Question

What is the name given to the process in which solutes are moved across a membrane against their concentration gradient? a. simple diffusion b. facilitated diffusion c. osmosis d. passive diffusion e. active transport

Short answer

The answer is (e) active transport.

Step-by-step solution

Analyzing Choices

First, it’s important to understand the characteristics of each term given as options in the question. \n\n(a) Simple diffusion is the process where a substance moves from an area of high concentration to an area of low concentration, without any energy input and across a semi-permeable membrane. It doesn't fit the definition given in the question. \n\n(b) Facilitated diffusion is similar to simple diffusion, but the difference is that it needs the help of transport proteins. Yet, no energy is required and it, too, follows the concentration gradient. Therefore, it doesn’t help in moving solutes against their concentration gradient. \n\n(c) Osmosis is the movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. It also follows the concentration gradient and doesn't require energy. \n\n(d) Passive diffusion is just another name for simple diffusion. Like the other options above, it also does not require energy and follows the concentration gradient. \n\nConsidering all the options stated, none of them fit the definition of moving solutes across a membrane against their concentration gradient except for (e).

Confirming the Answer

(e) Active transport is the only process which moves substances across a cell membrane from an area of lower concentration to an area of higher concentration against the concentration gradient. This process requires energy, usually in the form of ATP, because it is moving substances 'uphill'. Thus, this matches the process described in the question.

Key concepts

Concentration Gradient

In the context of biology, a concentration gradient refers to the gradual change in the concentration of solutes within a solution over a certain area. Think of it as a slope going from an area where those solutes are packed tightly together (high concentration) to regions where they are spread out (low concentration).
Cells often need to move substances against this slope, which is like carrying groceries up a set of stairs rather than letting them roll down. This process is pivotal in maintaining cell homeostasis and requires more than just the passive sliding down the gradient - it calls for an active mechanism.

Transport Proteins

Imagine transport proteins as specialized doorkeepers in the cell membrane. These doorkeepers don't just open and close to allow any passersby through. They are highly selective, and their primary role is to assist specific substances in crossing the cell membrane.
Some of these proteins help in facilitated diffusion, which is the transport of substances down their concentration gradient, but this process does not require energy. For moving substances against the gradient, a different type of doorkeeper is required. These are active transport proteins, which use energy to shuttle substances from areas of low concentration to areas of high concentration, defying the natural downhill flow.

Cell Membrane

The cell membrane, also known as the plasma membrane, is the gatekeeper of the cell, selectively letting substances in and out. Mainly composed of a double layer of lipids (fats) and embedded with various proteins, it maintains the internal environment of the cell by being selectively permeable.
Active transport takes place here, where the membrane employs transport proteins to 'pick up' ions or molecules outside the cell and 'drop them off' inside, working against the concentration gradient. It's a meticulously managed process ensuring that essential substances like glucose and ions maintain the proper balance for the cell's survival.

ATP

Adenosine triphosphate (ATP) is the universal energy currency of the cell. Think of it as a fully charged battery that is ready to power cellular processes. When cells require energy, ATP breaks down into adenosine diphosphate (ADP) and an inorganic phosphate, releasing energy.
This energy release is what fuels active transport mechanisms. Active transport proteins can use the energy from ATP to move substances into the cell against the concentration gradient - a vital process for cells to acquire nutrients and expel wastes, even when the process goes against the natural downhill flow of diffusion.