3.1.3 - Transport In Plants (set B - Transpiration And Translocation)

Question 1

Outline the role of water regarding the structure of plants?

Answer 1

• turgor pressure (hydrostatic pressure) due to osmosis provides a hydrostatic skelton to support the stems and leaves
• turgor drives cell expansion - force enables plants to force way through concrete ext

Question 2

Outline the role of water in the metabolism of plants?

Answer 2

• mineral ions and the products of photosynthesis are transported in aqueous solutions
• water is a raw material for photosynthesis

Question 3

Outline how root hair cells are adapted to be efficient exchange surfaces - give 4 ways and explain them?

Answer 3

• large SA:V ratio and large numbers of them
• each hair has a thin surface layer (just cell wall and cell-surface membrane) - short pathway for diffusion and osmosis
• concentration of solutes in cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
• microscopic in size - can penetrate easily between soil particles

Question 4

Explain how and why water moves into root hair cells?

Answer 4

Through passive process of osmosis - water moves from area of higher water potential to an area of lower water potential

• water potential of soil is high due to low concentration of dissolved minerals
• cells cytoplasm has low water potential due to lots of solvents (sugars,mineral ions ext)

Question 5

Outline the three different pathways for movement of water across the root?

Answer 5

• the symplast pathway (through cytoplasm)
• the apoplast pathway (through cell wall)
• vacuolar pathway (through cytoplasm and vacuoles)

Question 6

Explain what happens when the external water potential is lower than the water potential of the cell?

Answer 6

• low external water potential - causes water to move out of the cell
• plants can survive this for short periods - they can shrink the cell surface membrane away from the cell wall (cell is plasmolysed)

Question 7

Outline and explain the symplast pathway?

Answer 7

• movement of water through the living spaces of the cell cytoplasm across partially permeable plasma membrane
• moves from cell to cell through the plasmodesmata
• water is drawn through the plant - each cell is further away from the roots and has a lower water potential

Question 8

Explain 2 features of the symplast pathway which differs from the apoplast pathway?

Answer 8

• less water moves by symplastic pathway
• movement of water in the symplastic pathway is slower than the apoplastic pathway

Question 9

Outline and explain the apoplast pathway?

Answer 9

• movment of water through the cell wall and intracellular spaces by diffusion (not crossing partially permeable membrane)
• cohesive and tension forces acting on the cell wall pulls the water up the plant
• water may move directly from cell wall to cell wall
• fastest movement of water

Question 10

Outline the role of the Casparian strip in the apoplast pathway?

Answer 10

• when water reaches the endodermis, presence of a thick, waterproof, waxy band of suberin (casparian strip) within the cell wall blocks the apoplastic pathway

May help plant control which mineral ions reach the xylem

Question 11

Outline the vacuolar pathway?

Answer 11

• same as symplast pathway
• water moves through the cells vacuoles and cytoplasm through partially permeable plasma membrane
• slowest route
• water may move from cell to cell through the plasmodesmata

Question 12

Outline the evidence for the role of active transport in root pressure - give 3 points and explain them?

Answer 12

• some poisons such as cyanide affect mitochondria - prevents production of ATP, if cyanide is applied to root cells, the root pressure disappears
• root pressure increases with rise in temperature and falls with decrease in temperature - suggest chemical reactions involved
• if levels of oxygen or respiratory substrate falls, root pressure falls

Question 13

Outline and explain the role and formation of root pressure?

Answer 13

• active pumping of minerals into the xylem to produce movement of water by osmosis, results in root pressure
• root pressure gives water a push up the xylem

Question 14

What is transpiration?

Answer 14

• loss of water vapour (by evaporation and diffusion) from the surface of leaves and stems of a plant due to stomata opening for gas exchange
• around 99% of water absorbed by a plant is lost through evaporation (by process of transpiration)

Question 15

Outline the transpiration stream?

Answer 15

Water enters roots by osmosis and is transported up xylem to leaves (moves by osmosis across membranes and by diffusion in the apoplast pathway through leaf cells)

Water evaporates from the freely permeable cellulose cell walls of the mesophyll cells (in the leaves) into the air spaces - then moves along a concentration gradient into external air through stomata

Question 16

What is the role of guard cells?

Answer 16

Surround the stomatal opening - they open and close the stomata

Stomata open and close to control the amount of water lost by the plant

Question 17

Explain how adhesion and cohesion help move water up the xylem?

Answer 17

• adhesion - water molecules form hydrogen bonds with carbohydrates in walls of narrow xylem vessel
• cohesion - water molecule form hydrogen bonds with each other, sticking together

both of these effects result in water exhibiting capillary action

Question 18

Explain capillary action in regard to adhesion and cohesion cohesion-tension theory?

Answer 18

Process which invovles the combined effect of both - allows water to rise up a narrow tube against the force of gravity

• water drawn up xylem in a continuous stream to replace water lost via evaporation - this is transpiration pull

transpiration pull results in tension within xylem - helping move water across roots from the soil

Question 19

Outline and explain evidence for the cohesion-tension theory?

Answer 19

• changes in diameter of trees, tree shrinks in diameter when transpiration is at its highest so therefore tension in xylem vessel is also highest
• when xylem vessel broken (eg cutting flower stem) air is drawn in to the xylem, rather than water leaking out - plant can no longer move water up the stem as the continuous stream held by cohesive forces is broken

Question 20

Outline and explain how stomata open and closes - reference turgor pressure?

Answer 20

turgor-driven process

• when turgor is low the asymmetric configuration of the guard cell walls closes the pore
• when environmental conditons are favourable guard cells pump solutes by active transport, increasing turgor - cellulose hoops prevent swelling and allow extension lengthways (inner walls less flexible than outer walls causes cells to become bean-shaped, and open the pore)

Question 21

Outline and explain 2 ways temperature impacts the rate of transpiration?

Answer 21

• increase in temperature increases the kinetic energy of the water molecules - increases rate of evaporation from spongy mesophyll cells into air spaces of the leaf
• increase in temperature increases the concentration of water vapour that external air can hold before becoming saturated (decreases humidity and water potential)

Question 22

Explain how light impacts the rate of transpiration?

Answer 22

In light the stomata open for gas exchange, in the dark most stomata close

• increasing light intensity gives increasing number of open stomata - which increases rate of water vapour diffusing out (therefore increasing evaporation)

Question 23

Explain how humidity impacts transpiration?

Answer 23

A very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between inside of leaf and outside air

Question 24

Give 3 conditions things which impact transpiration?

Answer 24

• temperature
• light
• humidity

Question 25

What is translocation?

Answer 25

Transport of organic compounds/assimilates (products of photosynthesis) in the phloem from sources to sinks

• active process that requires energy ATP
• main assimilate transported is sucrose (glucose converted to sucrose for transport)

Question 26

Outline the main sources and sinks of plants?

Answer 26

sources
- green leaves and green stems
- storage organs (eg tubers)
- food stores in seeds

sinks
- roots that are growing or actively absorbing mineral ions
- meristems actively dividing
- developing seeds, fruits or storage organs

Question 27

Explain why sucrose is the main carbohydrate transported by plants?

Answer 27

Sucrose is not used in metabolism as readily as glucose, therefore less likely to be metabolised during the transport process

Question 28

outline the process of phloem loading - the apoplast route?

Answer 28

Assimilates travels through cell walls and inner cell spaces (apoplast) to the companion cells and sieve elements via diffusion (down a conc gradient)

• in companion cells active transport used to move sucrose into cytoplasm across the cell membrane - water carrying assimilates moves to areas of lower pressure (sinks) in a mass transport system

Question 29

Explain how active transport is used to move sucrose into the phloem - the apoplast route?

Answer 29

1- hydrogen ions (H+) activley pumped out of companion cell using ATP via protein pump

2 - hydrogen ions return down a conc gradient via a co-transporter protein

sucrose needs to be co-transported with H+ ions in order for H+ ions to get back in, down conc gradient

3 - increases sucrose conc in comparison cells - lowering water potential

4 - water moves via osmosis into companion cells increasing turgor pressure

5 - water carrying assimilates moves to area of lower pressure (sinks) in a mass transport system

Question 30

Explain the movment of assimilates passively via the symplast route?

Answer 30

Sucrose accumulates in sieve tubes, water moves via osmosis from an area of high potential to an area of recently lowered water potential (where sucrose is)

• pressure created causes sucrose to move along the phloem by mass flow

Question 31

Explain how companion cells are adapted to aid in the movement of sucrose via the apoplast route?

Answer 31

• many inner foldings in there cell membrane - gives an increased surface area for AT
• lots of mitochondria to supply ATP for the transport pumps

Question 32

Outline the process of phloem unloading?

Answer 32

Sucrose offloaded to cells which need it, by diffusion down a conc gradient

• loss of solutes from phloem causes water to move via osmosis to surrounding cells - water moves from higher potential to lower (some enters the transpiration stream)

Question 33

Outline evidence for translocation - give 3 points?

Answer 33

• microscopy allows us to see adaptions of companions cells for active transport (eg lots of mitochondria and inner folds)
• if mitochondria are poisoned, translocation stops - suggests its an active process which requires ATP
• flow of sugars in phloem is 10,000 x faster than diffusion alone - suggests there is an active process driving mass flow

Question 34

Explain what xerophytes are?

Answer 34

Plants which have evolved a wide range of adaptations that enable them to live in environments with very little water availability

• include plants which live in both hot and cold deserts

examples include cacti and marram grass

Question 35

Outline some ways xerophytes are adapted for conserving energy - give 3 points?

Answer 35

• thick waxy cuticle - 10% of water loss is through the cuticle, thick waxy cuticle minimises water loss
• sunken stomata - stomata located in pits, reduces air movement (reducing water vapour potential gradient) and therefore transpiration + paired with reduced number of stomata
• reduced leaves - reducing leaf area reduces water loss - thin needle like leaves have greatly reduced SA:V ratio, minimising the amount of water lost + paired with leaf loss when water is not available

Question 36

Explain how hairy leaves and curled leaves allow plants to conserve water?

Answer 36

Hairy leaves - for example on cacti and marram grass, which reduces the water vapour potential gradient and minimising loss of water by forming a microclimate

Curled leaves - greatly reduces water loss, by confining all of the stomata reducing the water vapour potential gradient and thus diffusion

Question 37

Outline the adaptions of the roots of xerophytes?

Answer 37

• long tap roots growing deep into the ground can penetrate several meters - can access water deep into the ground (for example marram grass which roots penetrate vertical meters into the sand)
• roots with large SA are able to absorb any available water

Question 38

Explain what hydrophytes are?

Answer 38

Plants that are adapted to live in conditions where water is abundant at the surface,submerged in or at the edges of bodies of water

example are water lilies

Question 39

Outline and explain some of the adaptions of hydrophytes - give 3?

Answer 39

• no waxy cuticle - maximises water loss, preventing oversaturation + paired with lots of stomata on the upper surface which maximises gas exchange
• reduced structure - due to water supporting the plant it requires less mechanical strength + small roots as water diffuses directly into stem (plant supported by water instead of roots)
• air sacs - enable leaves to float at the surface (where light intensity is strongest)

Question 40

Explain the role of Aerenchyma and Pneumatophores as adaptions for hydrophytes?

Answer 40

Aerenchyma - specialised parenchyma (packing) tissue which aids buoyancy and forms an efficient pathway for oxygen into the tissues important for the anoxic conditions (extremely low oxygen conditions)

Pneumatophores - special aerial roots which grow directly into the air which have many lenticels, allowing entry of oxygen into tissue