Transport in Plants

Lessons

Senior 5 · Biology

Transport in Plants

4 lessons

Lesson 4 of 4

Translocation, xerophytes and excretion in plants

25 min

What is an aphid drinking?

Look closely at the young shoots of a bean plant and you may find aphids: tiny insects that sit still for hours with their mouthparts pushed into the stem. The aphid's needle-like mouthpart, the stylet, goes straight into a single sieve tube. The sap inside is under pressure, so it is forced into the aphid, and so much arrives that the aphid passes out the excess as sticky "honeydew". Plant scientists have used this. They let an aphid feed, cut its body away from its stylet, and collect pure phloem sap from the stylet that is left in the stem. Such experiments show what the phloem carries, and that it is under pressure. This lesson explains how the pressure arises, and then looks at two other topics: how plants of dry places save water, and how plants get rid of their wastes.

Definition

Translocation: the movement of organic solutes, mainly sucrose and amino acids, through the phloem from one part of a plant to another.

Source: a part of the plant that releases sucrose into the phloem, such as a photosynthesising leaf, or a storage organ in a germinating seed. Sink: a part that removes sucrose from the phloem to use or store it, such as a growing root tip, a developing fruit or a cassava root.

Mass flow hypothesis (pressure flow hypothesis): the idea, put forward by Ernst Münch in 1930, that phloem sap flows from source to sink because of a difference in hydrostatic pressure between them.

Xerophyte: a plant adapted to live where water is scarce. Excretion: the removal of the waste products of metabolism.

Loading sucrose at the source

Sucrose is the form in which most plants carry sugar: it is soluble, but it is not a reducing sugar, so it does not react with other substances on the way. In many plants, companion cells load sucrose into the sieve tubes actively:

Proton pumps in the companion cell membrane use ATP to pump hydrogen ions (H⁺) out of the cell, creating a high concentration of H⁺ outside.
H⁺ ions diffuse back in through co-transporter proteins, and each carries a sucrose molecule in with it, against the sucrose concentration gradient.
The sucrose then diffuses from the companion cell into the sieve tube element through the plasmodesmata.

The mass flow hypothesis

At the source, loading sucrose into the sieve tube lowers its water potential. Water enters from the nearby xylem by osmosis, raising the hydrostatic pressure (the pressure potential) in the sieve tube.
At the sink, sucrose leaves the sieve tube and is used in respiration or converted to starch. The water potential of the sap rises, water leaves by osmosis, and the hydrostatic pressure falls.
The difference in pressure pushes the whole body of sap, with its dissolved solutes, along the sieve tube from source to sink, through the pores of the sieve plates.
The water returns to the xylem and is carried up again in the transpiration stream (Figure 7).

Water potential has two parts: $\psi = \psi_s + \psi_p$ , where $\psi_s$ is the solute potential (always zero or negative) and $\psi_p$ is the pressure potential, the hydrostatic pressure.

Worked example 1: the pressure difference that drives flow

The xylem sap next to both ends of a sieve tube has $\psi = -0.5\,\mathrm{MPa}$ . Phloem sap at the source has $\psi_s = -1.8\,\mathrm{MPa}$ ; at the sink, $\psi_s = -0.9\,\mathrm{MPa}$ . Assuming water moves until the phloem sap has the same water potential as the xylem, calculate the hydrostatic pressure at each end, and the difference.

Rearrange $\psi = \psi_s + \psi_p$ to $\psi_p = \psi - \psi_s$ .
Source: $\psi_p = -0.5 - (-1.8) = +1.3\,\mathrm{MPa}$ .
Sink: $\psi_p = -0.5 - (-0.9) = +0.4\,\mathrm{MPa}$ .
Difference: $1.3 - 0.4 = 0.9\,\mathrm{MPa}$ , from source to sink.

Interpretation: about nine times atmospheric pressure pushes the sap towards the sink. This is why phloem sap spurts from a cut aphid stylet.

Worked example 2: how fast is translocation?

A leaf of a bean plant is given carbon dioxide containing radioactive carbon-14 for 5 minutes. Radioactive sucrose is detected in the stem 1.2 m below the leaf 1.5 hours later. Calculate the mean rate of translocation, and compare it with diffusion.

$\text{rate} = \frac{\text{distance}}{\text{time}} = \frac{1.2\,\mathrm{m}}{1.5\,\mathrm{h}} = 0.80\,\mathrm{m\,h^{-1}}$
In lesson 1, diffusion of sucrose over 1.0 m was estimated to take about 30 years.

Conclusion: movement at nearly a metre an hour is far too fast for diffusion, so it must be a flow of solution: evidence for mass flow.

Weighing the evidence

Evidence that supports mass flow	Observations that it does not fully explain
Sap exudes from a cut aphid stylet under pressure, and faster nearer a source.	Sieve plates should slow the flow a great deal; why have them at all, if flow is purely passive?
Radioactive tracers move from leaves to sinks at up to about a metre per hour, far faster than diffusion.	Different solutes, such as sucrose and amino acids, can move at different speeds in the same phloem.
Removing a ring of bark (with the phloem) makes sugars build up above the ring, and the bark there swells.	Movement in two directions at once has been seen in the same vascular bundle, although probably in different sieve tubes.
Metabolic poisons that stop ATP production in the source stop translocation, as active loading predicts.	Companion cells along the whole path have many mitochondria, which suggests that energy is used along the way, not only at the ends.

Evaluation: mass flow explains the speed, the pressure and the source-to-sink direction, and is the accepted model. The objections show that it may not be the whole story; energy is also used along the path, for example to reload sucrose that leaks out of the sieve tubes.

Cognito: "A-Level Biology - Translocation & Mass Flow Hypothesis". Watch for how proton pumps and co-transporters load sucrose, and for the pressure difference between source and sink; compare it with Figure 7.

Xerophytes: saving water in dry places

Plants of deserts, dry savanna and windy coasts face the same problem: water is lost faster than it can be replaced. Xerophytes reduce transpiration, store water, or collect more of it.

Adaptation	How it saves water	Example
Thick waxy cuticle	Reduces cuticular transpiration	Aloe, prickly pear (Opuntia)
Sunken stomata, in pits	Moist air is trapped above the stoma, reducing the water vapour gradient	Pines, oleander
Hairs on the leaf surface	Trap a layer of still, humid air	Marram grass, inside its rolled leaf
Rolled leaves, stomata on the inside	Encloses humid air around the stomata	Marram grass
Leaves reduced to spines or tiny scales	Much smaller surface area for transpiration; the green stem photosynthesises	Cacti, candelabra tree (Euphorbia)
Succulent tissue	Stores water in stems or leaves for dry periods	Aloe, cacti
CAM photosynthesis	Stomata open at night, when it is cool and humid, and close by day	Pineapple, cacti
Deep or wide-spreading roots	Reach deep water or catch light rain quickly	Acacia trees of the savanna
Shedding leaves in the dry season	Removes the main transpiring surface	Many savanna trees

In Rwanda the driest areas are in the east, where savanna trees and succulent euphorbias grow. Crops matter too: pineapple, grown in several districts, uses CAM, which you met in Unit 6.

Excretion in plants

Plants make waste products, but they have no special excretory organs like kidneys. There are three reasons: their metabolic rate is much lower than an animal's, so wastes build up slowly; many wastes are recycled, such as the carbon dioxide from respiration, used in photosynthesis, and the oxygen from photosynthesis, used in respiration; and plants make little nitrogenous waste, because they build only the proteins they need.

Gases: excess oxygen (by day) and carbon dioxide (at night) diffuse out through stomata and lenticels.
Water: lost by transpiration and guttation.
Stored wastes: many wastes are made harmless and stored in vacuoles or in dead tissues. Examples are tannins in bark and in tea leaves, calcium oxalate crystals, resins in pines, latex in rubber trees and euphorbias, and alkaloids such as caffeine in coffee and tea and nicotine in tobacco.
Removal: wastes stored in leaves, bark and fruits are lost when these fall or peel off.

Many of these "wastes" also protect the plant, because they taste bitter or are toxic to herbivores, and people use several of them: rubber from latex, gum arabic from acacia, and caffeine from Rwanda's coffee and tea.

Warning

Common mistakes and safety

"Sucrose moves by diffusion in the phloem." It moves by mass flow of the whole solution; diffusion would be far too slow.
"Sinks are always below sources." A sink is any region that uses or stores sucrose: a fruit or shoot tip above the leaves is a sink too.
"Xerophytes have no stomata." They have stomata, often fewer and sunken, because they still need CO₂.
Safety: the milky latex of euphorbias is irritant and can harm the eyes and skin. Do not break their stems with bare hands.

Tip

Exam technique for mass flow: tell it as a chain: active loading at the source → water potential falls → water enters by osmosis → hydrostatic pressure rises → sap flows to the sink, where sucrose is removed and pressure is lower. Examiners award marks for each link, and for naming osmosis and hydrostatic pressure.

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Practice: translocation, xerophytes and excretion

Define the terms source and sink, giving one example of each.
Explain why plants transport sugar as sucrose rather than glucose.
Describe the role of H⁺ ions in loading sucrose into the phloem.
Radioactive sucrose travels 45 cm in 36 minutes. Calculate the rate in m h⁻¹.
At a source, $\psi = -0.7\,\mathrm{MPa}$ and $\psi_s = -2.1\,\mathrm{MPa}$ . Calculate $\psi_p$ .
Explain how sunken stomata and rolled leaves reduce water loss.
Give three reasons why plants do not need special excretory organs.
A potato tuber is a sink in its first season but a source when it sprouts. Explain this change.

Show answersHide answers

A source releases sucrose into the phloem, for example a photosynthesising leaf; a sink removes it to use or store, for example a growing root tip, a fruit or a tuber.
Sucrose is soluble but is not a reducing sugar, so it is less reactive and is not used up or changed on the way; it also has less effect on water potential per unit of sugar carried than two glucose molecules would.
Proton pumps use ATP to pump H⁺ out of the companion cell; H⁺ diffuses back in through co-transporter proteins, carrying sucrose in with it against its concentration gradient.
$\frac{0.45\,\mathrm{m}}{0.60\,\mathrm{h}} = 0.75\,\mathrm{m\,h^{-1}}$ .
$\psi_p = \psi - \psi_s = -0.7 - (-2.1) = +1.4\,\mathrm{MPa}$ .
Both trap still, moist air next to the stomata, which lowers the water vapour potential gradient between the leaf and the air, so water vapour diffuses out more slowly.
Low metabolic rate, so wastes build up slowly; gases are recycled between respiration and photosynthesis; little nitrogenous waste is made; wastes are stored in harmless forms and lost with leaves or bark.
While growing, the tuber receives sucrose and stores it as starch (a sink). When it sprouts, the starch is broken down to sucrose, which is loaded into the phloem and sent to the growing shoots (a source).

Key points

Translocation moves sucrose and amino acids in the phloem from sources to sinks, at up to about a metre an hour.
Mass flow: active loading at the source → water enters by osmosis → high hydrostatic pressure → flow to the sink, where sucrose is removed.
Xerophytes cut water loss with thick cuticles, sunken stomata, hairs, rolled or reduced leaves, succulence and CAM.
Plants excrete gases through stomata and lenticels, and store other wastes harmlessly, losing them with leaves and bark.

1What is an aphid drinking?

2Loading sucrose at the source

3The mass flow hypothesis

4Weighing the evidence

5Xerophytes: saving water in dry places

6Excretion in plants