9.1 Essential ideas
9.1.2 Transport in the phloem of plants
Phloem is the tissue through which the products of photosynthesis are translocated from the source of production – the leaves – to areas of growth and storage – the sinks, including fruits, seeds, stems and roots. Translocation in the phloem, unlike the transpiration stream in the xylem, is multidirectional, and requires active transport.
Structure of the phloem
- sieve tube elements – individual enucleated cells lined up from end to end and connected by sieve plates
- companion cells – nucleated cells positioned along the length of the sieve tube elements, attached by many plasmodesmata.
Figure 9.1.2a – Phloem elements
- Sugars are produced through photosynthesis in the mesophyll layers of leaves (source).
- Following production, sugars diffuse from the mesophyll through the companion cells of the phloem and are loaded into the sieve tube member.
- Loading in the sieve tube is an active process that requires ATP and a chemiosmotic mechanism using proton pumps and a symport protein.
- Loading results in a much higher concentration of solute inside the sieve tube member than in the mesophyll.
- High solute concentration in the sieve tube at the loading points causes water to enter the phloem by osmosis from surrounding tissues.
- This creates an area of high hydrostatic pressure at the source end.
- Hydrostatic pressure is relieved at the sink – as sucrose is removed, water exits the phloem by osmosis to surrounding tissues.
- Companion cells at the sink do not accumulate sugars – they are transported to the growing or storage organs.
- In this way, plant food always moves from source to sink.
- Normally, the source and sink are located relatively close together on a plant. For example, lower leaves will supply sugars to growing roots, and higher leaves to flowers and seeds.
- Define source, sink, translocation.
- Explain how structure and function are related in phloem.
- Identify the types of membrane transport involved in translocation.
Figure 9.1.2c – Comparison of xylem and phloem vessels
- Translocation is a process that requires active transport. Think about modified storage organs such as tubers (e.g. potatoes) or bulbs (e.g. onions), which act as either sources or sinks, depending on the season.
- Is it possible that a potato simultaneously acts as a source and a sink? Which factors influence when a potato acts as a source or a sink? How could you test it?
In the lab
Analyse data collected by aphid stylectomy. See: Page 9.2.4