Transport in Plants

The concept of Mass Flow

The mass flow hypothesis accounts for the movement of solutes (particularly sucrose) around plants. It depends on the movement of water through partially permeable membranes in response to differences in water potential and hydrostatic pressure.

In plants, sucrose is produced in leaves (called the source) and travels through phloem tubes to sites of utilization (called sinks).

The active transport of sucrose into phloem tubes sets up a difference in water potential (the cells of the phloem tube are more negative) so water moves into them (from the xylem) by osmosis.

This influx of water causes an increase in hydrostatic pressure forcing the water and solutes down the phloem tube to the sink regions where the sucrose is removed (again by active transport)

There is evidence both in for and against the mass flow hypothesis in this process and other hypotheses also exist to account for the movement of sucrose.

For further details of this topic click here

A rather more straightforward example of mass flow is in the movement of water up xylem vessels.

Here water enters the roots and moves up the xylem as a result of the lower pressure in the leaves caused by loss of water from the leaves. The loss of water from the leaves is called transpiration. The movement of water up a plant is analogous to the movement of water up a straw from which air is being sucked at the top.

For further details of this topic click here

Transport Structures in Plants

In humans the vascular tissues are the arteries, veins and capillaries. 

In plants the vascular tissues are the xylem (which transports water and ion from the roots to the rest of the plant) and phloem (which transport organic solutes, such as sucrose from he photosynthetic regions to the rest of the plant).

Xylem

Xylem tissue is made up of up to four different structures:

• vessels
• tracheids
• fibres
• xylem parenchyma

Vessels and tracheids 

• are the conductive structures - it is through these that water travels
• as they develop they become lignified. Lignin is a complex derivative of carbohydrates which provides strength to the cell

In this diagram (from http://www.cropsci.uiuc.edu/classes/cpsc121/images/FormFunction/xylem2.jpg) the cell on the left is a tracheid showing spiral lignification. Lignification is also found as rings or as interconnected regions of lignin (called scalariform if there are few interconnections or reticulate if the connections  are so numerous as to form a network).

The cell on the right is a vessel showing the perforation plate between adjacent cells.

• at maturity the living parts of the cell are lost (i.e. the cell "dies") and the cell assumes its conductive function
• water passes between adjacent cells through the pits found in the wall of the cells
• in vessels (as opposed to tracheids) water also passes between cells via the perforatio plates

This diagram (from http://www.sirinet.net/~jgjohnso/xylemlg.jpg) shows the passage of water between tracheids on the left and vessels on the right.

Note the tapering of the ends of the tracheids allowing a larger areas for transfer of water.

Of the other structures found in xylem tissue the fibres are strengthening cells while the xylem parenchyma are essentially packing cells but they also store starch and allow lateral movement of water and solutes through the vascular tissue

For further details of this topic click here

Phloem

Phloem tissue is more complex than xylem tissue and so is likely to bring more marks in an exam.

Phloem tissue is concerned with the transport of organic solutes from their source in the photosynthetic regions to the sinks in the sites of utilization.

The structures found in phloem tissue include:

• sieve tube elements
• companion cells
• phloem fibres
• phloem parenchyma

The phloem fibres and phloem parenchyma have the same roles as their xylem counterparts, acting, respectively, as; strengthening cells and packing, storage and lateral transport cells.

The sieve tube elements are the long-distance transport structures and are found in close association with companion cells.

Image from http://www.puc.edu/Faculty/Gilbert_Muth/botglosc.htm

Unlike xylem vessels and tracheids, the sieve tube elements do not completely lose their living components; some organelles remain in a thin layer of cytoplasm bordering the cells. However the nucleus and ribosomes are lost.

Between adjacent sieve tube elements are sieve plates which allow the passage of sap (the carbohydrate rich fluid present in phloem)

Each sieve tube element has an associated companion cell. The companion cells retain their nuclei and have large quantities of mitochondria enabling them to carry out much metabolic activity.

The sieve tube element and the companion cell are connected by numerous plasmodesmata. These are pores in the walls of the two cells allowing exchange of material. 

It is assumed that the companion cell provides enzymes and energy for the sieve tube elements

For further details of this topic click here

This site contains a lot of information about plant structure and function: http://www.sirinet.net/~jgjohnso/plants.html

Go to Biology Index