids. |
| Term | Meaning |
|---|---|
| Translocation | Movement of sucrose and amino acids in the phloem. |
| Source | Region where sugars enter the phloem. |
| Sink | Region where sugars leave the phloem. |
| Sucrose | The soluble sugar used for transport in plants. |
| Sieve tube | A living tube in phloem that carries sap. |
Section 15 of 15Exam technique
1
Saying a plant part is always a source or always a sink — a storage organ can be both at different times.
2
Saying translocation moves only downwards — it moves from sources to sinks in any direction.
3
Saying phloem carries water and minerals — it carries sucrose and amino acids.
4
Calling phloem tubes dead — they are living cells.
1
Saying translocation is in the xylem — it is in the phloem.
2
Thinking sugars move only down to roots — they move to wherever they are needed.
3
Saying translocation is passive — it needs energy.
4
Confusing glucose with sucrose as the transport sugar — plants transport sucrose.
Last updated: