Sap Transport
The Tule Tree, a Montezuma cypress (ahuehuete) in Santa María del Tule, Oaxaca, is about 40 meters tall. It lifts water from its roots to its crown without any heart to pump it. It does this with pure physics: the cohesion of water molecules and evaporation in the leaves. Sugar transport, on the other hand, does use energy. Two systems, two strategies.
Xylem and phloem: two saps, two directions
If you peel off just a thin strip of bark from a branch, a thick, sugary substance oozes out. In contrast, the liquid rising through the wood is almost pure water. Why do two such different fluids travel through separate pathways?
The two tissues are made of different cells, and the difference is related to their function:
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Xylem: tracheids and vessel elements. These are dead, hollow cells with thickened walls and no membrane or cytoplasm, so water passes through them like a pipe.
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Phloem: sieve tubes and companion cells. The sieve tubes are alive (without a nucleus) and connect to each other through sieve plates. The companion cells maintain them and help them load sugars.
In a diagram of the stem, the key to telling the vessels apart is the direction of the arrows. The vessel with an arrow pointing up (from the root to the leaves) is the xylem; the one with an arrow pointing down, or from the leaf toward an organ that consumes, is the phloem.
Differentiate between raw sap (xylem sap) and elaborated sap (phloem sap): what each transports, which vessel it travels through, and in which direction it moves.
Differentiate between raw sap (xylem sap) and elaborated sap (phloem sap): what each transports, which vessel it travels through, and in which direction it moves.
Indicate which cellular elements form the xylem (tracheids and vessel elements) and the phloem (sieve tubes and companion cells), and state a structural difference…
Indicate which cellular elements form the xylem (tracheids and vessel elements) and the phloem (sieve tubes and companion cells), and state a structural difference between them related to their function.
The diagram shows the two vascular tissues in the stem of a vascular plant. a) Indicate which number corresponds to xylem and which to phloem, justifying your answer by…
The diagram shows the two vascular tissues in the stem of a vascular plant. a) Indicate which number corresponds to xylem and which to phloem, justifying your answer by the direction of the arrows. b) Name the sap transported by each vessel and its composition. c) Explain which force drives the ascent through vessel 1.
The rise of xylem sap: cohesion and tension
What pulls the water up in a 40-meter tree if there is no pump? The answer is in the leaves.
Think of a drinking straw: when you suck at the top, the liquid rises because it is held together. Here the "suction" is provided by evaporation in the leaves. The steps are:
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Water evaporates in the leaves and exits through the stomata.
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That loss pulls water from the xylem in the leaf: a tension is created.
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Because of cohesion (hydrogen bonds), neighboring molecules follow the ones that rise.
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Because of adhesion to the xylem walls, the column does not break away or fall.
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The tension is transmitted all the way to the root, which absorbs more water from the soil.
Explain the cohesion-tension theory of xylem sap ascent: what role leaf transpiration, cohesion between water molecules, and adhesion to xylem walls play, and why water…
Explain the cohesion-tension theory of xylem sap ascent: what role leaf transpiration, cohesion between water molecules, and adhesion to xylem walls play, and why water can rise tens of meters without a pump.
Transpiration and the role of stomata
If you tie a transparent plastic bag around a lemon tree branch with leaves in the sun, in less than an hour its inner walls are covered with condensed water. Where does that water come from, and why does the plant benefit from losing it?
This explains what happens on a very hot, dry day, like in the north of the country in summer. If the plant loses too much water, it closes its stomata to save it. That slows transpiration, but it also blocks the entry of CO₂, and photosynthesis drops even if there is plenty of light.
Where does soil water enter the plant, and through which structures is most of it lost to the atmosphere? Name the process of water loss and explain its importance.
Where does soil water enter the plant, and through which structures is most of it lost to the atmosphere? Name the process of water loss and explain its importance.
During a very hot and dry summer day (maximum temperature of $38\ \text{°C}$ at $14{:}00\ \text{h}$), the following parameters are monitored in a well-illuminated crop…
During a very hot and dry summer day (maximum temperature of $38\ \text{°C}$ at $14{:}00\ \text{h}$), the following parameters are monitored in a well-illuminated crop plant:
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From $08{:}00$ to $11{:}00\ \text{h}$: transpiration and net $\text{CO}_2$ assimilation increase in parallel.
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From $12{:}00$ to $16{:}00\ \text{h}$: transpiration drops drastically by $70\ \%$ and net assimilation is reduced almost to zero, despite light intensity being at its peak.
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From $17{:}30\ \text{h}$ onwards: a slight recovery of both parameters is observed before dusk.
- a) Interpret the data by explaining what osmotic and ionic change guard cells undergo at midday and what hormonal signal triggers this process.
- b) Justify why stomatal closure, although vital to avoid desiccation, causes glucose synthesis to collapse despite abundant solar radiation, and predict what will happen to leaf temperature during those central hours of the day when transpiration is interrupted.
The transport of phloem sap: the Münch hypothesis
In a mango tree, if a complete ring of bark is removed, the area just above the cut swells with sugars and the roots below end up starving. How does the plant drive its nutrients from the leaves to distant organs?
What happens if a branch is girdled
Girdling removes the phloem (which is in the bark) but leaves the xylem (which is in the wood). It is used in fruit trees such as the mango to obtain larger fruits. If it is done on a branch, the sugars from its leaves cannot go down: they accumulate and the fruits on that branch, which are sinks, receive more. On the main trunk the result is different, and it unfolds as a chain of events:
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The sugars from the whole crown become trapped above the ring.
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The root, without sugars, cannot respire or grow, and it starves.
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Without a living root there is no absorption of water or salts, and the xylem stops being replenished.
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The leaves dry out and the tree dies. That the xylem is intact is not enough: without a living root there is nothing to lift.
Describe the pressure flow mechanism (Münch hypothesis) of phloem sap transport, explaining what happens at the 'source' (leaf) and at the 'sink' (root, fruit) and why…
Describe the pressure flow mechanism (Münch hypothesis) of phloem sap transport, explaining what happens at the 'source' (leaf) and at the 'sink' (root, fruit) and why this transport requires energy.
In horticulture, girdling (removal of a ring of bark) is sometimes practiced on a specific branch to obtain larger fruits, but if it is performed at the base of the main…
In horticulture, girdling (removal of a ring of bark) is sometimes practiced on a specific branch to obtain larger fruits, but if it is performed at the base of the main trunk, the tree eventually dies.
- a) Explain, according to the pressure-flow model (source-sink), why the fruits on the girdled branch increase in size and sugar content.
- b) A worker states: “The tree girdled at the trunk should not die, since the xylem remains intact and the leaves can still receive water and continue photosynthesizing normally.” Refute this argument by predicting the temporal sequence of physiological failures in roots and canopy that lead to the tree's death.
From the stem to the ecosystem: energy and matter
The sugars that travel through the phloem are the starting point for everything else: from them comes the energy of herbivores, carnivores, and decomposers. Ecosystems are sustained by a continuous input of solar energy and a constant recycling of matter.
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Energy flows in only one direction. The Sun provides the energy; plants capture it in photosynthesis. At each trophic level only a small fraction passes on to the next, on the order of 10% (a typical value, not an exact law). The rest is dissipated as heat in cellular respiration or remains in unconsumed matter. That is why there are fewer organisms at the higher levels.
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Matter circulates. Carbon atoms pass from atmospheric CO₂ into sugars, through the food webs, and return to the atmosphere through respiration and decomposition. Decomposers return to the soil the mineral salts that the root will absorb again, and the cycle closes.
With this you can predict the consequences of a disturbance. For example, if the trees (the producers) of an area are cut down, the energy entering the system drops and everything that depends on it is reduced. And if a lot of vegetation is burned, the carbon that was stored is released all at once.
Imagine that an air microbubble occurs in the xylem of a tree in summer and, at the same time, a gaseous microembolism occurs in an artery of a mammal. a) Explain why…
Imagine that an air microbubble occurs in the xylem of a tree in summer and, at the same time, a gaseous microembolism occurs in an artery of a mammal.
- a) Explain why the biophysical mechanism of crude sap transport (cohesion-tension theory) makes the xylem vulnerable to cavitation, and predict the immediate hydraulic consequences of the rupture of the water column in that vessel.
- b) Compare both transport systems by contrasting: the origin of the driving energy (solar/water potential gradient vs. metabolic/myocardial ATP), the nature of the conduits (lignified dead cells vs. living endothelium), and why a plant could not maintain its energy balance if it had to actively pump crude sap via active cellular transport.
Summary
Go back to the beginning: the 40-meter ahuehuete lifts water because its leaves transpire and the water column, held together by cohesion, is pulled up without spending energy. Its sugars, on the other hand, are distributed with energy and pressure through the phloem.