Loading

Nutrient Transport

Nutrient uptake (i.e., solute transport) is a cellular process for acquiring molecules from the cell environment that are needed to support cell growth, metabolism and cell maintenance.  

Key Concepts

  • Cells require many types of essential and non-essential nutrients.
  • Cells scavenge compounds present in low to high abundance from their environments and accumulate them intracellularly.
  • There are two distinct types of nutrient uptake:
  • Passive transport.  Passive transport does not require cell energy input. It occurs either by the passive diffusion of a molecule across the cell membrane, or by the facilitated diffusion of the molecule aided by a specialized membrane protein.
  • Active transport. Active transport of a nutrient requires a dedicated solute transport system and input of cell energy. There are several types of systems that are differentiated by the mechanism of molecule uptake, the energy source, and the types of proteins present. Examples include the ABC-type transporters, symporters, antiporters, and group translocation transporters.
  • Solute transport systems are also used to maintain intracellular ion levels, and to export cell waste materials and toxins.

 

General background on Nutrient Uptake

E. coli, like most microorganisms, thrives in environments that are often limiting for the many types of nutrients needed to support cell growth. To accumulate these molecules the cell employs dedicated nutrient uptake systems called solute transporters. Most of these require energy input in form of the proton motive force or ATP hydrolysis to drive nutrient uptake into the cell.

To accomplish nutrient uptake, the cell must overcome three major obstacles.

The number and types of nutrients used by the cell is large.  Some nutrients are essential and the cell cannot survive without them.

Many other nutrients are not essential for E. coli cell survival but they are acquired by the cell in order to conserve energy…

 

Specific information on E. coli Nutrient Uptake

I. Passive transport across membranes

Entry of solutes into the cell by simple diffusion is generally limited to a few types of molecules since the cytoplasmic membrane forms a hydrophobic barrier to most types of nutrients. The driving force for simple diffusion depends on the solute concentration gradient across the membrane.  The molecules will flow from the region of high concentration to the region of lower concentration. Thus, the concentration inside the cell can never be higher than the level outside the membrane.  

There are two types of passive transport systems called passive diffusion and facilitated diffusion. They differ in that one type employs a special protein to assist in moving the molecule across the membrane while the other does not. 

Facilitated vs Passive Diffusion

The solute molecule shown above (ammonia) enters the cell at a much faster rate via the ammonia-specific facilitator protein compared to passive diffusion.

1. Passive diffusion.

2. Facilitated diffusion.

There are two types of facilitator proteins called carrier proteins and porin proteins. The former are always located in the cytoplasmic membrane and the latter are always located in the outer membrane. Both proteins increase the rate of molecule diffusion across the membranes but they work in different ways.

    • Carrier proteins (uniporters)       

      Operation of a Uniport System

      The potassium ion transporter is an example of a uniport system. This animation demonstrates directional movement of K+ along a gradient and substrate specificity.

    • Porin proteins       

II. Active transport across the cytoplasmic membrane

The majority of the nutrients required for cell metabolism are taken up by active transport systems. This process is carried out by one or more proteins located in the cytoplasmic membrane or associated with it. All of these systems require the expenditure of cell energy supplied in the form of ATP, the proton motive force, or for some sugar transporters, by the high energy compound, phosphoenylpyruvate (PEP). Active transport systems are highly specific for an individual molecule or class of structurally related compounds.

Since active transport is driven by the expenditure of cell energy, it allows the cell to accumulate molecules to a much higher concentration inside the cell (i.e., the cytoplasm) relative to the outside of the cell (i.e., the periplasm, or cell exterior).   The synthesis and/or activity of many of these active transport systems are often regulated depending on the cellular needs.  

Types of Active Transport Systems

Active transport systems are divided into three general classes that are named according to their component parts or their mechanism of action.  In most cases the solute is transported from the periplasm to the cell interior (i.e., cytoplasm) in an unmodified form. In a few cases the solute is chemically altered during its transfer across the cytoplasmic membrane (e.g., group translocation).

  1. ABC transporters.       
    • In Gram-negative bacteria,       
    • The Gram-positive bacteria and the archaea       
    • Iron III transport in Gram negative bacteria.        
    • Acquisition of Fe3+ ions is a more complicated process than for uptake of most nutrients       

    Operation of ABC-type Transport System

    This ABC-type transport system imports solute molecules into the cell. The solute molecule shown in green is bound by the blue periplasmic binding protein which passes the solute to the cognate membrane spanning protein. The energy released by the hydrolysis of ATP to ADP and Pi (inorganic phosphate) drives the solute molecule into the cytoplasm.

  2. P-type ATPases.       
  3. Secondary transport systems.       

    The two types of secondary transport systems are defined by the directions of solute and ion movement (i.e., co-transported or antiported).

    • Symporters       
    • Antiporters       
  4. Operation of a Symport System

    The sugar disaccharide (green molecule) transporter is an example of a symport system. Sugar uptake is driven by a proton entering the cell (i.e., by a proton gradient or the pmf).

    Operation of an Antiporter System

    The Na+/H+ antiport system exchanges Na+ for H+. Sodium export is driven by the proton gradient (pmf).

  5. Group translocation systems.       
    • The phosphotransferase system (PTS).       
    • E. coli fatty acid transport.       

Operation of a PTS-type Transport System

The Glucose-specific PTS transporter is an example of a group translocation system. Here, glucose (the green hexagon) located in the cell periplasm space is bound by the membrane protein Enzyme IIC. Energy from PEP is used to drive glucose uptake via a phosphorylation cascade where glucose is ultimately phosphorylated as it enters the cell.

Summary:

  • Nutrients enter the cell via passive or active transport mechanisms.
  • Passive transport mechanisms do not require energy input and involve simple diffusion of solutes across the membrane.
  • Passive transport can occur with or without involvement of a protein to facilitate diffusion.
  • Active transporters always require energy input and involve dedicated transport proteins embedded in the cytoplasmic membrane.
  • The three general types of active transporters include the ABC transporters, the secondary transporters and the group translocation systems.
  • ABC transporters employ a periplasmic solute binding protein, a membrane intrinsic transport protein and an ATP hydrolyzing protein to drive solute uptake.
  • Secondary transporters import solutes either by co-transport (symport) or by molecule exchange (antiport of an ion).  The driving force for most secondary transporters is the proton motive force; alternatively, the gradient of the co- or anti- transported ion drives transport.
  • Group translocation systems modify the solute upon cell entry. PEP or ATP provide the energy for the modification reaction.
  • Molecules are accumulated against an increasing concentration gradient during active transport.

 

Credits:

Authored by Robert Gunsalus and Imke Schroeder
©The Escherichia coli Student Portal

This project acknowledges support from:
Peter Karp and coworkers at EcoCyc.org
NIH Grant Award GM077678 to SRI, International
Animations from surfrender

The UCLA Department of MIMG