Lecture 11: membrane transport in plant cells

Biological membranes

The biological membranes show similar permeability to water, O₂, CO₂ and glycerol as pure lipid bilayers. However, ions (particularly K⁺) and large polar molecules such as sugars are more permeant in biological membranes. The reason for this is that biological membranes contain transport protein molecules, which facilitate the transport of these substances.

A particular transport protein is usually highly specific for the kinds of substances it will transport. However, it will also leak a family of related substances (e.g. K⁺ transporters might also transport Rb⁺ and Na⁺, but very different substance such as Cl^- or sugars do not pass through).

Channels and Carriers

Channels are trans-membrane proteins that function as selective pores in the membranes. The size of the pore and the density of surface charge on its interior determine its transport specificity. Substances move through them under influence of thermodynamical forces (concentration and electrostatic gradients). The current flowing through particular type of channel, when the system is not in equilibrium, can be estimated using similar assumption of constant grady in the membrane as in derivation of eqn. (10.2):

   (11.1)

Where N is the number of channels, P is the permeability, c_i and c_o are concentrations of the ion on each side of the membrane, Dy is the p.d. across the membrane. This is the Goldman, Hodgkin Katz  (GHK) equation.

In carrier-mediated transport, the substance being transported is initially bound by an active site on the carrier protein. Binding leads to a conformational change of the protein, which exposes the substance to the solution on the other side of the membrane. Transport is complete when the substance dissociates from the carrier-binding site. Carrier-mediated active transport systems that move substances against their chemical or electrochemical gradients are called pumps. Energy has to be expended, usually from ATP hydrolysis. The binding and release of a molecule at a specific site on a protein are similar to the binding and release of molecules from an enzyme in an enzyme-catalyzed reaction and can be modeled the same way mathematically. Experimentally, channels and carriers can be distinguished by poisoning ATP production by metabolic inhibitors.

Cell compartments

To determine whether transport of common substances is active or passive, the concentrations are measured in cell compartments. In case of ions, the membrane potential difference has to be measured also and compared to that predicted by the Nernst potential. The cytosol (cytoplasm) and the vacuole are the most important intracellular compartments determining the ionic relations of plant cells. In mature plant cells, the central vacuole often occupies 90 % of the cell volume, enclosed by the tonoplast membrane,  and the cytosol is restricted to a thin layer around the cell periphery, enclosed by the plasmalemma membrane.  Potassium is accumulated passively by both compartments. Sodium is pumped actively out of cytosol into vacuole and outside. Excess protons are actively pumped out to maintain the cytosolic pH near neutrality, while vacuole tends to be more acidic. Anions are taken actively into the cytosol.

Electrogenic transport

Regulation of ion fluxes at the cell membrane is essential to ensure an adequate supply of nutrients for metabolic needs.  The cell can selectively switch on and off transporters by changing trans-membrane potential difference (p.d.).  Some of the p.d. is due to diffusion potential of K⁺. However, when we apply the Goldman equation, the experimental p.d.s are more negative. Where does this polarization come from?

Whenever ion moves out or into the cell unbalanced by the counter-ion of opposite charge a voltage is created across the membrane (similar to IR drop in a circuit). Such transport is called electrogenic. Carriers that mediate it are electrogenic pumps. ATP is hydrolyzed and the transporter becomes phosphorylated. In principle, any ion can be transported by an electrogenic ATPase, the transport specificity being determined by the protein's ion binding site. However, only a few electrogenic pumps have been identified.

In animal cells, Na⁺/K⁺ ATPase has binding sites for both Na⁺ and K⁺ and pumps three Na⁺ out of the cell for every two K⁺ pumped in. The imbalance leads to a slight negative charge inside the cell. However, as there are high passive fluxes of Na⁺ and K⁺ across animal membranes, the pump does not contribute to the membrane potential. Plants contain H⁺ ATPase, which drives H⁺ from the cytosol to the external medium, creating both pH gradient and a large inside negative membrane potential. Hansen et al (1981) constructed a scheme for the kinetics of H⁺ ATPases.  They divided the processes involved into potential-dependent with rate constants k_io and k_oi and potential independent ones with rate constants k_io and k_oi. The current flowing through the ATPase can be modeled:

 (11.2)

The equation can be curve-fitted to current-voltage data measured in charophyte cells Beilby (1984).

Electrogenic H⁺ transport occurs not only in plants but also in bacteria, algae, fungi and some animal cells, such as the gastric epithelium.

Ca⁺⁺ is kept very low in the cytosol and must be actively pumped out, but it is not known if the pump is electrogenic.  

Cotransport

The difference in pH on each side of the membrane (proton motive force) drives flow of protons down the electrochemical gradient, which is in turn coupled to uphill transport of other substances. The cotransporters can be symporters (both proton and transported substance moving in same direction) or antiporters (proton and transported substance moving in opposite directions). As the energy comes from the proton gradient rather than directly from ATP hydrolysis, it is secondary active transport.

Examples of active and passive transports

(Figures from “Plant Physiology” by Taiz and Zeiger)