13.4.4 Phloem: The Movement of Sap

Question 1

Phloem: The Movement of Sap

Answer 1

• The aqueous solution that flows through phloem is commonly known as phloem sap or sap. Sap consists mostly of sugar (sucrose) and can also contain minerals, amino acids, and hormones.
• Sucrose produced during photosynthesis can enter the phloem in two ways:
· The symplastic pathway—Sucrose travels in the cytoplasm through plasmodesmata.
· The apoplastic pathway—Sucrose travels through cell walls and must be actively pumped across the plasma membrane of the sieve-tube.
• A mechanism called pressure flow translocates sap through phloem. Pressure flow results from the osmosis of water into a hypertonic source. Water displaces the sap, moving it through the phloem towards a sink.

Question 2

sieve tube members and companion cells

Answer 2

• Sugar is transported through phloem, which typically consists of sieve tube members and companion cells (shown to the left). Sieve tubes are cells that meet end to end forming a long vessel. Cellular fluid flows through holes in the cell walls of sieve tube members. Companion cells adjacent to sieve tube members help regulate the flow of sap through the sieve tube.

Question 3

symplastic and apoplastic pathway

Answer 3

• Photosynthetic cells in the mesophyll of a leaf produce sugar that enters the phloem through the symplastic pathway, through the cytoplasm of cells connected by plasmodesmata. Some plants use the apoplastic pathway. Their companion cells have involuted cell walls, an adaptation that allows for more surface area for the movement of sucrose.

Question 4

cotransport

Answer 4

• The loading of sugar from mesophyll cells into sieve tube members and companion cells results in a high solute concentration near the source end of a sieve tube. As shown in the diagram, a cotransport mechanism actively pumps sucrose traveling through the apoplastic pathway into phloem cells.
• Cotransport proteins couple the diffusion of protons into the cell with the transport of sucrose molecules. Proton pumps use ATP to pump protons out of the sieve tube cell, creating a concentration gradient. This concentration gradient fuels the transport of sucrose into the sieve tube cell.

Question 5

pressure flow

Answer 5

• Sap is translocated through phloem by a mechanism called pressure flow. Pressure results from the osmosis of water into a hypertonic source, which displaces the sap, moving it through the phloem towards a sink.
• As the source builds up a high concentration of sugar, water from surrounding cells (the xylem) flows into the phloem by osmosis. As water moves into the phloem, pressure develops and is relieved by the displacement of sap towards sink cells. At the sink, water can be recycled back into the xylem.

Question 6

Which one of the following correctly represents a cotransport mechanism?

Answer 6

• Protons are pumped out of the cell, and a cotransport protein couples the diffusion of protons back into the cell with the transport of sucrose molecules.

Question 7

Concerning conducting tissue in plants, which one of the following is true?

Answer 7

• Phloem conducts mostly sugars.

Question 8

The transport of food materials, such as carbohydrates, in plants is called

Answer 8

• translocation

Question 9

Concerning the translocation of sugars and dissolved materials in the phloem, which one of the following is true?

Answer 9

• The source is hypertonic; relative to the sink.

Question 10

Phloem transports sap

Answer 10

• from sugar sources to sugar sinks.

Question 11

Phloem sap is composed of

Answer 11

• sucrose dissolved in water.

Question 12

The movement of sap from a sugar source to a sugar sink

Answer 12

• may translocate sugars from the breakdown of stored starch in the roots up to a developing shoot.

Question 13

Which one of the following cells is actively involved in the production of sugars?

Answer 13

• mesophyll cell

Question 14

Concerning a hypertonic cell, which one of the following is true?

Answer 14

• There is a net movement of water into the cell by osmosis.