xylem tubes

• Xylem vessels are long, hollow tubes made up of dead cells. The cells that make up the vessels have no end walls, so a continuous tube forms that water can move through, going upwards only. Xylem vessels contain pits. These allow water to move sideways between the vessels.
• Xylem is responsible for transporting water from roots to leaves
• Phloem consists of sieve elements which are living cells with large central vacuoles. They also have many pores called sieve plates on their cell walls. Sieve plates are perforated by small holes (called sieve areas) allowing substances to pass through them easily.
• Phloem consists of sieve elements which are living cells with large central vacuoles. They also have many pores called sieve plates on their cell walls. Sieve plates allow substances to pass through them easily. Phloem has companion cells which are small living cells next to the sieve elements. Companion cells control the movement of sugars into and out of the sieve element.
• Sieve plate - A perforated area in the wall of a sieve-tube member where protoplasmic strands extend across the opening.
• Sieve elements are joined together at their ends by companion cells. Companion cells are living cells that supply energy to the sieve element via plasmodesmata.
• The phloem transports sugars produced during photosynthesis around the plant. It does this using pressure flow. This means that as sugar is loaded into one part of the phloem it causes an increase in pressure. This forces the solution out of the other end of the phloem vessel.
• Companion cell - A small living cell adjacent to a sieve-tube member; it controls the passage of materials into and out of the sieve-tube member.
• Pressure flow occurs because there is always more sucrose being added than removed from the phloem. This creates a gradient of high pressure at the source and low pressure at the sink. As long as the gradient remains, the process continues.
• Parenchyma - Living cells found throughout the plant body; they store food reserves, manufacture chlorophyll, and carry out photosynthesis.
• Water moves from soil to root hair cells via osmosis. Water then enters the xylem vessels via osmosis as it is more concentrated outside than inside the vessel. This causes turgor pressure to build up within the vessel. As there are no end walls, this pressure forces water along the length of the vessel. Water moves up the stem because the concentration of dissolved minerals is higher at the base of the plant than at the top. Therefore, water moves up the plant against gravity due to cohesion-tension theory.
• Collenchyma - Cells with thickened primary walls containing cellulose fibers; they support young stems and leaves.
• Pressure flow occurs because there is no pumping mechanism in plants, so the only way to move fluids upwards against gravity is by creating a difference in hydrostatic pressure between two points.
• The phloem transports food around the plant using pressure flow. This means that as sugar is loaded into the phloem it causes an increase in pressure within the vessel. Water then moves into the phloem under osmosis until there is enough pressure to force the solution back down the phloem towards the root. As this happens more sugar is loaded into the phloem and the process continues.
• Pressure flow - The process by which water, minerals, and organic compounds move from source to sink along the length of a sieve tube due to differences in hydrostatic pressure caused by loading or unloading of solutes.
• Cohesion - water molecules stick together forming chains or columns. Tension - water molecules pull other water molecules behind them. Cohesion-tension theory explains how water travels up the xylem vessels without any energy input.
• Pressure flow is when the loading of sucrose into the phloem creates a positive pressure gradient along the length of the phloem. This forces the contents of the phloem downwards towards the root.
• Phloem loading - The movement of solutes (including sucrose) from the apoplast into the symplast of the sieve tube members.
• Ascending sap - Sap moving upwards in the xylem vessels towards the leaves.
• Transpiration pull is when water evaporates from leaves due to sunlight heating up the leaf surface. The loss of water pulls sap upwards along the xylem vessels towards the roots.
• Descending sap - Sap moving downwards in the xylem vessels away from the leaves.
• In the phloem, loading of sucrose into the sieve tube members creates a positive pressure gradient along the length of the phloem vessels. The high concentration of solutes in the sieve tube members leads to osmosis of water into these cells, increasing the volume of fluid within the sieve tube members.
• Phloem loading - The movement of solutes (e.g., glucose) from the cytoplasm of companion cells into the apoplast of sieve tube members.
• Apoplast - The pathway through which water moves between soil and roots or leaves.
• Sclerenchyma - Cells with lignified secondary walls that contain cellulose fibers; they strengthen woody tissue such as bark or fruit rinds.
• As more sucrose enters the sieve tube members, the pressure increases until it reaches a point where the force exerted by the pressure overcomes the frictional resistance caused by the viscosity of the sap. At this point, the sap flows downstream towards the sink tissue.
• Xylem is made up of tracheids and vessel elements. Tracheids are elongated dead cells with tapered ends. Vessel elements are longer and wider than tracheids but they still have tapered ends. Both types of cells have lignin deposits in their secondary cell walls making them strong and rigid.
• Sieve tube members - Long, thin dead cells joined together by perforated plates called sieves; they transport organic compounds such as glucose and amino acids.
• Xylem - Tissue composed of tracheids and vessel elements; it conducts water and mineral salts upward through the plant.
• Sclerenchyma - Cells with very thick secondary walls made of lignin; they provide mechanical strength to the plant.
• Cohesion-tension theory states that water molecules stick together (cohesion). When water evaporates from leaves, it pulls on neighbouring water molecules which pull on others until all the way back down to the roots. This creates tension in the xylem vessels.
• Tracheid - An elongate tube formed by the death and hardening of a single cell; found in gymnosperms.
• Sieve plates - Perforated membranes located at intervals along the length of a sieve tube element, allowing for the passage of fluid through small openings called sieve pores.