Transport in Plants, practice questions

B, Xylem, made of dead lignified vessels, carries water and minerals upward; phloem carries food.

B, The soil solution has a higher water potential than the cell sap, so water moves in by osmosis down the gradient.

C, Ions are usually less concentrated in the soil, so uptake needs active transport and energy from respiration.

B, The cohesion-tension pull created by transpiration at the leaves lifts water the full height; root pressure only pushes it part way.

A, Cohesion holds water molecules together and adhesion holds them to the walls, so the column does not break.

C, Higher humidity reduces the diffusion gradient of water vapour out of the leaf, slowing transpiration.

B, A sink, such as a growing shoot or storage root, uses or stores sugar; a source releases it.

B, The companion cell, rich in mitochondria, provides ATP for active loading of sugar at the source.

B, Root pressure forces water out of leaf tips when transpiration is low, such as on humid nights.

B, Phytoextraction stores the pollutant in the shoots, which are harvested and disposed of, removing it from the site.

• (a) Variable changed (manipulated): air movement / wind around the shoot.
• (a) Controlled variable: temperature, humidity, light intensity, or the same shoot and starting position (any one).
• (b) The bubble moves a greater distance in front of the fan.
• (b) Moving air carries away water vapour, keeping the diffusion gradient steep, so transpiration and water uptake increase.

• (a) Water-carrying tissue: xylem. Food-carrying tissue: phloem.
• (b) Feature 1: lignified walls - strengthen the vessel and prevent it collapsing under tension.
• (b) Feature 2: hollow tube with no cross-walls / dead empty cells - offers little resistance so water flows freely.
• (b) Accept: pits allow sideways movement of water between vessels.

• (a) Phytoremediation.
• (b) Phytoextraction: roots absorb the metal, it accumulates in the shoots, which are harvested and removed from the site.
• (b) Phytostabilisation: roots bind the metal in the soil, reducing its movement into groundwater, without removing it.
• (c) Only hyperaccumulator species can take up and tolerate high concentrations of that particular metal without being poisoned, so the plant must match the pollutant.

Xylem is made of dead, hollow, lignified vessels; phloem is made of living sieve tubes and companion cells.

Water travels up the xylem in the transpiration stream, driven by evaporation from the leaves.

The loss of water vapour from the leaves through the stomata, which creates the pull that draws water up.

Light, temperature, humidity and air movement change the rate of transpiration.

The movement of sugars in the phloem from sources (leaves) to sinks (growing or storage tissues).

Using plants to absorb and remove pollutants such as heavy metals from soil or water.

Root cells actively pump mineral ions into the xylem, lowering the water potential inside so water moves in by osmosis and builds up a positive pressure that pushes the sap partway up the stem. Root pressure is strongest at night or in humid conditions when transpiration is low, and it can be seen as guttation, small droplets forced out at leaf tips.

Evaporation of water from the leaf cells creates tension that pulls water upward through the xylem. Water molecules cling to each other by cohesion and to the lignified xylem walls by adhesion, so the pull at the top drags an unbroken column of water all the way from the roots without the column breaking.

A xylem vessel is formed from dead cells joined end to end with their end walls broken down, producing a continuous hollow tube. The walls are strengthened with lignin, which provides support and prevents the vessel from collapsing under the tension created during transpiration, while pits in the wall allow sideways movement of water.

Phloem tissue is built from living sieve tube elements joined end to end through perforated sieve plates, allowing sap to flow between them. Each sieve tube element is supported by an adjacent companion cell, packed with mitochondria that supply the energy needed to actively load sugar into the sieve tube.

At the source, such as a leaf, companion cells actively load sugar into the sieve tubes, lowering the water potential there so water enters by osmosis and raises the pressure. This pressure difference pushes the sugary sap through the sieve tubes towards a sink, such as a growing shoot or storage root, where sugar is removed and used or stored.

Root hairs greatly increase the surface area of the root for absorption. Water enters the root hair cells by osmosis, moving from the soil solution, which has a higher water potential, into the cell sap. Mineral ions are often at a lower concentration in the soil than in the root, so they are absorbed by active transport, which requires energy from respiration.

Each stoma is bordered by two guard cells that change shape as they become turgid or flaccid. When guard cells absorb water and become turgid, their uneven cell walls cause them to curve apart, opening the stoma; when they lose water and become flaccid, the stoma closes, directly controlling the rate of transpiration.

In phytoextraction, plant roots absorb pollutants such as heavy metals from soil or water and store them in the shoots, which can later be harvested and safely removed. In phytostabilisation, plant roots instead bind and immobilise pollutants within the soil, reducing their movement into groundwater without removing them from the site.

A hyperaccumulator is a plant species able to absorb and tolerate unusually high concentrations of a specific pollutant, such as a heavy metal, without being poisoned. Examples include ferns and certain grasses used to clean up soil contaminated with metals like arsenic or lead, making them useful tools in phytoremediation projects.

A potometer measures the rate of water uptake by a leafy shoot as a proxy for the rate of transpiration, tracked by timing the movement of an air bubble along a graduated capillary tube. Blue cobalt chloride paper, which turns pink as it absorbs moisture, can be pressed onto a leaf surface to compare the rate of water loss from the upper and lower epidermis.