Translocation in the phloem
Introduced| English | Meaning |
|---|---|
| translocation/trænsləʊˈkeɪʃn/ | The movement of dissolved sugars and other organic substances through a plant's phloem from sources to sinks. |
| source/sɔːs/ | In translocation, the organ such as a leaf that produces or releases assimilates. |
| sink/sɪŋk/ | A region of a plant that uses or stores assimilates delivered by phloem transport. |
| proton pump/ˈprəʊtɒn pʌmp/ | A membrane protein that uses energy to move hydrogen ions across a membrane against their electrochemical gradient. |
| cotransporter/ˈkɒtrənspɔːtə/ | cotransporter |
| hydrostatic pressure/ˌhaɪdrəˈstætɪk ˈpreʃə/ | The pressure exerted by a fluid at rest, including pressure within plant transport tissues. |
| mass flow/mæs fləʊ/ | The bulk movement of sap through phloem from source to sink under pressure. |
Delivering the sugar
- Translocation 运输 moves sugars (assimilates) through the phloem.
- It carries them from where they are made to where they are needed.
- It is driven by pressure, set up by active loading.
Translocation in the phloem transports:
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Phloem carries dissolved sugars (assimilates), unlike xylem which carries water and minerals.
Source 源 and sink 库
- A source is where the assimilate is made or released — e.g. a photosynthesising leaf.
- A sink is where it is used or stored — e.g. a growing root.
- Sucrose and amino acids flow from source to sink.
In translocation, a "source" is:
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A source makes/releases the assimilate (e.g. a leaf); a sink uses or stores it (e.g. a growing root).
Match each term to its meaning.
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Sugar is loaded at the source, flows by mass flow, and is unloaded at the sink.
Loading at the source
- Companion cells load sucrose into the sieve tube against its gradient — active transport:
- proton pumps 质子泵 pump hydrogen ions out,
- cotransporter 共转运体 proteins then bring sucrose back in with those ions.
- Loading sucrose lowers the water potential in the sieve tube, so water enters by osmosis — raising the hydrostatic pressure 静水压.
Sucrose is loaded into the sieve tube at the source by ______ transport (using proton pumps).
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Companion cells pump H⁺ out, then cotransporters bring sucrose in with the ions — active transport.
Loading sucrose at the source raises the pressure in the sieve tube because it:
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More sucrose lowers the water potential; water follows by osmosis, raising hydrostatic pressure.
Mass flow 集流
- At the sink, sucrose is removed, so water potential rises, water leaves, and pressure falls.
- This makes a pressure difference: high at the source, low at the sink.
- Sap flows from high to low pressure — mass flow.
Translocation by mass flow from source to sink
Translocation by mass flow
Step through how sugar moves. Loading at the source pulls water in and raises the pressure, pushing sap to the sink.
Put translocation by mass flow in order.
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Load → water in → high pressure → flow to the sink → unload.
Phloem can carry sugars both up and down the plant, unlike xylem which only carries water upward.
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Sugars flow from any source to any sink — so phloem transport can go up or down; xylem only carries water up.
You've got it
- translocation = moving assimilates (sucrose, amino acids) in phloem, source → sink
- source = made/released (leaf); sink = used/stored (root)
- loading: active transport (proton pump + cotransport) → lowers water potential → water in → pressure ↑
- mass flow: sap flows from high pressure (source) to low pressure (sink)