Mass transport in plants

What is transpiration?

The loss of water vapor through the stomata of leaves by evaporation

Factors that affect the rate of transpiration

1. Light intensity

- causes more stomata to open

- therefore larger SA for evaporation

2. Temperature

- more heat = more kinetic energy

- so molecules move faster

- therefore more evaporation

3. Humidity

- more water vapour in the air will make the water potential outside the leaf more positive

- therefore reduces the water potential gradient

4. Wind

- more wind will blow away humid air containing water vapour

- therefore marinating water potential gradient

Explain how the structure of xylem tissue is adapted to its function

1. Long tubes w no end walls

- continuous water columns

2. No organelles to obstruct flow

- allows easier water flow

3. Has lignin

- support

- waterproof

4. Pits in walls

- allows lateral movement

Describe the cohesion-tension theory of water transport in the xylem.

1. Evaporation of water molecules through the stomata by transpiration, which lowers the WP of leaf cells

2. Water molecules "stick" together by H-bonds, forming continuous water column

   • these H-bonds maintains column

3. Water pulled up the xylem, creating tension

4. Adhesion of water molecules to walls of xylem

Describe how a high pressure is produced in the leaves.

1. sucrose actively transported in the phloem

2. WP becomes lower as sugar enters phloem

3. Water enters phloem by osmosis

4. Increased volume of water causes increased pressure

What is adhesion in water?

the sticking of water molecules to the xylem wall

- causes capillarity

Explain 2 ways in which companion cells are adapted for the transport of sugars between cells

1. Mitochondria are present to release energy for active transport

2. Ribosomes make proteins which are needed for carrier proteins/enzymes

State and explain the ways in which sieve cells are adapted for mass transport

1. No/few organelles

2. Little cytoplasm

3. Hollow

4. Large vacuole

5. Thick walls

explanation for all:

• easier flow

• stronger resistance to pressure

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

translocation is how organic materials are transported around the plant

1. In source, sugars are actively transported in the phloem by companion cell

   • this lowers the WP of sieve tube & water enters by osmosis.

2. Increase in pressure causes mass movement towards the sink

3. Sugars used in root for respiration for storage

Give evidence for the mass flow hypothesis of translocation.

1. Cutting the stem of a plant results in phloem sap being released

   • therefore there must be hydrostatic pressure in the sieve tubes.

2. There is a higher sucrose concentration in the leaves than the roots.

Give evidence against the mass flow hypothesis of translocation.

1. The structure of sieve tubes seems to hinder mass flow

2. Not all solutes move at the same speed, as you would expect in mass flow.

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

• Plants are grown in the presence of radioactive CO2, which will be incorporated into the plant's sugars

- Can determine which tissue carries the radioactively labelled sucrose by:

1. take thin horizontal sections of plant tissue

2. Place against photographic film in dark for several hours and carry out autoradiography

   • we can see that the areas exposed to radiation correspond to where the phloem is.

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

1. 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 and the tissue below begins to die.

- Therefore sucrose must be transported in the phloem.

A student wanted to determine the rate of water loss per mm2 of surface area of the leaves of the shoot.

Outline a method she could have used to find this rate. You should assume that all water loss from the shoot is from the leaves.

1. draw around each leaf on graph paper and count the squares on both sides of the leaf

2. divide the rate of water loss by the total surface area of the leaf

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.

Suggest one reason why.

plants have roots