Plant Transport

Question 1

What is mass transport

Answer 1

The movement of materials over large distances, due to pressure differences

Question 2

What is transpiration?

Answer 2

The movement of water and dissolved minerals through the xylem tissue- from roots to cells

Question 3

What do cells require water for?

Answer 3

Photosynthesis
Maintaining turgid cells

Question 4

State and explain fove adaptations lf xylem vessels for transpiration

Answer 4

Made of dead cells- cells dont absorb the water, and lack of organelles reduces resistance

Lignified vessel walls cause cell contents to die- hollow lumen with no cutoplasm to reduce resistance

Vessel walls contain pits (tiny holes)- if vessel is damaged water can be moved laterally into adjacent vessels.

Vessles loose end walls- allows continuous columns of water to be formed

Contain lignin- strengthens walls agons tension, also makes them waterproof

Question 5

State and explain the theory of how water can move upwards from the roots in the xylem.

Answer 5

Cohesion tension theory:
- water molecules are polar
- they form weak hydrogen bonds with other molecules to stick together
- this is cohesion
- they form hydrogen bonds with xylem walls, this is adhesion

Question 6

Explain the stages of transpiration (5 marker)

Answer 6

• water is evaporated/transpired from the leaves and diffuses out the stomata
• water potential decreases in leaves
• water from xylem vessels replace lost water by osmosis
• negative pressure is created at top of xylem vessels
• water in xylem is now under tension, so is oulled up towards leaves
• cohesion and adhesion allows continuous columns of water

Question 7

What pieces of evidence are there for transpiration?

Answer 7

• tension can be measured during transpiration
• if a column of water is broken, air bubbles for, in xylem which break cohesion. Therefore no more upward movement of water occurs.
• respiratory inhibitors dont effect transpiration, as it does not require energy
• tree trunk diameter decreases when transpiring (measured using a dendrometer) as tension oulla xylem walls in

Question 8

Why is rate of transpiration in the xylem higher at midday?

Answer 8

There is more sunlight, so more stomata are open which increases the rate of evaporation. Water molecules also have more KE so move faster for faster diffusion.
More evaporation means water in xylem is under higher tension. Increases rate kf movement up the tree.

Question 9

What are xerophytes?

Answer 9

Plants that live in areas with very restricted and scarce water supplies, therefore must adapt to reduce water loss

Question 10

Name a xerophyte and how it is adapted for minimal water loss

Answer 10

Marram grass:
- thick waxy cuticle- increases diffusion pathway and reduces water loss from epidermis
- trapped humid air in air spaces inside leaf- increases water potential inside leaf, so water potential gradient decreases to reduce rate of diffusion
- hair on lower elidermis lf leaf- trapps water vapour between hairs to increase water potential…
Stomata are sinken in pits- allows water vapour to be held above stomata pore to increase water potential…

Question 11

Explain 4 more general adaptations of xerophytes

Answer 11

Small surface area to volume ratio of leaves- decreases rate of diffusion

Stomata mostly on underside of leaf, shere it is cooler- less heat energy for water evaporation

Stomata close in daylight to reduce transpiration during hottest part of day

Succulent leaves amd stems to store water

Question 12

What is translocation?

Answer 12

The mass transport of sugars and other organic solutes through the phloem, from sources to sinks

Question 13

Why is translocation important?

Answer 13

Not all cells (sinks) can photosynthesis, therefore cannot produce glucose for respiration. Tuerefroe translocation transports sucrose from cells that can photosynthesis to those that can’t

Question 14

Give the equation for the hydrolysis of sucrose

Answer 14

Sucrose + water —> a-glucose + fructose

Question 15

Why are reducing sugars converted to sucrose before translocation?

Answer 15

To prevent reducing sugars from being chemically alterered before reaching their destination. Because they are too reactive

Question 16

Describe the structure of the phloem and the benifits they bring

Answer 16

Made lf sieve tube elements and companion cells
Sieve tubes comnect to the next via sieve plates, which have pores to allow movement
Sieve tubes have little cytoplasm, a small number of mitochondria snd no other organelles.
Sieve tubes have cytoplasmic connections (plasmadesmata) with companion cells, as each companion cell has a nucleus, many mitochondria and other organelles needed for survival.

Question 17

What are sources?

Answer 17

Cells that ohotosynthesise to produce organic solutes. They have a very hugh comcentration of sugars

Question 18

What are sinks?

Answer 18

Cells that have no chloroplast so cannot photosynthesis, yet still need glucose for respiration.
They have low concentrations of sucrose

Question 19

Give an example of a source snd a sink

Answer 19

Source- mesophyll cells
Sink- root hair cell

Question 20

Explain the process of translocation

Answer 20

Sucrose are actively transported from the source to the phloem, using companion cells.
There is a high concentration of sucrose in the phloem at the source. This decreases the water potential.
Water moves from the xylem to the phloem by osmosis. This creates a high hydrostatic pressure.
Sucrose is actively transported from the phloem to the sink, using companion cells.
There is a lower concentration of sucrose in the phloem at the sink. This increases the water potential.
Water moves from the phloem into the xylem by osmosis. This creates a lower hydrostatic pressure, therefore forming a pressure gradient in the phloem.
Sucrose solution moves down pressure gradient by mass transport, in sieve tubes

Question 21

What pieces of evidence are there for mass flow of translocation?

Answer 21

There is a high hydrostatic pressure in the phloem, as shown when sap is released when cut

Conc of sucrose is higher at sources than at sinks

Companion cells have many mitochondria

Metabolic inhibitors can stop translocation in the phloem

Downward flow in the phloem occurs during the day, yet ceases at night/ when in shade

Increased sucrose conc in leaves causes conc to increase in the phooem a little later

Question 22

What pieces of evidence are there against mass flow of translocation?

Answer 22

Function lf sieve plates are still unclear, as they would hinder mass flow

Not all solutes move at the same speed, yet would do according to mass flow

Sucrose is delieverd at the same rate to all sinks, yet shouldnt due according to mass flow due to different comcentrationsl

Question 23

Explain in terms of the mass flow theory, why downward flow in the phloem ceases at night/ in shade

Answer 23

No photosynthesis, no sucrose produced
Sucrose is not actively transported into sieve tubes
Sucrose conc is not increased, water potential is not decreased
No water enters sieve tubes by osmosis from the xylem
A pressure gradient therefore cannot be created

Question 24

State two ways to investigate transport in plants

Answer 24

Ringing experiments and radioactive tracer experiments

Question 25

Describe how to investigate transport in plants using ringing experiments

Answer 25

Remove a complete ring of bark (containing phloem) yet leave the wood (containing xylem)
This prevents the downward movement of sugars as the phloem is removes. However has no effect on transpiration
Materials will accumulate where there is higher sucrose concs, (a5 the top) ye5 will not
Where there js lower sucrose comcentrations (below)

Question 26

Describe how to investigate the transport in plants using radioactive tracers

Answer 26

If a plant is exposed to an atmosphere with radioactively labelled carbon dioxide, it will use it to synthesis carbohydrates during photosynthesis
The movement of these carbohydrates can therefore be traced, by cutting tissue from different areas of plant into thin sections, then place photographic film over them.
The film will blacken when on the carbohydrates, revealing their location. This is autoradiography