Problem 6 - Biology of Humans: Concepts, Applications, and Issues

Question

Facilitated diffusion is a. the random movement of a substance from a region of higher concentration to a region of lower concentration. b. the movement of water across the plasma membrane. c. the movement of molecules across the plasma membrane against a concentration gradient with the aid of a carrier protein and energy supplied by the cell. d. the movement of a substance from a region of higher concentration to a region of lower concentration with the aid of a membrane protein.

Step-by-Step Solution

Answer

d

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 - Analyzing the Options

Examine each option to determine which best describes facilitated diffusion:a. Random movement of substances down their concentration gradient without assistance.b. Movement of water, which is actually osmosis.c. Movement requiring energy and carrier proteins, which describes active transport.d. Movement down a concentration gradient with the aid of a membrane protein, without energy.

Step 3 - Identifying the Correct Answer

Facilitated diffusion specifically involves the movement of a substance from high to low concentration with the help of membrane proteins, fitting option d.

Key Concepts

Passive Transport

Passive transport is a way for molecules to move across cell membranes without the need for energy from the cell. This means the cell does not use ATP (adenosine triphosphate) to facilitate the movement. Instead, molecules move due to natural processes. There are different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion.
In simple diffusion, molecules move directly through the lipid bilayer from an area of higher concentration to an area of lower concentration until an equilibrium is reached. Osmosis, on the other hand, specifically involves the movement of water molecules through a semipermeable membrane. Facilitated diffusion is a bit more specific. It requires the help of membrane proteins to move molecules from a region of high concentration to a region of low concentration.
Molecules that use passive transport often include oxygen, carbon dioxide, and small non-polar molecules. This process is crucial for maintaining the balance of substances inside and outside the cell, ensuring proper cell function and survival.

Membrane Proteins

Membrane proteins play an essential role in facilitated diffusion. These proteins act as channels or carriers that help move molecules across the cell membrane. Without these proteins, many substances would not be able to enter or leave the cell efficiently.
There are two main types of membrane proteins involved in facilitated diffusion:

• Channel proteins: These create a pore through the membrane, allowing specific molecules or ions to pass through. They act like gates that open and close to control the flow of substances.
• Carrier proteins: These bind to the substance they transport. Once bound, the carrier protein changes shape to move the molecule across the membrane.

Membrane proteins are highly selective, meaning they only allow certain substances to pass through. This selectivity is crucial for maintaining the correct internal environment of the cell, allowing necessary nutrients in and keeping harmful substances out.
For example, glucose, a vital energy source, often enters cells through facilitated diffusion using special carrier proteins called glucose transporters.

Concentration Gradient

The concentration gradient is a key concept in understanding facilitated diffusion. It refers to the difference in the concentration of a substance between two different areas. For example, there may be a high concentration of a specific molecule outside the cell and a low concentration inside the cell.
Molecules naturally move from areas of high concentration to areas of low concentration, a process driven by the desire to reach equilibrium. This means that the molecules will continue to move until the concentration is the same on both sides of the membrane.

• Gradient direction: In facilitated diffusion, molecules move 'down' the concentration gradient, meaning they move from high to low concentration.
• Equilibrium: Once molecules are evenly spread out, the concentration gradient no longer exists, and there is no net movement of molecules.

The concentration gradient is vital for many physiological processes. For instance, oxygen moves from high concentration areas in the lungs to lower concentration areas in the blood, providing vital oxygen to cells throughout the body. Similarly, waste products are expelled from cells where they are in high concentration to lower concentration areas outside the cell.