Transport In The Phloem

Question 1

What does the phloem transport?

Answer 1

Sugars + other organic substances produced by leaves in photosynthesis

Question 2

2 types of cells in the phloem

Answer 2

Sieve tube elements and companion cells

Question 3

Sieve tube element characteristics

Answer 3

Lined up end-to-end to all sap flow

Very little cytoplasm and no nucleus

Question 4

Companion cell characteristics

Answer 4

Control transport in sieve tube elements

Many mitochondria to help active transport of sap in translocation

Question 5

What is active loading?

Answer 5

The process of loading sucrose into the sieve tube element

Question 6

How does active loading work?

Answer 6

1. Hydrogen ions are pumped out from the companion cells to the surrounding leaf tissue creating a hydrogen diffusion gradient
2. Hydrogen ions diffuse back into the companion cells through co-transporter proteins bringing sucrose with them
3. High concentrations of sugar in the companion cells cause sucrose to diffuse into the sieve tube elements

Question 7

What is the source?

Answer 7

Where organic substances are created

Question 8

What is the sink?

Answer 8

Where organic substances are transported to and used

Question 9

What is the mass flow hypothesis?

Answer 9

Sucrose lowers the water potential of the source cell so water enters by osmosis causing an increase in the hydrostatic pressure
Sink cell is using up sucrose so there is a higher water potential causing water to move out by osmosis resulting in a decreased hydrostatic pressure
Since the source cell has a higher hydrostatic pressure than the sink cell a pressure gradient is created and the solution is forced towards the sink cell via the phloem

Question 10

What are the three steps of translocation?

Answer 10

1. Sucrose -> sieve tube element
2. Movement of sucrose in sieve tube element
3. Sucrose -> sink

Question 11

How does sucrose get to the sieve tube element in translocation?

Answer 11

• organic substances are created by photosynthesis in the chloroplasts of the leaves
• this creates a high sucrose concentration at source so sucrose diffuses down concentration gradient into companion cell via facilitated diffusion
• active transport of hydrogen occurs from the companion cell into the spaces within the cell walls using ATP
• this creates a concentration gradient so hydrogen will move down this gradient co transporting sucrose via carrier proteins into the sieve tube element

Question 12

How does sucrose move in the sieve tube element?

Answer 12

An increase in sucrose in sieve tube element lowers the water potential so water moves in from the xylem by osmosis
The increase in water volume in the sieve tube element increases the hydrostatic pressure causing sucrose/sap to be forced towards the sink

Question 13

How does sucrose get to the sink in translocation?

Answer 13

Sucrose is used up in respiration at the sin so more sucrose needs to be actively transported into the sink cell which causes the water potential to decrease so water from the STE moves into the sink cell by osmosis
The removal of water decreases the volume and therefore the hydrostatic pressure in the STE
Movement of soluble organic substances is due to differences in hydrostatic pressure between the source and the sin end of the sieve tube elements