Transport in Plants

The One Thing

Plants use two specialised transport systems: xylem carries water and mineral ions mainly upward from roots to leaves, while phloem translocates sucrose and other organic substances between sources and sinks. Water movement depends on osmosis, transpiration pull, cohesion, adhesion and pressure differences, while stomata regulate the balance between carbon dioxide uptake and water loss.

Reactions, Processes and Experiments

Key Terms

• Xylem: Lignified tubes that carry water and dissolved mineral ions from the roots to the stems and leaves. Movement is mainly upward.
• Phloem: Living tissue that transports dissolved sugars, especially sucrose, and other organic substances between sources and sinks.
• Transpiration: The loss of water vapour from a plant, mainly through the stomata in its leaves.
• Translocation: The movement of sucrose and other dissolved organic substances through the phloem from sources to sinks.
• Stoma: A pore, usually in the lower epidermis of a leaf, that allows gas exchange and water vapour loss.
• Guard cells: Specialised cells surrounding each stoma that change shape to open or close the pore.
• Root hair cell: A specialised root cell with a long extension that increases surface area for absorbing water and mineral ions.
• Osmosis: The net movement of water molecules through a partially permeable membrane from a region of higher water potential to a region of lower water potential.
• Water potential: A measure of the tendency of water to move. Water moves from higher water potential to lower water potential.
• Cohesion: The attraction between water molecules, helping maintain a continuous column of water in xylem vessels.
• Adhesion: The attraction between water molecules and xylem walls, helping water remain in narrow vessels.
• Transpiration pull: The upward force created when evaporation from leaves draws water through the continuous water column in xylem.
• Source: A plant organ or tissue where sucrose is produced or released, such as a photosynthesising leaf or a storage organ during mobilisation.
• Sink: A plant organ or tissue where sucrose is used or stored, such as roots, fruits, seeds, growing leaves or developing flowers.
• Lignin: A strong waterproof substance in xylem walls that supports the plant and prevents vessels collapsing.
• Vascular bundle: A group of transport tissues containing xylem and phloem, usually with supporting tissues.

Easily Confused

• Xylem and phloem: Xylem transports water and mineral ions mainly upward; phloem transports sucrose and other organic substances between sources and sinks, in different directions where required.
• Transpiration and translocation: Transpiration is the loss of water vapour from leaves; translocation is the movement of sucrose and other dissolved organic substances through phloem.
• Osmosis and active transport: Osmosis moves water down a water-potential gradient through a partially permeable membrane; active transport moves mineral ions using energy and carrier proteins.
• Cohesion and adhesion: Cohesion is attraction between water molecules; adhesion is attraction between water molecules and xylem walls.
• Transpiration and guttation: Transpiration is the main loss of water vapour, mainly through stomata; guttation involves the loss of small amounts of liquid water.
• Potometer measurement and direct transpiration measurement: A potometer measures water uptake as an indirect estimate of water loss, not transpiration directly.
• Source and sink: A source produces or releases sucrose; a sink uses or stores sucrose.
• Water potential and concentration gradient: Water potential describes the tendency of water to move; a concentration gradient describes a difference in concentration, relevant to the movement of mineral ions and water vapour.

What Gets Asked

• Explain how water enters a root hair cell by osmosis. Marks depend on identifying a partially permeable membrane and movement from higher water potential in the soil solution to lower water potential in the cell sap.
• Describe how mineral ions enter root hair cells by active transport. The required points are movement against a concentration gradient, energy from respiration and carrier proteins in cell membranes.
• Explain how xylem structure is related to its function. Relevant features are dead cells joined end to end, no cytoplasm, hollow continuous tubes and lignified walls that prevent collapse and provide support.
• Describe transpiration and explain how light intensity, temperature, air movement and humidity affect its rate. A common error is to state the direction of an effect without explaining evaporation, diffusion or the water vapour concentration gradient.
• Interpret or evaluate a potometer experiment. The key limitation is that a potometer measures water uptake indirectly because some absorbed water is used in photosynthesis, cell expansion and other processes.
• Explain translocation using the pressure-flow model. The sequence required is active sucrose loading, reduced water potential in phloem, osmotic entry of water from xylem, increased pressure and flow towards a sink.