mass transport in plants

Question 1

Process of transpiration

Answer 1

. Water evaporates/transpires out of the stomata;
. Water then moves down its water potential gradient from cells into air spaces;
. The movement of water creates tension. As one water molecule moves it pulls the other water molecule up;
. Water moves up the xylem from the roots in a continuous column;
. Column is maintained by cohesion - weak H bonds causing water molecules to stick together;

Question 2

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves. (6)

Answer 2

(In the root)
. Casparian strip blocks apoplast pathway / only allows symplast pathway;
. Active transport by endodermis;
. (Of) ions/salts into xylem;
. Lower water potential in xylem / water enters xylem by osmosis /down a water potential gradient;

(Xylem to leaf)
. Evaporation / transpiration (from leaves);
. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure;
. Adhesion / water molecules bind to xylem;
. (Creates continuous) column of water

Question 3

Root pressure moves water through the xylem. Describe what causes root pressure. (4)

Answer 3

. Active transport by endodermis;
. ions/salts into xylem;
. Lowers water potential (in xylem);
. (Water enters) by osmosis;

Question 4

Describe how a high pressure is produced in the leaves. (3)

Answer 4

. Water potential becomes lower/becomes more negative (as sugar enter phloem);
. Water enters phloem by osmosis;
. Increased volume (of water) causes increased pressure;

Question 5

Name a factor that can affect transpiration

Answer 5

Light (intensity) / temperature / air movement / humidity

Question 6

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.

Answer 6

. Seal joints / ensure airtight / ensure watertight;
. Cut shoot under water;
. Cut shoot at a slant;
. Dry off leaves;
. Insert into apparatus under water;
. Ensure no air bubbles are present;
. Shut tap;
. Note where bubble is at start / move bubble to the start position;

Question 7

Describe the mass flow hypothesis for the mechanism of translocation in plants.

Answer 7

. In source/leaf sugars actively transported into phloem;
. By companion cells;
. Lowers water potential of sieve cell/tube and water enters by osmosis;
. Increase in pressure causes mass movement (towards sink/root);
. Sugars used/converted in root for respiration for storage;

Question 8

Describe how a student could have used an eyepiece graticule to determine the mean diameter of stomata. (3)

Answer 8

1. Measure each stomata using an eye piece graticule;
2. Calibrate the eyepiece graticule against a stage micrometer;
3. Take at least 5 measurements and calculate a mean;