Question

Assertion: Transport saturates in facilitated diffusion. Reason: Facilitated diffusion occurs through protien molecule.

Short answer

The assertion is a true statement: Transport does saturate in facilitated diffusion when all carrier or channel proteins are fully engaged in the transport. The reason is also a true statement: Facilitated diffusion indeed happens through the help of protein molecules. But the reason given doesn't directly back the assertion statement. The statement would have been better supported, for instance, if the Reason mentioned how a limited number of these protein molecules leads to saturation in transport with an increasing number of substrate molecules.

Step-by-step solution

Understanding the components of facilitated diffusion

Facilitated diffusion is a type of passive transport. It doesn't require the input of energy, but instead relies on the natural movement of particles from an area of high concentration to an area of low concentration (down the concentration gradient). The special component here is the participation of protein molecules, which facilitate this process.

Understanding the statement 'Transport saturates in facilitated diffusion'

What does 'transport saturates' mean? In the context of facilitated diffusion, it typically refers to the point at which all carrier or channel protein molecules involved in the transport are fully occupied, all being used at the same time by substrate molecules. This means that the rate of transport doesn't increase further, despite the presence of more substrate molecules, because there are no free carrier proteins.

Assess the correlation between Assertion and Reason

The assertion states that 'Transport saturates in facilitated diffusion,' which is true as explained in Step 2. The reason given is because 'Facilitated diffusion occurs through protein molecules.' Now, although this is factually correct, the specific reason for transport saturation isn't entirely linked with this claim. The more precise reason would be that transport saturates because there are a limited number of carrier proteins that can get occupied during facilitated diffusion, which leads to saturation.

Key concepts

Passive Transport

Passive transport is a biological process that enables substances to move across cell membranes without the need for energy from the cell. This movement occurs naturally from an area of higher concentration to an area of lower concentration, following the path of least resistance. Think of it as a crowd dispersing into an open area; each individual wants their own space, so they spread out to find it.

Because passive transport does not require cellular energy, it is driven purely by the kinetic energy of the molecules in motion, which is dependent on temperature, molecule size, and the concentration gradient. An essential characteristic of passive transport is its ability to achieve equilibrium, where the concentration of a substance is uniform throughout, resulting in no net movement of molecules.

Concentration Gradient

The concentration gradient is a key factor in passive transport. It refers to the difference in the concentration of molecules across a space, such as across a biological membrane. Molecules naturally move from areas of high concentration, where there are many of the same type of molecules crowded together, to areas of low concentration, where there are fewer.

Imagine a hill: molecules at the top, where they're densely packed, will naturally move or 'roll down' to the bottom, where there's more room. This 'gradient' is the driving force behind diffusion. The steeper the gradient, the faster the movement until the slope is 'leveled out,' meaning until equilibrium is reached. A concentration gradient can exist for any substance within a solution or across a membrane and is foundational to understanding passive transport mechanisms such as facilitated diffusion.

Protein Molecules in Transport

Protein molecules are crucial in the transport of substances across cell membranes, serving as gatekeepers that manage what enters or exits a cell. In the context of passive transport, these proteins provide a pathway for molecules that otherwise couldn't freely pass through the cell's lipid bilayer due to factors such as size or polarity.

There are two primary types of transport proteins: channel proteins, which form open passageways for the quick movement of ions or water, and carrier proteins, which undergo a shape change to 'shuttle' molecules across the membrane. These protein 'corridors' are highly specific, admitting only certain substances, thus maintaining the necessary balance of molecules within the cell.

Carrier Proteins

Carrier proteins are specialized membrane proteins that bind to the specific molecules they transport, such as glucose or amino acids. Unlike channel proteins that create continuous openings, carrier proteins undergo a subtle change in their structure to engulf and then release the transported molecule on the other side of the membrane.

The 'saturation' mentioned in the exercise refers to the maximum rate of transport that can occur when all the carrier proteins are occupied. At this point, adding more of the substance won't increase the rate of transport – similar to a full bus that cannot take more passengers no matter how many are waiting. It's vital to note that the rate of transport will remain constant once saturation is reached, no matter how much the concentration gradient increases. This characteristic is essential in understanding the limitations and capacities of facilitated diffusion in cellular transport.