Phloem transport

Flashcards about plant translocation, covering topics such as pholem, xylem, and sugar movement.

What structures are found in leaves that provide support and transport?

Vascular bundles in midrib, network of veins gives lateral support to leaf.

What is translocation in plants?

The process where products of photosynthesis are transported from source to where they are used for growth or storage, the sink.

What is a sink in plant translocation?

Areas of the plant where products of photosynthesis are used for growth or storage. Roots, shoots, fruits

What is a source in plant translocation?

Leaf cell

What is the Mass flow theory?

The theory that explains translocation as passive movement, now understood to be an active process.

What evidence exists against the mass flow theory?

• Doesn’t explain companion cells, so it must be an active process

• High rate of oxygen consumption/translocation is slowed/stopped by potassium cyanide which is a respiratory inhibitor

• Doesn’t explain the amount of mitochondria in the companion cells so translocation must require an active mechanism

• Phloems have sieve pates which introduces resistance and creates a barrier to mass flow

• Sucrose and amino acids travel at different rates moving in different directions

How does translocation actively load sucrose into the phloem?

Loading of sucrose into phloem via H+ ions and co-transporters.

What is the first step in the translocation of sugars?

Hydrogen ions are actively transported out of companion cells, then return with sucrose down the concentration gradient (facilitated diffusion) via co-transporter proteins.

What is the second step in the translocation of sugars?

Sucrose diffuses into sieve tube elements through the plasmodesmata.

What is the third step in the translocation of sugars?

Water potential inside the sieve tube decreases so water moves into the sieve tube element by osmosis.

What is the fourth step in the translocation of sugars?

Hydrostatic pressure in the sieve tube at the source increases drives sugary fluid from source to sink from a high hydrostatic pressure to a low hydrostatic pressure.

What is the fifth step in the translocation of sugars?

Sucrose molecules move from the sieve tube into the surrounding cells by facilitated diffusion or active transport. Sucrose enters root cell to be used in respiration or to be converted into starch for storage.

What is the sixth step in the translocation of sugars?

Water moves out of the sieve tube by osmosis and hydrostatic pressure at the sink drops.

How do experiments help to support translocation?

• A feeding aphid may be anaesthetised with carbon dioxide and the body removed; their stylet remains attached to the to he plant and acts as a tube through which the contents of the sieve tube continue to be forced by hydrostatic pressure. A sample is taken at intervals, sucrose concentration

• Ringing experiment, a ring of bark is scraped away removing the phloem, the sugar trying to be transported down the stem is stopped by the ring, a bulge of sugar forms above the ring (accumulated sugars) suggesting sugar moves down the stem in the phloem

How are radioactive tracers used to study translocation?

• Autoradiography - Radioactive labelled CO2 is placed into a bag surrounding an illuminated individual leaf, source and sink leaves are placed firmly on photographic film in the dark, when the film is developed the presence of radioactivity shows up as a ‘fogging’ of the negatives

• In aphids a radioactive labelled Co2 is paced into a bag surrounding an illuminated individual leaf, co2 is incorporated into guards and transported into the phloem, aphids feeding on sugar in the phloem can be used to trace the movement of sugar in the plant from source to sink

C14 radioactive isotope is used

Both of these experiments demonstrate sugar transport in both directions up and down the stem.