Textbook Question
A substance moving across a membrane against a concentration gradient is moving by ________.
a. Passive transport
b. Osmosis
c. Facilitated diffusion
d. Active transport
e. Diffusion
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6. The Membrane
Concentration Gradients and Diffusion
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Which of the following would not diffuse through the plasma membrane by means of simple diffusion?
A
Carbon dioxide
B
Glucose
C
Water
D
Oxygen
Verified step by step guidance1
Understand the concept of simple diffusion: Simple diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration without the use of energy or transport proteins. It typically involves small, nonpolar molecules.
Identify the characteristics of molecules that can diffuse through the plasma membrane: The plasma membrane is composed of a phospholipid bilayer, which is permeable to small, nonpolar molecules such as gases (e.g., oxygen and carbon dioxide) and small polar molecules like water.
Examine the properties of each molecule listed: Carbon dioxide (CO₂) and oxygen (O₂) are small, nonpolar molecules, while water (H₂O) is a small polar molecule. Glucose, however, is a larger polar molecule.
Determine which molecule does not fit the criteria for simple diffusion: Glucose is a large polar molecule, which means it cannot pass through the plasma membrane by simple diffusion. It requires a transport protein to facilitate its movement across the membrane.
Conclude that glucose would not diffuse through the plasma membrane by means of simple diffusion, unlike carbon dioxide, water, and oxygen, which can diffuse directly through the membrane.
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