Transport in Plants

Transport in Plants

(a) explain the need for transport systems in multicellular plants in terms of size and surface area: volume ratio

Multicellular plants have a small surface area: volume ratio so diffusion would be too slow to provide necessary substances like water, minerals and sugars and to remove waste substances. Also multicellular plants are large so have a greater demand for substances. Therefore plants need transport systems to move substances to and from individual cells quickly.

(b)describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants


(c) describe, with the aid of diagrams and photographs, the structure and function of  xylem vessels, sieve tube elements and companion cells

(d) define the term transpiration

Transpiration is the evaporation of water from a plant’s surface, especially the leaves.

Transpiration involves 3 processes:

  1. Water leaves the xylem and passes to the mesopyll cells by osmosis.
  2. Water evaporates from the surface of the mesophyll cells, to form water vapour, into the air spaces.
  3. The water vapour potential in the leaf is higher than outside, so water molecules will diffuse out of the leaf.

(e) explain why transpiration is a consequence of gaseous exchange

It is a side effect of gas exchange as a plant needs to open its stomata to let in carbon dioxide so that it can produce glucose by photosynthesis, but it also lets water out as there’s a higher concentration of water inside the leaf than in the air outside water moves out of the leaf down the water potential gradient when the stomata is open.

(f) describe the factors that affect transpiration rate

(g) describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates

A potometer is used to estimate the rate of water loss. It is not an exact measure, as it actually measures the rate of water uptake by a cut shoot. It is important to make sure there are no air bubbles inside the apparatus. Water lost by the leaf is replaced from the water in the capillary tube. The movement of the meniscus at the end of water column can be measured.

You need to remember:

  • Cut a shoot underwater to prevent air from entering the xylem
  • Cut a shoot at a slant to increase the surface area available for water uptake
  • Dry the leaves
  • Use non-wilting shoots
  • Allow time for equilibrium and for it to acclimatise
  • Note where meniscus is at start and end of time period

(h) explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment


(i) describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata


  1. Water enters from the soil to the root hair and epidermis through osmosis – from a higher water potential (soil) to the most negative water potential (xylem)
  2. Water enters the cortex by the apoplast pathway between cell walls, the symplast pathway through plasmodesmata and the vacuolar pathway
  3. Water enters the endodermis which has a Casparian strip which blocks the apoplast pathway so water must be transported by the symplast pathway allowing selective mineral uptake
  4. Water enters the xylem and minerals are moved using active transport which reduces the water potential in the xylem creating a water potential gradient. Water can’t pass through to the cortex again as the endodermis is blocked

(j) explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream

  1. Minerals are actively transported into the xylem vessels. This lowers the water potential in the xylem and water follows by osmosis.
  2. Root pressure pushes some of the water upwards.
  3. Water evaporates from the surface of the leaf by transpiration and water is lost.
  4. The water must be replaced as water moves out of the xylem into the leaf, creating a low hydrostatic pressure, and a pressure gradient, and thus tension.
  5. Water molecules are attracted to each other by forces of cohesion creating a continuous column of water so that water can be moved by mass flow, pulled upwards by tension from above.
  6. Water molecules are also attracted to the walls of the xylem by forces of adhesion and causing capillary action.

(k) describe with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration

A xerophyte is a plant that is adapted to reduce water loss so that it can survive in very dry conditions e.g. marram grass, cacti etc.


(l) explain translocation as an energy-requiring process transporting assimilates especially sucrose, between sources (e.g. leaves) and sinks (e.g. roots, meristems)

Translocation is the movement of assimilates (e.g. sugars – sucrose) through the phloem tissue and is an energy-requiring process. It moves from source to sink.

  • Source: where the assimilates are made/came from (e.g. leaves)
  • Sink: where the assimilates are used/stored (e.g. roots, meristems)

(m) describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and evidence for and against this mechanism

How are assimilates loaded into the phloem?

  1. ATP is used by the companion cells to actively transport hydrogen ions out of their cytoplasm and into the surrounding tissue
  2. This sets up a diffusion gradient as there are more hydrogen ions outside the cell than inside, and the hydrogen ions diffuse back into the companion cells.
  3. Diffusion happens through cotransporter proteins – they allow hydrogen ions to bring sucrose molecules into the companion cells.
  4. As the concentration of sucrose molecules builds inside the companion cells, they diffuse into the sieve tube element through the numerous plasmodesmata.


Movement of Sucrose Along the Phloem:

Evidence For and Against Mass Flow: