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

Verified step by step guidance

1. Understand the concept of a concentration gradient: A concentration gradient occurs when there is a difference in the concentration of a substance across a membrane, with one side having a higher concentration than the other.
2. Recognize the term 'against a concentration gradient': This means the substance is moving from an area of lower concentration to an area of higher concentration, which requires energy input because it goes against the natural flow of diffusion.
3. Review the definitions of the options provided: a) Passive transport involves movement along the concentration gradient without energy input. b) Osmosis is the movement of water across a membrane. c) Facilitated diffusion is passive transport with the help of transport proteins. d) Active transport requires energy to move substances against the concentration gradient. e) Diffusion is the passive movement of molecules along the concentration gradient.
4. Identify the correct process: Since the problem specifies movement against the concentration gradient, the process must involve energy input, which eliminates passive transport, osmosis, facilitated diffusion, and diffusion.
5. Conclude that the correct answer is active transport, as it is the only process that moves substances against the concentration gradient using energy, typically in the form of ATP.

Key Concepts

Concentration Gradient

A concentration gradient refers to the difference in the concentration of a substance across a space or membrane. Substances tend to move from areas of higher concentration to areas of lower concentration, which is a fundamental principle in understanding how molecules move within biological systems.

Active Transport

Active transport is the process by which substances move across a cell membrane against their concentration gradient, requiring energy, typically in the form of ATP. This mechanism is crucial for maintaining cellular homeostasis and allowing cells to uptake necessary nutrients or expel waste products.

Passive Transport

Passive transport is the movement of substances across a cell membrane without the use of energy, occurring along the concentration gradient. This includes processes like diffusion, osmosis, and facilitated diffusion, where molecules move from areas of higher concentration to lower concentration until equilibrium is reached.