Dr. Syed Abdullah Gilani Solute Transport Dr. Syed Abdullah Gilani

Passive transport (diffusion) occurs spontaneously down a chemical potential gradient. Passive transport on a carrier is sometimes called facilitated diffusion, although it resembles diffusion only in that it transports substances down their gradient of electrochemical potential, without an additional input of energy. Active transport occurs against a chemical potential gradient. It is not spontaneous and requires cellular energy. Chemical potential of any solute is defined as the sum of the concentration, electric, and hydrostatic potentials, and the chemical potential under standard conditions.

Ion concentrations in the cytosol and the vacuole are controlled by passive (dashed arrows) and active (solid arrows) transport processes.

Potassium is accumulated passively by both the cytosol and the vacuole Potassium is accumulated passively by both the cytosol and the vacuole. When extracellular K+ concentrations are very low, K+ is taken up actively. Sodium is pumped actively out of the cytosol into the extracellular spaces and vacuole. Excess protons, generated by intermediary metabolism, are also actively extruded from the cytosol. This process helps maintain the cytosolic pH near neutrality, while the vacuole and the extracellular medium are generally more acidic by one or two pH units. All the anions are taken up actively into the cytosol. Calcium is actively transported out of the cytosol at both the cell membrane and the vacuolar membrane, which is called the tonoplast

Transport Proteins Transport proteins facilitate the passage of selected ions and other polar molecules. There are three major categories of transport proteins: Channel protein Carrier protein Pump proteins

1. Channel Proteins - Enhance Ion and Water Diffusion across Membranes Channels are transmembrane proteins that function as selective pores through which molecules or ions can diffuse across the membrane. Transport through channels is always passive. Channel transport is limited mainly to ions or water because the specificity of transport depends on pore size and electric charge more than on selective binding. Channel proteins have structures called gates that open and close the pore in response to external signals. Signals that can regulate channel activity include membrane potential changes, ligands, hormones, light, and posttranslational modifications such as phosphorylation.

2. Carrier Proteins - Bind and Transport Specific Substances carrier proteins do not have pores that extend completely across the membrane. In transport mediated by a carrier, the substance being transported is initially bound to a specific site on the carrier protein. carriers are highly selective for a particular substrate to be transported. Carriers therefore specialize in the transport of specific organic metabolites. Binding causes a conformational change in 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’s binding site.

3. Pumps The membrane proteins that carry out primary active transport are called pumps. Most pumps transport ions, such as H+ or Ca2+. Ion pumps can be further characterized as either electrogenic or electroneutral. Electrogenic pumps are primary active transporters that hydrolyze ATP and use the resulting energy to transport ions across biological membranes, causing net electric charge across the membrane. Electroneutral transport involves no net movement of charge.

Active transport

Active transport includes primary and secondary active transport. Primary Active Transport Is Directly Coupled to Metabolic or Light Energy Secondary Active Transport Uses the Energy Stored in Electrochemical-Potential Gradients. There are two types of secondary transport: symport and antiport. Symport (and the protein involved is called a symporter) because the two substances are moving in the same direction through the membrane. Antiport (facilitated by a protein called an antiporter) refers to coupled transport in which the downhill movement of protons drives the active (uphill) transport of a solute in the opposite direction

Hypothetical model for secondary active transport

In the initial conformation, the binding sites on the protein are exposed to the outside environment and can bind a proton. This binding results in a conformational change that permits a molecule of S to be bound. The binding of S causes another conformational change that exposes the binding sites and their substrates to the inside of the cell. Release of a proton and a molecule of S to the cell’s interior restores the original conformation of the carrier and allows a new pumping cycle to begin.

Two examples of secondary active transport coupled to a primary proton gradient.