Transport in plants - Module 3

Active loading

The process by which hydrogen ions are actively pumped out of companion cells using ATP, before diffusing down a concentration gradient, back into the cells via co-transporter proteins, whilst carrying sucrose.

Adhesion (water movement)

The formation of hydrogen bonds between carbohydrates in the xylem vessel walls and water molecules. This contributes to the capillarity of water and transpiration pull.

Apoplast route

One of two pathways by which water and minerals move across the root. Water moves through intercellular spaces between cellulose molecules in the cell wall.

Casparian strip

A waterproof strip surrounding the endodermal cells of the root that blocks the apoplast pathway, forcing water through the symplast route.

Cohesion (water movement)

The formation of hydrogen bonds between water molecules. This contributes to the capillarity of water and transpiration pull.

Cohesion-tension theory

The model that explains the movement of water from the soil to the leaves, in a continuous stream.

Companion cells

The active cells of the phloem located adjacent to the sieve tube elements. They retain their nucleus and organelles, producing ATP for metabolic processes in both themselves and the sieve tube elements.

Dicotyledonous plants

Plants that produce seeds that contain two cotyledons. They have two primary leaves.

Hydrophytes

Plant that are adapted to live and reproduce in very wet habitats, e.g. water lilies.

Phloem

A living plant transport vessel responsible for the transfer of assimilates to all parts
of the plant. The phloem consists of sieve tube elements and companion cells.

Plasmodesmata (phloem)

Small pores between adjacent sieve tube elements and companion cells that allow communication and the exchange of materials.

Potometer

An apparatus used to measure water uptake from a cut shoot.

Root hair cells

Specialised cells responsible for the uptake of water and minerals from the soil. They have long hair-like extensions known as root hairs, which are adapted as exchange surfaces.

Sieve plates

The perforated end walls of sieve tube elements that allow plant assimilates to flow between cells unimpeded.

Sieve tube elements

The main cells of the phloem. They are elongated cells laid end-to-end with sieve plates between. They contain few organelles.

Sinks (plants)

The regions of a plant that remove assimilates e.g. roots, meristem, fruits.

Sources (plants)

The regions of a plant that produce assimilates e.g. leaves, storage organs.

Symplast route

One of two pathways by which water and minerals move across the root. Water enters the cytoplasm through the plasma membrane and moves between adjacent cells via plasmodesmata. Water diffuses down its water potential gradient by osmosis.

Translocation

The movement of organic compounds in the phloem, from sources to sinks.

Transpiration

Water loss from plant leaves and stems via diffusion and evaporation. The rate of transpiration is affected by light, temperature, humidity, air movement and soil-water availability.

Transpiration stream

The flow of water from the roots to the leaves in plants, where it is lost by evaporation to the environment.

Vascular bundle

The vascular system in herbaceous dicotyledonous plants. It consists of two transport vessels, the xylem and the phloem.

Vascular system

A network of transport vessels in animals and plants.

Xerophytes

Plant that are adapted to live and reproduce in dry habitats where water availability is low, e.g. cacti and marram grass.

Xylem

A non-living plant transport vessel responsible for the transfer of water and minerals from the roots to the shoots and leaves