Solute Transport 1

What is the difference between a solute and a solvent?

1. the substance dissolved;

2. the substance that dissolves it.

In plant cells, water is the main solvent.

What are examples of solutes that plants absorb or transport?

Mineral ions such as K+, Ca2+, nitrate, and phosphate; sugars, amino acids, hormones, and pigments.

What is the main sugar transported over long distances in most plants?

Sucrose.

What is passive transport?

Movement down a chemical or electrochemical gradient without a direct input of metabolic energy.

What is active transport?

Movement against a chemical or electrochemical gradient that requires an energy source.

What two factors make up an ion's electrochemical gradient?

Its concentration gradient and the electrical potential difference across the membrane.

What happens to net solute movement at equilibrium?

There is no net movement because the driving forces balance. Individual particles can still move in both directions.

How does preferential K+ diffusion create a diffusion potential in the KCl example?

K+ leaves compartment A faster than Cl-; A becomes relatively negative and B relatively positive, creating a voltage difference.

What is membrane potential, and what causes it?

The voltage difference across a membrane, produced by an unequal distribution of electrical charge on its two sides.

What membrane-potential range is given for plant cells?

About -60 to -240 mV, with the cell interior negative relative to the outside.

What does the Nernst equation calculate?

An ion's equilibrium potential: the membrane voltage at which its electrical and concentration gradients balance, producing no net movement of that ion.

How can membrane potential be measured?

Using microelectrodes placed inside and outside the cell and connected to a voltmeter.

What is the tonoplast?

The membrane surrounding the vacuole.

What types of solute transport occur across the plasma membrane and tonoplast?

Both passive and active transport.

Why does the vacuole act as a solute sink?

It accumulates a high concentration of solutes, creating a solute gradient and lowering solute potential, which favors water entry.

Why are biological membranes more permeable to ions than a pure lipid bilayer?

They contain transport proteins that provide routes for ions to cross the membrane.

Why do O2 and CO2 cross a pure lipid bilayer relatively easily?

They are small, nonpolar molecules that can diffuse through the lipid portion of the membrane.

What are the three main types of membrane transport proteins?

Channels, carriers, and pumps.

How do ion channels transport solutes?

They form selective pores that allow ions to move passively down their electrochemical gradients.

What can control the opening and closing of ion channels?

Changes in membrane voltage, binding of a chemical ligand, or mechanical stimulation such as membrane stretching.

Which transport proteins generally allow the fastest movement of ions?

Ion channels; many ions can pass rapidly through an open pore.

What ion do K+ channels selectively transport, and what is their basic structure?

Potassium ions; their helical protein subunits form a selective pore, described in the slides as an inverted teepee.

How does a voltage-gated K+ channel sense membrane voltage?

A region containing positively charged amino acids responds to voltage changes and controls the channel's gate.

What do inward K+ channels do in root cells?

Allow K+ to enter the cytoplasm from the soil solution when the electrochemical gradient favors entry.

How does K+ entry into guard cells promote stomatal opening?

K+ accumulation lowers solute potential; water enters by osmosis, guard-cell turgor increases, and the stomatal pore opens.

How do outward K+ channels in guard cells promote stomatal closure?

K+ leaves the guard cells; water follows, turgor decreases, and the stomatal pore closes.

What is the role of outward K+ channels in xylem parenchyma cells?

They release K+ into the xylem, contributing to xylem loading and solute gradients.

What does a current-voltage graph show about an ion channel?

How ion current changes with membrane voltage, helping reveal the direction of ion movement and the voltages at which channels conduct.

What do negative and positive K+ currents indicate?

Negative current indicates K+ entering the cell; positive current indicates K+ leaving the cell.

What happens to net K+ current when membrane voltage equals the K+ equilibrium potential (EK)?

Net K+ current is zero because the electrical and concentration driving forces balance.

How does a carrier protein move a solute across a membrane?

It binds a specific solute, changes shape, releases the solute on the other side, and returns to its original configuration.

How does a carrier differ structurally from a channel?

A channel forms a continuous pore when open; a carrier changes shape to expose its binding site to opposite sides in turn.

Can carrier proteins perform passive and active transport?

Yes. Downhill transport is passive; moving a solute against its electrochemical gradient requires an energy source.

What are examples of substances transported by carriers?

Sugars, amino acids, nitrate, phosphate, magnesium, and hormones such as auxin.

What do ATP-driven membrane pumps do?

Use energy directly from ATP to move solutes against their electrochemical gradients; this is primary active transport.

What are the products when ATP is hydrolyzed to power a pump?

ADP and inorganic phosphate (Pi), with energy released to drive transport.

What do proton pumps transport, and why is this important?

They transport H+ against its electrochemical gradient, helping regulate pH and establish gradients that can power other transporters.

How does changing H+ concentration affect pH?

Increasing H+ concentration lowers pH; decreasing H+ concentration raises pH.

Where are P-type and V-type H+-ATPases typically located in plant cells?

P-type H+-ATPases are in the plasma membrane; V-type H+-ATPases are in the tonoplast.

Which membranes listed in the lecture contain proton-transport systems?

The plasma membrane, tonoplast, chloroplast thylakoid membrane, and mitochondrial inner membrane.

In which direction do the illustrated plasma-membrane and tonoplast H+ pumps move protons?

The plasma-membrane pump moves H+ out of the cytosol into the apoplast; the tonoplast pump moves H+ from the cytosol into the vacuole.

How does primary active transport differ from secondary active transport?

Primary transport uses an energy source such as ATP directly; secondary transport uses energy stored in an ion's electrochemical gradient.

How does coupled secondary transport move a solute uphill?

The downhill movement of one substance, often H+, supplies the energy to move another against its electrochemical gradient.

What is a symporter?

A transporter that couples the movement of two different substances in the same direction across a membrane.

What is an antiporter?

A transporter that couples the movement of two different substances in opposite directions across a membrane.

What are examples of H+-coupled symport at the plasma membrane?

H+ entering the cell together with sucrose, an amino acid, nitrate, or phosphate.

How does the illustrated plasma-membrane Na+/H+ antiporter work?

H+ moves into the cytosol down its gradient while Na+ moves out of the cell.

How does the illustrated tonoplast Ca2+/H+ antiporter work?

Ca2+ moves from the cytosol into the vacuole while H+ moves from the vacuole into the cytosol.

What happens during the H+-solute symport cycle shown in the slides?

H+ binding favors solute binding; the protein changes shape, releases both into the cytoplasm, and returns to its original configuration.

What makes a pump electrogenic?

It produces a net movement of electrical charge across a membrane, contributing to membrane potential.

What role does phosphate play in the illustrated phosphorylation-driven pump cycle?

ATP transfers phosphate to the pump, changing its shape; after solute release, phosphate is released on the cytoplasmic side rather than transported across.

What do the illustrated vacuolar ABC transporters use for energy, and what can they transport?

They use ATP and can move substances such as anthocyanin-related compounds and cadmium complexes into the vacuole.

What solutes are shown using H+-coupled antiporters in the tonoplast?

Na+, Ca2+, Cd2+, Mg2+, hexoses, and sucrose.

How do the apoplastic and symplastic routes of solute movement differ?

The apoplastic route follows cell walls and intercellular spaces; the symplastic route follows connected cytoplasm through plasmodesmata.

Why must apoplastically moving solutes cross a membrane before passing the endodermis?

The Casparian strip blocks the cell-wall route, forcing solutes into the symplast through selective membrane transport proteins.

What is xylem loading?

The transfer of mineral ions and other solutes from living root cells into the xylem for transport through the plant.

What transport pattern is illustrated by the root ion-loading diagram?

Ions are actively accumulated into root cells and can then be released down an electrochemical gradient into the xylem.

What does depletion of radioactive tracer from compartment A measure in the root experiment?

Uptake of the labeled ions from the surrounding solution into the root.

What does radioactive tracer appearing in compartment B measure in the root experiment?

Xylem loading and transport of the labeled ions through the root segment; this is compared with uptake from compartment A.

Why can H+-coupled symport be active transport even though H+ moves downhill?

The H+ gradient supplies energy to move the other solute uphill; ATP was used indirectly to establish the H+ gradient.