Transport in plants

• Plants require a transport system to ensure that all the cells of a plant receive a sufficient amount of nutrients.
• The combined action of xylem tissue, which enables water as well as dissolved minerals to travel up the plant in the passive process of transpiration, and phloem tissue, which enables sugars to reach all parts of the plant in the active process of translocation, ensures that all the cells of a plant receive a sufficient amount of nutrients.
• Xylem and phloem are components of the vascular bundle, which serves to enable transport of substances as well as for structural support.
• The xylem vessels are arranged in an X shape in the centre of the vascular bundle, enabling the plant to withstand various mechanical forces such as pulling.
• The X shape arrangement of xylem vessels is surrounded by endodermis, an outer layer of cells which supply xylem vessels with water.
• An inner layer of meristem cells known as the pericycle is also part of the vascular bundle.
• Xylem is located on the inside in non-wooded plants to provide support and flexibility to the stem.
• Phloem is found on the outside of the vascular bundle.
• There is a layer of cambium in between xylem and phloem, which are meristem cells involved in production of new xylem and phloem tissue.
• The vascular bundles form the midrib and veins of a leaf.
• Dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards, which are involved in transport and support.
• Xylem vessels transport water and minerals, and also serve to provide structural support.
• Xylem vessels are long cylinders made of dead tissue with open ends, therefore they can form a continuous column.
• Xylem vessels contain pits which enable water to move sideways between the vessels.
• Xylem vessels are thickened with a tough substance called lignin, which is deposited in spiral patterns to enable the plant to remain flexible.
• Phloem vessels are tubes made of living cells involved in translocation, the movement of nutrients to storage organs and growing parts of the plant.
• Phloem vessels consist of sieve tube elements and companion cells.
• Sieve tube elements form a tube to transport sugars such as sucrose, in the dissolved form of sap.
• Companion cells are involved in ATP production for active processes such as loading sucrose into sieve tubes.
• Cytoplasm of sieve tube elements and companion cells is linked through structures known as plasmodesmata, which are gaps between cell walls that allow communication and flow of substances such as minerals between the cells.
• As a result of that, the sucrose diffuses out of the companion cells down the concentration gradient into the sieve tube elements through links known as plasmodesmata.
• The action of these two forces in combination is known as the cohesion-tension theory, which is further supported by capillary action where the forces involved in cohesion cause the water molecule to adhere to the cellulose in the cell wall of the xylem, thus pulling water up.
• As sucrose enters the sieve tube elements, the water potential inside the tube is reduced, therefore causing water to enter via osmosis, as a result increasing the hydrostatic pressure of the sieve tube.
• When water reaches the endodermis, it encounters a layer of suberin known as the Casparian strip, which cannot be penetrated by water.
• The flow of water is maintained with the help of surface tension of water and the attractive forces (hydrogen bonding) between water molecules known as cohesion.
• In the pathway of water, water doesn't pass through any plasma membranes, carrying dissolved mineral ions and salts.
• The push of water upwards is aided by root pressure, which is where the action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis, thus pushing it upwards.
• Water moves down the sieve tube from an area of higher pressure to an area of lower pressure.
• H+ ions are facilitated diffusion involving cotransporter proteins which allows the returning H+ ions to bring sucrose molecules into the companion cells, thus causing the concentration of sucrose in the companion cells to increase.
• Eventually, sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus increasing the water potential in the sieve tube.
• This in turn means that water leaves the sieve tube by osmosis, as a result reducing the pressure in the phloem at the sink.
• Water moving in the xylem up the stem is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient.
• The proton pumps of companion cells use ATP to transport hydrogen ions into the surrounding tissue, thus creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cells.
• Once water has moved across the endodermis, it continues down the water potential gradient from cell to cell until it reaches a pit in the xylem vessel, which is the entry point of water.
• Translocation is an energy requiring process which serves as a means of transporting assimilates (such as sucrose) dissolved in water in the phloem between sources which release sucrose such as leaves and sinks.
• Sucrose enters the phloem in a process known as active loading.
• In order for water to cross the endodermis, the water that has been moving through the cell walls must enter the symplast pathway.
• Transpiration is the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves.
• Water evaporates from the surface of the leaf and then moves into the atmosphere via diffusion as water vapour.
• Xerophytes roll their leaves to reduce the exposure of lower epidermis to the atmosphere, thus trapping air.