Problem 7 - Campbell Biology Concepts & Connections

Question

Facilitated diffusion across a membrane requires ____ and moves a solute ____ its concentration gradient. a. transport proteins ... up (against) b. transport proteins ... down c. energy and transport proteins ... up d. energy and transport proteins ... down

Step-by-Step Solution

Answer

b

Step 1 - Understanding Facilitated Diffusion

Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special transport proteins.

Step 2 - Identifying the Required Components

Note that facilitated diffusion requires transport proteins to assist the passage of solutes across the cell membrane.

Step 3 - Considering the Solute Movement

Facilitated diffusion moves substances along their concentration gradient, from areas of higher concentration to areas of lower concentration, without the use of energy.

Step 4 - Choosing the Correct Option

By analyzing the options: (a) involves moving solute up the gradient which is not passive, (c) and (d) incorrectly involve energy. Hence, (b) is the correct answer as it appropriately describes facilitated diffusion.

Key Concepts

Understanding Passive Transport

Passive transport is a way cells move substances across their membranes without using any energy. It relies on natural processes. Substances move from areas of high concentration to areas of low concentration.
This movement is like rolling a ball down a hill. The ball moves without any push. In cells, molecules move across the membrane through processes like diffusion, osmosis, and facilitated diffusion. These processes help cells maintain balance and function properly.

In facilitated diffusion, certain molecules need help to move across the membrane. This help comes from transport proteins, which act like channels or carriers, guiding molecules through the membrane.

Role of Transport Proteins

Transport proteins are essential for facilitated diffusion. These proteins are located within the cell membrane and assist in moving substances that can't pass through on their own. There are two main types of transport proteins: channel proteins and carrier proteins.

• Channel proteins form pores or tunnels that allow specific molecules to flow through. They are like doorways in the cell membrane, permitting certain substances to pass as long as they fit the channel's criteria.
• Carrier proteins, on the other hand, change shape to transport molecules. They bind to the substrate on one side of the membrane and release it on the other side.

These proteins are highly specific, meaning that each one only allows certain molecules to pass. This specificity ensures that cells efficiently control what enters and leaves them, maintaining internal balance without expending energy.

What is a Concentration Gradient?

A concentration gradient refers to the difference in the concentration of a substance across a space. In biology, it's often the difference in concentration of a solute across a cell membrane. Substances tend to move naturally from areas of high concentration to areas of low concentration. This movement continues until the concentration is equal on both sides.
You can think of a concentration gradient like a river. Water flows from higher elevations to lower elevations naturally. In cells, molecules move down their concentration gradient through diffusion, including facilitated diffusion.

The concentration gradient is the driving force behind passive transport. Because it does not require energy, substances can move across membranes efficiently based on this gradient alone. Understanding how these gradients work helps explain many physiological processes in the body.