7. Transport in plants

Question 1

What is mass transport

Answer 1

the movement of materials over large distances, due to pressure differences

Question 2

What is phloem tissue

Answer 2

The transport of organic substances from the leaves to other parts of the plant

Question 3

What is xylem tissue

Answer 3

the transport of water and dissolved minerals, from the soil, through the roots, the stem and to the leaves.

Question 4

What is transpiration

Answer 4

the process of water movement through a plant and its evaporation from leaves.
The continuous columns of water that move from the soil through the roots, stems and leaves to the air are known as the transpiration stream.
consequence of gas exchange in leaves

Question 5

What happens in transpiration

Answer 5

Open stomata allow the exchange of O2 and CO2
Water will be lost

Question 6

What is the movement of water through the leaf

Answer 6

-Open stomata, water vapour diffuses from the air psaces in the leaf through the stomata down a WP gradient - transpiration
-Water evaporates from mesophyll cells into air spaces forming water vapour which builds in the air spaces
-This reduces tje WP of the mesophyll cells surrounding the air spaces
-results in a QWP gradient across the leaf, from xylem to mesphyll cells
-Water drawn out of xylem vessel in leaf and diffuse into the mesophyll cells to replace lost water

Question 7

What are the structures of a xylem cell

Answer 7

-Cell walls contain lignin
-Lignified vessels walls cause the cell to die
-Walls contain tiny holes called pits
Vessels lose their end walls

Question 8

What is th function of lignin in xylem cell walls

Answer 8

Strengthens the xylem walls agaisnt the tension within them and makes them waterprood

Question 9

What is the function of xylem cells being dead

Answer 9

Leaves a hollow lumen with no cytoplasm that offers little resistance to the mass flow of water and minerals

Question 10

What is the funtion of pits in xylem vessels

Answer 10

If a vessel becomes blocked or damaged, the water can be diverted laterally, so the upward movement of water can continue in an adjacent vessel

Question 11

Why is the function of no vessel walls in xylem

Answer 11

They form a continuous column for water movement from root to leaves

Question 12

Why is the thinkening of xylem walls a spiral

Answer 12

Use less material and less wasteful
Use less material therefore allows xylem to be flexible

Question 13

How does water have cohesion

Answer 13

Polar bonds with many weak H bonds making them stick together

Question 14

How does water have adhesion

Answer 14

Water forms H bonds with the walls of xylem

Question 15

What is the cohesion tension theory

Answer 15

-Transpiration occurs is leaves. Water is evapourated from the mesophyll cells of the leaf, reducing their WP. Water vapour forms in air spaces then diffuse out through stomata
- WP gradient is formed across leaf. Water leaves xylem vessels in leaf and diffuses into mesophyll cells by osmosis
-Creates negative pressure/tension at the top of xylem vessel
-Remaining water is xylem is under tension and is pulled up to leaves
-Continuous columns of water are maintained due to cohesion between water molecules
-adhesion to the walls of xylem. Creates an inward pull on the vessel walls as water is pulled up, xylem vessel decreases in diameter

Question 16

How can the cohesion tention theory be powered

Answer 16

No ATP
Heat energy required t evaporate water from leaves

Question 17

What evidence is there for the cohesion tenion theory

Answer 17

-Tension has been measured in xylem as plants respire
-If xylem is broken, air bubbles in xylem form and stops any further upward movement of water in that vessel as air bubbles prevent cohesion
-Respiratory inhibitors dont inhibit this process
-Diameter of trees decreases when transpiring and more so when temps and light intensity is higher - measured using a dendrometer

Question 18

How does air being drawn into a broken xylem vessel support the CTT

Answer 18

Fact taht air is draw in means water in xylem must be under tension rarther than under pressure (pushed from bottem)

Question 19

Why is a trunk diameter lowest at noon

Answer 19

Stomata are open
Transpiration rates are high
Increased tension
so water column pulled up xylem faster
so walls of xylem pulled in and diameter decreases

Question 20

What are xerophtes

Answer 20

Found in any region where water is scarce
Have addition adaptations to further reduce water loss through transpiration
They have structural adaptations which allow them to survive in dry conditions by either reducing transpiration loss or storing water

Question 21

What is marram grass

Answer 21

Colonises sand and is the main speices found on sand dunes
Xerophyte

Question 22

What adaptations does marram grass have to reduce water loss

Answer 22

Leaf rolled up
Thick waxy cuticle
Trappend humid air with high WP
Hairs on lower epidermis of leaf
Stomata sunken in pits

Question 23

How does trapped humid air help to limit water loss

Answer 23

Water vapour trapped within rolled leaf
Reduces WP gradient between spaces inside leaf and atmosphere
A lower rate of diffusion from stomata occurs

Question 24

How does hairs on lower epidermis help limit waterloss

Answer 24

Water vapour trapped between thin hairs
Reduces WP gradient between air spaces inside leaf and the atmosphere
Lower rate of diffusion and less transpiration occurs

Question 25

How does stomata sunken pits limit water loss

Answer 25

Water vapour is held above the stomatal pore so reducing the WP gradient between air spaces and atmosphere
Lower rate of diffusion and less transpiration

Question 26

How does a thick waxy cuticle help limit water loss

Answer 26

Reduces water loss from epidurmis
Greater thickness increases length of the diffusion pathway for water to reach the atmosphere
Decreases rate of diffusion of water through cuticle

Question 27

How does the reduction in the surface area to volume ratio of leaves limit water loss in xerophytes

Answer 27

The samller the SA:V the slower the rate of diffusion.
Some xerophytes like cacti have spines rather than broad, flat lieaves

Question 28

why does stomata confined to underside of leaf help limit water loss in xerophytes

Answer 28

Underside of leaves is often cooler so less heat energy to evaporate water.

Question 29

why does daylight closure of stomata help limit water loss in xerophytes

Answer 29

reduces transpiration during the hottest part of the day

Question 30

How do succulent stems or leaves help limit water loss in xerophytes

Answer 30

storage of water

Question 31

What is translocation

Answer 31

The process by which soluble organic molecules and some mineral ions are transported around the plant as sap

Question 32

Why is it necessary to transport sucrose and amino acids from one part of the plant to another?

Answer 32

Only leaves photosynthesis to produce sugars but all parts of the plant requires sugars for respiration.
Amino acids are required for protein synthesis.

Question 33

Why might non-reducing sugars such as sucrose be transported in the phloem rather than reducing sugars such as glucose?

Answer 33

Reducing sugars are too reactive and would be chemically altered before arriving at their destination

Question 34

What are phloem tissues made of

Answer 34

-Made up of sieve tube elements
Long thin cells araanged end to end. With companion cells

Question 35

What is the structure of phloem tissue

Answer 35

-Each sieve tube element links to the next via a sieve plate which is perforated with pores.
-Sieve tube has little cytoplasm, no nucleus, no vacuole and few organelles other than a small number of mitochondria.
-Sieve tubes are alive because of cytoplasmic connections (plasmodesmata) with the companion cell.
-Each companion cell has a nucleus, many mitochondria and other organelles.

Question 36

What does translocation do

Answer 36

moves organic solutes from ‘sources’ to ‘sinks’ by mass transport.

Question 37

What are sources

Answer 37

· Where the organic solutes are produced and are therefore at a high concentration.
· The source for sucrose is usually the mesophyll cells of the leaves where it is formed by the condensation of fructose and glucose

Question 38

What are sinks

Answer 38

· Where organic solutes are used up and are therefore at a low concentration
-Sinks are the other parts of the plant, especially the growing points (meristems) of roots, stems, flowers and leaves where the sucrose is hydrolysed to glucose and fructose and then respired to provide metabolic energy in the form of ATP.
-Fruits, seeds, roots and other storage organs act as sinks when sucrose is converted into starch and stored.

Question 39

What are the 3 parts of translocation

Answer 39

• The transfer of sucrose from photosynthesising tissue to sieve tubes
  -Mass flow of sucrose through sieve tube elements.
  -Transfer of sucrose from the sieve tube elements into sink cells

Question 40

How does the mass flow theory transfer sucrose from photosynthesising tissue to sieve tubes

Answer 40

· Active transport is used to pump the sucrose from mesophyll cells into companion cells and then into the sieve tube elements of the phloem. This is known as active loading.
· requires ATP and special carrier proteins in the cell membranes of companion cells.
·increases the concentration of sucrose in the sieve tubes in this region.

Question 41

How does mass flow of sucrose through sieve tube elements occur

Answer 41

·increased conc of sucrose in sieve tubes lowers the WP of the sieve tubes, so water enters by osmosis from xylem and companion cells.
· increases volume within sieve tubes around the source, which increases the hydrostatic pressure.
· At sinks, sucrose is either used up in respiration or converted to starch for storage, so sucrose conc of sinks is low.
· Sucrose is actively transported from sieve tube elements through companion cells into sink cells.
· increases the WP of the sieve tube cells so water leaves sieve tubes by osmosis entering sink cells (and xylem).
· decreases volume within sieve tubes near the sink, decreases the hydrostatic pressure.
· As a result of water entering the sieve tube elements at the source and leaving at the sink, there is a hydrostatic pressure gradient between source and sink.
· there is a mass flow of sucrose solution down pressure gradient in the sieve tubes.

Question 42

How does transfer of sucrose from the sieve tube elements into sink cells happen

Answer 42

Sucrose is unloaded from the phloem by active transport from sieve tube elements into sink cells via companion cells.

Question 43

Whaat evidence is there supporting the mass flow theory

Answer 43

· high hydrostatic pressure in phloem as shown by the release of sap when they are cut.
· conc of sucrose is higher in leaves (source) than in the roots (sink).
· Downward flow in phloem occurs in daylight, but ceases when leaves are shaded, or at night.
· Increases in sucrose levels in leaf are followed by increases in the phloem a little later.
· Metabolic inhibitors and/or a lack of oxygen inhibit translocation of sucrose in phloem.
· Companion cells have many mitochondria.

Question 44

What is a possible conclusion of the ringing experiment

Answer 44

removing the bark reduced the mass of carbohydrate transported to the lower part of the stem in the phloem.

There is a positive correlation; BUT correlation does not infer causation
The results do not prove that there is a downward movement of carbohydrate in the phloem
There could be other factors affecting the results.
For example it could be that the carbohydrate is transported in the xylem, but the xylem was damaged in the ringing process
The experiment has also been carried out on only one species of plant;

Question 45

What is the radioactive tracer experiment

Answer 45

plant is exposed to an atmosphere containing radioactively labelled carbon dioxide 14CO2
carbohydrates are synthesised by photosynthesis using the radioactively labelled carbon
movement of carbohydrates in plant can be traced by cutting tissue from different areas of plant into thin sections, placing photographic film over specimens
film blackens where is exposed to radioactive isotope, revealing location of labelled carbohydrates
process is called autoradiography