Module 3.3 Transport in Plants

Question 1

Xylem tissue

Answer 1

Transports water + soluble mineral ions upwards

Question 2

Phloem tissues

Answer 2

Transport assimilates such as sugars up or down

Question 3

Vascular bundles in the root

Answer 3

Found in centre

Xylem in X shape

Phloem between arms of xylem’s X shape

Ring of endodermis around vascular bundle

Meristomatic pericycle located on inside of endodermis

Question 4

Vascular bundles in the stem

Answer 4

Found near outer edge of stem

Xylem on inside of vascular bundle

Phloem on outside

Cambium between xylem and phloem (meristomatic cells)

Question 5

Vascular bundles in leaves

Answer 5

Form veins

Xylem above phloem

Xylem is wider and often stained darker

Question 6

Leaf structure🍃

Answer 6

Waxy cuticle
Upper epidermis
Palisade layer
Xylem 
Phloem
Spongy mesophyll 
Air spaces
Lower epidermis
Stomata 
Guard cells

Question 7

🌟Transpiration process

Answer 7

Water evaporates from mesophyll cells (surface) in leaf - water vapour formed

Water vapour diffuses from high water pot. to lower water pot. out the leaf via the stomata

Water drawn from mesophyll cells via symplast/apoplast pathways - replaces water lost (osmosis down water pot. grad.)

Water moves out of xylem vessels via osmosis to replace water lost

Low hydrostatic pressure at top of xylem
Water moves from high pressure (roots) to low pressure down pressure gradient under tension i.e. Water moves from roots to top of xylem

This movement is mass flow

Cohesion between water molecules causes long unbroken column of water - transpiration stream

Question 8

Water moving up the xylem is helped by

Answer 8

Root pressure
Capillary action
The transpiration pull

Question 9

Adaptions of root hair cell

Answer 9

Large SA for osmosis and mineral uptake

Thin walls = short diffusion path

Lots of mitochondria = lots of energy for active trans.

Lots of aquaporins for uptake of water

Question 10

Why does water enter the root hair cell?

Answer 10

Minerals are actively transported in

This decreases the water pot. of the root hair cell below that of the soil

Water move in via osmosis down a water pot. grad. and across the plasma membrane

Question 11

Apoplast pathway

Answer 11

Water travels through gaps between the cellulose fibres

Minerals transported

Doesn’t cross membranes!

Question 12

Symplast pathway

Answer 12

Water travels through the cytoplasm

Can move through plasmodesmata

Question 13

Vacuolar pathway

Answer 13

Basically the exact same as the symplast pathway but water can also cross through vacuoles

Question 14

The casparian strip

Answer 14

On the cell walls of the cells of the endodermis

Made of suberin

Block apoplast pathway

Water must take symplast pathway

Minerals actively transported through via carrier proteins

Water pot. lowered

Water moves in via osmosis

Question 15

🌟Xylem vessel structure

Answer 15

Continuous hollow tubes (no contents) - less resistance to water flow, more space

Walls impregnated with lignin - strengthens walls (prevents collapse), waterproofs wall (reduces lateral movement of water), Increases adhesion - increases capillarity

Spiral pattern of lignin - flexibility

Bordered pits - allows lateral movement of water to get around a blockage

Narrow lumen - more effective capillary action

Question 16

🌟Structure of sieve tube elements in phloem tissues

Answer 16

Small cytoplasm + most organelle absent - less resistance, more space

Sieve plates - allows sucrose through

Joined end to end to form tube - continuous transport

Bi-directional flow - sucrose can go up and down

Living - active processes can take place

Question 17

🌟Structure of companion cells in phloem tissue

Answer 17

Lots of mitochondria - lots of respiration, allows active processes to occur e.g. active loading of sucrose into sieve tubes

Nucleus - controls companion cell and sieve tube element

Plasmodesmata - allows continuation of cytoplasm between companion cell and sieve tube element

Question 18

Water always moves from

Answer 18

A region of higher water potential to a region of lower water potential

Question 19

The importance of transpiration

Answer 19

Transports minerals up the plant

Maintains cell turgidity

Supplies water for growth, cell elongation and photosynthesis

Keeps the plant cool on a hot day

Question 20

🌟How does light intensity affect rate of transpiration?

Answer 20

In light, the stomata open to allow gaseous exchange for photosynthesis. Higher light intensity increases the rate of transpiration because there will be a larger surface area for water to evaporate out

Question 21

🌟How does temperature affect the rate of transpiration?

Answer 21

It increases the rate of transpiration in three ways:
1) increases the rate of evaporation from the cell surfaces - the water vapour potential in the leaf rises

2) increases the rate of diffusion through the stomata because the water molecules have more kinetic energy
3) decreases the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf

Question 22

🌟How does relative humidity affect rate of transpiration?

Answer 22

Higher humidity will decrease the rate of water loss. There will be a smaller water vapour potential gradient between the air spaces in the leaf and the outside air.

Question 23

🌟How does wind affect rate of transpiration?

Answer 23

Wind will carry away water vapour as soon as it has diffuses out of the leaf. This maintains a high water vapour potential gradient.

Question 24

🌟How does water availability affect rate of transpiration?

Answer 24

If there is little water in the soil then the plant cannot replace the water that it has lost.

Question 25

🌟How to set up a potometer

Answer 25

Set it up underwater

Get rid of all air bubbles

Cut the stem underwater and at an angle

Put the stem into the potometer and take the potometer out of the water, ensuring it is full of water and has no air bubbles

Dry the leaves

Allow time to acclimatise

Introduce an air bubble into the capillary tube

Measure rate of transpiration e.g. by how long it takes the bubble to move a set distance

Question 26

Most terrestrial plants are adapted to reduce water loss by…

Answer 26

Having a thick waxy cuticle - reduces water loss due to evaporation through the epidermis

Stomata often found on the underside of the leaf - reduces evaporation

Most stomata closed at night - no light for photosynthesis

Deciduous plants (i.e. plants that lose their leaves in winter) - temp. may be too low for photosynthesis

Question 27

🌟Adaptions of xerophytes

Answer 27

Leaf rolled - traps air inside, air becomes humid, water vapour collects, inc. water potential in space, dec. water potential grad. between air space and outside the leaf, reduces water loss

Thick waxy cuticle on upper epidermis - hydrophobic, reduces evaporation

Stomata in sunken pits - reduces air movement, air becomes humid, water vapour collects, inc. water potential in space, dec. water potential grad. between air space and outside the leaf, reduces water loss

Hairs - traps air inside, air becomes humid, water vapour collects, inc. water potential in space, dec. water potential grad. between air space and outside the leaf, reduces water loss

Spongy mesophyll - v dense, few air spaces, less SA for water loss

Small leaves - small SA, fewer stomata, less evaporation, less transpiration

Stomata shut in day but open at night - warmer during the day, more transpiration occurs during the day so shutting the stomata in the day reduces this loss

Question 28

Cacti adaptations 🌵

Answer 28

Corrugated surface - allows expansion to store even more water

Spines NOT leaves - reduces SA and total SA, less water lost by transpiration

Green stem - for photosynthesis

Roots - widespread, max. water uptake if it rains

Question 29

Adaptions of hydrophytes

Answer 29

Air spaces (large) - buoyancy, keeps leaves afloat so they are in the air and can absorb sunlight

Stomata on upper epidermis - exposed to air for gas exchange

Leaf stem has many large air spaces - buoyancy, allows O2 to diffuse quickly to the roots for aerobic respiration

Question 30

Active loading (how sucrose enters the phloem from the source)

Answer 30

H+ ions actively transported out of the companion cell using ATP

This creates a conc. grad. for H+ ions into surrounding tissue

The high conc. of H+ ions causes facilitated diffusion back into the companion cell. Sucrose is carried w/H+ ions through the cotransporter (protein) and into the plasma membrane. This process actively loads sucrose

There is a higher conc. of sucrose in the companion cell compared to the sieve tube element causing it to diffuse through the plasmodesmata into the sieve tube element down the conc. grad.

Question 31

Any growing part of a plant =

Answer 31

A sink e.g. Flowers or growing tips

Question 32

Starch storage molecules

Answer 32

Can be a source and a sink

Question 33

🌟How sucrose moves along the phloem at the source

Answer 33

Sucrose actively loaded into sieve tube elements @source

Reduces water pot. in sieve tube element

Water enters sieve tube elements by osmosis

Hydrostatic pressure of sieve tube element near source increases

Question 34

🌟How sucrose moves along the phloem at the sink

Answer 34

Sucrose unloaded at the sink by diffusion or active transport and used in respiration or is stored

Water pot. of sieve tube element increases

Water moves into the sink from the sieve tube element via osmosis down the water pot. grad

The hydrostatic pressure in the sieve tube element near the sink decreases

Water in the sieve tube element at the source moves down the hydrostatic pressure gradient from source to sink

This creates a mass flow of sucrose and other assimilates along the phloem either up or down the plant

Question 35

🌟If a ring is cut around the bark of a tree, a swelling may occur above the ring. Why?

Answer 35

The phloem is in the bark so sucrose cannot pass the cut

The area above the cut acts as a sink so water moves into cells

Damage causes more cell divisions to produce cells to store sugars

Cuts cause infection

Question 36

🌟Evidence for translocation: How we know the phloem is used

Answer 36

Radioactively labelled CO2 supplies for photosynthesis appear in the phloem

Aphids that feed on plant stems insert their “mouth” into the phloem

Sugars collect above the ring when a tree is ringed to remove the phloem

Question 37

🌟Evidence for translocation: ATP is needed

Answer 37

Companion cells have many mitochondria

Translocation stops if a poison stopping ATP production is given

Flow of sugars is v high - ATP must be used - diffusion would be much slower

Question 38

🌟Evidence for translocation

Answer 38

The pH of companion cells is higher than the surrounding cells (and H+ ions reduce pH)

The conc. of sucrose is higher in the source than in the sink

Question 39

🌟Evidence against translocation

Answer 39

Not all solutes in the phloem move @ the same rate

Sucrose is moved to all parts of the plant at the same rate and doesn’t move to places with the lowest conc. faster

The role of sieve plates is unclear

Question 40

🌟Why can a potometer not accurately measure rate of transpiration?

Answer 40

Transpiration = loss of water by evaporation via the stomata

A potometer measures rate of water uptake to replace water loss

Some water will be used by the plant for photosynthesis and other processes rather than all evaporating from the leaves - not actual rate measured

Uptake by the detached shoot may not be the same as the uptake for the whole plant

Question 40

Precautions for setting up a potometer to get a valid reading/readings?

Answer 40

Set up underwater so no air bubbles

Ensure shoot is healthy

Cut stem underwater

Ensure equipment is watertight

Allow time to acclimatise

Keep conditions constant

Dry leaves

Question 41

🌟Summarise how water moves from the soil to the xylem

Answer 41

Minerals actively transported into root hair cells through carrier proteins

Water moves via osmosis from the soil into root hair cells across the cell surface membrane through aquaporins down the water potential gradient

Water transported via the apoplast pathway or via the symplast pathway (cytoplasm, through plasmodesmata)

At the endodermis, the casparian strip (made of suberin) blocks the apoplast pathway –> forces water to enter via the symplast pathway

Water pot. most neg. in the xylem due to active transport of minerals into it –> causes water to move into the xylem from the cells of the endodermis and cortex

Question 42

🌟How does number of leaves affect water loss by transpiration?

Answer 42

More leaves = more water loss

Larger SA over which water can evaporate out of plant

More stomata

Question 43

🌟How does number and size of stomata affect water loss via transpiration?

Answer 43

More/bigger stomata = more water loss

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

Question 44

🌟How does the waxy cuticle affect water loss via transpiration?

Answer 44

The waxy cuticle reduces water loss

Hydrophobic

Question 45

🌟Translocation/active loading

Answer 45

H+ ions actively transported (ATP required) out of companion cells

Diffusion gradient of H+ ions created

H+ ions move back into companion cells via facilitated diffusion through a co-transporter carrier protein along w sucrose

Sucrose has been actively loaded into the companion cell

High conc. of sucrose in the companion cell compared to the sieve tube element –> diffuses into sieve tube element down conc. grad. through plasmodesmata

Question 46

🌟How the sieve tube elements are adapted to allow mass flow to occur

Answer 46

Elongated elements, joined end to end to form a column

Sieve plates with pores in end walls - allow sucrose through

Little cytoplasm + no nucleus - less resistance to transport