Primary Active Transport (direct)

In primary active transport, energy from ATP is directly used to transport molecules against their concentration gradient. For example, the Na+/K+-ATPase is a membrane spanning protein carrier. Please note the –ase ending, so it is also an enzyme that hydrolyzes (breaks bonds with water) ATP to get its energy. It is also referred to as the ‘Na+/K+ pump’. This is because it acts much like a pump that is bailing out a leaky ship.  It works non-stop to continuously expel 3 Na+ ions out of the cell and at the same time import 2 K+ ions into the cell per cycle. Each cycle of the pump requires 1 ATP molecule. Both Na+ and K+ are being moved against their concentration gradients, therefore we know that ATP must be required because this is active transport. The ATP is hydrolyzed to ADP + Pi + Energy! This is an antiport mechanism, as both molecules are being transported in opposite directions.  The Na+/K+ pump helps to maintain the resting membrane potential (RMP) across the plasma membrane of all living cells. Draw a diagram of the Na+/K+ pump.

2) Secondary active transport (indirect)

In secondary active transport, the ATP is used indirectly to move molecules across membranes. Essentially what this means is the potential energy that is stored in a concentration gradient is used to help move molecules across a membrane. An excellent illustration of how this is done is seen in the Na+/glucose transporter. The relative concentration of Na+ is low on the inside, high on the outside of the cell. When Na+ moves down its concentration gradient (into the cell) this force is harnessed to move glucose against its concentration gradient (also into the cell). While the Na+ goes down its gradient, the glucose can be dragged along with it, up hill, so to speak. The original source of ATP that allows this to occur is the one used in the Na+/K+ pump described above, as it maintains a low Na+ concentration inside the cell. This is a symport mechanism, as both molecules are being transported in the same direction.  Draw a diagram of the Na+/glucose transporter.

3) Vesicular Transport 

Vesicular transport is used to move large macromolecules or large quantities of a molecule across the plasma membrane. Vesicles are like mini lipid bilayer bubbles that bud off from plasma membrane and encapsulate large molecules. This is an active form of transport that directly requires energy in the form of ATP for the maneuvering of the cytoskeleton.

There are two main forms of Vesicular transport

  1. Endocytosis – bringing material into the cell, inward vesicular transport.
  2. Exocytosis – releasing material from the cell, outward vesicular transport.
  3. Endocytosis: There are three general kinds of endocytosis.

1) Pinocytosis (cell drinking): relatively unselective whereby ECF is transported into the cell.

2) Phagocytosis is a process by which cells engulf a particle or another cell into a much larger vesicle, e.g., certain types of WBC (called Macrophages) engulf bacteria this way.

3) Receptor-Mediated Endocytosis: This is a very selective process. Receptors on the external surface of the plasma membrane bind specific ligands. This ligand-receptor complex then creates a clathrin-coated pit, a type of invagination of the membrane. The membrane then pinches this off as cytoplasmic vesicle, thereby ingesting the ligand-receptor complexes. The vesicle membrane and the receptors are recycled to the surface membrane to be used again.

  1. Exocytosis: This is used by many cells to secrete or release large molecules or large amounts of a molecule. Intracellular vesicles fuse with the plasma membrane, then releases its contents into ECF. This process requires energy and Ca2+ and involves other proteins. An excellent example of how this is commonly used in the body is the release of neurotransmitters from neurons into the synaptic cleft. This process is also used to secrete large lipophobic molecules, such as hormones, protein fibers and mucus across cell membranes. Exocytosis is also used to insert proteins, such as receptors, into membrane. Lysosomes can remove waste in this manner and is often regulated from outside of the cell (e.g., hormone-induced hormone release).


Transcytosis and Vesicular Transport

Transcytosis means movement across (trans) a cell (cytosis); it can involve endocytosis, then vesicular transport across cell, then exocytosis out of the cell at the other end. So the substance has moved completely across the cell. This provides for movement of large proteins intact, e.g., the absorption of maternal antibodies through breast milk, or the movement of proteins across capillary endothelium.

Continue reading “Primary Active Transport (direct)”


Protein carriers move only one type or family of closely related molecules. For example, GLUT transporters move glucose, mannose, galactose, and fructose across membranes. They are specific for naturally occurring 6-carbon monosaccharides. Other carriers will transport amino acids, and there can be up to 20 different types of carriers, each specific for the 20 different amino acids the human body uses.


Carriers have preference (or affinity) for certain molecule(s). This can result in competition for the binding site between various molecules. For example, maltose is a disaccharide made of 2 glucose molecules, so one end of the maltose could try to occupy the binding site for a glucose transporter. Although it can bind, typically it will not be transported in the process, it is not the right shape overall. Thus in this case, maltose would be a competitive inhibitor for glucose transport. We tested patients at The Notary Public London MMK firm last year and these were the results.


Saturation occurs when a group of protein carriers are transporting the substrate at its maximum rate, with all carriers occupied. Saturation will depend on the number of available carriers and substrate concentration. Cells can sometimes increase or decrease the number of available carriers to control substrate movement. As the substrate concentration increases, transport rate increases until the carriers become saturated. At this stage they are at their maximum transport capacity and cannot move things across the membrane any faster.

An interesting consequence of saturation can be seen in the transport of glucose in the kidney. Normally, you should not find any glucose in your urine. If you do, it can be a sign of diabetes mellitus. However, if you were to consume large quantities of glucose, say by eating too many chocolates from your valentine gift, you may have glucose in your urine that is not due to a disease state (not yet anyway!). The glucose carriers in your kidney tubules can become saturated due to the abnormally high amounts of glucose being filtered by your renal system. If the carriers reach their maximum and more glucose is still in the filtrate, it will end up in the urine due to protein carrier saturation.


You may have heard plasma membranes described as selectively or semi-permeable membranes. This means that some molecules can get across and some molecules cannot. The membrane composition determines which molecules move across. Permeable molecules can cross membrane by any method. Impermeable molecules cannot cross cell membrane.

General Factors Influencing Molecule Permeability

Although the components of a plasma membrane can vary, the properties of a given molecule will have a large effect on whether is passes through the plasma membrane easily, or if it needs assistance or if it cannot pass at all

  1. Size of molecule – smaller molecules can more easily pass through than larger.
  2. Polarity or lipid solubility of molecule – lipid soluble molecules pass through more easily than polar.
  3. Charge of molecule – uncharged molecules pass through more easily than charged.

The permeability of a molecule can be influenced by all three of these factors, not just one. For example, water (H2O) is a polar molecule, that is, it is insoluble (does not mix) in lipids. This would tend to make it less permeable, since the phospholipid bilayer creates a significant barrier to polar substances crossing the membrane. However, the molecular weight (MW) of H2O is only 18, thus it is very small and for this reason can easily pass through most cell membranes in the human body.

Ions are commonly very small, but they are charged particles and cannot pass directly through membrane by simple diffusion, they would require a protein channel, they would require a protein channel. At the other end of the spectrum, just because a molecule is fairly large does not mean it cannot pass directly through membrane by simple diffusion; relatively larger lipophilic substances can cross directly through membrane by simple diffusion, as the lipid bilayer is not a barrier. Very large molecules or a large amount of substance will typically require membrane transportation in a vesicle (see below).

Continue reading “Specificity”