Theory of the movement of substances in the phloem (1 Viewer)

[BlueGas]

What's a simple yet detailed answer if there was a question asking you to describe the theory responsible for the movement of substances in the phloem?

 

[BlueGas]

Bump.

 

[Chris_S]

PHLOEM:
- Movement of organic molecules, eg sugars, amino acids and hormones, in the phloem is called translocation.
- Materials are transported both up and down the stem. Materials are distributed especially to the growing points and reproductive structures, including developing fruits and seeds
- Flow of materials in the phloem is an active process that requires energy
- It is thought to occur by a mechanism called the source-path-sink system and is driven by a gradient generated osmotically
- The source-path-sink THEORY:
- In the plants, there are SOURCES of nutrients, e.g. leaf cells are the sources of glucose. As the glucose builds up, the cells transport the glucose by active transport into the phloem tubes, by 2 ways:
- SYMPLASTIC LOADING: Sugars and nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmodesmata joining adjacent cells (NOTE: Plasmodesmata have not been found in all plants)
- APOPLASTIC LOADING: Sugar and nutrients move along the cell walls to the sieve tube. Then they cross the cell membrane by active transport.
- As sugars enter the phloem the concentration of phloem sap increases and the water decreases. This causes the entry of water by osmosis from the surrounding cells. This resulting pressure causes water and dissolved solutes to flow towards a SINK.
- A sink is a region of the plant where sugars and other nutrients are actively begin removed from the phloem. As sugars move out of the phloem, water flows out with them. This reduces the pressure in the sieve cells at the sink region.

 

[BlueGas]

PHLOEM:
- Movement of organic molecules, eg sugars, amino acids and hormones, in the phloem is called translocation.
- Materials are transported both up and down the stem. Materials are distributed especially to the growing points and reproductive structures, including developing fruits and seeds
- Flow of materials in the phloem is an active process that requires energy
- It is thought to occur by a mechanism called the source-path-sink system and is driven by a gradient generated osmotically
- The source-path-sink THEORY:
- In the plants, there are SOURCES of nutrients, e.g. leaf cells are the sources of glucose. As the glucose builds up, the cells transport the glucose by active transport into the phloem tubes, by 2 ways:
- SYMPLASTIC LOADING: Sugars and nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmodesmata joining adjacent cells (NOTE: Plasmodesmata have not been found in all plants)
- APOPLASTIC LOADING: Sugar and nutrients move along the cell walls to the sieve tube. Then they cross the cell membrane by active transport.
- As sugars enter the phloem the concentration of phloem sap increases and the water decreases. This causes the entry of water by osmosis from the surrounding cells. This resulting pressure causes water and dissolved solutes to flow towards a SINK.
- A sink is a region of the plant where sugars and other nutrients are actively begin removed from the phloem. As sugars move out of the phloem, water flows out with them. This reduces the pressure in the sieve cells at the sink region.

I've go the same meaning in my notes but I'm looking for 1-1.5 paragraphs that speak about the overall process of the movement of substances in the phloem. Just because if I memorised the information you mentioned I might not be able to structure properly if a question was asked about this theory.

 

[andreismolnikov]

Here's my attempt at a relatively simple answer to an exam question such as:

Describe the current theory for the movement of materials through the phloem of a plant.

Organic molecules, including amino acids, sugars and hormones are transported around the plant through the phloem, according to the source to sink or pressure flow theory. Parts of this process occur by active transport and require energy.

Firstly, sources of organic molecules, such as the leaf cells, release these nutrients which move into the phloem. As this goes against a concentration gradient, they must move by active transport.

This causes an increase in the concentration of phloem sap, leading water to enter the phloem by osmosis. Pressure builds on the phloem sap at the source, and the water moves from source to sink, i.e. from high concentration to low concentration.

At the sink, which may be reproductive structures or fruits, the molecules are again actively removed from the phloem. This also requires energy.

Thus phloem involves active transport at either end, but movement of the sap itself occurs passively. Also note that unlike xylem, direction of nutrient travel in the phloem can change seasonally as the energy needs of the plant change, e.g. fruits may not be produced during winter.