Module 3.3 - Transport In Plants

Question 1

The Casparian strip

Answer 1

On cell walls of cells of endodermis
Strip of waterproof material (suberin)
Blocks apoplast pathway between cortex and xylem - water must now take symplast pathway

Question 2

Root pressure

Answer 2

The push from the water entering the xylem vessels in the roots
Doesn’t move water far

Question 3

Capillary action

Answer 3

Adhesion of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel

Question 4

Transpiration pull

Answer 4

Water loss due to transpiration causes tension at the top of the xylem which pulls the water up the vessel by mass flow (from high to low hydrostatic pressure)
Most of the driving force

Question 5

Define transpiration

Answer 5

The loss of water by evaporation out of plant’s leaves via the stomata

Question 6

What does a potometer measure?

Answer 6

Water uptake

Question 7

Process of setting up a potometer

Answer 7

Cut a healthy shoot under water (stop air entering xylem vessels)
Cut shoot at a slant (increase surface area)
Check there are no air bubbles in the potometer
Insert shoot into potometer under water
Remove potometer from water and ensure joints are airtight
Dry leaves
Keep conditions constant
Allow time for shoot to acclimatise
Shut screw clip
Keep ruler fixed and record position of air bubble on scale
Start timing and measure distance moved per minute

Question 8

Why is a potometer not an exact measure of the rate of transpiration?

Answer 8

Transpiration is the loss of water by evaporation from leaves
A potometer measures water uptake to replace loss
Some water may be used (e.g. in photosynthesis) rather than all evaporating from the leaves
Also uptake by detached shoot may not be the same as that of the whole plant

Question 9

How number of leaves affects transpiration rate

Answer 9

More leaves = more water loss

Larger SA over which water can evaporate out of plant (often more stomata)

Question 10

How number and size of stomata affects transpiration rate

Answer 10

More/bigger stomata = more water loss

Larger SA over which water can evaporate out of plant via stomata

Question 11

How a waxy cuticle affects transpiration rate

Answer 11

Waxy cuticle present = less water loss

Reduces water evaporating from leaf surface as it is hydrophobic

Question 12

How light affects transpiration rate

Answer 12

Lighter conditions = more water loss
Stomata open wider in light (to allow gas exchange for photosynthesis), if they are open then larger SA for water to evaporate out of

Question 13

How temperature affects transpiration rate

Answer 13

Higher temperature = ore water loss

More KE, water evaporates faster and water vapour diffuses out of leaf faster

Question 14

How humidity affects transpiration rate

Answer 14

Higher humidity = less water loss

Air is more saturated with water so shallower water potential gradient

Question 15

How wind affects transpiration rate

Answer 15

More windy = more water loss
Carries water vapour that has just diffused from leaf away, making the air immediately surrounding the leaf less saturated and maintaining a steeper water potential gradient

Question 16

How water availability affects transpiration rate

Answer 16

More water in soil = more water loss

Cannot replace the water that is lost

Question 17

Adaptations of dichotomous plants

Answer 17

Waxy cuticle reduces water loss through epidermis as it is hydrophobic
Stomata on underside of leaf to reduce evaporation (due to direct heating from the sun)
Stomata close at night (no light for photosynthesis)
Deciduous plants lose leaves in winter (when may not be able to photosynthesise)

Question 18

Define xerophyte

Answer 18

A plant that is adapted to reduce water loss by transpiration so that it can survive in very dry/said conditions

Question 19

Adaptations of xerophytes

Answer 19

Epidermis covered in hairs
Thicker waxy cuticle
Small leaves/needles
Sunken stomata (in pits)
Curled leaves
Small air spaces in mesophyll
Stomata shut in day, open in night

Question 20

Define hydrophyte

Answer 20

A plant that is adapted to living in water or where the ground is very wet

Question 21

Adaptations of hydrophytes

Answer 21

Many large sir spaces in leaf (keeps leaves afloat so they are in the air and can absorb sunlight)
Stomata are on upper epidermis (exposed to air to allow gaseous exchange)
Leaf stem has many large air spaces (helps with buoyancy, allows oxygen to diffuse quickly to roots for aerobic respiration)

Question 22

Define translocation

Answer 22

The transport of assimilates between the sources and sinks of a plant in the phloem tissue
This requires energy

Question 23

Define source

Answer 23

Where sucrose and other assimilates are loaded into the phloem (e.g. leaf)

Question 24

Define sink

Answer 24

Where sucrose and other assimilates are unloaded from the phloem (e.g. flower)

Question 25

Process of sucrose being actively loaded into the phloem from the source

Answer 25

H+ ions are actively transported (requires ATP) out of the companion cells
This produces a diffusion gradient for the H+ ions
They move back into the companion cell via facilitated diffusion through co-transporter carrier proteins with sucrose
Sucrose has been actively loaded into the companion cell
There is a high concentration of sucrose in the companion cell compared to the sieve tube element, so it diffuses into it down the concentration gradient through the plasmodesmata

Question 26

How sucrose moves along the phloem at the source

Answer 26

Sucrose is actively loaded into the sieve tube elements at the source
This reduces the water potential in the sieve tube element
Water enters the sieve tube elements by osmosis
This increases the hydrostatic pressure in the sieve tube element near the source

Question 27

How sucrose moves along the phloem at the sink

Answer 27

Sucrose is unloaded at the sink by diffusion (or active transport) and used in respiration/stored
This increases the water potential in the sieve tube element
Water moves into the sink via osmosis down the water potential gradient
This reduces the hydrostatic pressure in the sieve tube element near the sink
Water in the sieve tube element at the source moves down the hydrostatic gradient from source to sink
This creates a flow which carries the sucrose and other assimilates along the phloem, via mass flow either up or down the plant

Question 28

Sieve tube elements adaptations

Answer 28

Elongated elements joined end to end to form a column
Sieve plates with pores in end walls allow sucrose through
Little cytoplasm and no nucleus - less resistance to transport

Question 29

Evidence for phloem used in translocation

Answer 29

Radioactively labelled COw supplied for photosynthesis appears in phloem
Aphids feeding on plant stems insert mouthparts into phloem
Sugars collect above ring when tree is ringed to remove phloem

Question 30

Evidence for ATP required in translocation

Answer 30

Cap pan ion cells have many mitochondria
Translocation is stopped is a poison which stops ATP production is given
Flow of sugars is very high that ATP must be used - much faster than would be possible with diffusion

Question 31

Evidence for mechanism of translocation

Answer 31

pH of companion cells is higher than surrounding cells (H+ ions reduce pH)
Concentration of sucrose is higher in source than sink

Question 32

Evidence against mechanism of translocation

Answer 32

Not all solutes in phloem move at same rate
Sucrose moved to all parts of plant at same rate and doesn’t go to places with lowest concentration faster
Role of sieve plates is unclear

Question 33

Phloem tissue

Answer 33

Dissolved assimilates (e.g. sugars) travel up or down the phloem tissue

Question 34

Xylem tissue

Answer 34

Water and soluble mineral ions travel upwards in xylem tissue

Question 35

How are the xylem and phloem arranged in the roots?

Answer 35

Central core of xylem in the shape of an X
Phloem between arms of X-shaped xylem
Arrangement of xylem and phloem provides strength to withstand pulling forces experienced by the roots
Endodermis is sheath of cell’s surrounding vascular tissue
Pericycle between endodermis and vascular tissue containing meristem cells

Question 36

How are the xylem and phloem arranged in the stem?

Answer 36

Vascular bundles found near outer edge of stem, providing strength and flexibility to withstand bending forces
Xylem towards inside of each vascular bundle
Phloem towards outside of each vascular bundle
Cambium in between xylem and phloem containing meristem cells

Question 37

How are the xylem and phloem arranged in a leaf?

Answer 37

Vascular tissues form the veins of a leaf

With each vein, the xylem is found on top of the phloem

Question 38

Apoplast pathway

Answer 38

Water passes through spaces in cells walls and between cells
Water moves by mass flow, not osmosis
Dissolved mineral ions and salts can be carried with water

Question 39

Symplast pathway

Answer 39

Water enters cell cytoplasm through plasma membrane

Water passes through plasmodesmata from one cell to the next

Question 40

Vacuolar pathway

Answer 40

Water enters cell cytoplasm through plasma membrane but is not confined to cytoplasm of cells
Water can enter and pass through vacuoles as well

Question 41

Importance of transpiration

Answer 41

Transports useful mineral ions up the plant
Maintains cell turgidity
Supplies water for growth, cell elongation and photosynthesis
Supplies water to keep plant cool