Mass transport in plants

• The mass flow hypothesis is used to explain the movement of substances through phloem.

  Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube.

  1. Sucrose actively transported into phloem;

  2. Reducing water potential

  3. Water moves into phloem by osmosis from xylem;
• Describe the cohesion-tension theory of water transport in the xylem.

  1. Water lost from leaf because of transpiration / evaporation of water molecules through stomata

  2. Lowers water potential of mesophyll

  3. Water pulled up xylem creating tension;

  4. Water molecules cohere by hydrogen bonds;

  5. forming continuous water column;

  6. Adhesion of water molecules to walls of xylem;
• Describe the mass flow hypothesis for the mechanism of translocation in plants.

  1. In source sugars actively transported into phloem;

  2. By companion cells;

  3. Lowers water potential of sieve tube and water enters by osmosis;

  4. Increase in pressure causes mass movement towards sink

  5. Sugars used in root for respiration or used for storage
• Give ways in which the structure of starch is similar to cellulose.

  1. Are polysaccharides

  2. Contain glucose / carbon, hydrogen and oxygen;

  3. Contain glycosidic bonds;

  4. Have 1−4 links;

  5. Hydrogen bonding within structure.
• Give ways in which the structure of starch is different from cellulose.

  1. Contains alpha glucose;

  2. Helical /not straight;

  3. 1,6 bonds

  4. Monomers same way up;

  5. No H-bonds between molecules;

  6. No macrofibrils
• Organic compounds synthesised in the leaves of a plant can be transported to the plant's roots. This transport is called translocation and occurs in the phloem tissue of the plant.

  One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots.

  Describe how a high pressure is produced in the leaves.

  1. Water potential becomes lower / becomes more negative as sugar enters phloem;

  2. Water enters phloem by osmosis;

  3. Increased volume of water causes increased pressure.
• Contrast the processes of facilitated diffusion and active transport.

  1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins;

  2. Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP;

  3. Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient.
• Describe the mass flow hypothesis for the mechanism of translocation in plants.

  1. In source sugars actively transported into phloem;

  2. By companion cells;

  3. Lowers water potential of sieve cell / tube and water enters by osmosis;

  4. Increase in pressure causes mass movement towards sink;

  5. Sugars used / converted in sink for respiration or for storage.