Mass transport in plants Flashcards – AQA A-Level Biology - (7402)

In what form does xylem transport water through the stem to the leaves?

As continuous columns in hollow, lignified vessels.

How does transpiration reduce water potential in a leaf?

Water evaporates from mesophyll cell surfaces and is lost through stomata.

How is tension created in the xylem of a leaf?

Water is drawn from the xylem to replace water lost from the leaf, creating negative pressure.

How does cohesion enable tension to pull water up the xylem?

Hydrogen bonds hold water molecules together, so tension pulls up a continuous column.

What role does adhesion play in water transport through xylem?

Adhesion between water and vessel walls helps maintain the water column.

How does the pull generated in leaves draw water into the xylem from the roots?

The pull is transmitted down the continuous water column through the plant.

According to cohesion-tension theory, what drives water movement through xylem?

Transpiration, rather than pumping by the xylem vessels.

What does phloem translocate between sources and sinks?

Organic substances, particularly sucrose.

What is the distinction between a source and a sink in phloem transport?

A source supplies substances; a sink uses or stores them.

How does the mass flow hypothesis explain movement through sieve tubes?

Phloem sap moves in bulk flow from source to sink down a hydrostatic pressure gradient.

How does sucrose loading at a source increase pressure in sieve tubes?

Loading lowers water potential, so water enters from the xylem by osmosis, increasing hydrostatic pressure.

How does sucrose unloading at a sink lower pressure in the phloem?

Unloading raises water potential, so water can leave the phloem, lowering hydrostatic pressure.

Which steps in mass flow can require metabolic energy, and what drives the bulk-flow step?

Sucrose loading and unloading can require metabolic energy; bulk flow is driven by the pressure gradient.

What two observations support the mass flow hypothesis?

Pressure differences between sources and sinks, and movement of labelled organic substances through phloem.

What is a limitation of evidence supporting the mass flow hypothesis?

It does not by itself establish every detail of the mechanism.

What observation about the rates of transported substances must be evaluated against the mass flow hypothesis?

Substances can move at different apparent rates.

Why does metabolic inhibition of phloem transport not necessarily show that bulk flow itself requires metabolic energy?

The metabolic dependence may reflect loading or unloading, rather than bulk flow.

What is a tracer in a plant transport experiment?

A detectable label incorporated into a substance so its movement through the plant can be followed.

What can be used to label organic substances in plant tracer experiments?

Radioactive isotopes.

What technique can detect the distribution of a radioactive label in plant tissues?

Autoradiography.

What can observations at different times reveal in a plant tracer experiment?

Where labelled material moves and which tissues are associated with its transport.

What tracer observation provides evidence of translocation?

A label appearing in phloem and moving from a source towards a sink.

Why can tracer results alone not prove the physical mechanism driving transport?

They show a correlation between the label and particular tissues or locations, not the mechanism driving movement.

What should be considered when interpreting evidence from a plant tracer experiment?

What was labelled, where and when it was detected, and whether the evidence distinguishes phloem transport from other explanations.

How can tracer results contribute to evaluating the mass flow hypothesis?

They can be assessed alongside other evidence.

How is a ringing experiment performed on a plant stem?

A ring of bark containing phloem is removed, while the xylem is left largely intact.

What can happen above the ring in a ringing experiment?

Organic substances can accumulate, often causing swelling.

How can ringing affect the supply of organic substances to tissues below the ring?

Those tissues may receive less of them.

What does the accumulation of organic substances above a ring suggest about phloem?

It supports the conclusion that phloem transports organic substances.

Why must changes observed after ringing be attributed to a cause cautiously?

Removing the ring damages tissues and can have effects beyond interrupting phloem.

What transport mechanism is not proved by a ringing experiment alone?

Pressure-driven mass flow.