The part of the plant that produces the organic substances is the source. An example of the source is the leaf. Glucose produced in the leaf is converted to sucrose by photosynthesis, which is then transported to the sink, which is the part that lacks the nutrients. Phloem loading is the process by which substances to be transported by translocation are loaded into the phloem. Phloem loading may occur either actively or passively and can undertake two important pathways, namely, apoplastic pathway and symplastic pathway. The apoplastic pathway uses membrane-bound transporter proteins to move sugars against the concentration gradient, that is, by active transport, whereas the symplastic pathway allows diffusion of the sugar molecules by creating a pressure gradient.
Pressure Flow or Mass Flow Hypothesis:
The mass flow hypothesis was proposed by Munch. This hypothesis is used to explain the translocation of sugars to various parts of the plant and is thought to be the most accepted model to explain the concept. The pressure flow hypothesis can be explained in five steps.
Plants prepare glucose by the process of photosynthesis. This glucose is then converted to sucrose.
Sucrose enters the phloem sieve tube cells by active transport. This movement is aided by the companion cells.
Xylem vessels are placed next to the phloem vessels. Water enters the xylem by endosmosis, which causes osmotic pressure to build up.
Due to the rise in the osmotic pressure, the phloem SAP tends to move from higher to lower pressure. Osmotic pressure is low in the sink.
The sucrose that reaches the sink moves out of the phloem by active transport again.
Active Transport in Phloem:
As sugars move through the phloem, water enters into the companion cells by osmosis. Since water cannot be compressed at normal temperature and the cell walls are strong and rigid, it leads to pressure build up. Such a pressure gradient causes water to move from a region of higher pressure to a region of lower pressure. Once sucrose reaches the sink, it is converted to another form of sugar called starch and thus decreases the osmotic pressure.
Usually, sugars are loaded into the phloem sieve tubes by active transport. Companion cells help in the process of loading sugars into the sieve tubes. If phloem loading is by the symplastic pathway, materials pass through plasmodesmata and into the sieve tube, whereas for the apoplastic pathway, materials are loaded with the help of transporter proteins. The apoplastic pathway uses ATP for loading and is an active transport process. During active transport, hydrogen ions are transported out of the cells by proton pumps. Thus, it leads to an increase in the concentration of hydrogen ions outside the cell. Less proton or hydrogen ions inside the cell and more protons outside the cells result in a proton gradient, which provides the energy for the movement of sucrose.
Sucrose is the most abundant solute present in the phloem SAP. Glucose has to be transported to sucrose since sucrose is a disaccharide and glucose is a monosaccharide. Sucrose has greater efficiency in storing energy than glucose. Also, since glucose is highly reactive, there may be chances that it forms reactive intermediates during transport in the phloem. Hence, glucose is converted to sucrose during transport in the phloem. A co-transporter protein transports sucrose into the phloem.
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