3.3

• 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.
• The vascular bundles form the midrib and veins of a leaf, with dicotyledonous leaves having a network of veins, starting at the midrib and spreading outwards, 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, 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.
• Sucrose enters the sieve tube elements in the phloem through links known as plasmodesmata.
• 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.
• The flow of water is also maintained with the help of surface tension of water and the attractive forces between water molecules known as cohesion.
• Sucrose enters the phloem in a process known as active loading where 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.
• In order for the water to cross the endodermis, the water that has been moving through the cell walls must now enter the symplast pathway.
• Translocation is an energy requiring process which serves as a means of transporting assimilates such as sucrose in the phloem between sources which release sucrose such as leaves and sinks which remove sucrose from the phloem.
• Water moves down the sieve tube from an area of higher pressure to an area of lower pressure.
• The mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where they are needed.
• When the water reaches a part of the root called the endodermis, it encounters a layer of suberin which is known as the Casparian strip, which cannot be penetrated by water.
• 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.
• In the apoplast pathway, water doesn't pass through any plasma membranes, therefore it can carry dissolved mineral ions and salts.
• 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.
• Once it has moved across the endodermis, the water 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.
• The apoplast pathway is a water movement pathway through the water filled spaces between cellulose molecules in the cell walls.
• The push of water upwards is aided by the 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.
• 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, gaps between cell walls which allow communication and flow of substances such as minerals between the cells.
• 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.
• Carbon dioxide enters, while water and oxygen exit through a leaf’s stomata.
• The rate of transpiration can be investigated with the help of a potometer where water vapour lost by the leaf is replaced by water in the capillary tube.
• Xerophytes contain hairs and pits which serve as a means of trapping moist air, thus reducing the water vapour potential.
• Xerophytes also roll the leaves to reduce the exposure of lower epidermis to the atmosphere, thus trapping air.
• Minerals are also absorbed through the root hair cells by active transport, as they need to be pumped against the concentration gradient.
• Transpiration also involves evaporation from the surface of mesophyll cells into intercellular spaces and diffusion of water vapour down a water vapour potential gradient out of the stomata.
• Water enters through root hair cells and moves into the xylem tissue located in the centre of the root as a result of a water potential gradient, due to the dissolved substances in the cell sap.