Translocation

The glucose that is made in photosynthesis needs to be transported to other parts of the plant where it’s needed - this happens in the phloem vessels in a process called translocation. Scientists aren’t 100% certain how this happens but their best guess is a theory called the mass flow hypothesis.

Translocation

Translocation is the movement of dissolved substances, such as sucrose and amino acids, from parts of the plant where the substances are made to other parts of the plant where they’re needed. Translocation takes place in the phloem - transport vessels made up of two types of cell, sieve tube elements and companion cells. The parts of the plant which make these substances are referred to as sources (e.g. the leaves) and the parts of the plant which store or use the substances are called sinks (examples include bulbs and roots). When sucrose reaches a sink, it is converted into starch for carbohydrate storage. This maintains a concentration gradient between the source and the sink, so that more sucrose moves into the source. Translocation is an active process, so if respiration is reduced or inhibited (e.g. using a respiratory toxin), translocation will be impaired.

The Mass Flow Hypothesis

The mass flow hypothesis is a theory which attempts to explain how solutes are transported from source cells into sinks through the phloem. It isn’t concrete, but it is the best-accepted theory we currently have based on the available evidence. It states that mass flow of solutes takes place in the phloem in the following stages:

• Sucrose moves from companion cells into sieve tube elements by active transport.

• This reduces the water potential of the sieve tube element.

• Water moves into the phloem by osmosis, which increases the hydrostatic pressure.

• There is a pressure gradient with high hydrostatic pressure near the source cell and lower hydrostatic pressure near the sink cells.

• Solutes move down the pressure gradient towards the sink end of the phloem.

• Solutes move into sink cells and are converted into other molecules (e.g. starch).

• The removal of solutes increases the water potential at the sink end, causing water to move out of the phloem by osmosis. This maintains the hydrostatic pressure gradient between the source and the sink.

Active Loading

You also need to know how sucrose is actively transported into the sieve tube elements in the first place. This process involves the companion cells which are associated with each sieve tube element:

• The companion cell actively transports hydrogen ions into the surrounding cells.

• This creates a hydrogen ion gradient between the surrounding cells and the companion cell.

• Hydrogen ions move back into the companion cell down their concentration gradient through a co-transporter protein.

• Whenever a hydrogen ion moves through the co-transporter, a sucrose molecule is also transported into the companion cell, against its concentration gradient.

• The same process occurs to transport sucrose from the companion cell into the sieve tube element.

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