Membrane transport mechanisms

Membrane transport mechanisms

Cells constantly exchange solutes (e.g., nutrients, wastes, and respiratory gases) with the interstitial fluid.

The transport of solutes across cell membranes is fundamental to the survival of all cells, and the transport mechanisms are therefore present in all cells. Specializations in membrane transport mechanisms often underlie tissue function.

For example, excitable tissues express many voltage-sensitive membrane transport systems that account for the ability to generate and propagate electrical signals.

1. The electrochemical gradient

The electrochemical gradient is the driving force for substance, which is a combination of the membrane voltage (electrical gradient) and the concentration different (chemical gradient).

• Electrical gradient is the force that causes the movement of ions towards the opposite charge.
• Chemical gradient is the force that causes the movement of solutes in the direction of lower concentration.

For example, to most resting cells have gradients:

2. Classification of membrane transport mechanisms

The membrane transport mechanisms are divided into two major compartments:

• passive solute transport,
• active solute transport.

Passive solute transport can only occur along a favorable electrochemical gradient.

Active transport uses cellular energy, unlike passive transport, which does not use cellular energy.

Active transport can only occur against a electrochemical gradient.

Classification of membrane transport systems based on the use of cellular energy

3. Passive transport

Passive transport does not require energy of ATP hydrolysis or coupling to another solute.

 There are three types of passive transport:

1.            Simple diffusion through lipid bilayer
2.            Simple diffusion through channel (ion channels and  aquapores )
3.            Facilitated diffusion through  uniporter

Passive transport

Diffusion of lipid-soluble substances (e.g., gases – O2 and CO2) may occur directly through the plasma membrane. 

Transport of ions and small molecules (e.g. water) more frequently occurs via membrane-spanning proteins, which serve as pores or carriers through the lipid bilayer. The pores are divided into two major  types: ion channels and  aquapores (to the osmosis).

There are many examples of passive transport through membrane pores and carriers, the most numerous of which are ion channels. 

Ion channels have the following general components:

1) A pore region, through which ions diffuse.

2) A selectivity filter within the pore, causing the channel to be highly selective for a particular ion (e.g., Na+ channels).

3) A gating mechanism that opens and closes the channel. In the closed state, no ions flow through a channel but the channel is available for activation. In the open state, ions flow down their electrochemical gradient. Channel gates may be controlled by one of the following mechanisms: membrane voltage (voltage-gated channels); chemicals (ligand-gated channels); mechanical forces in the membrane (e.g., stretch-activated channels).

Osmosis 

is water movement that is driven by a water concentration gradient across a membrane.

•            Water concentration is expressed in terms of total solute concentration; the more dilute a solution, the lower its solute concentration and the higher its water concentration. When two solutions are separated by a semipermeable membrane (i.e., one that allows the transport of water but not solutes), water moves by osmosis away from the more dilute solution.

Illustration of osmotic water movement across a semipermeable membrane. B. The concept of osmotic pressure.

Osmolarity is an expression of the osmotic strength of a solution and is the total solute concentration. Two solutions of the same osmolarity are termed isotonic (isosmotic ). A solution with a greater osmolarity than a reference solution is said to be hypertonic , and a solution of lower osmolarity is described as hypotonic .

An isotonic solution has the some effective osmolarity as the cells and causes no net water movement; a hypotonic solution has a smaller effective osmolarity than the cells and causes cells to swell; a hypertonic solution has a larger effective osmolarity than the cells and causes cells to shrink.

For example, if a patient is intravenously infused with a hypotonic solution, the ECF tonicity is initially decreased and water moves into the ICF by osmosis (cells swell).

Conversely, if a hypertonic solution is infused, the ECF tonicity initially increases and water moves out of the ICF (cells shrink).

Passive transport of small molecules (e.g. glucose) can also occur via carrier proteins called uniporters, which selectively bind a single solute at one side of the membrane and undergo a conformational change to deliver it to the other side. Solute transport via uniporters is called facilitated diffusion because it is faster than simple diffusion.

4. Active transport

Classification of active transport systems

Active transport requires energy (generally adenosine triphosphate (ATP) hydrolysis or coupling to another solute).

There are three types of active transport :

1.Primary active transport

2.Secondary active transport

3.Vesicular transport

Primary active transport occurs via membrane proteins that directly couple ATP hydrolysis to solute movement. Example the Na+/K+-ATPase (known as the “sodium pump”).

Secondary active transport

Secondary active transport couples the transport of two or more solutes together. In secondary active transport, energy is used to develop a favorable electrochemical driving force for one solute, which is then used to power the transport of other solutes (e.g., the inwardly directed Na+ gradient is used to drive glucose uptake from the intestine).

Cotransporters (symporters) couple the movement of two or more solutes in the same direction. Example of Na+-driven cotransporters include Na+/glucose uptake in the intestine.

Exchangers (antiporters) couple the movement of two solutes in the opposite direction. Example Na+-driven exchangers include Na+/Ca2+ and Na+/H+ exchange, which are important for maintaining low intracellular [Ca2+] and [H+], respectively.

Vesicular transport.

Movement of macromolecules occurs in membrane-limited vesicles; macromolecules enter cells by endocytosis and exit cells by exocytosis.

There are three types of endocytosis:

1)Pinocytosis is the ingestion of small particles and ECF that occurs in most cells.

2)Phagocytosis is the uptake of large particles (e.g., microorganisms)  that occurs in specialized immune cells.

3)Receptor-mediated endocytosis allows uptake of specific molecules and occurs at specialized areas of membrane called clathrin-coated pits (e.g., uptake of cholesterol bound to low-density lipoproteins).

Exocytosis is the export of soluble proteins into the extracellular  space. Such proteins are synthesized in the cell and packaged into intracellular vesicles. When vesicles fuse with the plasma membrane, the soluble proteins are secreted and the vesicle membrane is incorporated in the plasma membrane.