Mass transport in plants

Where does gas exchange take place in plants?

The stomata

When are stomata open?

During the day

Which chemical reactions take place during the day in plants?

Photosynthesis and respiration

When are stomata closed?

During the night

Which chemical reactions take place during the night in plants?

Respiration only

What 3 adaptations do plants have for efficient gas exchange?

• numerous stomata for gas exchange

• air spaces in spongy mesophyll

• thin and broad so large surface area

What is transpiration?

The evaporation of water through the stomata down a water potential gradient

What are 4 ways you can increase the rate of transpiration?

• Increasing the light intensity

• Increasing temperature

• Increasing wind intensity

• Decreasing humidity

Why does increasing light intensity increase the rate of transpiration?

• It increases the number of open stomata as they are needed for gas exchange

• Water leaves the leaf through open stomata via osmosis

Why does increasing temperature increase the rate of transpiration?

Increases the kinetic energy of water molecules

Why does increasing wind intensity increase the rate of transpiration?

Decreases the water vapour around the leaf, increasing the water potential gradient

Why does decreasing humidity increase the rate of transpiration?

Decreases the water vapour around the leaf, increasing the water potential gradient

What does negative pressure mean?

A pressure lower than atmospheric pressure

Explain the tension part of the cohesion-tension theory of water transport in the xylem.

• The loss of water lowers the water potential in mesophyll cells

• This pulls water up a continuous column in the xylem

• This puts the xylem under tension- there is a negative pressure within it

Explain the cohesion part of the cohesion-tension theory of water transport in the xylem.

Water molecules adhere to xylem walls and each other due to hydrogen bonding

Which atoms are attracted to each other in hydrogen bonding in water molecules?

A partially positive hydrogen and a partially negative oxygen

What are xerophytic plants?

Plants that have evolved other adaptations to minimise water loss

What are 4 adaptations xerophytic plants have to minimise water loss?

• Hairs on leaves

• Rolled leaves

• Stomata in pits

• Thick waxy cuticle

How do hairs on leaves help xerophytic plants minimise water loss?

They trap water vapour in outside air, decreasing water potential gradient

How do rolled leaves help xerophytic plants minimise water loss?

They trap water vapour in outside air, decreasing water potential gradient

How do stomata in pits help xerophytic plants minimise water loss?

They trap water vapour in outside air, decreasing water potential gradient

How does a thick waxy cuticle help xerophytic plants minimise water loss?

It increases diffusion distance and reduces evaporation

What are potometers used for?

To measure a plant’s rate of water uptake

What is a plant’s rate of water uptake proportional to?

Its rate of transpiration

How do you use a potometer to measure a plant’s rate of water uptake?

The distance an air bubble moves in a given time

What are 2 limitations of using a potometer to measure a plant’s rate of transpiration?

• Calculated rate doesn’t account for water uptake in the roots (the plant’s roots are removed)

• Not all water is lost from the plant via transpiration

What is the word equation for photosynthesis?

carbon dioxide + water —> glucose + oxygen

What is sap?

Solution of dissolves sugars and amino acids found in plants

What is translocation?

The transport of dissolved sugars from a source to a sink in plants via the phloem

What are sources?

The parts of the plant where substances are produced

What are sinks?

The parts of the plant where substances are needed (they can’t produce these substances themselves)

In what direction does translocation take place?

It is bidirectional- any direction as long as it is from a source to a sink

Give an example of a part of the plant which can act as both a source and a sink.

The roots

Why can the roots be described as a source?

They absorb useful substances from the soil

Why can the roots be described as a sink?

The phloem transports substances to the roots for storage

What do plants transport carbohydrates in the form of?

Sucrose

Why do plants transport carbohydrates as sucrose?

• It’s more energy efficient to transport substances as a single molecule (rather than two molecules of glucose and fructose)

• Glucose is reactive, so may react with other substances in the phloem if transported on its own

Describe the structure of the phloem.

• Long, tube-like structure

• Made up of sieve tube elements arranged end to end

• Sieve plates with pores

• Companion cells sat between sieve tube elements and other plant cells

Which major organelle do sieve tube elements not contain?

A nucleus

Why do sieve tube elements not contain a nucleus?

To allow the maximum amount of space in the phloem for sucrose solution to pass through (increasing how much sucrose can be transported at a time)

Why are sieve tube elements still living cells, despite not containing a nucleus and other organelles?

Companion cells provide the sieve tube elements with energy

What are the two experiments that scientists used to discover the role of the phloem?

• Ringing experiments

• Tracer experiments

How is a ringing experiment carried out?

Remove a ring from a tree/plant containing the bark and phloem, leaving the xylem intact

What are the 3 things you observe in a ringing experiment?

• The plant swells directly above the ring

• Everything above the ring continues to survive and grow

• Parts of the plant below the ring begin to die

Why does the plant swell directly above the ring in a ringing experiment?

Substances that would otherwise be transported to the roots accumulate above the ring

What does the swell directly above the ring in a ringing experiment contain?

A high concentration of sucrose

Why do the roots die in a ringing experiment?

They are unable to perform photosynthesis, so must obtain substances from translocation

Why do the parts of the plant above the ring in a ringing experiment continue to survive and grow?

Leaves carry out photosynthesis, and the substances produced are transported to other places in the upper part of the plant

What happens in a tracer experiment?

Plants absorb a radioactive substance issued by a scientist, and it is identified in the plant using a special scanner

What name is given to the radioactive substance absorbed by the plant in a tracer experiment?

A tracer

Why would using radioactive carbon as a tracer allow you to see what happens in the phloem?

• The plant absorbs radioactive carbon as carbon dioxide

• The plant uses this carbon dioxide in photosynthesis to create radioactive sucrose

• This is then tracked as it travels through the phloem

Explain the Mass Flow Hypothesis for the movement of sucrose in a plant.

Due to a hydrostatic pressure gradient, there is a mass flow of sucrose from an area of high hydrostatic pressure to an area of low hydrostatic pressure

What is hydrostatic pressure?

The pressure exerted by a fluid

Which cells, in order, does sucrose travel through during translocation?

Source cell, companion cell, sieve tube element, companion cell, sink cell

What are the stages in the movement of sucrose from photosynthesising cells to the sieve tube element? (6 marks)

• Sucrose is produced in photosynthesising cells

• Protons are actively transported from the cytoplasm of the companion cell to the cell wall space using ATP

• This creates a steep proton gradient

• H+ ions, along with sucrose, move from the cell wall of the companion cell back to the cytoplasm of the companion cell via co-transport

• This is through carrier proteins known as co-transport proteins

• Sucrose then enters sieve tube elements via facilitated diffusion

What are the stages in the mass flow hypothesis for the movement of sucrose in the phloem from a source cell to a sink cell? (6 marks)

• Sucrose is co-transported with protons from the companion cell to the sieve tube element at the source cell

• This decreases the water potential in the sieve tube element at the source cell

• Water moves down a water potential gradient from the xylem to the sieve tube element via osmosis

• This increases the hydrostatic pressure in the sieve tube element at the source cell

• Sucrose solution moves down a hydrostatic pressure gradient to sink cell end via mass flow

• Sucrose is actively transported into the sink cell via the companion cell

Why can’t sucrose just cross the cell membrane from photosynthesising cells to companion cells?

It is large and polar

What are the 3 key features of the Mass Flow Hypothesis?

• Water moves in and out of the phloem by osmosis

• Pressure moves the sucrose solution

• It involves active processes that use ATP

What are four pieces of evidence which support the Mass Flow Hypothesis?

• If you cut a plant at the phloem, sap leaks out

• As rate of respiration increases, rate of translocation increases

• Sap near a source has a higher concentration of sucrose than sap near a sink

• Companion cells have lots of mitochondria

Explain why sap leaking out of the phloem supports the mass flow hypothesis.

It shows that sucrose is under pressure in the phloem and moves due to this pressure

Explain why translocation rate increasing with respiration rate supports the mass flow hypothesis.

• More ATP is produced from a higher rate of respiration

• If this increases the rate of translocation, it shows translocation involves active processes requiring ATP

Explain why sap having a higher concentration of sucrose near a source supports the mass flow hypothesis.

Shows that water moves in and out of the phloem by osmosis, as there are different water potentials between the source and sink

Explain why companion cells having lots of mitochondria supports the mass flow hypothesis.

Mitochondria produce ATP from aerobic respiration, shows that translocation involves active processes requiring ATP

What are three pieces of evidence against the mass flow hypothesis?

• Speed of substances

• Function of sieve plates

• Sucrose delivery

Explain the speed of substances piece of evidence against the mass flow hypothesis.

Experiments show that dissolved sucrose travels faster than dissolved amino acids, but you would expect the substances to all travel at the same speed under pressure

Explain the function of sieve plates piece of evidence against the mass flow hypothesis.

The presence of sieve plates hinders mass flow (even though they have holes), but you would expect the phloem to provide maximum space for the sucrose to travel through

Explain the sucrose delivery piece of evidence against the mass flow hypothesis.

Sucrose doesn’t always travel to areas of low concentration, so movements may not be entirely due to differences in water potential.