Mass Transport in Plants - A7

Question 1

What does the xylem do?

Answer 1

Transports water and mineral ions in solution.
Substances move up the plant from roots to leaves

Question 2

What does the phloem do?

Answer 2

Transports organic substances like sucrose. It is bidirectional.

Question 3

What is the structure if the xylem?

Answer 3

Very long, tube-like structures formed from dead cells joined end to end.(continuous column of water)

Question 4

What is the benefit of the xylem having no cytoplasm or organelles?

Answer 4

It allows for easier flow of water.

Question 5

How does lignin help the xylem?

Answer 5

It strengthens the xylem by helping with support and withstanding tension

Question 6

What do pits in the walls of the xylem allow for?

Answer 6

It allows for lateral movement.

Question 7

What is the cohesion tension theory?

Answer 7

-Water evaporates from mesophyll cells
-this lowers the water potential of the mesophyll cells
-water moves from the xylem into the mesophyll cells via osmosis
-this creates a tension
-water is pulled up the xylem as a continuous column of water
-because of the cohesion(caused by hydrogen bonding) between water molecules

Question 8

Define transpiration.

Answer 8

When water evaporates from mesophyll cells to the stomata of the leaf

Question 9

What are the factors affecting transpiration rate?

Answer 9

light intensity, temperature, air movement/wind, humididty

Question 10

How does light intensity affect transpiration rate?

Answer 10

The lighter it is the faster the rate if transpiration(+ve correlation)
-stomata open when it’s light to let in CO2 for photosynthesis. So more water vapour can diffuse out. When dark, stomata close so there is little transpiration.

Question 11

How does temperature effect transpiration rate?

Answer 11

positive correlation.
-warmer molecules have more kinetic energy so they evaporate from the cells inside the leaf faster
-increases the water potential gradient making water diffuse out faster.

Question 12

How does air moevement effect transpiration rate?

Answer 12

positive correlation
-air movement blows away water molecules from around the stomata- increasing the water potential gradient - increased diffusion rate

Question 13

How does humidity effect the rate of transpiration?

Answer 13

As humidity rises, transpiration rate slows
-if air is wet around a plant the water potential gradient is decreased - sower rate of diffusion

Question 14

What are the properties of water?

Answer 14

Metabolite, solvent, large latent heat of vaporisation, high specific heat capacity, cohesive, adhesive

Question 15

What effect does water being a metabolite have?

Answer 15

-needed in photosynthesis and hydrolysis
-condensation reactions - releases a molecule of water as a new bond is formed
-hydrolysis reactions - requires a water molecule to break a bond

Question 16

What is the effect of water being a solvent?

Answer 16

-allows transport of ionic substances because water is also polar
-the ions get totally surrounded by water molecules - they dissolve(hydration shell)

Question 17

How does a large latent heat of vaporisation effect water?

Answer 17

-buffers temp. change
-provides a cooling effect through evaporation
-takes a lot of energy to break hydrogen bonds->so water has a high latent heat of vaporisation->so lots of energy needed to evaporate water
-useful for living organisms->use water loss through evaporation to cool down without losing too much water

Question 18

What effect does specific heat capacity have on water?

Answer 18

-buffers change in temperature
-hydrogen bonds can absorb heat energy- takes a lot of energy to heat water
-means living organisms don’t experience rapid temp. changes
-good habitat as stable temp.

Question 19

Describe cohesion as a property of water.

Answer 19

-cohesion=attraction between water molecules
-water is very cohesive because the water molecules are polar
-supports columns of water(in xylem)
-high surface tension - insects can sit on surface

Question 20

What does it mean if water is adhesive?

Answer 20

It is attracted to other water molecules.

Question 21

What is a potometer?

Answer 21

Potometer - apparatus used to estimate transpiration rate

Question 22

What do potometers measure?

Answer 22

Measures uptake of water by a plant, which is assumed to be the same amount lost by a plant

Question 23

What are potometer estimates used for?

Answer 23

Used the estimates to investigate how different factors effect transpiration.

Question 24

Why can it only be an estimate when using a potometer?

Answer 24

-This is only an estimate because some water is used in a reaction such as in photosynthesis and in cells for supporting the plant
-some water is also produced during respiration
-water is used in hydrolysis reactions
-some water may leak from the equipment during the experiment
-some is used for support/turgidity

Question 25

What are the steps for using a potometer?

Answer 25

1)cut a shoot underwater to prevent air getting into the xylem(so the continuous column of water isn’t broken)
2)cut at a slant to increase surface area for water uptake
3)assemble the potometer underwater(to prevent air bubbles in equipment) and insert the shoot(removed from water but keep the end of the capillary tube in water)
4)check the apparatus is watertight and airtight
5)dry the leaves- so stomata aren’t blocked. Allow the shoot to acclimatise and then shut the tap.
6)remove the capillary tube from the water until 1 air bubble forms and put the tube back into the water
7)record the start position of the bubble
8)time and record the distance moved by the bubble per unit time e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate.
9)only ever change one variable, control all others.

Question 26

What is the function of the reservoir on a potometer?

Answer 26

Function of the reservoir - returns air bubbles back to the start

Question 27

Why is glucose transported into sucrose?

Answer 27

Glucose is made during photosynthesis but is converted into sucrose in order to be transported in the phloem

Question 28

What is the structure of the phloem?

Answer 28

Arranged in tubes with sieve tube elements and companion cells.

Question 29

What are sieve tube elements?

Answer 29

Sieve tube elements are living cells that form the tube- no nucleus and few organelles so doesn’t impede the flow of sucrose(sap)

Question 30

What are companion cells?

Answer 30

companion cells- for each sieve tube element. They carry out living functions for sieve cells e.g. providing energy for active transport of solutes

Question 31

What is the benefit of having end walls with holes?

Answer 31

allow sucrose/sap through

Question 32

What is translocation?

Answer 32

-the movement of solutes(amino acids, sugars - sucrose) - sometimes called assimilates - requires energy(active transport)
-moves solutes from ‘source’ to ‘sink’
-source is where assimilates are produced(high conc.)
-sink is where assimilates are used up(low conc.)
-enzymes:enzymes maintain a conc. gradient from the source to the sink by changing the solutes at the sink > ensures a lower conc. of solutes at the sink.

Question 33

What is the mass flow hypothesis?

Answer 33

-sucrose is actively transported into the sieve tube elements via the companion cells from source cells in the leaf
-lowers the water potential in the sieve tube elements
-water moves into the sieve tube elements by osmosis - increasing the volumes of fluid
-increases the hydrostatic pressure in the sieve tube, creating a pressure gradient
-causing mass flow of sucrose towards the sinks in the roots
-sucrose is actively transported into sink cells in the roots via the companion cells where it is used in respiration/stored as starch
-increases the WP of the sieve tube elements
-water leaves by osmosis into the xylem - maintains the pressure gradient

Question 34

What is the evidence for radioactive tracers?

Answer 34

-translocation of solutes can be modelled using radioactive tracers > done by supplying part of a plant(often a leaf) with an organic substance(sucrose) that has a radioactive label, then tracking it’s movement
-the movement of these substances can be tracked using a technique called autoradiography
-the plant must be killed(freezing in liquid nitrogen)
-place the plant on photographic film, wherever the film turns black, radioactive substances are present