Transport in plants

Joanna George-Johnson, Contributor

Just like animals plants also need to have substances transported throughout its structure.

The system in plants is much simpler than in animals. Here are a few of the differences:

• Whereas humans and other animals have a heart plants do not have a pump.
• Substances are carried in solution in water.
• One type of vessel carries water and minerals and a different type carries food.
• There is no circulatory system.

Carriage of water and minerals

Water and mineral salts are carried in plants through special vessels called xylem.

Xylem vessels are well-suited for this job. (See table below).

Therefore xylem looks like pipes attached end to end throughout the entire plant.

Water passes through the root hairs in the soil by the process of osmosis. Minerals also enter through the same route but by the process of active transport.

Once inside the root hairs, the water makes its way to the centre of the root where the xylem vessels are. The water then rises up the xylem and makes its way to the leaves.

The stream of water from root to leaves is known as the transpiration stream. The process by which the excess water exits the leaves is known as transpiration. It is affected by a few environmental factors.

Various factors will increase the rate of transpiration.

• Increasing light intensity causing full stomata opening.
• Increasing temperature increases kinetic energy of H2O molecules causing faster movement.
• Decreasing humidity means the atmosphere can hold more water, thus this increases the concentration gradient between the atmosphere and the leaves.
• Increased air movement (wind) removes water saturated air from around leaves. This in turn increases the concentration gradient between the atmosphere around the leaf and the leaves.
• Decreased stomata density prevents overlap of diffusion cells.

Carriage of food

Food, primarily sucrose is transported by the vascular tissue called phloem from a source to a sink*.

*Sink: area in the plant where the concentration of sucrose is low.

Unlike transpiration's one-way flow of water, food in phloem can be transported in any direction needed so long as there is a source of sugar and a sink that can be used to store or remove the sugar.

The source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, the sink, needs energy for growth and development of the photosynthetic apparatus.

Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported. The movement of such substances in the plant is called translocation.

As glucose is made at the source (by photosynthesis for example) it is converted to sucrose (a dissacharide). The sugar is then moved into companion cells and into the living phloem's sieve tubes by active transport.

Water moves into the phloem by osmosis from the adjacent xylem.

Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar, converting it into energy, starch, or cellulose.

Phloem tissue

Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. These cells are alive, but they lose their nuclei and other organelles, and their cytoplasm is reduced to strands around the edge of the cells. The centre of these tubes is empty. Each sieve tube cell is associated with one or more companion cells, normal cells with nuclei and organelles. These companion cells are connected to the sieve tube cells by special pores and provide them with proteins and other nutrients.

Diagram of Phloem tissue

Questions

• Draw and label a diagram of a longitudinal section through a xylem vessel.
• List the ways in which phloem is adapted for its function and explain how these adaptations help.
  
  

  These students work on a graphic during their art class at Denbigh High School. -Photo by Michael Lee

Joanna George-Johnson teaches at Ardenne High School.