phloem part 2

Why are grape trees missing an outer layer of bark?

Removes the phloem so sugar can’t move from top to bottom. End up getting bigger and sweeter fruit

Photosynthate translocates from

Source to sink.

How do we know that photosynthate moves from source to sink.

C14 isotope is added into one leaf and fixes CO2. Track the C14 throughout plant and is accumulated into the new baby leaves.

Original leaf- source

Baby leaf- sink

Sieve elements

Living cells in the phloem of vascular plants that transport of sugars and organic compounds throughout plant

Sieve tube elements

Individual cells that make up sieve tube found in angiosperms

Sieve cells

Found in gymnosperms and are relatively unspecialized

Sieve element organelles

Lack nuclei, vacuoles, Golgi bodies, ribosomes, microfilaments, and microtubules. Rich in P-proteins. Metabolically supported by companion cells

P-proteins

Structural proteins found in sieve tubes responsible for rapid sealing of the sieve tube following injury to prevent photosynthate loss.

Companion cells

Form a functional unit with sieve elements to support the long distance transport of sugars and organic compounds

Parenchyma cells

Essential for photosynthesis, storage of food and water, and plant repair and growth

Sieve pores

Connect neighbouring Sieve elements To form the conducting sieve tubes of the phloem. Developed from plasmodesmata

sieve plates

Collection of differentiated sieve pores. In angiosperms. Larger pores found on the end walls

Sieve areas

In gymnosperms. Not open channels Smooth ER membranes blocking pores

Phloem loading part 1

Sucrose accumulates in mesophy cells

Phloem loading part 2

Pre-phloem transport. Sucrose moves in metoprolol cells next to sieve elements of the small veins

Phloem loading part 3

Phloem loading. Sugars are transported into sieve elements and companion cells

Phloem loading types

Apoplastic loading, symplastic loading with oligomer trapping, passive symplastic loading

Apoplastic loading

Translocate sucrose almost exclusively. Sucrose enters the apoplast between phloem parenchyma cells and companion cells via transporters and is carried against a concentration gradient into the companion cell-sieve element complex by sucrose-proton symporters. sucrose is then transported towards the sinks. Few plasmodesmata connections between SE-CC complex and surrounding cells

What kind of active transport is used in Apoplastic loading

Secondary active transport

What kind of companion cells used in Apoplastic loading

Transfer companion cells that have many ingrowths on the side opposite the sieve element. More SA = more transport of photosynthate

Oligomer trapping symplastic loading

Translocate oligosaccharides (raffinose and stachyose). Abundant plasmodesmata connections between sieve element-companion cell complex, and surrounding cells. Concentrate sugar in phloem cells to generate the driving force for long distance transport. Sucrose combines with another sugar to make an oligosaccharide which follows its concentration gradient into the sieve element. It doesn’t go back into the bundle sheet cell because the plasmodesmata is too small.

What kind of companion cells in oligomer trapping symplasmic loading

Intermediary companion cells. Between bundle sheath cell and sieve element

Passive symplastic loading

Abundant plasmodesmata connections all the way from mesophyll to the sieve element-companion cell complex. High sugar concentrations in the leaves allow passive transport through PD to sieve element. High [sugar] = more +ve pressure = turgor

What kind of companion cells are used in passive symplasmic loading

Ordinary companion cells

Materials translocates in phloem

Transports sugars, amino acids, organic acids, proteins, and minerals at high velocities. Much faster than diffusion

Pressure gradient at source

Flow of phloem sap is driven by pressure gradient between source and sink. High pressure at source would mean low pressure at sink

What established the pressure gradient

Phloem loading at the source and unloading at the sink

In what ways does the solute and water potential change when a pressure gradient is established

Accumulation of sugars in sieve elements generates low (-ve) solute potential causing a drop in water potential. In response water enters the sieve element causing an increase in pressure potential

Pressure flow model

A passive mechanism where bulk flow of phloem sap is driven by an osmotically generated pressure gradient between source and sink

Is energy still required in the pressure flow model

Yes it’s required in sources and sinks for synthesis and consumption of photosynthate which is required for active phloem loading and unloading. Also required to maintain necessary cellular and anatomical structures.

Pressure flow part 1

Transpiration stream moves up the xylem (-0.6 to -0.8, high to low water potential). Active phloem loading into sieve elements decreases solute and water potential. Water enters sieve element from xylem and results in a high turgor pressure

Pressure flow part 2

Pressure drive bulk flow of water solute from source to sink in sieve elements. Phloem unloading solutes into sink cells, increases the solute and water potential. Water flows out of sieve element and into xylem, resulting in lower turgor pressure