Exchange and Transport - Plants

Question 1

What is the role of the xylem?

Answer 1

Transport water and mineral ions unidirectionally.

Question 2

What is the xylem composed of, and which components are living?

Answer 2

Vessels, tracheids, xylem fibres. (lignified)

Xylem parenchyma

Question 3

Why are most cells in the xylem dead?

Answer 3

No nucleus, cytoplasm, end walls preventing flow.

Question 4

What is the purpose of pits in the xylem?

Answer 4

Allows water to move laterally to supply cells/tissues with water.

Question 5

What is the benefit of the xylem having a wide lumen?

Answer 5

Allows more water to flow.

Question 6

What strengthens and waterproofs the xylem?

Answer 6

Spiral lignin

spiral shape is flexible and strong

Question 7

What is the role of the phloem?

Answer 7

Transport photosynthetic products(sucrose+amino acids) multidirectionally.

Question 8

What is the phloem comprised of, and which components are living?

Answer 8

Sieve tube elements, companion cells, phloem parenchyma.
Phloem fibres (lignified)

Question 9

Describe the sieve tube element/companion cell arrangement.

Answer 9

Nuclei and almost all organelles are in the companion cells which are connected to the sieve tube elements via plasmodesmata.
This decreases the obstruction of flow through the column of elements.

Question 10

What is parenchyma?

Answer 10

Packing tissue with a support/storage function.

Question 11

Draw a transverse cut of a plant root and stem.

Answer 11

See bio book.

Question 12

What is transpiration?

Answer 12

The loss of water vapour via evaporation from the aerial parts of a plant.
Water leaves cells in the spongy mesophyll layer.
Water evaporates into spaces.
Water diffuses out of the leaf, via stomata, down a water potential gradient.

Question 13

What is the transpiration stream?

Answer 13

Water moving up the xylem vessels in a continuous stream.

Question 14

What role does transpiration play in the transpiration stream?

Answer 14

Transpiration reduces pressure at the top of the xylem, causing hydrostatic pressure

Question 15

What causes water to move up the xylem as a continuous stream?

Answer 15

Water molecules pull each other up the xylem through water cohesion due to hydrogen bonding.
Water molecules also move up the xylem through capillary action (water adhesion).

Question 16

What causes water adhesion in the xylem?

Answer 16

The lignin in the walls of xylem vessels.

Question 17

How does water first enter a plant?

Answer 17

By osmosis (down a water potential gradient) through root hair cells.

Question 18

What are the three ways water travels through root hair cells?

Answer 18

Apoplast pathway (cell wall)
Symplast pathway (cytoplasm)
Vacuolar pathway (vacuole)

Question 19

What is the casparian strip?

Answer 19

Present in the cell walls of endodermal cells (at the endodermis).
Contains suberin (waxy and waterproof).
Forces water to use symplast pathways.

Question 20

What is the purpose of the casparian strip?

Answer 20

Forces water to pass through a partially permeable membrane which filters out any potentially toxic solutes.

Question 21

What environmental factors affect transpiration? (5)

Answer 21

Light intensity
Humidity
Temperature
Wind speed
Soil water availiability

Question 22

When do stomata open and why only then?

Answer 22

In high light intensity, so that water is not wasted when photosynthesis rate is slow.

Question 23

What are xerophytes? Can you give some examples?

Answer 23

Plants with structural and physiological adaptations that allow them to survive in dry conditions.
e.g: cacti, marram grass

Question 24

What are 2 examples of the features of a xerophyte?

Answer 24

Thick waxy cuticle
Sunken stomata (reduces air movement so less water loss)

Question 25

What are hydrophytes? Can you give some examples?

Answer 25

Plants with structural and physiological adaptations that allow them to survive in wet conditions. e.g: water lilies

Question 26

What are 4 examples of the features of a hydrophyte?

Answer 26

Thin/no waxy cuticle Always open stomata (on upper surfaces) Less structure (water supports them) Wide flat leaves (for floating)

Question 27

What is translocation?

Answer 27

The transport of assimilates through the plant in the phloem tissue.

Question 28

What are the assimilates that are transported via translocation?

Answer 28

Organic solutes, molecules the plant has made such as the products of photosynthesis. e.g: sucrose, amino acids

Question 29

What are the production sites in translocation?

Answer 29

(Sources) leaves, storage organs

Question 30

What are the destination sites in translocation?

Answer 30

(Sinks) roots, meristem

Question 31

Why was the pressure flow hypothesis for translocation necessary?

Answer 31

The rate of assimilate movement is too fast to be caused only by diffusion.

Question 32

Outline active loading at the source (translocation).

Answer 32

Companion cell uses ATP to pump H+ ions to the surroundings. H+ diffuses back in via cotransporter proteins, bring sucrose with it. Concentration of sucrose rises in companion cells. Sucrose diffuses into sieve tube elements via plasmodesmata.

Question 33

Outline mass flow (translocation).

Answer 33

Sucrose is actively loaded at the source, reducing water potential. Water enters by osmosis, increasing hydrostatic pressure. Water with dissolved sucrose moves towards sink from source down hydrostatic pressure gradient. Sucrose is actively transported out of the phloem sieve tubes by surrounding sink cells, increasing water potential in the sieve tube. Water moves out by osmosis, reducing hydrostatic pressure.

Question 34

What happens during phloem unloading (translocation) and why?

Answer 34

Sucrose is actively transported out of sieve tube elements by companion cells and then converted into glucose or starch. This conversion happens to maintain the concentration gradient between the contents of the phloem and surrounding sink cells.

Question 35

What is the name of the protein which allows sucrose to diffuse into companion cells along with H+ ions?

Answer 35

Cotransporter protein

Question 36

What piece of equipment could be used to measure the transpiration rate of plants?

Answer 36

A potometer