3.3.4.2 - Mass Transport in Plants

Give 2 tissues for transport in plants

• xylem

• phloem

What does the xylem transport?

water, and mineral ions

What does the phloem transport?

assimilates

Describe the structure of the xylem?

xylem vessels

• dead cells

• no end walls

• no nuclei

• walls lignified

• lignin arranged in spirals or broken rings

• bordered pits

Why does the xylem have no end walls with no nuclei or cytoplasm?

to allow for a continuous column of water

What are the xylem walls thickened with?

lignin

WHat is the function of the lignified xylem walls?

prevents the tubes from collapsing and waterproofs the walls. also provides strength amd flexibility

Describe the pattern of the lignin in xylem walls?

spiral or network of broken rings

What does annular lignin mean?

spiral

What does reticulate lignin mean?

broken rings

What is the advantage of bordered pits in xylem?

water can pass between adjacent bessels or part of a plant

What theory explains the movement of water up a plant?

tension-cohesion

Describe the cohesion-tension theory?

1. water evaporates from the leaf surface in transpiration

2. water moves into cells in the leaf down a WPG in osmosis

3. causing low HS pressure near the leaf

4. at the roots, mineral ions are actively transported into the xylem, lowering WP so osmosis moves water in

5. high HS pressure at root

6. causes tension in the xylem

7. because water molecules are cogesive there is a column of water pulled upwards

In what process does water leave a leaf?

transpiration

What does transpiration cause in the xylem?

tension

Why does transpiration cause tension in the xylem?

water moves out of xylem down a water potential gradient

Why is there low HS pressure at the xylem at leaves?

water moves out by osmois

Why is there high hydrostatic pressure at the roots in xylem?

water moves in by osmois

Why are water molecules cohesive?

hydrogen bonding between them

Due to tension and cohesion what is formed in the xylem?

a continuous column of water

When water is pulled upwards in the xylem, what happens at the root?

the water is taken up and replaced

Draw a diagram of the cohesion-tension theory?

Define transpiration

the loss of water vapour from the aerial parts of the plant such as through the stomata

Where does transpiration mainly occur?

the leaves

Give 3 processes involved in water movement through the leaf

1. osmosis from the xylem to the mesophyll cells

2. evaporationf rom the surface of mesophyll cells into intercellular spaces

3. diffusion of water vapour from the intercellular spaces out through stomata

Why is water replaced as it is lost?

due to the continuous stream

Draw a diagram of transpiration?

In what state is water lost in transpiration?

water vapour

What is transpirtaion important for?

1. water is required in the leaves for photosynthesis

2. water is required to enable cells to grow

3. keeps cells turgid

4. ensures usefull mineral ions are pulled up the plant

5. evaporation of wayer keeps plant cool

Give 8 factors that affect transpiration?

1. number of leaves

2. number, size and position of stomata

3. waxy cuticle

4. light intensity

5. temperature

6. humidity

7. wind movement

8. water availability

As number of leaves increases, what happens to rate of water loss and why?

it increases as there is greater surface area for diffusion

As number of stoma (and size) increase, what happens to rate of water loss and why?

increases as surface area for diffusion of water vapour increass

If stoma are placed on the lower epidermis and not the upper, what jappens to water vapour loss and why?

less water vapour loss as more shaded

As waxy cuticle gets thicker, what happens to rate of water loss and why?

rate of water loss decreases as it prevents evaporation

As temperature increases, what happens to rate of water loss and why?

increases as the kinetic energy of the water molecules is greater

As humidity increases, what happens to rate of water loss and why?

decreases as there is a smaller water potential gradient

As wind movement increases, what happens to rate of water loss and why?

increases as water vapour is removed so greater water potential gradient

As water availability decreases, what happens to rate of water loss and why?

decreases as the stomata close

Draw a graph for rate of transpiration and humidity

Draw a graph for rate of transpiration and temperature

What is xerophyte?

plants which can survive in dry places such as deserts and sand dunes

Give 8 examples of adaptations of xerophytes?

1. smaller leaves

2. thick waxy cuticle

3. stomata close when very hot

4. hairs on leaf surface

5. stomata sunk in pits

6. rolling of leaves

7. loss of leaves

8. high salt in cells

Why does having smaller leaves reduce water loss?

smaller surface area

Why does having a thicker waxy cuticle reduce water loss?

reduces evaporation

Why does the stomata closing when very hot reduce water loss?

reduces water loss when water availability is low

Why does having hairs on leaf surface reduce water loss?

traps an layer of water vapour- smaller water potential gradient

Why does having sunk stomata reduce water loss?

traps an layer of water vapour- smaller water potential gradient

Why does having rolling leaves reduce water loss?

traps an layer of water vapour- smaller water potential gradient

Why does having less/no leaves reduce water loss?

no/less SA for water loss

Why does having high salt concentration reduce water loss?

decsreases WP in cells so smaller WPG

Draw a diagram of a potometer

What equipment is involved in a potometer?

• lead shoot

• water reservoir

• screw clip

• scale

What does a potometer measure?

water uptake

Why isnt a potometer an accurate measure of transpiration rate?

water is used for other things, e.g cell turgidity

Give the steps to set up a potometer

1. cut the healthy shoot underwater

2. cut shoot at a slant

3. check aparatus is full of wayer

4. insert shoot into aparatus

5. remove potometer from water

6. ensure airtight joints

7. dry leaves

8. keep conditions constant

9. allow shoot to acclimatise

10. screw clip

11. keep ruler fixed

12. measure

Why do you cut the shoot underwater?

to prevent air entering xylem

Why do you cut the shoot at a slant?

increase surface area

Why do you ensure all joints are airtight in a potometer?

to precent air lock

What conditions need to be kept constant when using a potometer?

• temperature

• humidity

• light

• wind

How do you calculate the rate of transpiration?

(pi*r² *d/time)

What do phloem tissue consist of?

• sieve tubes elements

• companion cells

Draw a diagram of phloem

Describe the composition of sieve plate cells

• little cytoplasm, few organelles and no nucleus

• no lignified walls

• cross-walls at intervals called sieve plates

• sieve plates connect elements

• sieve plates allow assimilates to flow

• plasmodesmata link the cytoplasm of companion cells and sieve tube elements

What exists between sieve tubes?

sieve plates

What are the purpose of sieve plates?

they allow assimilates to flow easily

What are the names for the gaps in the cell membrane and wall between sieve tubes and companion cells?

plasmodesmata

What are plasmodesmata?

gaps/ pores that connect the cytoplasm of the companion cells and the sieve tube elements

Describe the cytoplasm of sieve tubes

there is little of it

Describe the cytoplasm of companion cells?

dense

How are companion cells adapted?

they have a large nucleus, many mitchondria, many proteins in the plasma membrane and many ribosomes

Why do companion cells require many mitochondria?

produce ATP for active transport

Define translocation?

the movement of assimilates from source to sin. It is an energy-requiring process and happens in the phloem

Give 4 examples of assimilates

• amino acids

• sucrose

• solutes

Does translocation require ATP?

yes

Define source

the part of the plant that releases the assimilates (leaf)

Define sink

the part of the plant that recieves the assimilates

What is the best theory for translocation called

mass flow hypothesis

What is the first step of mass flow hypothesis

active loading

what is the purpose of active loading

to load the sucrose into the phloem

what are the steps of active loading

• the companion cells use ATP to actively transport hydrogen ions out of their cytoplasm and into surrounding tissue

• this sets up a concentration gradient so hydrogen ions diffuse back into the companion cells via co-transport with sucrose via co transporter proteins

• This sets up a concentration gradient so sucrose diffuses from the companion cells into the sieve tube elements via plasmodesmata

What type of diffusion is that via the plasmodesmata

NOT FACILITATED - no proteins

What are the steps of mass flow hypothesis?

1. sucrose is actively loaded into the sieve tube element

2. this reduces the water potential at the sieve tube

3. this causes water to move into the sieve tube element by osmosis from the surrounding tissue

4. this increases hydrostatic pressure at the sieve tube element

5. at the sink, the solutes ate used, leaving the sieve plate via active transport or diffusion

6. this decreases the water potential in the sieve tube so water moves out by osmosis, reducing the hydrostatic pressur

7. water, and its dissolved assimilates, moves from source to skin down a hydrostatic pressure gradient

What might assimilates be used for at the sink?

• converted into starch

• respiration

Give 4 pieces of evidence that supports mass flow hypothesis

• use of aphids

• ringing a tree

• radioactive traces

• metabolic inhibitors

What are aphids

insects that feed on a plant stem

How does the use of aphids provide evidence for mass flow?

• the aphid sylet is in the phloem

• the sap flows our quicker closer to the leaves (the source), showing greater hydrostatic pressure

• ecidence there is a hydrostatic pressure gradient

What is ringing a tree?

removing a ring of bark that includes the phloem but not xylem

How does ringing a tree provide evidence for mass flow hypothesis

a bulge forms above the ringed area providing evidence that there is a downward flow of sugars

why does a bulge form above the ringed area

the sugars can’t pass the ringed area, increasing water potential

How are radiactive tracers used to provide evidence for mass flow hypothesis?

they can be used to track the organic substances

Which radioactive tracker is used

carbon dioxide containing radioactive isotope C14, this is incorporated into organic substances , eg sucrose, which is moved around via translocation

How do metabolic inhibitors provide evidence fot msss flow hypothesis?

translocation is stoped when a metabolic inhibitor is used (prevents formation of ATP), as it stops it proves that active transport is involved

Give 2 pieces of evidence against mass flow?

• sugar travels to many different sinks not just the one with lowest hydrostatic pressure

• sieve plates would create a barrier to mass flow