Mrs H bio 7 Mass Transport in Plants

Question 1

give the 4 main properties of water

Answer 1

1.metabolite in many metabolic reactions (e.g. condensation and hyrolysis
2.important solvent in which metabolic reactions occur as it has a relatively high heath capacity, buffering temp changes
3.relatively large latent heat of vaporisation, providing a cooling effect with little water loss through evaporation
4.strong cohesion due to H bonds which supports columns of water in tube like transport cells of plants and produces surface tension where water meets air

Question 2

why is water good as a solvent?

Answer 2

they are charged with O2 atoms being slightly - and H atoms being slightly +, these opposite charges attract forming H bonds. Because it is charged, it is a very good solvent
charged or polar molecules like salts, sugars, amino acids dissolve readily in water and are called hydrophilic
uncharged or nonpolar molecules like lipids dont dissolve well in water and are called hydrophobic

Question 3

why are H bonds in water useful?

Answer 3

H bonds are weak meaning htey can break and form spontaneously at the temps found in living cells without needing enzymes

Question 4

how can water resist temp changes?

Answer 4

H bonds between water molecules can absorb a lot of energy meaning it has a high specific heat capacity

Question 5

what does a high specific heat capacity mean?

Answer 5

takes a lot of energy to heat up

Question 6

why is water’s high specific heat capacity useful?

Answer 6

-does not change temp easily
-constant temp useful for living organisms because doesnt experience rapid temp changes
-good habitat as temp underwater is likely to be more stable then temp on land
-water inside organisms fairly stable allowing it to maintain internal body temp

Question 7

what is cohesion?

Answer 7

tendency of molecules within a substance to “stick together”

Question 8

why is water cohesive?

Answer 8

water molecules are very cohesive as they are polar H bonds

Question 9

why is water’s cohesion useful?

Answer 9

helps water to flow which is useful for transporting substances like in the xylem
also gives water high surface tension allowing small organisms like pond skaters to “walk” on pond surfaces

Question 10

what is transpiration?

Answer 10

water is obtained by the plant from the soil via their roots and passes through the roots to the stem where the xylem transports it to the leaves. water then leaves through stomatal plants by diffusion

Question 11

list all 8 properties of water

Answer 11

polar, solvent, reactive, metabolite, high latent heat of vaporisation, high specific heat capacity, cohesive and adhesive

Question 12

how does water move through a plant?

Answer 12

1. minerals move into the roots via active transport against the conc gradient
   2.this lowers the water potential of the roots so water enters the roots via osmosis down the water potential gradient
   3.water moves through the cortex
   4.water evaporates from the spongy mesophyll into air spaces due to the heat from the sun and out the stomata via transpiration
2. this pulls the water up the xylem in a continuous column due to tension as water moves from the narrow xylem to the spongy mesophyll cells via osmosis
   6.also though cohesion, water molecules stick to each other and adhesion causes them to stick to the walls of the xylem

Question 13

give 3 adaptations of the xylem

Answer 13

1. long cells/ hollow tubes with no end walls and 2contain lignin which waterproofs them and they have thick walls to withstand water pressure
2. no cytoplasm so no obstruction and easy flow

Question 14

what 4 factors increase the rate of transpiration?

Answer 14

1. light
   2.temp
   3.humidity
   4.air movement

Question 15

how does light intensity affect transpiration?

Answer 15

stomata open in the light and close in the dark, rate of transpiration in higher in the light

Question 16

how does temp affect transpiration?

Answer 16

^ in temp means ^ in rate of transpiration as temp increasing causes diffusion to occur quicker due to increased kinetic energy

Question 17

how does humidity affect transpiration?

Answer 17

air spaces in leaf are saturated with water vapour + the air outside the leaf contains much less so the higher conc gradient between inside and out the higher the rate of diffusion as water leaves down the conc grad

Question 18

how does air movement affect transpiration?

Answer 18

air movement over a leaf moves water vapour away from the stomatal pores which increases water potential gradient between inside and outside increasing the rate of transpiration

Question 19

how are Xerophyte plants adapted to reduce water loss?

Answer 19

1. reduced no. of stomata
   2.stomata in pits
   3.hairs to trap water
   4.rolled leaves
   5.leaves reduced to spines
   6.thick waxy cuticles

Question 20

what is translocation?

Answer 20

movement of sugars and ions through the phloem

Question 21

give an adaptation of the phloem

Answer 21

seive cells

Question 22

what are seive cells always accompanied by and why?

Answer 22

companion cells that contain lots of mitochondria and ribosomes to provide proteins and ATP that seive cells cannot

Question 23

where does the mass flow theory of translocation say that substances move from and to?

Answer 23

source to the sink

Question 24

describe the steps of the mass flow hypothesis for translocation in plants

Answer 24

1. at source cells, sucrose is actively transported into the phloem by companion cells which lowers the water potential in the seive tubes of the phloem causing water to move from a high WP to a low WP by osmosis
   2.movement of water into phloem creates a high pressure near the source and therefore a lower pressure in the phloem next to the sinks therefore, moving down the pressure gradient, sucrose and water are transported down the phloem
2. at sink cells sucrose is transported into them via AT using carrier proteins to be used for respiration or stored as starch
   4.removal of sucrose from phloem means higher WP in seive tube and there is a higher WP in phloem than in xylem so water moves into the xylem via osmosis

Question 25

what are the 3 experiments to show the mechanism of translocation?

Answer 25

puncture experiment
ringing experiment
radioactive tracer experiment

Question 26

what does the puncture experiment involve?

Answer 26

phloem is punctured by a tube and sap flows out showing that there is a high pressure inside the phloem
xylem is punctured then air is sucked in showing that there is a low pressure in the xylem
demonstrates that water is pulled up the xylem whereas sap is pushed down the phloem

Question 27

what does the ringing experiment involve?

Answer 27

the phloem is selectively removed by cutting a ring in the stem just deep enough whilst still not cutting the xylem. after a week there is swelling above the ring but the leaves are unaffected. evidence that sugars were transported down from source to sink

Question 28

what does the radioactive tracer experiment involve?

Answer 28

radioactive isotopes can be used to trace precisely where different compounds are being transported from and to and can also measure rate of transport. traced using photographic film called autoradiograph. it involves a plant growing with a leaf exposed to CO2 containing the radioactive isotope 14C which will be taken up during photosynthesis and in sucrose. plant is then forzen in liquid nitrogen to kill it and placed on photographic film in the dark.

Question 29

what were the results of the radioactive tracer experiment?

Answer 29

that sucrose is only transported in the phloem and that sucrose can be transported to any cell that required it

Question 30

describe the process of using a potometer

Answer 30

1.leafy shoot cut underwater and care taken to prevent water getting on the leaves
2.potometer completely filled with water to make sur there are no air bubbles
3. using a rubber tube, leafy shoot is fitted to the photometer under water
4. potometer is removed from under the water and all joints are sealed with waterproof jelly
5. an air bubble is introduced in the capillary tube
6.as transpiration occurs, water moves through the capillary tube and the bubble of air moves with it
7.distance moved over a period of time is recorded and mean is calculated of a no. of repeats
8. vol of water lost over a period of time can be calculated

Question 31

use the cohesion-tension theory to suggest why the diameter of a trunk was at its minimum when the flow rate was highest

Answer 31

adhesion of water molecules to the walls of the xylem results in tension as water is pulled up the stem, pulling in the walls

Question 32

suggest why pieces of leaf tissue examined under a microscope were very thin

Answer 32

single/ few layers of cells so light can pass through

Question 33

give two reasons why a student counted the no. of stomata in several parts of each leaf during an investigation

Answer 33

1. distribution may not be uniform so is representative
2. to obtain a reliable mean