Transport In Plants Flashcards
What is cambium
Cambium is a layer of unspecialised cells that divide giving rise to more specialised cells that in turn form both the xylem and phloem
What is the structure of the xylem
The xylem starts off as living tissue. The first xylem to form is called the protoxylem. It is capable of stretching and growing because the walls are not fully lignified. The cellulose microfibrils in the walls of the xylem vessels are laid down more or less vertically in the stem which increases the strength of the tube and allows it to withstand compression forces from the weight of the plant pressing down on it. As the stem ages increasing amounts of lignin are laid down in the cell walls. As a result the cells become impermeable to water and other substances. The tissue becomes stronger and more supportive but the contents of the cells die. This lignified tissue is known as metaxylem. The end walls between the cells largely break down so the xylem forms hollow tubes running from the roots to the tip of the stems and leaves.
How are minerals and water transported up the plant
They are transported in the transpiration stream. Water moves out of the xylem into the surrounding cells either through unlignified areas or through specialised pits in the walls of the xylem vessels. The lignified xylem vessels are very strong and play an important supportive role in the stems of plants. In smaller non woody plants support mainly comes from the turgid parenchyma cells in the centre and the scleremchyma and Collenchyma. As woody plants grow older more xylem tissue is lignified to increase support.
What is the structure of the phloem
They have phloem sieve tubes which are made up of many cells joined together to make very long tubes that run from the highest shoots to the end of the roots. Phloem cells don’t become lignified. The walls between the cells become perforated to form specialised sieve plates and the phloem contents flow through the holes in these plates. As the gaps in the sieve plates form the nucleus, the tonoplast and some of the other organelles break down. The phloem sieve tube becomes a tube filled with phloem sap and the mature phloem cells have no nucleus. They survive because they have closely associated cells called companion cells. The companion cells are very active cells that have all the normal organelles and they are linked to the sieve tube elements by plasmodesmata. The cell membrane of companion cells have many infoldings that increase the SA over which they can transport sucrose into the cell cytoplasm and they have many mitochondria to supply the ATP needed for active transport.
How do plants absorb water from the soil
Water is absorbed mainly by the younger parts of the roots where the majority of the root hairs are found. These are extensions of the membranes of the outer cells of the root and they greatly increase the SA for absorption. There are 2 alternative routes in the xylem vessel for the uptake for water. In the symplast pathway water moves by diffusion down the conc gradient from the root hair cells to the xylem through the interconnected cytoplasm symplast of the cells of the root system. It moves through the plasmodesmata gaps in the cellulose cell walls that allow strands of cytoplasm to pass through them so the contents of the 2 cells are in contact. In the apoplast pathway water is pulled by the attraction between water molecules across adjacent cell walls from the root hair cell to the xylem. Because of the loose open network structure of cellulose up to half of the volume of the cell wall can be filled with water. As water is drawn into the xylem attraction between molecules ensures that more water is pulled across from the adjacent cell wall and so on. Minerals are drawn through the apoplast pathway too. The water moves across the cells of the root in the cell walls until it reaches the endodermis which contains a waterproof layer called the casparian strip. Once they reach this they enter the cytoplasm of the cell temporarily. Minerals may need to enter the cytoplasm up a conc gradient involving active transport
How is translocation of water achieved
The movement of water in the xylem of plants depends on transpiration which is the loss of water vapour from the surface of the plant mainly from the leaves. Water evaporates from the cellulose walls of the spongy mesophyll cells into air spaces. The water vapour moves through open stomata into external air along a diffusion gradient. Each leaf has a layer of still air around it. The thickness of this layer varies with the wind speed. The water vapour diffuses through this still layer before it is swept away by the mass of moving air. The amount of water lost by a plant due to transpiration can be surprisingly large. When water is lost by transpiration from the leaves it moves by osmosis across the leaf from cell to cell all the way from the xylem. When molecules of water leave the xylem to enter a cell by osmosis this creates tension in the column of water in the xylem and this tension is transmitted all the way down to the roots. This is due to cohesion of the water molecules . Because of their polar nature and the hydrogen bonds that form between them water molecules ‘stick together’ giving the column
