3.3.4.2 Mass Transport in Plants

Question 1

How does water out through the stomata?

Answer 1

Humidity of the atmosphere is less than that of the air spaces next to the stomata
When stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of surrounding mesophyll cells
By changing the size of the stomatal pores, plants can control their rate of transpiration

Question 2

What is the function of xylem tissue?

Answer 2

It transports water and soluble materials from the roots to the leaves

Question 3

How does water move across the cells of a leaf?

Answer 3

Mesophyll cells lose water to the air spaces by evaporation due to heat
Cells have a lower water potential so water enters via osmosis from neighbouring cells
Loss of water from neighbouring cells lowers water potential
They take water from neighbouring cells
This creates a water potential gradient where water moves to the mesophyll then into the atmosphere

Question 4

How does water move up the xylem stem?

Answer 4

Water evaporates from mesophyll cells due to heat
Water molecules form hydrogen bonds and stick together- cohesion
Water forms a continuous column across mesophyll cells and down the xylem
As water evaporates, more molecules are drawn up via cohesion (transpiration pull)
Transpiration puts the xylem under tension which creates negative pressure (cohesion tension theory)

Question 5

What is the structure of the xylem?

Answer 5

Long cells
Have thick cells containing lignin
Lignin waterproofs walls of cells and strengthens them
Cells die from lignin which forms a long tube
Lignification is not complete and pores (called pits or bordered pits) allow water to move between vessels or into living parts
Lignin forms rings or spirals around the vessel and strengthen the tube

Question 6

What is the apoplastic pathway?

Answer 6

Movement between walls of neighbouring cells

Question 7

What is the symplastic pathway?

Answer 7

Movement through plasma membranes and plasmodesmata to cytoplasms from cell to cell

Question 8

What is the vacuolar pathway?

Answer 8

Same as the symplastic pathway but also through vacuoles

Question 9

What are the effects of the cohesion tension theory?

Answer 9

In daytime there is greater transpiration which creates a higher tension thats pulls the xylem walls inward and shrinks the trunk Therefore the trunk is wider at night
Xylem vessel is broken which allows air to enter it. No water is drawn up because the continuous column of water is broken and cohesion is stopped
Or the xylem vessel is broken so water does not leak because air is drawn in instead which creates tension

Question 10

Is transpiration pull an active/passive process?

Answer 10

Passive process and xylem tissue is dead so cannot actively move water

Question 11

Is transpiration an active/passive process?

Answer 11

An active process as heat from the sun evaporates water from the leaves

Question 12

How does xylem tissue withstand the pressure of cohesion tension?

Answer 12

It is strengthened with lignin

Question 13

What is transpiration?

Answer 13

The loss of water vapour from upper parts of the plant

Question 14

What is the endodermis?

Answer 14

A layer of cells surrounding the xylem

Cells move minerals by active transport into the xylem

Question 15

What is capillary action in plants?

Answer 15

Adhesion of water to xylem vessels as they are narrow

Question 16

What affects the rate of transpiration?

Answer 16

Leaf number, number/size/position of stomata, cuticle, light, temperature, humidity, wind, water availability

Question 17

What is translocation?

Answer 17

The process by which organic molecules and some mineral ions are transported from one part of a plant to another

Question 18

What is the function of phloem tissue?

Answer 18

It transports biological sugars from sites of production, sources, to the places where they are stored or used, sinks

Question 19

What is the structure of the phloem?

Answer 19

Sieve tube elements
Long thin structures arranged end to end
Walls are perforated to form sieve plates
Associated with the sieve tube elements are cells called companion cells

Question 20

What molecules does the phloem transport?

Answer 20

Organic: sucrose and amino acids
Inorganic: phosphate, potassium, chloride, magnesium ions
Sucrose is the most common translocated process

Question 21

In which direction do phloem tissues transport material?

Answer 21

As sinks can be above or below the source, translocation occurs in either direction

Question 22

What is the function of companion cells?

Answer 22

They transport products of photosynthesis to sieve tube elements through plasmodesmata

Question 23

What is the tissue which transports biological molecules?

Answer 23

Phloem

Question 24

What is a source or sink in the phloem transport system?

Answer 24

A source is the site where sugars are produced, e.g. the leaf
A sink is where they are used directly or stored for future use. Sinks can be anywhere in the plant

Question 25

In which direction are substances moved in the phloem?

Answer 25

Both ways as the sink can be above or below the source

Question 26

Which kinds of substances does the phloem transport?

Answer 26

Organic molecules including sucrose and amino acids

Inorganic ions such as potassium, chloride, phosphate and magnesium ions

Question 27

Is mass flow an active or passive process in the phloem system?

Answer 27

Passive process but occurs as a result of the active transport of sugars do it classed as active
This means it is affected by temperature and metabolic poisons

Question 28

How does sucrose move into companion cells?

Answer 28

Sucrose is manufactured in chloroplasts (source)

Sucrose diffuses down a concentration gradient by facilitated diffusion into companion cells

Question 29

How do hydrogen ions move into companion cells?

Answer 29

They are actively transported

Question 30

How does hydrogen move into sieve tube elements?

Answer 30

Hydrogen ions use carrier proteins to move into sieve tube elements

Question 31

How is sucrose moved into sieve tube elements?

Answer 31

Sucrose is transported along with hydrogen ions via co-transport proteins

Question 32

How does water move into the sieve tubes?

Answer 32

Sucrose produced by chloroplasts is actively transported into sieve tubes in step 1
This decreases water potential in the phloem
Water moves into the phloem by osmosis from a high water potential to a low water potential

Question 33

How does water move into the sink?

Answer 33

At the sink, sucrose is used up or converted to starch for storage
This decreases sucrose content so sucrose is actively transported into the sink
This reduces the water potential at the sink
The low water potential causes water to move into the sink from sieve tubes by osmosis
This lowers hydrostatic pressure in the sieve tubes

Question 34

What is the mass flow theory?

Answer 34

High hydrostatic pressure at the source and low at the sink

Mass flow of sucrose solution down this hydrostatic gradient

Question 35

What are the stages of transport or organic substances in the phloem? (No explanation)

Answer 35

Sucrose moves into companion cells
Hydrogen ions move into companion cells
Hydrogen moves into sieve tube elements
Sucrose moves into sieve tube elements
Water moves into the sieve tubes
Water moves into the sink

Question 36

What is the evidence supporting the mass flow hypothesis?

Answer 36

Concentration of sucrose is higher at source than sink
Downward movement of phloem occurs in light but ceases in darkness
Increases in leaf sucrose levels is linked to an increase in phloem sucrose
Companion cells contain many mitochondria

Question 37

What evidence contradicts the mass flow hypothesis?

Answer 37

Function of sieve plates is unclear as they hinder mass flow
Not all solutes move at the same speed
Sucrose is delivered at roughly the same rate to all regions

Question 38

What are the steps of a ringing experiment?

Answer 38

Section of the outer layers of the plant are removed (protective layers and phloem)
After time, the region above will swell
The swelling is caused by the sugars of the phloem accumulating above the ring
Some tissues below the ring wither and die
This is due to the interruption of the flow of sugars below the ring
This proves that the phloem is responsible for transporting sugars

Question 39

What are the steps of a tracer experiment?

Answer 39

Radioactive isotope used for tracing: e.g 14C
14C is used by the plant to make carbon dioxide
14C isotope is used to make sugars
Radioactive sugars can be traced as they move through the plant by x-rays

Question 40

What is the evidence of translocation of organic molecules occurs in the phloem?

Answer 40

When the phloem is cut, a solution of organic molcules flow out
Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time
The removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring and their disappearance below it

Question 41

What are the xerophyte adaptations in the leaves?

Answer 41

Hairs so ‘trap’ water vapour
and water potential gradient 
decreased;
Stomata in pits/grooves so
‘trap’ water vapour and water 
potential gradient decreased;
Thick (cuticle/waxy) layer so
increases diffusion distance;
Waxy layer/cuticle so reduces 
 evaporation/transpiration.
Rolled/folded/curled leaves so
 ‘trap’ water vapour and water 
 potential gradient decreased;
Spines/needles so reduces 
 surface area to volume ratio