Mass Transport in plants

Question 1

Explain the cohesion tension theory.

What creates a ticking affect
Therefore..

Answer 1

• Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion).
• The surface tension of the water also creates this ticking affect. Therefore as water is lost through transpiration, more can be drawn up the stem.

Question 2

What are the three components of phloem vessels?

Answer 2

• Sieve tube elements
• Companion cells
• Plasmodesmata

Question 3

Name the process whereby organic materials are transported around the plant.

Answer 3

Translocation

Question 4

How does sucrose in the leaf move into the phloem?

Answer 4

Sucrose enters companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions.

Sucrose sieve tube elements through the plasmodesmata.

Question 5

How do phloem vessels transport sucrose around the plant?

Answer 5

As sucrose moves into the tube elements, water potential inside the phloem is reduced. This causes hydrostatic pressure. Water moves along the sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed.

Question 6

Give evidence for the mass flow hypothesis of translocation.

3

Answer 6

• Sap is released when a stem is cut, therefore there must be pressure in the phloem.
• There is a higher sucrose conc in the leaves than the roots
• Increasing sucrose levels in the leaves results in increased sucrose in the phloem

Question 7

Give evidence against the mass flow hypothesis of translocation.

3

Answer 7

• The structure of sieve tubes seems to hinder mass flow.
• Not all solutes move at the same speed, as they would in mass flow.
• Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose conc first.

Question 8

How can ringing experiments be used to investigate transport in plants?

Answer 8

The bark and phloem of a tree are removed in a ring, leaving behind the xylem. Eventually the tissues above the missing ring swells due to accumulation of sucrose as the tissue below begins to die. Therefore sucrose must be transported in the phloem.

Question 9

How can tracing experiments to be used to investigate transport in plants?

Answer 9

Plants are grown in the presence of radioactive CO2, which will be incorporated into the plants sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is.

Question 10

Xylem

Answer 10

The tissue that transports water in the stem and leaves of plants.

Question 11

Features of xylem vessels

Answer 11

No end walls
Walls lined with lignin
Vessel elementd

Question 12

Steps Involved in Water Transport in the Xylem

Answer 12

1. Water is evaporated from leaves through transpiration
2. Loss of water leaves the cell and causes tension in the xylem
3. Water is pulled up the xylem by tension
4. Water molecules also form hydrogen bonds with each other via cohesion
5. Water is pulled up the xylem by cohesion
6. Water diffuses through the roots to replace the water that is lost from the bottom of the stem

Question 13

Phloem

Answer 13

A type of tissue found in plants used to transport organic substances from where
they are made to where they are needed.

Question 14

Structure of Phloem:

Answer 14

Sieve tube elements
Companion cells
Sieve plates

Question 15

Translocation

Answer 15

the process where sugars produced in photosynthesis are transported from the leaves to other parts of a plant.

Question 16

During translocation, what type of cell is sucrose co-transported into?

Answer 16

Companion cells

Question 17

Sieve tube elements

Answer 17

Forms a tube to transport sucrose in the dissolved form sap

Question 18

Companion cells

Answer 18

Involved in the production of ATP for active loading of sucrose into sieve tubes

Question 19

Plasmodesmata

Answer 19

Gaps between cell walls where the cytoplasm links allowing substance to flow