In AP Bio, a concentration gradient is the difference in concentration of a substance between two regions. Molecules move down the gradient (high to low) during passive transport, and cells spend ATP to pump molecules up the gradient during active transport.
A concentration gradient is just a difference in how much of something exists in one spot versus another. If there's a lot of glucose outside a cell and very little inside, you've got a gradient. Substances naturally drift from where they're crowded (high concentration) to where they're sparse (low concentration) until things even out at equilibrium.
That "downhill" movement is the whole engine behind diffusion and facilitated diffusion (topics 2.6 and 2.7). Per EK 2.6.A.2, large polar molecules slide down their gradient through transport proteins with no energy input. Going the other direction is the catch. Moving a substance against its gradient, from low to high, costs energy, which is why active transport burns ATP (EK 2.8.A.1). The Na⁺/K⁺ pump is the classic example: it builds and maintains gradients that the cell can later cash in.
Concentration gradients live in Unit 2: Cells and tie together four topics: facilitated diffusion (2.6, 2.7), tonicity and osmoregulation (2.7), mechanisms of transport (2.8), and cell compartmentalization (2.9). The core learning objective is AP Bio 2.7.A, which asks you to explain how concentration gradients affect movement of molecules across membranes. You also need 2.6.A (how molecular structure determines what crosses) and 2.8.A (how energy moves things against a gradient). The big idea here is that life depends on controlled movement of stuff across membranes, and gradients are the rule that decides which direction is "free" and which costs ATP.
Keep studying AP Biology Unit 2
Diffusion and Facilitated Diffusion (Unit 2)
Diffusion is just molecules following a concentration gradient downhill. Facilitated diffusion is the same downhill move, but for charged or large polar molecules that need a channel or transport protein to get through the membrane. Either way, no ATP is required because the gradient does the work.
Active Transport and Electrochemical Gradients (Unit 2)
Active transport is gradients in reverse. The Na⁺/K⁺ pump spends ATP to push ions uphill, creating both a concentration gradient and a charge difference. Stack those together and you get an electrochemical gradient, which is what really drives ion movement across a polarized membrane.
Osmosis and Water Potential (Unit 2)
Osmosis is a concentration gradient applied to water. Water moves from high water potential to low (from hypotonic to hypertonic), which is the same high-to-low logic. The water potential equation ψ = ψₚ + ψₛ just lets you put numbers on which way water will flow.
Cell Compartmentalization (Unit 2)
Membrane-bound organelles let a cell maintain different concentrations in different compartments at once (2.9). Without those internal membranes holding gradients apart, competing reactions would mix and the cell couldn't keep, say, a low pH in a lysosome separate from the cytoplasm.
Expect concentration gradients in MCQ stems that hand you two numbers and ask which way something moves. A classic setup gives 10 mM glucose outside and 2 mM inside, then tells you ATP is depleted, and asks what happens. The trick: glucose still moves down its gradient by facilitated diffusion because that doesn't need ATP. Another version pairs a gradient with a membrane potential (like K⁺ at -70 mV) and tests whether you can reason about the electrochemical gradient, not just the concentration one. On FRQs, the 2021 long free-response on polycystic kidney disease asked you to connect water movement to ion movement across membranes, which is gradient reasoning applied to a real disease. You need to identify the gradient direction, decide whether the move is passive or active, and justify whether ATP is required.
A concentration gradient only tracks how much of a substance differs between two regions. An electrochemical gradient adds charge into the picture. For ions like Na⁺ and K⁺, both the concentration difference AND the membrane voltage push on them, so the net direction depends on both forces combined, not just concentration alone.
A concentration gradient is the difference in a substance's concentration between two regions, and molecules naturally move from high to low until they reach equilibrium.
Moving down a concentration gradient (diffusion and facilitated diffusion) requires no energy, while moving up against it requires ATP through active transport.
Facilitated diffusion uses channel or transport proteins to move charged ions and large polar molecules down their gradient with no energy input (EK 2.6.A.2).
The Na⁺/K⁺ pump uses ATP to build and maintain ion gradients and the membrane potential (EK 2.8.A.1).
Osmosis is a concentration gradient for water, where water moves from high water potential (hypotonic) to low water potential (hypertonic).
For ions, you must consider the electrochemical gradient, which combines the concentration difference with the membrane's charge.
It's the difference in concentration of a substance between two regions. Substances move from areas of high concentration to areas of low concentration until they reach equilibrium, and this drives diffusion, facilitated diffusion, and osmosis in Unit 2.
No. Moving down a gradient (high to low) is passive and needs no energy, which is why diffusion and facilitated diffusion are free. Only moving up a gradient (low to high) through active transport costs ATP, like the Na⁺/K⁺ pump does.
A concentration gradient only accounts for how much of a substance differs between regions. An electrochemical gradient adds the membrane's charge, so for ions like Na⁺ and K⁺ you have to consider both concentration and voltage together to predict which way they actually move.
Osmosis is a concentration gradient applied to water. Water moves from high water potential (hypotonic, low solute) to low water potential (hypertonic, high solute), which is the same high-to-low logic just described in terms of water and solutes.
Yes, if it's moving high to low through a transport protein. Facilitated diffusion doesn't need ATP, so depleting energy won't stop downhill glucose movement. ATP only matters when glucose has to be pumped against its gradient.
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