Biology Midterm 2

 girdling a woody plant will

kill a plant

Girdling

 removing a strip of bark all the way around a tree. Removes material all the

way to the wood (xylem).

Girdling Includes removal of

 the phloem and the vascular cambium.

Girdling of the trunk leads to

swelling of the bark above the girdle, and shrinkage below

the girdle.

Phloem-mediated translocation is mainly

organic molecules (including the products of

photosynthesis), although some inorganics are also translocated.

The organic molecules are the products of

 assimilation (“assimilates”) from inorganic

precursors (CO2 and inorganic N)

The organic molecules are the products of assimilation (“assimilates”) from inorganic

precursors (CO2 and inorganic N) 2 examples

soluble carbohydrates (and sometimes sugar alcohols); from photosynthesis

-amino acids (and sometimes some other organic N compounds); from inorganic N

Sucrose

most common carb

Pinitol

 a sugar alcohol

sometimes found in phloem

Glutamine

 an amino acid

commonly found in phloem

Phloem is found in close association with

xylem

Phloem is found  2 places

either side of the vascular cambium (woody plant stems), or, vascular bundles (herbaceous plant stems), leaves of woody and herbaceous plants

Vascular bundles in

herbaceous stems

Leaves have

Vascular Bundles

 Leaf vascular bundles are the 

veins of a leaf.

A vascular bundle is composed 

xylem & phloem

A vascular bundle is surrounded by

bundle sheath

cells.

Unlike xylem, phloem is 2

alive at maturity, and the cells exhibit turgor.

Vascular bundles in the stem of 

sunflower (Helianthus).

Sunflowers are 

herbaceous eudicots (leaves have net-like/reticulate venation).

sieve element 2

sieve tube member,

sieve tube element

stacked sieve elements

= 

a sieve tube 

companion cell 

does “phloem

loading” and “unloading”

sieve elements don’t have

a

nucleus

plasmodesmata

between a

companion cell and a sieve

element

P protein aka

slime

P protein

stringy

protein that is not translocated;

P protein runs between 

sieve

elements, through the

plasmodesmata

 Phloem is

alive at maturity

Phloem cells exhibit

turgor

Sieve elements =

sieve tube members

Sieve elements  are aligned

end to end to make sieve tubes

sieve tubes

sort like pipes

made of living cells

Sieve plate

with plasmodesmata, occur between

adjacent sieve elements

Translocation occurs via

the sieve tubes

Translocation is what kind of flow

bulk

Pressure-driven

bulk flow (compare to tension-

driven flow in the xylem)

Companion cells

responsible for loading solute

into the sieve elements and unloading from the

sieve elements.

At maturity, companion cells have a 

nucleus

sieve elements are

anucleate

the nucleus of the

adjacent companion cell controls 

the sieve cell

metabolism

Sieve elements have all the other

standard

components of a living cell

Phloem-mediated translocation

is from 

source to sink.

Translocation is via the

sieve

tubes

sieve tubes are formed

from

sieve elements

sieve elements aka

sieve tube elements

 In leaves, the sieve elements are

smaller than the companion cells.

Products of photosynthesis must

travel from the _ to the _

MC

SE

SE

sieve element

CC

companion cell

PP

phloem parenchyma

VP

 vascular parenchyma

MC

mesophyll cell

mesophyll cell

site of

photosynthesis

Phloem-mediated translocation is from _ to _

source

sink

source

where “assimilate” is made

sink

where that something “assimilate” is used or stored

The source for photosynthate (typically soluble carbohydrates) is 2

mature leaves or

almost mature leaves

Sinks for photosynthate include: 3

-roots

-developing leaves

-developing fruits

The source for organic N (typically as amino acids) may be 3

roots or may be leaves or

may be both

Sinks for organic N include: 3

-developing leaves

-developing fruits

-roots, if the source is in the leaves

Only water-soluble materials (solutes) can be

moved

Can’t move  2

lipids (hydrophobic) or starch (hydrophilic, but insoluble).

 Lots of long, stringy _ in each

sieve element.

protein (P-protein)

P-protein is not 

translocated

P-protein runs through the 

pores/plasmodesmata

in the sieve plates.

Damage to a sieve element (e.g. due to insect feeding) leads

to

pressure release

Damage to a sieve element (e.g. due to insect feeding) leads

to pressure release 2 examples

formation of a slime plug in the adjacent sieve element

seal off the damaged sieve element.

The presence of P-protein makes

it difficult to 

sample phloem.

Pressure release during phloem

sampling leads to

immediate

plugging (slime plugs)

Solution

mini-hypodermic

needle, aka aphid “stylet”

(needle-like mouthpart).

Let the aphid start feeding on

the

phloem

Puncturing the phloem with the

stylet does not lead to 

significant

pressure release (no slime plugs

are formed)

Lastly

 cut off the aphid and

collect the phloem sap.

Phloem translocates 

many products of

assimilation (= “assimilates”

Phloem translocates many products of

assimilation (= “assimilates”), including 2

sugars (C assimilation)

amino acids (N assimilation).

very high undefoliated leaf sheaths

sucrose concentration

Münch Pressure Flow Hypothesis 2

phloem-mediated translocation

1930- now

Münch Pressure Flow Hypothesis is based on the concept of

 phloem loading of solutes at the source and phloem

unloading of solutes at the sink.

Water follows the solutes →

ΔP along the phloem.

In this model, it is important to keep in mind that

the source is different from the

phloem at the source, and the sink is different from phloem at the sink.

phloem loading is

loading of solutes from the source to the phloem at the

source

phloem unloading is 

unloading of solutes from the phloem at the sink to

the sink

ΔP drives

 the bulk flow through the sieve tube.

Bulk flow of water through the sieve tube carries

dissolved solutes from source to

sink

whole point of phloem is to 

 move dissolved stuff

water movement is simply

a mechanism to have

phloem move dissolved stuff;

Osmometers are used to

measure

solute potential (ΨS)

Modern osmometers are

electronic

Old school osmometers are what

Münch had in mind in the 1930s.

Two solutions are separated by a

selectively permeable membrane

 determine the ΨS of solution 2 by

what happens to solution 1.

Assume that [Solution 1] > [Solution 2]. 2

→ water will diffuse (osmosis) from Solution 2 to Solution 1

→ “h” will increase

Water follows 

solute

phloem loading increases

ψP of the phloem at the

source

phloem unloading decreases

ψP in the phloem at the sink

phloem loading increases ψP of the phloem at the

source, and phloem unloading decreases ψP in the phloem at the sink maintains a

ΔP along the phloem,

maintaining a ΔP along the phloem, results in

pressure-driven bulk flow of

water and the solutes dissolved in the water.

The water is carrying the

solutes

plant is “trying” to

move the solutes.