mass transport in plants

Question 1

what do these two tissues transport:
-xylem
-phloem

Answer 1

xylem:
-water and mineral ions in solution, substances move up the plant from the roots to the leaves

phloem:
-organic substances like sugars up and down the plant like sucrose

Question 2

describe the structure of the xylem.

Answer 2

-long, tube-like structures formed from dead cells joined end to end
-no end walls, making an uninterrupted tube that allows water to pass up easily

Question 3

describe how cohesion and tension allows water to move up the xylem.

Answer 3

-water evaporates from the leaves at the top of the xylem
-this creates tension which pulls more water into the leaf
-water molecules are cohesive so when some are pulled into the leaf, others follow meaning the whole column of water in the xylem from the leaves down to the roots move upwards
-water then enters the stem through the roots

Question 4

what is transpiration?

Answer 4

evaporation of water from a plants surface, especially the leaves

-water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf

-when the stomata opens, water moves out of the leaf down a water potential gradient

Question 5

how does temperature affect the rate of transpiration?

Answer 5

-the higher the temperature, the faster the rate of transpiration
-warmer water molecules have more energy so they evaporate from the cells inside the leaf faster
-this increases the water potential gradient between the inside and outside of the leaf making water diffuse out of the leaf faster

Question 6

how does light intensity affect the rate of transpiration?

Answer 6

-the lighter it is, the faster the transpiration rate
-this is because the stomata opens when it gets light to let in CO2 for photosynthesis

Question 7

how does humidity affect the rate of transpiration?

Answer 7

-the lower the humidity, the faster the rate of transpiration
-if the air around the plant is dry, the water potential gradient between the leaf and the air is increase

Question 8

how does wind affect the rate of transpiration?

Answer 8

-the windier it is, the faster the rate of transpiration
-lots of air movement blows away water molecules from around the stomata which increases the water potential gradient

Question 9

what is a potometer?

Answer 9

a piece of apparatus used to estimate transpiration rates.

-it measures water uptake by a plant but its assumes water uptake by the plant is directly related to water loss by the leaves

Question 10

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Answer 10

TO BE DELETED

Question 11

describe the structure of the phloem including the roles of sieve tube elements and companion cells.

Answer 11

-formed from cells arranged in tubes
-sieve tube elements and companion cells are important cell types in the phloem

-sieve tubes are living cells that form the tube for transporting solutes, they have no nucleus and few organelles
-this means there is a companion cell for each sieve tube element that carry out living functions for them such as providing the energy needed for the active transport of solutes

Question 12

what is translocation?

Answer 12

movement of solutes (such as amino acids and sugars like sucrose) to where they are needed in a plant
solutes are sometimes called assimilates
-its an energy requiring process
-moves substances from source to sink (source= where assimilates (solutes) are produced, sink= where assimilates are used up)

Question 13

how is a lower concentration maintained at the sink vs the source during translocation?

Answer 13

enzymes maintain the concentration gradient by changing the solutes in the sink like breaking them down or making them into something else.

Question 14

describe the mass flow hypothesis.

Answer 14

1. source
   -active transport is used to load the solutes from companion cells into the sieve tubes of the phloem at the source, lowering the water potential in the sieve tubes so water enters the tubes by osmosis.
   -this creates a high pressure inside the sieve tubes at the source end of the phloem
2. sink
   -solutes are removed from the phloem to be used up
   -increases water potential inside the sieve tubes so water also leaves the tubes by osmosis
   -lowers the pressure in the sieve tubes
3. flow
   -result is a pressure gradient from the source end to the sink end
   -this gradient pushes solutes along the sieve tubes towards the sink
   -when they reach the sink the solutes will be used such as in respiration or stored like starch

PRESSURE GRADIENT FORCES SOLUTES DOWN

Question 15

what is the mass flow hypothesis?

Answer 15

the best supported theory of how solutes are transported from source to sink in translocation

Question 16

what is the supporting evidence for the mass flow theory?
describe:
-rings of bark
-aphids
-metabolic inhibitor

Answer 16

supporting:
-if a ring of bark including the phloem is removed from a woody stem, a bulge forms above the ring
-the fluid from the bulge has a higher concentration of sugars than the fluid below the ring
-this is because the sugars cannot move pas the area where the bark has been removed, evidence of a downward flow of sugars

-pressure in the phloem can be investigated using aphids (pierces the phloem than their bodies are removed leaving the mouthparts behind, allowing the slap to flow out)
-the sap flows out quicker nearer the leaves than further down the stem
-evidence of a pressure gradient

-if a metabolic inhibitor (stops ATP production) is put in the phloem, translocation stops
-evidence active transport is used

Question 17

what are some objections to the mass flow theory?

Answer 17

-sugar travels to many different sinks, not just the ones with the highest water potential
-sieve plates would create a barrier to mass flow, a lot of pressure would be needed for the solutes to get through at a reasonable rate

Question 18

what is some evidence of translocation from radioactive tracers?

Answer 18

-supply’s the plant with an organic substance that has a radioactive label, then tracks its movement
-the movement of these substance can be tracked using a technique called autoradiography to reveal where the radioactive tracer has spread to in a plant
-the plant is killed
-wherever the film turns black, the radioactive substance is present
-the results demonstrate the translocation of substances from source to sink over time