9.2 Transport in the Phloem of Plants

The active loading of sucrose into phloem sieve tubes near the fruit will create a hydrostatic pressure gradient that will facilitate mass flow between the leaves and fruit.

The movement of water molecules in the phloem sieve tubes from the leaves to the fruit will contribute to the hydrostatic pressure gradient between them.

The movement of organic compounds into the phloem sieve tubes near leaves will increase the hydrostatic pressure in the sieve tubes and create a pressure gradient between leaves and fruit.

The unloading of organic compounds from the phloem sieve tubes at the fruit will increase the osmolarity of the sieve tubes and create a hydrostatic pressure gradient between the leaves and fruit.

0.4

1.5

24

90

Aphids produce more oligosaccharides when consuming high sucrose phloem sap in order to increase honeydew production in their guts.

Aphids produce more oligosaccharides when consuming high sucrose phloem sap in order to decrease the osmolarity of the gut contents.

Aphids produce more oligosaccharides when consuming high sucrose phloem sap in order to increase the osmolarity of the gut contents.

Aphids produce more oligosaccharides when consuming high sucrose phloem sap in order to prevent an excess of water from moving into the cells.

Loading of organic compounds in the apoplast pathway cannot occur due to a lack of proton pump activity.

Loading of organic compounds in the apoplast pathway cannot occur due to non-functional membrane transport proteins.

Loading of organic compounds through the symplast pathway cannot occur due to a lack of proton pump activity.

Unloading of organic compounds cannot occur at the source due to a lack of membrane transport proteins.

III → II → IV → I

III → I → IV → II

I → III → IV → II

I → IV → III → II

I only

II and III only

I, II and IV

III and IV only

Sucrose moves from the companion cell into the sieve tube through plasmodesmata

Sucrose is carried against its concentration gradient into the companion cell through a co-transporter protein

H⁺ ions are pumped out of companion cells by proton pumps

Transfer cells contain many mitochondria to provide energy for the proton pumps to work

I only

I and II only

I, II and IV

I, III and IV

A high solute concentration in phloem sieve tubes due to the loading of sucrose

The incompressibility of water molecules

The rate at which sucrose is converted to starch in storage tissue

Rigid plant cell walls made of cellulose

Xylem is removed from the vascular bundles, so water and mineral salts cannot be transported to the leaves

Phloem is removed from the vascular bundles, so sugars cannot be transported to the roots

Xylem is removed from the vascular bundles, so sugars cannot be transported to the roots

Phloem is removed from the vascular bundles, so water and mineral salts cannot be transported to the leaves

The hydrostatic pressure at a sink is low due to the buildup of sucrose in the phloem sieve elements

The hydrostatic pressure at a sink is high due to water moving into phloem sieve elements by osmosis

There is a high hydrostatic pressure at the source due to the incompressibility of water

There is a low hydrostatic pressure at the source due to a low solute concentration in the sieve tube elements

It provides a way to track the distribution of molecules between sources and sinks in a plant

To determine the order in which carbon compounds are generated

It provides a way to determine the rate of carbon fixation in a plant

Radioactive molecules contribute to the hydrostatic pressure in xylem vessels