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Write an account of the transport of organic substances in the plant.

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Write an account of the transport of organic substances in the plant. Phloem tissue is made up of four main elements Sieve tubes are living, tubular cells that disintegrate during development. They are connected end to end, up and down the entire plant. The end cell walls have perforations in them to make sieve plates. The cytoplasm is present but in small amounts and in a layer next to the cell wall. It lacks a nucleus and most organelles so there is more space for solutes to move. The cell walls are made of cellulose so solutes can move laterally a well as vertically. ...read more.


If the cells are fully turgid and tightly packed as they normally are, parenchyma helps to maintain the shape and firmness of the plant. The process called phloem translocation, involves the movement of organic substances around the plant. It requires energy to create a pressure difference and so is considered an active process. Sucrose is loaded into the phloem at a source, usually a photosynthesising leaf. For this to occur, hydrogen ions are pumped out of the companion cell using ATP. This creates a high concentration of hydrogen ions outside the companion cell. Sucrose is moved into companion cells by active transport, against the concentration gradient. ...read more.


This lowers the water potential of the sieve element so water enters by osmosis. At another point sucrose will be unloaded from the phloem into a sink (e.g. root). It is likely that the sucrose moves out by diffusion and is then converted into another substance to maintain a concentration gradient. Again, water will follow by osmosis. This loading and unloading results in the mass flow of substances in the phloem. There is evidence to support this theory; the rate of flow in the phloem is about 10,000 times faster than it would be if it were due only to diffusion. The pH of the phloem sap is around 8 (it is alkaline due to loss of hydrogen ions). There is also an electrical potential difference across the cell surface (negative inside due presumably to the loss of positively charged ions). ...read more.

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