Translocation

Question 1

What vessel transports organic substances around the plant?

Answer 1

The phloem vessel

Question 2

Structure of the phloem vessel?

Answer 2

Composed of the sieve tubes: contains sieve plates and has no end walls
Next to companion cells which carry out living functions for the sieve tubes as they have few organelles, specifically respiration

Question 3

Process of transporting organic substances around a plant name?

Answer 3

Translocation

Question 4

What organic substances are transported in translocation

Answer 4

Amino acids and sugars

Question 5

Assimilates

Answer 5

The organic substances (amino acids and sugars, esp sucrose) which are transported in translocation

Question 6

Source

Answer 6

Where assimilates are produced in a plant such as in a leaf in photosynthesis
At a high concentration here

Question 7

Sink

Answer 7

Where assimilates are used up in a plant such as storage organs or meristems in the roots and stem and leaves (needed for metabolic processes)
They are at a lower concentration here due to being broken down or converted to something else for these processes

Question 8

The mass flow hypothesis

Answer 8

The idea of how translocation occurs in a plant to transport organic substances: movement of these from source to sink
Still contradicting evidence so is a hypothesis

Question 9

Mass flow hypothesis

Answer 9

The theory of how assimilates in the source are transported to the sink in a plant using a hydrostatic pressure gradient

Question 10

What happens at the source in mass transport?

Answer 10

Active transport of solutes into sieve tubes over companion cells from the source
Decreases water potential in sieve tubes near source as a result
So water moves into sieve tubes from xylem (from area or high water potential to lower one) by osmosis = high hydrostatic pressure at the source

Question 11

What happens at the sink in mass flow?

Answer 11

The solutes are removed from sieve tubes by moving into the sink over companion cells and used up (by active transport)
Increases water potential at sink end in the sieve tubes as a result of
So water moves by osmosis down its water potential gradient from sieve tubes into the xylem
Causes lower hydrostatic pressure at sink end of sieve tubes

Question 12

So as a result, how is the fluid moving from source to sink?

Answer 12

Down the hydrostatic pressure gradient to carry assimilates where they are needed

Question 13

Evidence against mass flow hypothesis

Answer 13

Could be gravity
Sieve plates
Multiple different sinks present in a plant

Question 14

How do sieve plates contradict mass flow hypothesis?

Answer 14

Because they would be a barrier to mass flow so a lot of pressure would be needed for solutes to flow at reasonable rate

Question 15

Evidence to support mass flow

Answer 15

Ringing experiments
Metabolic inhibitors effect
Aphids

Question 16

Ringing experiments

Answer 16

Remove bark from a cutting thus remove phloem in a ring
Forms bulge above rung which has a high conc of sucrose due to accumulation after phloem was cut thus shows they are transported by phloem (because they can’t move into phloem below)
Decrease in water potential above = moves into cells

Question 17

Metabolic inhibitors as evidence for mass flow

Answer 17

Stops ATP production to release energy: no more active transport of substances into the sieve tubes from source or out into sink

Question 18

Using aphids as support for mass flow

Answer 18

Cut their body off their mouth piece when it uses it to feed on sugars from phloem
So the fluid continues to run
The fluid runs faster (higher hydrostatic pressure) at source than the sink (lower hydrostatic pressure)