Active and passive transport are processes that move ions, molecules, and other materials across cell membranes. The key difference is that active transport requires cellular energy, while passive transport does not. Passive transport moves substances down their concentration or electrochemical gradients, while active transport uses energy to move substances against these gradients or to transport materials in vesicles. Together, these processes regulate the movement of oxygen, water, ions, nutrients, and waste products into and out of cells.
Key Takeaways: Active and Passive Transport
- Active transport requires cellular energy, while passive transport does not.
- Passive transport moves substances down their concentration or electrochemical gradients. Active transport can move substances against these gradients.
- The main types of passive transport are simple diffusion, facilitated diffusion, osmosis, and filtration.
- Active transport includes primary active transport, secondary active transport, and bulk transport (endocytosis and exocytosis).
- Primary active transport uses energy directly, usually from ATP. Secondary active transport uses energy stored in electrochemical gradients.
- Transport proteins, including channels, carriers, and pumps, help move substances across cell membranes.
- Both active and passive transport are essential for maintaining cellular homeostasis, absorbing nutrients, and removing waste.
Overview of Transport Processes
Concentration and Electrochemical Gradients
A concentration gradient is a difference in the concentration of a substance between two regions. Substances naturally tend to diffuse from higher to lower concentration because of random molecular motion.
For charged particles, such as sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺) ions, movement also depends on electrical charge. The combined effects of concentration differences and electrical potential form an electrochemical gradient.
- Moving down a gradient: Substances move in the direction favored by the gradient. This is passive transport.
- Moving against a gradient: Substances move in a direction that requires an energy source. This is active transport.
For example, the sodium-potassium pump uses ATP to maintain different sodium and potassium concentrations inside and outside animal cells. These gradients are essential for nerve signaling, muscle contraction, and secondary active transport.
Passive Transport
Passive transport moves ions and molecules down the concentration gradient using their natural kinetic energy and entropy — no energy input from the cell is needed. Examples of species that move via passive transport are gases, water, small molecules, and some ions. The main types of passive transport are diffusion, facilitated diffusion (sometimes called facilitated transport), osmosis, and filtration.
- Diffusion is the movement of particles from higher concentration to lower concentration. Water, gases, and small molecules diffuse across a plasma membrane. An example is the movement of carbon dioxide into or out of a cell.
- Facilitated diffusion or carrier-mediated osmosis is the movement of molecules across the membrane using special membrane-embedded transport proteins. An example is absorption of glucose into cells. Glucose is a large enough molecule that it needs a channel that aids its entry into cells. (Note: Some forms of glucose movement involve active transport.)
- Osmosis is the movement of water across a semipermeable membrane. Water freely enters and exits cells via osmosis so its concentration is the same on both sides of the membrane.
- Filtration is the movement of water and dissolved solutes across a membrane through pores. Hydrostatic pressure from the cardiovascular system helps molecules permeate the membrane. For example, the Bowman’s capsule in the kidneys filters albumins, but larger proteins cannot pass.
Active Transport
Active transport uses cellular energy to move substances across membranes against their concentration or electrochemical gradients. It allows cells to accumulate nutrients, maintain ion gradients, and regulate internal conditions. Active transport also includes energy-dependent vesicular processes that move larger materials into or out of cells. The three main categories are primary active transport, secondary active transport, and bulk transport.
- Primary active transport or direct active transport primarily uses ATP hydrolysis or NADH reduction to transport ions and molecules across a membrane. Metal ions (Na+, K+, Mg2+, Ca2+) need ion pumps or channels for crossing membranes.
- Secondary active transport or coupled transport (cotransport) uses energy for transporting molecules, except the process is not directly coupled to ATP. The energy comes from the potential difference achieved by pumping ions into or out of the cell. There are two types of cotransporters. Symporters pump two chemical species in the same direction across a membrane. One species moves with the concentration gradient, providing energy to move the other species. Antiporters pump two species of ions or solutes in opposite directions across the membrane. The energy of the one molecule moving from high to low concentration transports the movement of the other species against the gradient.
- Bulk transport moves materials into and out of cells using vesicles. Endocytosis and exocytosis are forms of bulk transport. Endocytosis surrounds materials with a vesicle and brings the vesicle inside the cell. Exocytosis, in contrast, releases enclosed materials to the outside of the cell. The two main types of endocytosis are pinocytosis (“cell drinking”), which involves liquids, and phagocytosis (“cell eating”), which engulfs solids.
Difference Between Active and Passive Transport
This table summarizes the key points regarding active and passive transport and the differences between them.
| Feature | Active Transport | Passive Transport |
|---|---|---|
| Definition | Uses cellular energy to transport materials against gradients or through vesicles | Moves substances down gradients or by pressure-driven flow without requiring cellular energy |
| Energy | Requires energy directly or indirectly | Does not require cellular energy input |
| Direction | Can move substances against concentration or electrochemical gradients | Moves substances down their gradients; filtration follows a pressure gradient |
| Types | Primary active transport, secondary active transport, endocytosis, exocytosis | Simple diffusion, facilitated diffusion, osmosis, filtration |
| Transported Materials | Ions, glucose, amino acids, proteins, large particles | Gases, water, ions, glucose, other small molecules |
| Transport Proteins | Uses pumps and cotransporters; bulk transport uses vesicles | Simple diffusion needs no protein; facilitated diffusion uses channels or carriers |
| Examples | Sodium-potassium pump, sodium-glucose cotransport, phagocytosis | Oxygen diffusion, GLUT-mediated glucose transport, osmosis |
| Importance | Maintains ion gradients, accumulates nutrients, and enables secretion and uptake of large materials | Supports gas exchange, water balance, nutrient movement, and waste removal |
Similarities Between Active and Passive Transport
Despite their differences, active and passive transport perform many of the same functions:
- Both move materials across cell membranes.
- Both help maintain cellular homeostasis.
- Both transport nutrients, ions, or other substances needed for cell function.
- Both can involve specialized membrane proteins.
- Both contribute to processes such as nutrient absorption, waste removal, and cellular communication.
Cells commonly use both transport mechanisms together. For example, the sodium-potassium pump maintains an ion gradient through active transport, while ion channels allow ions to move down their electrochemical gradients through passive transport.
Active and Passive Transport Worksheet
Test your understanding of active and passive transport with these worksheets. Download and print the worksheets for personal use or as class assignments.
Frequently Asked Questions
Does passive transport require ATP?
No. Passive transport does not directly consume ATP or require an energy input from the cell. It relies on existing concentration, electrochemical, or pressure gradients. However, cells sometimes use ATP-dependent processes to establish the gradients that later drive passive transport.
Is osmosis active or passive transport?
Osmosis is passive transport because water moves across a selectively permeable membrane without cellular energy input. Water moves according to its water potential gradient, which depends on factors such as solute concentration and pressure.
Can the same molecule use both active and passive transport?
Yes. The transport mechanism depends on the molecule, its concentration or electrochemical gradient, and the transport proteins available. For example, glucose enters some cells through GLUT carrier proteins by facilitated diffusion, while intestinal cells can absorb glucose using sodium-glucose cotransport, a form of secondary active transport.
References
- Jahn, Reinhard; Südhof, Thomas C. (1999). “Membrane Fusion and Exocytosis”. Annual Review of Biochemistry. 68 (1): 863–911. doi:10.1146/annurev.biochem.68.1.863
- Reese, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B. (2014). Campbell’s Biology (10th ed.). United States: Pearson Education Inc. ISBN 978-0-321-77565-8.
- Rosenberg, T. (1948). “On accumulation and active transport in biological systems. I. Thermodynamic considerations”. Acta Chem. Scand. 2: 14–33. doi:10.3891/acta.chem.scand.02-0014
- Sadava, David; H. Craig Heller; Gordon H. Orians; William K. Purves; David M. Hillis (2007). “What are the passive processes of membrane transport?”. Life : The Science of Biology (8th ed.). Sunderland, MA: Sinauer Associates. ISBN 9780716776710.
- Srivastava, P. K. (2005). Elementary Biophysics : An Introduction. Harrow: Alpha Science Internat. ISBN 9781842651933.