Botany 4400/5400

Lecture 22

6 March 2006

Reading: Chapter 6, Taiz and Zeiger's Plant Physiology


III. Transport of Water and Solutes

F. Transport of Solutes

All living cells are bounded by a biological membrane consisting of a phospholipid bilayer with many kinds of proteins in and on the bilayer. The function of the membrane is to create an internal space that can be made different from the outside environment, which is a fundamental requirement for life.

Solutes move across biological membranes in different ways, depending on their chemical and physical characteristics. Non-polar, uncharged solutes diffuse readily across the phospholipid bilayer, especially if they are small. Such solutes include O2, CO2, and many lipids. Other kinds of molecules that are large or charged require transport proteins to move across the lipid bilayer. These include ions (e.g. K+, Na+, Ca++, PO4+, NO3-), and large polar molecules such as amino acids and sugars. Water is small but polar so it would be expected to diffuse through the lipid bilayer at a slow rate. Water is observed to move through biological membranes quickly, however, and it is now understood that channel proteins called "aquaporins" assist its movement.

1. Modes of Solute Transport

There are many kinds of transport proteins and the movement of solutes through them is divided into several different functional categories.

a. Faciitated diffusion

Movement of solutes through a transport protein with their charge and concentration gradients is called facilitated diffusion, (facilitated because the solute cannot pass the lipid bilayer without the protein). Such movement does not require ATP, either directly or indirectly. The two types of transport proteins that engage in facilitated diffusion are channels and carriers.

Channels are essentially pores that allow only one, or at most two, types of solutes to move through them in one direction, either into or out of the cell. Channels are often"gated", meaning they can be open or closed. Some kinds of channels are said to be "voltage gated", which means that they open or close depending on the electrical difference across the membrane.

Carriers are proteins that allow diffusion of solutes across a biological membrane but they differ from channels in that they transiently bind the solute they are specific for as they move it across the membrane.

b. Active transport

Active transport is the movement of a solute by a transport protein against its concentration or charge gradient. Active transporters require ATP, either directly or indirectly. They can be divided into two groups: "pumps" and "co-transporters".

Pumps are transport proteins that use ATP as an energy source to pump solutes against their concentration and/or charge gradients. Two kinds of pumps are common in plant cells, the H+ ATPases and the Ca++ ATPases. The former pump protons from the symplast into the apoplastic space, acidifying it and generating a charge difference across the membrane such that the interior of the cell is more negatively charged than the exterior. Ca++ ATPases continuously pump Ca++ out of the cytosol into the central vacuole or apoplastic space and thus keep the cytosolic concentration of Ca++ very low. Because they use ATP directly, the operation of pumps is described as "primary active transport".

Co-transporters also move solutes across biological membranes against their concentration and/or charge gradients but they do not hydrolyze ATP directly. Instead, the passage of a second solute through a co-transporter with its charge and concentration gradients provides the free energy needed to move the first solute against its charge or concentration gradients. For example, sucrose is taken up by plant cells against its concentration gradient by sucrose co-transporters that simultaneously allow protons to pass into the cell with their charge and concentration gradients. Similarly, Na+ is exported from plant cells against its charge and concentration gradients by co-transporters that simultaneously allow protons to pass into the cell with their charge and concentration gradients. The protons used for this co-transport have been pumped into the apoplast by the H+ ATPases described earlier. Thus, co-transporters require ATP to operate but do not use ATP directly. For this reason, the operation of co-transporters is often called "secondary active transport"

Co-transporters are further divided into "symporters" and "antiporters". In symporters, the two solutes move in the same direction. In antiporters, the two solutes move in opposite directions.

Co-transporters and secondary active transport are illustrated in Figures 6.9 and 6.10 of your text. Figure 6.11 is a nice pictorial summary of common plant cell transporters.

G. Stomatal Regulation

In order for plants to take up CO2 for photosynthesis, they must expose the moist surfaces of their leaf mesophyll cells to the air. In doing so, water is lost by evaporation. The evaporation of water from leaves is called transpiration. Under some circumstances, transpiration of water from leaves may act to cool them and prevent damage from high ambient temperature. In general, however, transpiration is neutral or bad for plants. It is an unavoidable loss of water as the plant photosynthesizes.

To minimize transpiration, movement of gases into or out of a leaf is controlled by the stomata. The stomata are small pores in the leaf epidermis that can be opened or closed. Stomatal opening is highly regulated by multiple mechanisms so as to minimize transpiration. Transpiration is minimized even under conditions of high ambient temperature. (Stomata close at high temperature. They do not open in order to cool the leaf).

1. Structure of stomata

Stomata are composed of two guard cells. These cells have walls that are thicker on the inner side than on the outer side. This unequal thickening of the paired guard cells causes the stomata to open when they take up water and close when they lose water. A diagram of stomata is shown on page 408 of your text. The opening and closing of stomata is governed by increases or decreases of solutes in the guard cells, which cause them to take up or lose water, respectively.

2. Responses of stomata to the environment

In general, stomata open by day and close at night. During the day, photosynthesis requires that the leaf mesophyll be exposed to the air to get CO2. At night, the stomata close to avoid losing water when photosynthesis is not occurring.

During the day, stomata close if the leaves experience a lack of water, such as during a drought.

The opening or closing of stomata occur in response to signals from the external environment.

Light = Stomata open

Dark = Stomata close

High CO2 inside leaf = stomata close

Low CO2 inside leaf = stomata open

Drought stress = stomata close

Closure of stomata by drought is caused by abscisic acid, a plant hormone that is synthesized in response to drought. Abscisic acid overrides other signals and closes stomata when saving water is more important than photosynthesis.