Biology: Translocation of Solutes

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Questions and Answers

What is the direction of water movement in the root hairs?

From the vacuole to the soil water

From the soil water into the vacuole (correct)

From the root hairs to the cortex

From the cortex to the root hairs

How many possible pathways can water and dissolved minerals follow after entering the plant through the root hairs?

Four

One

Three (correct)

Two

What is the term for the pathway of water and dissolved minerals movement along and between cell walls, through intercellular spaces?

Symplast

Vacuolar

Apoplast (correct)

Plasmodesmata

What is the mechanism by which water uptake occurs in the parenchyma cells of the cortex?

Osmosis Signup and view all the answers

What is the term for the pathway of water and dissolved minerals movement through the sap vacuoles, from one vacuole to the next?

Vacuolar Signup and view all the answers

What are the structures that connect cells and allow for the Symplast pathway?

Plasmodesmata Signup and view all the answers

What is the direction of ion movement in the cytoplasm of the root cortex?

Along the water potential gradient Signup and view all the answers

Where does water uptake primarily occur in the plant?

In the parenchyma cells of the cortex Signup and view all the answers

What is the term for the passive movement of solutes in plants?

Mass flow Signup and view all the answers

In the mass flow model, what is the role of turgor pressure?

To push solutes to the sink Signup and view all the answers

What happens to the water potential at the sink when sucrose is removed?

It increases Signup and view all the answers

What is the limitation of the mass flow hypothesis?

It does not explain bidirectional movement of solutes Signup and view all the answers

What is the alternative mechanism of solute transportation proposed to overcome the limitations of the mass flow hypothesis?

Cytoplasmic streaming Signup and view all the answers

What is the role of protein filaments in the sieve tubes in the mechanism of cytoplasmic streaming?

To transport solutes along the sieve tubes Signup and view all the answers

Where does the energy for the process of phloem transportation come from?

Both sieve tubes and companion cells Signup and view all the answers

What is the direction of solute movement in the phloem?

From source to sink Signup and view all the answers

What is the purpose of using a refractometer in measuring water potential?

To measure the change in refractive index of the solution Signup and view all the answers

What is the characteristic of an isotonic solution?

It causes no change in refractive index Signup and view all the answers

What is the purpose of preparing a graded series of sucrose solutions?

To find the isotonic solution for the tissue Signup and view all the answers

How long are the plant tissues allowed to equilibrate in the solution?

1 to 1½ hours Signup and view all the answers

What is plotted against the molarities of sucrose solutions?

The ratio of initial/final refractometer readings Signup and view all the answers

Who developed the pressure bomb technique for estimating xylem water tension?

Scholander et al. Signup and view all the answers

What is the purpose of using compressed air in the pressure bomb technique?

To gradually increase the pressure in the chamber Signup and view all the answers

What is the main difference between the walls of the guard cells that border the stoma pore and the thinner outer walls?

The inner walls are thicker and less elastic than the outer walls. Signup and view all the answers

What is the limitation of the method using change in refractive index?

It has the same limitations as the gravimetric method Signup and view all the answers

What is the characteristic of the connection between the subsidiary cells and the guard cells?

There are no symplastic connections between the subsidiary cells and the guard cells. Signup and view all the answers

What is the main function of the changes in turgor in the guard cells?

To regulate the opening and closing of the stomata. Signup and view all the answers

What is the main ion involved in the active transport of ions into the guard cells?

Potassium (K+) Signup and view all the answers

What is the energy source used to power the active pumping of protons out of the guard cells?

ATP (Adenosine triphosphate) Signup and view all the answers

What is the result of the active transport of ions into the guard cells?

An increase in the solute potential of the guard cells. Signup and view all the answers

What is the role of the electrochemical gradient in the guard cells?

It allows K+ to flow passively into the guard cells. Signup and view all the answers

What is the orientation of the cellulose microfibrils in the guard cells that allows them to expand outward?

Radial orientation. Signup and view all the answers

What is the main reason for the development of an electrical potential gradient between the two solutions?

One solution has more of a certain type of ion than the other. Signup and view all the answers

What determines the passage of ions across the membrane?

The size of the ions. Signup and view all the answers

What is the result of the movement of permeable ions across the membrane?

K+ and Cl¯ are in equimolar amounts on both sides. Signup and view all the answers

What is the characteristic of each side of the membrane at equilibrium?

Electrically neutral within itself. Signup and view all the answers

Which ion is highest in concentration on side 1?

K+. Signup and view all the answers

What is the purpose of the selectively permeable movement of chloride ions?

To maintain electrical neutrality. Signup and view all the answers

What is the result of the electrical gradient influx?

The passage of ions across the membrane is promoted. Signup and view all the answers

What is the role of the anionic protein (Pr-) in the Donnan equilibrium?

It allows the passage of certain charged ions. Signup and view all the answers

Study Notes

Translocation of Solutes

Mass flow is the phenomenon of passive movement of solutes, highest in the leaves (source) and lowest in the roots (sink).
The mechanism of mass flow is demonstrated by a model where two reservoirs are immersed in water, and solutes are pushed from the source to the sink by turgor pressure, with water being forced out of the sink by hydrostatic pressure.

Mechanism of Phloem Transportation

The mass flow hypothesis is limited by the lack of explanation for bidirectional movement of solutes within the same sieve tube.
The mechanism of cytoplasmic streaming provides further explanation for transportation of solutes, where solutes are transported by streaming along protein filaments of the sieve tubes.
The energy for the process comes from the sieve tube and companion cells.

Measurement of Water Potential

Refractive index (RI) is measured quantitatively with a refractometer to determine the water potential (Ψ) of a solution.
A graded series of sucrose solutions is prepared, and their refractive index (RI) is measured using a refractometer.
Plant tissues are introduced into the range of solutions, and after equilibration, the refractive index (RI) of each solution is re-determined.
The ratio of initial/final refractometer readings is plotted against molarities of sucrose solutions to determine the water potential (Ψ).

Concept of Water Potential

Pressure bomb technique is used to estimate xylem water tension, where a leafy shoot or leaf petiole is quickly cut and placed in a pressure chamber.
The pressure in the chamber is gradually increased by compressed air until the sap/water just returns to the severed ends of the xylem vessels.

Water Relations of Whole Plant

Water and dissolved minerals entering the plant through root hairs can follow three possible pathways: apoplast, symplast, and vacuolar.
The apoplast pathway is along and between cell walls, through intercellular spaces between them.
The symplast pathway is through the cytoplasm, from cell to cell via plasmodesmata strands.
The vacuolar pathway is through the sap vacuoles, from one vacuole to the next by osmosis.

The Ascent of Sap

Primarily, water uptake occurs by osmosis along a water potential gradient from the soil solution into the parenchyma cells of the root cortex.
The walls of the guard cells that border the stoma pore are thicker and less elastic than the thinner outer walls.

Physiology of the Stomata

There are no symplastic connections between the subsidiary cells and the guard cells.
Mechanisms of stomata opening involve changes in turgor in the guard cells, which regulates the opening and closing of the stomata.
The active transport of ions (principally Potassium K+) into the guard cells from the neighboring epidermal cells alters the solute potential and water potential, causing water to move in by osmosis.

Absorption of Mineral Salts

The Donnan equilibrium is a state of balance where an electrical potential gradient develops between two solutions due to uneven distribution of ions.
The passage of some ions across the membrane is promoted, and the net result at equilibrium is that K+ and Cl¯ will be in equal molar amounts together from one side to the other.

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Description

Quiz on the mechanism of phloem transportation, mass flow of solutes, and translocation process in plants.