Title: InkBiology / Plant transport and mineral nutrition
On every note of this deck, unless a note says otherwise:
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Front: DEFINE
Define: Apoplast pathway.
Back: Movement of water through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell.

Front: DEFINE
Define: Translocation.
Back: The transport of assimilates, mainly sucrose, through the phloem from a source to a sink.

Front: CHECK YOURSELF
A student measures water uptake by a leafy shoot in a potometer. She records 2.4 mm min⁻¹ of bubble travel in still air at 20 °C, and 0.9 mm min⁻¹ after sealing a clear plastic bag loosely around the shoot. She concludes that the bag 'stopped the plant transpiring'. Evaluate her conclusion and explain the result.
Back: The conclusion is wrong on two counts. The rate fell to 0.9 mm min⁻¹, which is a reduction of 62.5%, not a stop. The shoot went on taking up water throughout.
And the potometer was measuring uptake, not transpiration, so no reading it produces can be described directly as a transpiration rate.
The explanation is the gradient. Water vapour leaving the leaves accumulated inside the bag, so the humidity of the air around the shoot rose and its water potential rose with it, towards the water potential of the saturated air inside the leaf. A shallower gradient across the stomata means slower diffusion out, so less water is lost and less is drawn in to replace it.
It does not fall to zero because the air in the bag never quite reaches saturation, and even air at 95% relative humidity sits far below the leaf in water potential.
A fair criticism of the method: the bag also cut the light reaching the leaves slightly and raised the temperature inside it, so humidity was not the only variable that changed. A better design would humidify the air around an unbagged shoot instead.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Water is pushed up the stem by root pressure.
Back: Root pressure reaches perhaps 100 to 200 kPa, enough for ten or twenty metres, and it disappears if the roots are chilled. Xylem sap in a tall tree at midday is under tension, not pressure: the column is pulled from the leaf, not pushed from the root.

Front: CHECK YOURSELF
A plant is supplied with carbon dioxide containing the carbon-14 isotope for one hour in bright light. Sections cut from the stem four hours later are placed against photographic film. The film darkens over the phloem of the vascular bundles and not over the xylem. In a second plant treated identically, a ring of bark had been removed from the stem an hour before the carbon dioxide was supplied; in that plant the film darkens above the ring but not below it. Explain both results.
Back: The labelled carbon dioxide was fixed in photosynthesis, so the carbon-14 ended up in sugars made in the leaves. Those leaves are sources, and the sugar is loaded into the sieve tubes as sucrose.
The film darkens where the radioactive isotope is, so darkening over the phloem and not the xylem shows that the labelled sucrose travelled in the phloem. This is what separates the two tissues experimentally rather than by assertion.
In the ringed plant, removing the bark removed the phloem while leaving the xylem intact. Sucrose loaded above the ring cannot pass the gap, so it accumulates in the phloem above it and the film darkens there.
Below the ring there is no continuous sieve tube from the leaves, so no labelled sucrose arrives and the film stays clear. The xylem below the ring is undamaged and still carrying water upwards, which is why the plant does not wilt immediately even though its roots are being starved.
One caution if you are asked to evaluate. This shows the route sucrose takes and shows that it needs intact phloem, and it says nothing at all about what drives the movement, because mass flow, or anything else, would give the same autoradiograph.

Front: DEFINE
Define: Stoma.
Back: A pore in the epidermis, bounded by two guard cells, through which gases and water vapour pass.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Transpiration is how a plant transports water and minerals to its leaves, so the pull is the point of it.
Back: It is a cost, not a purpose. A plant loses 95 to 99 per cent of the water it takes up, and no system evolves whose main output is losing nearly everything it collects. Stomata open for carbon dioxide and water leaves through the same pores.

Front: DEFINE
Define: Transpiration.
Back: The loss of water vapour from the aerial parts of a plant, mainly through the stomata, by evaporation from the mesophyll cell walls followed by diffusion out of the leaf.

Front: DEFINE
Define: Sieve plate.
Back: The perforated end wall between two sieve tube elements, through which the sap passes.

Front: DEFINE
Define: Sieve tube element.
Back: A living cell of the phloem, joined end to end with others into a tube; at maturity it has no nucleus, few organelles and only a thin layer of cytoplasm around the edge.

Front: DEFINE
Define: Companion cell.
Back: A cell alongside each sieve tube element with dense cytoplasm, a nucleus and many mitochondria, connected to it by numerous plasmodesmata and responsible for loading it and keeping it alive.

Front: CHECK YOURSELF
A gardener waters a wilted potted plant thoroughly and it recovers within an hour. A second wilted plant, watered identically, does not recover, and when the pot is tipped out the compost is waterlogged and smells stale. Explain both results in terms of water movement into and through the root.
Back: In the first plant, watering raises the water potential of the soil solution towards zero, so it is again higher than that inside the root hair cells. Water enters by osmosis, crosses the cortex by the apoplast and symplast routes, passes the endodermis into the xylem, and is carried up by the transpiration pull. The mesophyll cells regain turgor and the leaves stiffen.
In the second plant the compost is waterlogged, so the air spaces between the particles are full of water and the roots are short of oxygen. Aerobic respiration falls, less ATP is available, and the active transport of mineral ions into the root hair cells slows.
With less solute inside, the root cells' water potential rises towards the soil's, the gradient collapses, and uptake slows even though the plant is standing in water. Water uptake is passive, but it depends on an active process to keep the gradient in place, which is why over-watering kills as reliably as drought.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Sugars are actively transported along the phloem from the leaves to the roots.
Back: Active transport happens at the ends, loading at the source and unloading at the sink. A mature sieve tube element has no nucleus and few organelles and could not power transport along a metre of stem. In between it is bulk flow down a pressure gradient.

Front: DEFINE
Define: Root hair cell.
Back: An epidermal cell of the root extended into a long narrow projection, giving a large surface area for the absorption of water and mineral ions.

Front: DEFINE
Define: Symplast pathway.
Back: Movement of water through the cytoplasm of the cells, passing from one to the next through plasmodesmata.

Front: DEFINE
Define: Transpiration stream.
Back: The continuous movement of water from the soil, through the root and up the xylem, to replace the water lost from the leaves.

Front: DEFINE
Define: Casparian strip.
Back: A band of suberin in the walls of the endodermal cells that is impermeable to water, so the apoplast route is blocked and water must cross a plasma membrane.