Transport in flowering plants

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Transport in flowering plants

Plants are less active than animals so their cells don’t need materials so quickly. Also, the shape and structure of plants means that oxygen and carbon dioxide can diffuse into and out of cells. Substances such as glucose molecules and nitrate ions move naturally from a region of higher concentration to one of lower concentration by diffusion. Water diffuses through a selectively permeable membrane from a region of higher water potential to one of lower water potential, a process known as osmosis. These vital processes are essential but adequate only for a transport of molecules and ions over short distances within a living system. Diffusion by itself is not sufficient; it is often too slow, possibly in the wrong direction and inadequate over longer distances. The evolution of flowering plants has seen the development of efficient transport systems, which have enabled plants to colonise the land, to increase their size and to develop an established habit. Plants have two transport systems; the xylem carries water and dissolved minerals and the phloem carries the ‘food’ materials that the plant has made. These enable the movement of water, organic molecules and ions over long distances.  

Xylem and phloem tubes are found together in a group called a vascular bundle.

Xylem vessels run from the roots, up through the stem and out into each leaf. The xylem is made of hollow dead cells joined end to end. The cells don’t contain any cytoplasm or nuclei. The cell walls are made of cellulose and lignin, which is very strong and helps keep the plant upright. The ends of the cells have disappeared so that a long continuous tube is formed. There are two types of xylem cells involved in the transport of water, tracheids and vessel elements. Both types of cell have cell walls thickened with lignin. Lignin strengthens the wall enabling it to provide support. It also waterproofs the cell so that it can conduct water.

A new xylem is formed each year, and the old xylem is pushed to the centre and becomes compacted. The xylem at this point is almost entirely lignin (this is wood). This is the structure of the xylem:

The xylem vessels also allow sideways flow of water through the vessels. This is done by bordered pits. Xylem vessels can transport water even when a plant is dead. This is because they work passively. Metabolic poisons e.g. potassium cyanide (KCN) do not prevent transport of water in plants.

The phloem is also made up of cells joined end to end. The ends of the cells are not completely broken down and form a kind of sieve (called a sieve plate). Each of the cells is called a sieve tube element. These cells contain strands of cytoplasm, but no nucleus, and there is no lignin in the cell wall but there is cellulose. Each sieve tube element has a companion cell attached to it. The companion cells have a nucleus, organelles and dense cytoplasm, and provide the sieve tube elements with some of their requirements. This is the structure of the phloem:

The plant to make proteins, oils and other organic substances uses carbohydrates made in the leaves during photosynthesis. Organic food material, particularly sugar, is transported in the phloem tubes to where it is needed. This is called translocation. The sap inside phloem tubes contains lots of sugar. Moving it is an active process that uses energy.

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Food, primarily sucrose is transported by the vascular tissue called phloem from a source to a sink.

Unlike transpiration's one-way flow of water sap, food in phloem sap can be transported in any direction needed so long as there is a source of sugar and a sink able to use, store or remove the sugar.

The source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, the sink, need energy for growth and ...

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