Difference between facilitated diffusion and secondary active transport in cells

Question

Specifically, what is the difference between facilitated diffusion carrier processes (passive transport) and secondary active transport co-transport processes (active transport)?

They seem to be the same thing - doesn't the passive process with the carrier transports substance $x$ against its gradient together with substance $y$ that is moving down its electrochemical gradient?

Similarly, in the active process, substance $x$ is transported against the gradient, and substance $y$ down its electrochemical gradient?

I don't see how energy differentiates the active transport in this case from the passive transport.

Answer 1

Short answer
Facilitated diffusion is a passive process in which membrane channels mediate the transport of polar, or big molecules that are not solvable in the cell membrane. Co-transport, on the other hand, is active transport, as it depends on the electrochemical gradient of ions across the cell's membrane, particularly Na⁺. Because ATP or other energetic compounds are not directly involved in co-transport, it is referred to as secondary active transport. The electrochemical ion gradient is mainly established by the Na⁺,K⁺-ATPase.

Background
As commented upon by others, your question is not entirely clear and the best way to approach this question is by defining the various terminologies and physiological mechanisms used in your question:

1. Active transport needs an energy source such as ATP. For example, the Na⁺,K⁺-ATPase is an enzyme that pumps Na⁺ and K⁺ ions against their concentration gradients into the cytosol (K⁺) or out of the cell (Na⁺). Note that both Na⁺ and K⁺ are transported against their chemical gradients (and Na⁺ also against its electrical gradient as the cell is negative). Therefore your statement that [...] in the active process, substance x is transported against the gradient, and substance y down its electrochemical gradient is untrue, and likely the cause of your confusion.
2. Passive transport is mediated by diffusion, which typically occurs through a concentration gradient. Only small, relatively hydrophobic molecules are able to diffuse across a phospholipid bilayer at significant rates. Examples are gases (such as O₂ and CO₂) in the lungs, and small polar but uncharged molecules (such as H₂O and ethanol) (Cooper, 2000).
3. Facilitated diffusion is an example of passive diffusion, but aided by membrane-spanning channel proteins that span the lipid bilayer. Therefore the particles (molecules or ions) do not have to dissolve in the cell membrane's hydrophobic lipid bilayer, allowing hydrophylic and larger molecules (carbohydrates, ions) to be transported into the cell. No external source of energy is needed and travel across the membrane in the direction determined by their concentration gradients and, in the case of charged molecules, by the electric potential across the membrane (Cooper, 2000).
4. Co-transport is active transport, where the energetically unfavorable transport of a particle against its electrochemical gradient is facilitated by the co-transport of a favorable one down its gradient. In other words, multiple particles are transported so as to neutralize the sum of the transport. For example, sodium-dependent glucose tranporters transport glucose into the cell against the concentration gradient (unfavorable) and Na⁺ is transported along with it into the cell which is a favorable transport (Haraki & Inagaki, 2012). This is an example of a symporter. An example of an antiporter is the Na⁺/Ca²⁺ antiporter found in the heart to transport Ca²⁺ out of the cell against a steep concentration gradient and electrical gradient, while 3 Na⁺ is imported into the cell to balance the unfavorable Ca²⁺ transport. Note that all these mentioned co-transporters and many others are dependent on the Na,K-ATPase that ultimately generates the Na⁺ and K⁺ gradients by pumping Na⁺ out of the cell and K⁺ into the cell using ATP (Freeman, 2000). This generated membrane potential works like a battery being charged, and co-transporters discharge it by using the energy stored by importing Na⁺. Hence, it is referred to as secondary active transport, as no direct energy input from ATP is needed, but instead it utilizes the electrochemical gradient created by the Na⁺,K⁺-ATPase.

These considerations should clarify your issues and answer your question on what co-transport exactly is. Specifically, the glucose transporter mentioned, GLUT1 as you refer to, is a symporter that mediates secondary active transport.

<sub>References
- Cooper, The Cell, 2000
- Haraki & Inagaki, J Diabetes Invest 2012; 3: 352-3
- Molecular Cell Biology, 2000</sub>

Answer 2

"Specifically, what is the difference between facilitated diffusion carrier processes (passive) and secondary active transport co-transport processes (active)?"

Facilitated diffusion creates alternative paths for polar/charged substances to move down their electrochemical gradients across the hydrophobic interior of the membrane, thus greatly speeding up their movement into the cell.

Secondary active transport is a form of active transport where the transport of a substance AGAINST its electrochemical gradient (endergonic) is coupled to the movement of another substance DOWN its electrochemical gradient (exergonic), thus providing energy to transport the first substance against its electrochemical gradient.

The key difference is really that passive transport does not require energy whereas active transport does.

According to wikipedia the GLUT1 you mention is a uniporter that facilitates the diffusion of glucose into the cell, so it is a passive transporter. It does not require energy as it simply helps glucose move down its concentration gradient.

Answer 3

It's an old article I know, but after a bit of thinking my answer would be that while facilitated diffusion is a single step process (not counting the confugurational changes within the CARRIER* protein of course), secondary active transport couples a step of facilitated diffusion with the movement of another molecule/ion in the direction against its (electro)chemical gradient. There's both a passive and an active displacement. (*It's a carrier, not a channel.)

Answer 4

The difference between the two definitions is not scientifically justified. Here is an example to clarify this point. A facilitated transport through a carrier A will dissipate the gradient of a substance a. This dissipation will consume energy indirectly if the gradient for that substance is maintained by an active (ATP-dependent) pump. Hence a voltage-gated sodium channel dissipates the sodium gradient maintained by the Na/K ATPase. We think of these sodium ion channel as facilitating the movement of Na down its gradient but in fact they also need the energy indirectly provided to maintain the Na gradient. Now let us consider a coupled transport, Na+/glu for instance. The favorable transport of Na down its gradient largely overcomes the energy cost of the transport of glu against its gradient. Our textbooks commonly justify that this type of transport is secondarily active because it needs the energy necessary to maintain the Na gradient. But isn't the same thing for the voltage-gated sodium channel? If there is no Na gradient, then the transport of glucose by the Na/glu cotransporter will not exist, same thing for the Nav sodium channel: there would be limited flux of sodium ions without the Na gradient (there will be a little due to the membrane potential).