7.8 - Transport of organic molecules in the phloem

Question 1

What is translocation

Answer 1

The process by which organic molecules and some mineral ions are transported from one part of a plant to another

Question 2

Describe the structure of the phloem

Answer 2

• Phloem is made up of sieve tube elements, long thin structures arranged end to end.
• Their end walls are perforated to form sieve plates.
• Associated with the sieve tube elements are cells called companion cells.

Question 3

Having produced sugars during photosynthesis, the plant transports them from the sites of production, known as….

Answer 3

• Sources
• then to the places where they will be used directly or stored for future use
• known as sinks.

Question 4

Describe how the location of the sinks influences the direction of translocation

Answer 4

• As sinks can be anywhere in a plant
• sometimes above and sometimes below the source
• it follows that the translocation of molecules in phloem can be in either direction.

Question 5

What molecules does the phloem transport

Answer 5

• sucrose
• amino acids
• inorganic ions (such as potassium, chloride, phosphate and magnesium ions)

Question 6

What are the 3 phases of ‘Mass flow theory’

Answer 6

1. Transfer of sucrose into sieve elements from photosynthesising tissue
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from the sieve tube elements into storage or other sink cells

Question 7

Describe Transfer of sucrose into sieve elements from photosynthesising tissue

Answer 7

• Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts.
• The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells.
• Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP.
• These hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements.
• Sucrose molecules are transported along with the hydrogen ions via co-transporter proteins

Question 8

Describe Mass flow of sucrose through sieve tube elements

Answer 8

• The sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes as described in the previous phase.
• This causes the sieve tubes to have a lower (more negative) water potential.
• As the xylem has a much higher (less negative) water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them. At the respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage.
• These cells therefore have a low sucrose content and so sucrose is actively transported into them from the sieve tubes lowering their water potential.
• Due to this lowered water potential, water also moves into these respiring cells, from the sieve tubes, by osmosis.
• The hydrostatic pressure of the sieve tubes in this region is therefore lowered.
• As a result of water entering the sieve tube elements at the source and leaving at the sink, there is a high hydrostatic pressure at the source and a low one at the sink.
• There is therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes

Question 9

Describe Transfer of sucrose from the sieve tube elements into storage or other sink cells

Answer 9

The sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells

Question 10

Describe active loading at the source for mass flow

Answer 10

• the cytoplasm of companion cells are dense in mitochondria, which are the organelles responsible for producing ATP via aerobic respiration
• The hydrolysis of the ATP molecules releases energy that is used to actively pump protons (H+) out of the companion cells and into the surrounding tissue
• this creates a proton gradient across the membrane of the companion cells. Via co-transporter proteins, the protons and sucrose (or amino acids) move into the companion cells
• the protons move with their conc. gradient whilst the sucrose moves against the gradient
• following the increase in conc, of sucrose in the companion cells, this assimilate will then diffuse into adjacent sieve tube elements through the plasmodesmata
• the presence of sucrose in the sieve tube elements lowers the water potential which causes water to move into the phloem from the surrounding tissue by osmosis. The sucrose dissolves in the water to form cell sap and the hydrostatic pressure at the source increases
• the hydrostatic pressure at the sink is low

Question 11

What is a plasmodesmata

Answer 11

Fine strands of cytoplasm that connect adjacent plant cells

Question 12

Explain what happens at the sink in terms of removal

Answer 12

• the cells surrounding the phloem use sucrose for a number of processes
• sucrose may be stored (as starch) or used in metabolic processes like respiration
• as sucrose is constantly being used by these cells, the sucrose conc, decreases and therefore the sucrose diffuses from the phloem to the cells
• As a result of the movement of sucrose, the water potential in the phloem increases and so water moves from the high water potential in the phloem to the lower water potential in the surrounding cells by osmosis
• Ultimately, this causes the hydrostatic pressure at the sink to decrease

Question 13

What is the evidence supporting mass flow hypothesis

Answer 13

Question 14

What is some evidence that contradicts mass flow hypothesis

Answer 14