If you wish to download it, please recommend it to your friends in any social system.Phloem has sieve tubes (columns of specialized cells) separated by perforated walls called sieve plates.
Sieve tube cells are closely associated with companion cells. Typically solutes are transported from source to sink, but this can happen in reverse if necessary. Transport in the phloem can be in either direction, unlike the xylem which is one way. Energy is required to maintain the gradients, therefore translocation is an active process. Table taken from pg. This data shows: sucrose is the most abundant carbohydrate, particularly in the sap (phloem) Remember, sucrose is not is in respiration, it is a good carbohydrate for transport because it will not be metabolized in this form. Diagram shows the movement of sucrose across a sieve tube membrane. Sucrose concentration gradient is established by active transport. H ions are actively transported out of the companion cells from surrounding tissues using ATP. Build up of H ions then flow down the concentration gradient, through a co-transport protein (protein able to carry more than one thing, in this case H and sucrose). In the companion cell, sucrose is converted into an oligosaccaride (a simple polysaccaride) in order to maintain the sucrose concentration gradient (other species) Plasmodesma singular Diagram compares the two routes. Water from surrounding tissues is drawn into the companion cells via osmosis due to the high concentrations of carbohydrates in the companion cells. The cell walls are rigid, and water is not easily compressed therefore the pressure increases in these cells creating a pressure gradient. In sink cells, sucrose is removed from the phloem and used as an energy source or converted into starch. The loss of solutes, reduces osmotic pressure so water leaves the companion cells via osmosis and returns to the transpiration stream (aka. Sieve tube cells must be living because they require membrane proteins to maintain concentration gradients by active transport. Plasmodesmata connect sieve tube cells to companion cells Sieve tube cells Companion cells Rigid cell wall necessary to support pressure gradients Separated from one another by sieve plates help to ensure a steady flow of sap in the phloem. Perform many geneticmetabolic functions of the sieve tube cells. Many mitochondria, supporting active transport Infolding plasma membrane (increases surface area) Sieves tube cells and companion cells are closely related. To use this website, you must agree to our Privacy Policy, including cookie policy.
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