9.2: Transport in the phloem

Sunflower stem? Root? Leaf?

Sunflower stem? Root? Leaf?

Comparison of xylem and phloem tubes

The Phloem Found within the leaf, stem and root of the plant. Phloem has sieve tubes (columns of specialized cells) separated by perforated walls called sieve plates. Sieve tube cells are closely associated with companion cells.

Translocation Defined as: the transport of organic solutes within a plant, including: sugars and amino acids. Typically solutes are transported from source to sink, but this can happen in reverse if necessary. Transport in the phloem can be in either direction, unlike the xylem which is one way. Transport is regulated using pressure gradients. Energy is required to maintain the gradients, therefore translocation is an active process.

Photosynthetic tissue: Mature green leaves Green stems Sources Sinks Photosynthetic tissue: Mature green leaves Green stems Storage organs, that are unloading their stores: Storage tissues in germinating seeds Tap roots or tubers at the start of the growing season Roots that are growing or absorbing mineral ions using energy from cell respiration Parts of the plant that are growing or developing food stores: Fruits Seeds Leaves Tap roots e.g. carrot/tubers e.g. potato Examples of sources and sinks

Phloem loading Active transport is required to load organic compounds into phloem sieve tubes at the source. Table taken from pg. 414. This data shows: sucrose is the most abundant carbohydrate, particularly in the sap (phloem) Remember, sucrose is not is in respiration, it is a good carbohydrate for transport because it will not be metabolized in this form.

Apoplast route: sucrose travels: mesophyll cells  companion cells or sieve cells, there a sucrose transport protein actively transport sucrose into phloem tube (not all species do this) Diagram below (see pg. 413) shows how energy for this process is produced. Diagram shows the movement of sucrose across a sieve tube membrane. Sucrose concentration gradient is established by active transport. H+ ions are actively transported out of the companion cells from surrounding tissues using ATP. Build up of H+ ions then flow down the concentration gradient, through a co-transport protein (protein able to carry more than one thing, in this case H+ and sucrose). This allows sucrose to enter the tube.

Symplast route: sucrose travels between cells by using connections called plasmodesmata. In the companion cell, sucrose is converted into an oligosaccaride (a simple polysaccaride) in order to maintain the sucrose concentration gradient (other species) Plasmodesma = singular Diagram compares the two routes.

Translocation: role of water pressure gradients & pressure. Water from surrounding tissues is drawn into the companion cells via osmosis due to the high concentrations of carbohydrates in the companion cells. As water moves, pressure changes take place. The cell walls are rigid, and water is not easily compressed therefore the pressure increases in these cells creating a pressure gradient. Water flows, high  low pressure. In sink cells, sucrose is removed from the phloem and used as an energy source or converted into starch. The loss of solutes, reduces osmotic pressure so water leaves the companion cells via osmosis and returns to the transpiration stream (aka. Xylem) Diagram on next slide

Osmosis is driven by concentration gradients of water molecules. But, high pressures in the phloem interfere with this process allowing solutes to be transported in the phloem.

DBQ on page 415

Answers to DBQ pg. 415

Sieve tubes Sieve tubes are composed of columns of specialized cells known as sieve tube cells. These cells are living. Sieve tube cells must be living because they require membrane proteins to maintain concentration gradients by active transport. Plasmodesmata connect sieve tube cells to companion cells Sieve tube cells Companion cells Rigid cell wall necessary to support pressure gradients Separated from one another by sieve plates help to ensure a steady flow of sap in the phloem. Perform many genetic/metabolic functions of the sieve tube cells. Many mitochondria, supporting active transport Infolding plasma membrane (increases surface area) Sieves tube cells and companion cells are closely related

Read info page 417. See above.

DBQ page 418

Answers to DBQ page 418

DBQ page 419

Answers to DBQ page 419

DBQ page 420

Answers to DBQ page 420