Mass transport in plants

Question 1

Xylem

Answer 1

Tissue that transports water in the stem and leaves of plants

Question 2

The cohesion-tension theory of water transport in the xylem against gravity via the transpiration stream

Answer 2

Water evaporates from the leaves via the open stomata due to transpiration which reduces water potential in the cell and increases the water potential gradient.

Water is drawn out of the xylem, creating tension, and cohesive forces between water molecules pull water up as a column.

Water lost enters the roots via osmosis and it moves up, against gravity. Since water is cohesive, it sticks to the edges of the column

Question 3

Phloem

Answer 3

The tissue that transports organic substances in plants

Question 4

Translocation

Answer 4

The movement of solutes from source to sink/one place to another eg sugars from photosynthesis in the leaves are transported to the site of respiration

Question 5

Source

Answer 5

Photosynthesising cells

Question 6

Sink

Answer 6

Respiring cells

Question 7

The mass flow hypothesis for the mechanism of translocation in plants: at the source

Answer 7

At the source, there is a high concentration of solute. Active transport loads solutes from companion cells to sieve tubes of the phloem, lowering the water potential inside of the sieve tubes.

Water enters the sieve tubes by osmosis from the xylem and companion cells which increases pressure inside sieve tubes at the source end.

Question 8

The mass flow hypothesis for the mechanism of translocation in plants: at the sink

Answer 8

At the sink, there is a low concentration of solute as solutes are removed to be used up.

This increases the water potential inside the sieve tubes and so water leaves tubes via osmosis, lowering pressure inside sieve tubes

Question 9

The mass flow hypothesis for the mechanism of translocation in plants: mass flow

Answer 9

There is a pressure gradient from the source to the sink which pushes solutes from the source to the sink.

Solutes are then used or stored at the sink eg respiration

While mass flow is a passive process, it occurs as a result of the active transport of sugars, so the process as a whole is active.

Question 10

Adaptations of the phloem

Answer 10

Sieve tube elements have no nucleus and few organelles

Companion cell for each sieve tube element to carry out the living functions for the sieve cells ie for active transport of solutes

Question 11

The use of tracers and ringing experiments to investigate transport in plants

Answer 11

Supply plant with a radioactive tracer such as 14C in CO2 to a photosynthesising leaf by pumping the radioactive CO2 into a container surrounding the leaf

14C is incorporated into the organic substances produced by the leaf eg sugars via photosynthesis. Organic substances then undergo translocation

The plant is killed and placed in a photographic film, film turns black where the radioactive substance is present (autoradiography)

Identifies where radioactive substance has moved to and thus where the organic substances have moved to via translocation from source to sink

Can show this over time by taking autoradiographs at different times

Question 12

What does a potometer do

Answer 12

A potometer estimates the transpiration rate by measuring water uptake (assuming that water uptake is directly related to water loss in the leaves)

Question 13

Potometer method

Answer 13

Cut a shoot underwater to prevent air entering xylem and interrupt water flowing in column (transpiration would stop)

Assemble potometer with capillary tube end submerged in a beaker of water. Insert the shoot underwater.

Ensure apparatus is watertight and airtight. Dry leaves and allow time for the shoot to acclimatise

Shut off tap to reservoir and remove the end of the capillary tube from the water beaker until one air bubble has
formed, then put the tube back into the water.

Record the position of the air bubble. Use a stopwatch to record time e.g. one minute.

Record the distance moved per unit time. The rate of the air movement is an estimate of the transpiration rate

Change one variable at a time and keep all other variables constant (wind, humidity, light and temperature)

Question 14

Environmental factors that affect the rate of transpiration: light

Answer 14

The higher the light intensity, the greater the rate of transpiration (positive correlation)

The stomata open in light to let in CO2 for photosynthesis, which allows more water to evaporate faster.

Stomata close in the dark and the transpiration rate is slower

Question 15

Environmental factors that affect the rate of transpiration: temperature

Answer 15

The higher the temperature, the faster the transpiration rate (positive correlation)

Water molecules gain kinetic energy as temperature increases so they move faster and water evaporates faster

Question 16

Environmental factors that affect the rate of transpiration: humidity

Answer 16

The lower the humidity, the faster the transpiration rate (negative correlation)

As humidity increases, the water potential of the air increases as there are more water molecules. This decreases the water potential gradient from leaf to air and so water evaporates slower

Question 17

Environmental factors that affect the rate of transpiration: wind

Answer 17

The windier, the faster the transpiration rate (positive correlation)

Wind blows away water molecules from the stomata decreasing the water potential of the air around the stomata, increasing the water potential gradient. Water evaporates faster