Sugar Transport

How is glucose transported from the leaves to the rest of the plant?


A12.15 Sieve tube cells
The phloem tissue is responsible for transporting the glucose produced during photosynthesis to the rest of the cells in the plant. All cells need glucose to produce energy for life functions. This energy is produced through cellular respiration in the mitochondria.

The phloem tissue is made up of sieve tube cells that have perforated ends. These perforated ends allow the cytoplasm to flow from one cell to the next. These cells do not have a nucleus, so they must be connected with companion cells that do contain a nucleus. These companion cells control the function of the sieve tube cells by using carrier proteins and active transport to move glucose into the sieve tube cells. Since the concentration of glucose is now higher in these cells, water follows through osmosis to try to even out the concentration. This mixture of water and glucose is then transported down the phloem following a theory called the pressure-flow theory.

The pressure-flow theory suggests that as more water and glucose enter the sieve tube cells in the leaves, the water and glucose that is already in the sieve tube cells get pushed farther down the phloem tissue. Due to the pressure behind them, the water and glucose look for a way to move out of the sieve tube cells to an area of lower concentration. They find this area in parts of the plant that need glucose. In these locations, the glucose is actively transported out of the cells into the adjacent cells that need it. Again, due to osmosis, the water follows the glucose out of the phloem and into the surrounding cells.
 

A12.16 Pressure-flow theory
The sugars are then used for life functions by the cells or stored as starches or oils in the roots, stems, or fruits of the plant. The water leaves the plant through transpiration, gets pulled into another cell through osmosis, or enters the xylem to go back up to the leaves.

Due to the glucose and water leaving the phloem where they are needed, the pressure is always less in those locations. This means the glucose and water are constantly being pushed from the leaves to the locations where they are needed the most.

  Digging Deeper


A12.17 Venus flytrap

Carnivorous plants eat meat to get the nutrients they otherwise would not have. This is an adaptation to living in environments with poor soil. There are a variety of ways the plants digest the meat they trap, but once it is digested, it is absorbed and transported along with the rest of the nutrients that are absorbed in the soil. Go to the following link for more information on carnivorous plants.

Learn More

  Read This

Please read pages 320 and 321 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on how glucose is transported through the phloem. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Why would damage to the phloem potentially kill a plant?
Damage to the phloem would stop the transportation of glucose. Since glucose is used by all cells for cellular respiration, the cells would start to die if they were not able to get it. The energy created during cellular respiration is used in many different important cell functions, such as active transport, protein synthesis, and digestion.

  1. Describe the structure of the phloem tissue.
The phloem tissue is made up of a bunch of sieve tube cells lined up end to end. The ends of these cells are perforated, so the cytoplasm can flow from one cell to another. Each cell has a companion cell attached to it that directs the function of the sieve tube cell. These cells make long tubes that run from the leaves to the rest of the plant. There are many of these tubes bundled together along with the xylem tissue to make up the vascular tissue.

  1. Draw a diagram that shows how glucose moves from the leaves to the rest of the plant. Be sure to include the type of cell transport used at each location.
Active transport is used in the leaf; glucose is actively transported from the palisade cells into the phloem. The water follows the glucose into the phloem through osmosis. When the glucose reaches the low pressure area, it is actively transported out of the phloem into the cells that require it. Water again follows through osmosis.


A12.16 Pressure-flow theory