Transported substances in plants

· Mineral ions and organic compounds can be transported in plants dissolved in water.
· Xylem transports water and dissolved mineral ions from roots to leaves.
· Phloem transports assimilates dissolved in water, such as sucrose and amino acids.
· Transport is needed because substances are often absorbed or produced in one part of the plant but used or stored elsewhere.

Water uptake: soil to xylem

· Water enters root hair cells from the soil by osmosis, down a water potential gradient.
· Water moves across the cortex to the xylem by the apoplast pathway and symplast pathway.
· Apoplast pathway = water moves through cell walls and spaces between cells, not through cytoplasm.
· Cell walls contain cellulose, which helps water move by adhesion.
· Lignin in xylem walls makes vessels waterproof and strengthens them against collapse.
· Symplast pathway = water moves through the cytoplasm of cells via plasmodesmata.
· At the endodermis, the Casparian strip blocks the apoplast pathway.
· The Casparian strip contains suberin, a waterproof material that forces water into the symplast pathway.
· This means water must pass through selectively permeable cell surface membranes, helping control which ions enter the xylem.

This diagram compares the apoplast pathway through cell walls with the symplast pathway through cytoplasm and plasmodesmata. It is useful for explaining why the Casparian strip forces water into living cells before it reaches the xylem. Source

Transpiration

· Transpiration = loss of water vapour from aerial parts of a plant, mainly through stomata in leaves.
· Water evaporates from the internal surfaces of mesophyll cells into the air spaces of the leaf.
· Water vapour then diffuses out of the leaf through the stomata into the atmosphere.
· Transpiration depends on a water potential gradient between moist leaf air spaces and the drier external air.
· Transpiration creates a pull that helps move water up the xylem from roots to leaves.
· Exam phrasing: transpiration involves evaporation from internal leaf surfaces, followed by diffusion of water vapour to the atmosphere.

Cohesion-tension and transpiration pull

· Water moves up xylem by the cohesion-tension mechanism.
· Evaporation of water from mesophyll cell walls lowers water potential in the leaf.
· Water is drawn from the xylem into leaf cells, creating tension in the xylem.
· This tension produces a transpiration pull, pulling the continuous column of water upwards.
· Cohesion = water molecules stick to each other by hydrogen bonds.
· Cohesion keeps the water column continuous so it can be pulled up the xylem.
· Adhesion = water molecules stick to cellulose in xylem cell walls.
· Adhesion helps resist gravity and supports upward movement of water.
· Xylem vessels are strengthened by lignin, preventing collapse under tension.

This image shows how cohesion between water molecules and adhesion to xylem walls help water move upwards. Link it to hydrogen bonding, transpiration pull, and the continuous water column in xylem. Source

This diagram connects xylem transport with transpiration at the leaf. It is useful for showing that water movement in xylem is driven by water loss from leaves. Source

Xerophytic leaf adaptations

· Xerophytes are plants adapted to dry habitats where water loss must be reduced.
· In exam drawings, label structures clearly and link each feature to reduced transpiration.
· Thick waxy cuticle reduces evaporation from the leaf surface.
· Sunken stomata trap moist air, reducing the water potential gradient between the leaf and air.
· Hairs around stomata trap moist air and reduce air movement.
· Rolled leaves trap humid air inside the leaf, reducing diffusion of water vapour.
· Reduced leaf surface area lowers the area available for evaporation.
· Few stomata reduce the number of pores through which water vapour can diffuse.
· Multiple epidermal layers can increase the diffusion distance for water loss.
· In annotated drawings, use large, clear labels and explain the adaptation, not just identify it.

Assimilates, sources and sinks

· Assimilates = organic substances made by the plant, especially sucrose and amino acids.
· Assimilates move in phloem sieve tubes dissolved in water.
· A source is a region where assimilates are produced or released into the phloem.
· Examples of sources: photosynthesising leaves, storage organs when releasing stored food.
· A sink is a region where assimilates are used or stored.
· Examples of sinks: roots, growing shoots, flowers, fruits, seeds, storage organs when accumulating food.
· Phloem transport can occur up or down the plant, depending on the positions of sources and sinks.
· Movement of assimilates in the phloem is called translocation.

Phloem loading by companion cells

· Companion cells transfer assimilates, especially sucrose, into phloem sieve tube elements.
· Companion cells contain many mitochondria to provide ATP for active transport.
· Proton pumps use ATP to actively pump H⁺ ions out of companion cells.
· This creates a proton gradient: high H⁺ concentration outside the companion cell.
· H⁺ ions diffuse back into companion cells through cotransporter proteins.
· The cotransporter carries sucrose into the companion cell together with H⁺.
· Sucrose then moves into the sieve tube element through connections between the companion cell and sieve tube.
· Loading sucrose lowers the water potential in the sieve tube.
· Water enters the sieve tube from nearby xylem by osmosis, increasing hydrostatic pressure at the source.

This diagram is useful for explaining how sucrose loading can involve proton pumps, H⁺ gradients, and cotransporter proteins. It helps connect companion cell activity to sucrose entry into the phloem. Source

Mass flow in phloem

· Mass flow in phloem is movement of sap down a hydrostatic pressure gradient from source to sink.
· At the source, sucrose is loaded into sieve tubes.
· This lowers the water potential inside sieve tubes.
· Water enters from xylem by osmosis, increasing hydrostatic pressure.
· At the sink, sucrose is removed from the sieve tube for use or storage.
· Removal of sucrose increases the water potential in the sieve tube.
· Water leaves the sieve tube, lowering hydrostatic pressure at the sink.
· Sap moves from high hydrostatic pressure at the source to low hydrostatic pressure at the sink.
· The movement is a bulk flow of solution, not diffusion of individual sucrose molecules.
· Key exam phrase: phloem sap moves by mass flow down a hydrostatic pressure gradient from source to sink.

This image summarises the pressure-flow hypothesis for phloem transport. It shows how loading at a source and unloading at a sink create the pressure gradient that drives mass flow. Source

Xylem vs phloem transport

· Xylem: transports water and mineral ions from roots to leaves.
· Phloem: transports assimilates, such as sucrose and amino acids, from sources to sinks.
· Xylem movement is driven mainly by transpiration pull and the cohesion-tension mechanism.
· Phloem movement is driven by mass flow down a hydrostatic pressure gradient.
· Xylem relies on hydrogen bonding, cohesion, adhesion, and lignified vessels.
· Phloem loading requires ATP, proton pumps, cotransporter proteins, and companion cells.
· Xylem flow is generally from soil → roots → leaves → atmosphere.
· Phloem flow can be upwards or downwards, depending on source and sink locations.

Common exam mistakes to avoid

· Do not say transpiration is just “water leaving the plant”; specify evaporation from internal leaf surfaces and diffusion through stomata.
· Do not confuse cohesion with adhesion: cohesion = water to water; adhesion = water to cellulose.
· Do not say the Casparian strip is made of lignin; it contains suberin.
· Do not say xylem transports sucrose; sucrose is transported in phloem.
· Do not describe phloem translocation as simple diffusion; it occurs by mass flow driven by hydrostatic pressure differences.
· Do not forget that proton pumps require ATP and are found in companion cells.
· In xerophyte questions, always link each feature to reduced water loss by transpiration.