Membrane Transport
Substances cross the cell membrane via passive transport (no energy) or active transport (requires ATP).
Substances move across cell membranes in different ways, depending on whether the cell needs to spend energy to make it happen. Transport processes fall into two broad categories: passive transport and active transport.
Key Takeaways
Passive transport (simple diffusion, facilitated diffusion, osmosis) requires no energy and moves substances down their concentration gradient.
Facilitated diffusion is passive but still protein-mediated, using channels or carriers.
Tonicity determines water movement: hypertonic shrinks a cell, hypotonic swells it, isotonic keeps it stable.
Osmotic pressure (π = iMRT) is a colligative property — it depends on particle number, not identity.
Active transport requires energy and moves substances against their gradient; primary active transport uses ATP directly (e.g., the sodium-potassium pump), secondary active transport uses an existing gradient (symport or antiport).
Endocytosis (pinocytosis, phagocytosis) and exocytosis move bulk material via vesicle formation and fusion.
Passive vs. Active Transport: The Big Picture
Passive transport doesn't require any energy input from the cell. Instead, it relies on the natural movement of molecules from an area where they're more concentrated to an area where they're less concentrated — driven by a concentration gradient.
Active transport, in contrast, requires the cell to use energy — usually in the form of ATP — to move substances against their concentration gradient, from an area of low concentration to an area of high concentration.
Feature | Passive Transport | Active Transport |
|---|---|---|
Energy required? | No | Yes (ATP, direct or indirect) |
Direction relative to gradient | With the gradient (high → low) | Against the gradient (low → high) |
Examples | Simple diffusion, facilitated diffusion, osmosis | Sodium-potassium pump, sodium-glucose symporter |
Passive Transport
There are three main types of passive transport: simple diffusion, facilitated diffusion, and osmosis.
Simple Diffusion
Simple diffusion is the movement of small, nonpolar molecules directly through the lipid bilayer, without help from proteins. Oxygen and carbon dioxide are good examples — they're small and nonpolar, so they slip easily between the lipid tails.
Facilitated Diffusion
Some substances can't cross the membrane on their own and need help — that's where facilitated diffusion comes in. It's still passive (no energy required), but it relies on specific membrane proteins to help molecules move across. These proteins can be channels, which form pores that let molecules or ions flow through, or carriers, which bind to the molecule, change shape, and release it on the other side. Large polar molecules like glucose, and ions such as potassium and sodium, often rely on this type of transport.
Osmosis and Tonicity
Osmosis is another type of passive transport: the movement of water across a semipermeable membrane. Water moves from the side with lower solute concentration to the side with higher solute concentration, working to balance concentrations on both sides of the membrane — driven by differences in solute concentration, requiring no energy.
The effect of osmosis on a cell depends on the tonicity of the surrounding solution:
Solution | Solute vs. Cell Interior | Water Movement | Cell Result |
|---|---|---|---|
Hypertonic | Higher outside | Water leaves the cell | Cell shrinks |
Hypotonic | Lower outside | Water enters the cell | Cell swells, may burst |
Isotonic | Equal | Balanced in/out | Normal size maintained |
In a hypertonic solution, the surrounding fluid has a higher solute concentration than the cell interior; because water moves toward the higher solute concentration, water leaves the cell, causing it to shrink. In a hypotonic solution, the surrounding fluid has a lower solute concentration than the cell interior, so water enters the cell, causing it to swell and potentially burst. In an isotonic solution, solute concentration is equal inside and outside the cell, so water moves in and out at equal rates and the cell maintains its normal size.
Osmotic Pressure (π = iMRT)
Osmotic pressure quantifies the force that drives water movement during osmosis. It's a colligative property, meaning it depends only on the number of dissolved particles in a solution — not their identity. Whether the solute is salt, sugar, or any other molecule, osmotic pressure depends solely on how many particles are present.
Osmotic pressure is calculated using the equation:
π = iMRT
π = osmotic pressure
i = van't Hoff factor (the number of particles a solute dissociates into)
M = molarity
R = the gas constant
T = absolute temperature in Kelvin
This equation shows that osmotic pressure increases with the number of dissolved particles, their concentration, and the temperature. In living cells, osmotic pressure doesn't act against gravity, as in the classic fluid-column example — it acts against the cell membrane. This means cells must carefully balance solute concentrations inside and outside to prevent excessive water gain or loss, which is essential for maintaining cell structure and function.
Active Transport
Active transport moves solutes against their concentration gradient — from lower concentration to higher concentration. Because this works against the natural flow of diffusion, it requires an input of energy, either directly from ATP or indirectly from the energy stored in another gradient.
Primary Active Transport
Primary active transport uses ATP directly to power the transport process. Specialized membrane proteins — often transmembrane enzymes called ATPases — hydrolyze ATP into ADP and inorganic phosphate, releasing energy. That energy is then used to change the shape of the transporter protein and move molecules across the membrane. A key example is the sodium-potassium pump, which uses ATP to move sodium ions out of the cell and potassium ions in, both against their concentration gradients.
Secondary Active Transport: Symport and Antiport
Secondary active transport doesn't use ATP directly. Instead, it uses the energy stored in an existing electrochemical gradient created by primary active transport — the movement of one particle down its gradient powers the movement of another particle against its gradient.
Feature | Primary Active Transport | Secondary Active Transport |
|---|---|---|
Energy source | ATP, directly | An existing electrochemical gradient |
Mechanism | ATPase hydrolyzes ATP to move a solute | One particle's downhill movement drives another's uphill movement |
Example | Sodium-potassium pump | Sodium-glucose symporter |
If both particles move in the same direction across the membrane, it's called symport. If they move in opposite directions, it's called antiport. For example, the sodium-glucose symporter uses the sodium gradient to drive glucose uptake into cells.
Bulk Transport: Endocytosis and Exocytosis
In addition to transport through pumps and carriers, cells move larger volumes of material through bulk transport processes — like endocytosis and exocytosis — which rely on vesicle formation and fusion with the cell membrane.
Endocytosis occurs when the cell membrane folds inward (invaginates) and engulfs material, bringing it into the cell inside a vesicle. This can take different forms: pinocytosis, the uptake of fluids and dissolved substances, and phagocytosis, the engulfing of large particles or solids, such as cellular debris or microbes.
Exocytosis is the reverse process: vesicles formed inside the cell fuse with the cell membrane and release their contents into the extracellular environment. This is how cells secrete hormones, neurotransmitters, and other important substances.
Common MCAT Mistakes
Calling facilitated diffusion "active" because it uses proteins. Facilitated diffusion still requires no energy input and still moves molecules down their concentration gradient — only the mechanism (channels/carriers) distinguishes it from simple diffusion, not the energy source.
Mixing up hypertonic and hypotonic water movement. In a hypertonic solution, water leaves the cell (the cell shrinks) because water moves toward the higher solute concentration outside; in a hypotonic solution, water enters and the cell swells — the solution's name describes the outside environment relative to the cell.
Treating osmotic pressure as dependent on solute identity. Osmotic pressure (π = iMRT) is a colligative property — it depends only on the number of dissolved particles (via the van't Hoff factor and molarity), not on what the solute actually is.
Confusing symport with secondary active transport in general, or forgetting antiport is also secondary. Both symport (same direction) and antiport (opposite directions) are types of secondary active transport — the defining feature of secondary active transport is the energy source (an existing gradient, not direct ATP use), not the direction the two particles move.
MCAT-Style Concept Check
Question: A transport protein uses the inward sodium gradient (maintained by the sodium-potassium pump) to move glucose into the cell against its own concentration gradient, with both sodium and glucose entering the cell in the same direction. Which type of transport does this describe?
A) Primary active transport
B) Secondary active transport, specifically symport
C) Secondary active transport, specifically antiport
D) Facilitated diffusion
Answer: B
Explanation: This describes secondary active transport because the energy driving glucose movement comes from an existing electrochemical gradient (the sodium gradient), not directly from ATP hydrolysis. Because sodium and glucose move in the same direction across the membrane, this specific arrangement is a symport. Option A is wrong because primary active transport requires direct ATP hydrolysis by the transporter itself, which isn't happening here — the sodium-potassium pump did that work earlier to build the gradient. Option C is wrong because antiport involves the two particles moving in opposite directions, not the same direction. Option D is wrong because facilitated diffusion can't move a solute against its concentration gradient; only active transport can.
FAQ
What's the difference between passive and active transport?
Passive transport requires no energy and moves substances down their concentration gradient (simple diffusion, facilitated diffusion, osmosis). Active transport requires energy, usually from ATP, to move substances against their concentration gradient.
How does tonicity affect a cell?
In a hypertonic solution, water leaves the cell and it shrinks. In a hypotonic solution, water enters the cell and it swells, potentially bursting. In an isotonic solution, water moves in and out at equal rates and the cell maintains its normal size.
What's the difference between primary and secondary active transport?
Primary active transport uses ATP directly, hydrolyzed by an ATPase, to move a solute against its gradient (e.g., the sodium-potassium pump). Secondary active transport uses the energy stored in an existing electrochemical gradient instead of ATP directly, moving one particle down its gradient to power another particle's movement against its gradient (symport or antiport).
What's the difference between symport and antiport?
Both are forms of secondary active transport. In symport, both particles move in the same direction across the membrane (e.g., the sodium-glucose symporter). In antiport, the particles move in opposite directions.
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