Transport of Water and Assimilates

Question 1

Bulliform cells vs motor cells

Answer 1

Bulliform cells: control the folding of the leaf

Motor cells: Faster mechanism than bulliform cells because of the rapid movement of ions and water exiting the cells via aquaporins

Question 2

Phloem: What are sieve tubes consisted of?

Answer 2

Consisted of sieve elements:

Elongated cells that are connected to each other via sieve plates (end walls with large pores)

Question 3

Phloem: Why do sieve elements lose their nucleus and vacuoles?

Answer 3

In order to maintain a tube system with low resistance

Question 4

Phloem: Explain companion cells

Answer 4

Found beside the sieve tubes, companion cells contain all organelles: nucleus, vacuoles, and chloroplasts

Question 5

What controls water movement in a sieve tube?

Answer 5

Aquaporins found in the plasma membrane

Question 6

What is Munch’s pressure flow hypothesis?

Answer 6

Having a high concentration of organic substances in the phloem of a source tissue creates a diffusion gradient to draw water into the cells (by osmosis) from the adjacent xylem.

Sugars are unloaded at the sink tissue and water exits as well.

Question 7

Phloem: apoplasmic loading

Answer 7

• Active transport of sugars from cell wall (apoplast) into companion cell
• Sugars are unable to leak back into the bundle sheath because of the lack of plasmodesmata
• Done by HERBS

Question 8

Phloem: symplasmic loading

Answer 8

• Sugars flows from bundle sheath into the phloem via plasmodesmata
• Done by TREES

Question 9

Define callose

Answer 9

Formation that closes up pores of a sieve plate

Question 10

What is phloem sap contained of?

Answer 10

Sugars (20%), amino acids, RNA, viruses, hormones, other types of molecules

Question 11

What is used to collect phloem sap?

Answer 11

Aphids

Question 12

What is the cohesion-tension theory? (dixon and joly, 1894)

Answer 12

Transpiration is the main of water movement in teh xylem:

The atmosphere and soil are both pulling on the water column, but the atmosphere pulls harder, moving the water up. Thanks to strong cohesion and adhesion, water is pulled up into the leaves against gravity. This results in water under great tension and prone to turning into vapor.

Question 13

Explain embolism

Answer 13

Drought and frost can create air bubbles in the vessel. If the sap around the bubble is under tension, it could expand and the vessel embolizes: unable to transport water

Question 14

Explain guttation

Answer 14

Water is pushed out of the xylem through root pressure, causing water droplets to form on leaf tips. This happens when there is very little transpiration

Question 15

What happens during root pressure?

Answer 15

Pressure strong enough to collect sap at the bottom of a tree, but too weak to refill vessels. This occirs before leaves emerge in the spring