Transport in Plants, worked answers

A xylem vessel is made of **dead, empty cells** joined **end to end** with the **cross-walls broken down**, forming a **continuous hollow tube** through which water flows with little resistance. The wall is thickened with **lignin**, which **supports** the plant and **prevents the vessel from collapsing** under the **tension** created during transpiration. **Pits** in the wall allow water to move **sideways** between vessels. Because the cells are **dead and contain no cytoplasm**, there is no barrier to the upward flow of water.

dead cellsend to endcross-walls broken downhollow tubeligninsupportprevent collapsepits

The **soil solution has a higher water potential** than the **cell sap** in the root hair, so water moves in by **osmosis** across the **partially permeable membrane**, down the **water potential gradient**. The root hair's **long, thin shape increases surface area** for faster absorption. Water then passes **from cell to cell across the cortex** down a water potential gradient until it reaches the **xylem** at the centre of the root. **No energy is needed** because the movement is passive.

higher water potentialosmosispartially permeablewater potential gradientsurface areacortexxylempassive

Mineral ions such as **nitrate and potassium** are usually at a **lower concentration in the soil** than **inside the root cell**. They therefore cannot diffuse in, because diffusion moves substances **down** a concentration gradient. Instead they are absorbed by **active transport**, which moves ions **against the concentration gradient** using **energy from respiration**. This is why uptake slows when the soil lacks oxygen and respiration falls.

lower concentration in soilagainst concentration gradientactive transportenergyrespiration

Water **evaporates** from the wet cell walls inside the leaf and **diffuses out through the stomata** as **transpiration**, creating a **tension (pull)** at the top of the **xylem**. Water molecules are held together by **cohesion** and stick to the **lignified xylem walls** by **adhesion**, forming a **continuous, unbroken column**. The tension is transmitted down this column, so water is **pulled up** from the roots. Because the column does not break, the pull can lift water to the top of a tall tree.

transpirationevaporationtension/pullcohesionadhesioncontinuous columnxylemstomata

At the **source**, **companion cells** use **energy** to **actively load sugar** into the **sieve tubes**, **lowering the water potential** there. **Water enters by osmosis** from the xylem, **raising the hydrostatic pressure** at the source. At the **sink**, **sugar is unloaded** and used or stored, water leaves, and the **pressure falls**. Sap therefore flows by **mass flow** from the **high-pressure source** to the **low-pressure sink**. The direction can change because a part of the plant can be a source at one time and a sink at another.

sourcesinkcompanion cellactive loadinglower water potentialosmosishigh pressure to low pressuremass flow

In **phytoextraction**, roots **absorb the pollutant** and transport it to the **shoots**, which are then **harvested and removed**, so the pollutant is **taken out of the site**. In **phytostabilisation**, roots **bind and immobilise the pollutant in the soil**, **reducing its movement into groundwater**, but the pollutant **stays in the ground**. Both use **hyperaccumulator** plants able to **tolerate high concentrations** of the pollutant, but only phytoextraction removes it. The choice depends on whether the aim is to remove or merely contain the pollutant.

phytoextractionabsorb and harvest shootsremoved from sitephytostabilisationimmobilise in soilreduce groundwater movementhyperaccumulator

Each stoma is bordered by **two guard cells** with **unevenly thickened walls** (thicker on the inner side). When guard cells **absorb water and become turgid**, they **curve apart**, **opening the stoma** and allowing more water vapour to escape, so **transpiration increases**. When they **lose water and become flaccid**, they straighten and the **stoma closes**, reducing water loss. In this way the guard cells **directly control the rate of transpiration** and help prevent excessive water loss.

two guard cellsuneven wall thickeningturgidstoma opensflaccidstoma closescontrol transpiration