Cells use active transport proteins to move specific molecules across the cell membrane against their concentration gradient by using energy from adenosine triphosphate (ATP).

Introduction / Context:
Active transport proteins are essential components of cell membranes. This question checks whether you understand that these proteins move substances across the membrane against the concentration gradient and that this process requires energy, usually supplied by ATP. Understanding active transport is important for topics such as nerve impulses, kidney function, and nutrient absorption in the intestine.

Given Data / Assumptions:

• Cells have protein molecules embedded in their plasma membrane.
• These proteins can use cellular energy to move substances.
• The direction of movement can be against the concentration gradient.

Concept / Approach:
Active transport is defined as the movement of ions or molecules across a membrane from a region of lower concentration to a region of higher concentration. Because this movement goes against the natural tendency of diffusion, it cannot occur spontaneously and therefore requires an input of energy. In cells, this energy usually comes from ATP. Active transport proteins, such as ion pumps or carrier proteins, bind specific substances and change their shape to move them across the membrane.

Step-by-Step Solution:
Step 1: Recall that diffusion and osmosis move substances from high concentration to low concentration without energy, which is called passive transport. Step 2: Active transport is the opposite in terms of direction. It moves substances from low concentration to high concentration, which is described as moving against the concentration gradient. Step 3: Because this movement is uphill, the cell must spend energy. Transport proteins often use ATP, which is split to release energy that powers a change in protein shape. Step 4: Therefore, active transport proteins specifically help to move selected ions or molecules from low to high concentration using energy.

Verification / Alternative check:
A classic example is the sodium potassium pump in animal cells. It moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients. This pump clearly requires ATP. This example confirms that active transport proteins are designed to move substances from lower to higher concentration using energy rather than allowing simple diffusion.

Why Other Options Are Wrong:

• Option B: Passive diffusion from high to low concentration does not require energy and does not need active transport proteins.
• Option C: Genetic information storage is the role of DNA in the nucleus, not membrane transport proteins.
• Option D: Lipid synthesis occurs mainly in the smooth endoplasmic reticulum, not in active transport proteins of the plasma membrane.

Common Pitfalls:
Students sometimes confuse active and passive transport and think any movement through a protein is active transport. In reality, many channels allow passive flow without energy. Only when substances are moved against the concentration gradient with energy use is it called active transport. Another common mistake is forgetting that ATP is usually the energy source for these proteins.

Final Answer:
The correct role of active transport proteins is to move selected substances from low concentration to high concentration across the membrane using energy.