Adaptions for transport (plants)

Question 0

What is the path of least resistance in the cortex and why and what is the name of this pathway?

Answer 0

When the water remains in the cell walls. This is because it does not have to move through a cell membrane. This method of transport is known as apoplastic.

Question 1

When water moves, what “path” does it take?

Answer 1

The path of least resistance.

Question 2

By what means does water move in the apoplast

Answer 2

It moves by capillarity and moves very quickly.

Question 3

What is the second method of water movement in the cortex?

Answer 3

They simplastic pathway or simplast. This involves water remaining in the cytoplasm, moving around the vacuole and from one cell to another through plasmodesmata.

Question 4

Label this diagram of a root.

Answer 4

Question 5

Label this diagram of a sectioned root.

Answer 5

Question 6

Complete this diagram outlining water transport across the cortex

Answer 6

Question 7

Describe the structure of the endodermis.

Answer 7

This is a single layer of cells forming a ring around the vessels in the centre of the root. Each cell has a waterproof band called the casparian strip running completely around the cell.

Question 8

What is the function of the casparain strip?

Answer 8

This forms a barrier to water movment through the apoplast and so forces the water to enter the symplast.

Question 9

What is the purpouse of forcing water to enter the symplast?

Answer 9

This gives the plant control over water uptake from the soil.

Question 10

How is water uptake controlled by the endodermis?

Answer 10

This is acheived using K⁺ pumps in the membranes of the cells of the endodermis. This active transport of K⁺ ions from the cortex lowers the water potential in the endodermis relative to that of the cortex and causes water to enter the endodermis by osmosis. This in turn pulls water from the soil faster as water clings to water by cohesion.

Question 11

Deifine transpiration.

Answer 11

The movment of water in xylem vessels from the roots to the leaves and out to the air through the stroma by evaporation.

Question 12

Define cohesion.

Answer 12

A force of attraction between like molecules. e.g between water molecules.

Question 13

Define adhesion.

Answer 13

A force of attraction between unlike molecules e.g water and xylem vessel walls.

Question 14

Outline the tension theory of transpiration.

Answer 14

1. wet cell walls in the leaf evaporate water into air spaces between spoungy measophyll cells.
2. The air outside is dry compared with the air inside the leaf and so water evaporates through the stomata, drawing water out from the air spaces in the leaf.
3. Pressure in the xylem water is lower than atmospheric pressure because the water is under tension
4. As water moves up the xylem it is pulled from the root and the soil down the pressure gradient. This is called the transpiration stream.

Question 15

Label these diagrams of leaf structure.

Answer 15

Question 16

What is the function of the waxy cuticle?

Answer 16

This reduces water loss.

Question 17

What is the upper epidermis.

Answer 17

A protective transparant covering

Question 18

What is the role of the pallasade measophyll?

Answer 18

This is the primary site of photosynthesis in the leaf.

Question 19

What is the function of the spoungy mesophyll?

Answer 19

This allows gas exchange between the palisade layer and the air outside.

Question 20

How does the tension theory of transpiration require the force of cohesion?

Answer 20

Because without this force the water column would snap and bubbles would develop in the xylem water, preventing transpiration.

Question 21

How does the tension theory of transpiration depend on the force of adhesion?

Answer 21

Adhesion prevents the water column from falling down.

Question 22

Outline the structure of xylem vessels/tissue.

Answer 22

Xylem consists of of a series of xylem cells which are bonded end to end with one another. The cells themselves are hollow tubular structures which are perforated, allowing water to seep into surrounding tissues laterally. They are not living and have a lignified cell wall.

Question 23

What are the properties of lignin?

Answer 23

• It is hydrophobic - so waterproof
• it is mechanically very strong
• It is the second most abundant organic molecule on earth (cellulose is the first)

Question 24

What is transpiration powered by and why is this an advantage?

Answer 24

Energy from the sun i.e heat. This is advantagous because it means that the plant provides no energy for transpiration because it is a purely physical process.

Question 26

What are the four factors affecting transpiration rate?

Answer 26

1. Air temprature
2. humidity
3. windspeed
4. light intensity.

Question 27

How does increasing temprature affect transpiration rate?

Answer 27

Higher temprature increases the transpiration rate in direct proportion.

Question 28

How does humidity affect transpiration rate?

Answer 28

The lower the humidity the higher the transpiraton rate. - Inverse proportionallity.

Question 29

How does windspeed affect transpiraton rate?

Answer 29

The higher the windspeed the higher the rate of transpiration.

Question 30

How does light intensity affect transpiration rate?

Answer 30

As light intensity increases the stomata open and transpiration rate goes up

Question 31

What are xerophytes?

Answer 31

These are plants adapted to dry conditions.

Question 32

What are Hydrophytes?

Answer 32

These are plants adapted to living in water.

Question 33

Outline some of the adaptions seen in xerophytes.

Answer 33

Stomata can be found in deep grooves in the epidermis. this provides a more humid microclimate immediately outside the stomata, reducing transpiration rate. They have thicker waxy cuticles to further reduce waterloss. some plants like marram grass have stomata pointing inwards on a curved leaf.

Question 34

Outline some of the adaptions of hydrophytes.

Answer 34

For these plants water loss is no longer a problem. Because of this true water plants like pondweeds (e.g elodea) have no cuticle, functionless stomata, no xylem and the roots are used for anchorage and mineral uptake only. They get CO₂ and water from the water around them.

Question 35

Define translocation.

Answer 35

The movment of organic substances from sources to sinks.

Question 36

What is a source in translocation.

Answer 36

A site of manufacture of mobilisation of organic molecules.

Question 37

What is a sink in translocation?

Answer 37

These are places where organic molecules are used or stored. e.g the growing regions of the plant.

Question 38

In what kind of tissue does translocation occur?

Answer 38

Phloem tissue.

Question 39

Label this diagram of a vascular bundle.

Answer 39

Question 40

Label this diagram of a primary stem.

Answer 40

Question 41

What are the two types of cells that make up phloem tissue?

Answer 41

Sive tubes and companion cells.

Question 42

Are the cells in phloem living or dead?

Answer 42

Living.

Question 43

Outline the structure of a sieve tube cell.

Answer 43

Mature Sieve tubes have no nucleus, ribosomes or golgi and have small mitochondria contained in a very thin layer of peripheral cytoplasm which is bounded by the plasma membrane. in the centre of the cell ther is a large lumen. On either end they have a sieve plate perforated with pores, through which run strands of phloem protien. The portion of its cell walls which is adjacent to companion cells contains plasmodesmata for nutrient exchange.

Question 44

How are the cells arranged in phloem tissue?

Answer 44

Sieve tubes run one after another end to end to create a ‘Pipe like’ tube. Each sieve tube cell is associated with its own companion cell.

Question 45

What is the function of the seive tube cells?

Answer 45

These transport solutes such as sucrose.

Question 46

How are sieve tubes adapted to their function?

Question 47

What is the function of the companion cells?

Answer 47

Companion cells controll the sieve tubes by communicating with them using messenger molecules which pass through the plasmodesmata. Companion cells have dense cytoplasm, a nucleus and all the major organelles.

Question 48

What four techniques provide evidence for translocation?

Answer 48

1. Tree ringing experements
2. Autoradiography
3. Radioactive labelling
4. Translocation rate experement.

Question 49

How do tree ringing experements provide evidence for translocation?

Answer 49

This involves removing a circular ring of bark, together with the underlying phloem. Phloem cells repair this break by blocking the pores in the sieve plates by secreteing callose. Over a period of days the tree swells just above the ring and is found to be full of sucrose. This suggests that translocation takes place in the phloem.

Question 50

What would happen if the tree ringing experement was carried out in early spring - before the leaves appeared?

Answer 50

The tree would swell below the ring.

Question 51

How has autoradiography provided evidence for translocation?

Answer 51

This uses C¹⁴O₂ which is delivered to a leaf inside a plastic bag. The plant is allowed to photosynthesise for 48 hours, after which stem sections are cut and placed on photographic film . The pattern of exposure corresponds to the location of phloem in the stem.

Question 52

How have radioactive labelling experements provided evidence for translocation?

Answer 52

Plant leaves are once again exposed to C¹⁴O₂ for 48 hours. Plant tissue samples are taken and radioactivity is measured and expressed as a proportion of the radioactivity in the source leaf. This shows that translocation occurs to the nearest sink. However it also shows bi-directional flow.

Question 53

What is bi-directional flow?

Answer 53

This means that phloem tissue can translocate up and down the stem at the same time.

Question 54

How have translocation rate experements provided evidence for translocation?

Answer 54

Using greenfly mouthparts the rate of translocation has been shown to average about 1000mmhr^-1. Because ths is about 60,000 times faster than diffusion it suggests that transloation uses allot of energy.

Question 55

What are the two main theories for the mechanism by which translocation takes place?

Answer 55

1. The mass flow theory of translocation
2. The protien fillament theory of translocation.

Question 56

Outline the mass flow theopry of translocation.

Answer 56

1. Phloem vessels in the leaves are loaded with sugar using energy from ATP whilst phloem vessels in the roots(sinks) have sugar removed
2. This results in water entering the entering phloem vessels in the leaves by osmosis, creating a hydrostatic pressure gradient from the leaves to the roots (sinks)
3. the pressure gradient causes the water to move. - this is called mass flow.

Question 57

What are the problems associated with the mass flow theory of transpiration?

Answer 57

This accounts for the need for energy in translocation but cannot explain bidirectional flow or the rate of translocation (1mhr^-1 )

Question 58

Outline the protien fillament theory of translocation.

Answer 58

• Hollow, contractile, protien fillaments pass through the sieve pores, connecting one tube to the next. These fillaments are also found within the cytoplasm of the seive tube.
• contraction of these fillaments accellerates the movment of solute in the phloem vessels across the seive plates. Movment through the seive tube is due mainly to momentum built up by this accelleration.
• This process of accelleration accounts for the rate of translocation, bidirectional flow and the need for energy. - this is therefore the correct one.