M2.2.4 - Transport of Nutrients in Plants

Question 1

Transpiration

Answer 1

Evaporation of water through the stomata
- Causes water loss → differential pressure in the cell walls of mesophyll cells
- Transpiration pull + surface tension of water → negative pressure in the leaf, pulling water from the xylem

Question 2

Water potential gradient

Answer 2

A concentration gradient formed between the plant and its environment.
- Less water potential in the plant → more water from the roots drawn up through the xylem

Question 3

Tension

Answer 3

• Water vapour concentration outisde the leaf < inside the leaf → water vapour diffuses out of the leaf
• Loss of water → tension outside the mesophyll cells → water moves out into the intercellular spaces
• Loss of water from the mesophyll cells → water compensates by flowing out of the veins into the mesophyll cells
• Causes water to be pulled up from the xylem into the leaf
• This increases the tension of the water column in the xylem, drawing more water from the roots

Question 4

Cohesion

Answer 4

Cohesion: attraction of water molecules to one another
* Hydrogen bonds form between the polar ends of the water molecule → intermolecular forces hold the molecules together → water molecules pull one another along in a continuous stream
* Therefore, any tension/pull in the xylem would result in the whole column be moving up.

Question 5

Adhesion

Answer 5

• Xylem is hydrophilic → water clings to the walls of the xylem
• Adhesive forces between water molecules and xylem wall causes water to rise up the sides (capillarity)
• Continuous stream of water is formed from the combination of cohesive and adhesive forces
• The narrower the xylem, the bigger the impact of capillarity

Water, mineral ions and other water soluble substances

Question 6

Root pressure

Answer 6

• Continual influx of water and ions creates a small amount of root pressure → solution in xylem moves upwards
• Pressure not sufficient to life water and ions very high

Question 7

Source and Sink Theory

Answer 7

• High and low pressure regions due to water diffused from the xylem in the phloem drives the movement of substances
• Movement occurs from region of high pressure (source) to region of low pressure (sink)
• The flow of substances is continuous because sucrose is continually being added at one end and removed at another

Sugar, hormones and other organic substances

Question 8

Source

Answer 8

Where the substances are produced (leaves) or enter the plant
- Energy is required to actively pump sugars into the phloem tissue → concentrated solution in the phloem → draws water into the phloem from the xylem by osmosis due to hypotonicity
- This creates a region of high pressure at the source

Question 9

Sink

Answer 9

Where the substrate can be stored - where is is needed
- Energy used to actively remove sugars from phloem
- This creates a dilute solution → water leaves the phloem for the xlyem via osmosis due to hypertonicity → forms a low pressure region

Question 10

Importance of phloem-xylem proximity

Answer 10

Proximity allows for the diffusion of water into and out of the phloem (concentration gradient and osmosis) → creates areas of high pressure and low pressure, driving substrate movement

Question 11

Xylem practical

Answer 11

Findings: under the microscope, the xylem was stained red. This is because the solution the celery specimen was placed in contained red-dye, and the xylem took up this solution. This indicates that xylem is responsible for the transport of fluid and is passive