Photosynthesis leads to production of reduced carbohydrates. G3P is exported from chloroplasts to the cytoplasm, where it is used to synthesize transportable sugars mainly sucrose, but also other sugars such as stachyose, raffinose, etc. These sugars are exported from the “source” mesophyll cells and transported to the “sink” tissues where they are used for metabolism (growing tissues) or storage. Over long distances, transport is passive along a pressure gradient causing bulk flow of the solution carrying the sugars.
Sugars move primarily through the phloem. Phloem is a complex tissue made up of: sieve cells (tubes) c ompanion cells fibers parenchyma Compounds transported in the phloem: photosynthate (sugars) amino acids hormones viruses nutrients
Pressure Flow Hypothesis Sugars move passively down a concentration gradient, or are actively transported in some cases. Accumulation of sugar at the source end of the phloem osmotically attracts in water, increasing pressure in the sieve tubes at the source. This pressure ‘pushes’ the phloem sap toward a region of lower pressure – the sap moves by bulk flow in the phloem, down a pressure gradient. Water circulates between phloem and xylem tubes.
The content of phloem sap, as well as its pH and pressure, were discovered by ingenious experiments using aphids. Aphids insert their stylets into the sieve tubes, allow the phloem sap to pass through their bodies, and utilize the good parts. Experimenters used aphid stylets (without bodies) as ‘taps’ into the phloem to discover how it works.
Mechanisms of solute transport Sucrose transport across the PM in general terms: relies on active transport of protonsout of the cell by the H+ ATPase and H+/sucsymport into the cell Transport is passive when it occurs DOWN an electrochemical gradient; Transport is active when it occurs UP and electrochemical gradient.
Phloem sap can move symplastically (staying entirely within the cytoplasm, connected by plasmodesmata cell-to-cell; Or, sugars can be transported by a combination of apoplastic and symplastic transport. For example: companion cells at the source can utilize active sucrose uptake to concentrate sucrose in the source phloem; sink cells may also utilize active sucrose uptake across the PM instead of relying on plasmodesmatal connections.