Active and passive transport are two processes for moving ions and molecules into and out of cells. The key difference between them is that active transport requires the input of energy, while passive transport does not. Active transport moves ions and molecule against the concentration gradient (from lower to higher concentration), while passive transport moves chemicals from higher to lower concentration. Together, these processes transport oxygen, water, ions, nutrients, and waste products across plasma membranes.
Overview of Transport Processes
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 moves molecules against the concentration gradient or against polar repulsion. Types of chemicals moved via active transport include sugars, amino acids, and ions (against the concentration gradient). The main types of active transport 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.
|Active Transport||Passive Transport|
|Definition||Active transport moves molecules from low to high concentration, requiring energy (ATP).||Passive transport moves molecules from high to low concentration and does not require energy.|
|Types||Endocytosis, exocytosis, ion and protein pumps||diffusion, facilitated diffusion, osmosis, filtration|
|Transported Molecules||proteins, ions, complex sugars, other large molecules, cells||water, oxygen, carbon dioxide, lipid-soluble molecules, small sugars|
|Functions||Transports molecules so there can be more inside the cell than outside||Maintains dynamic equilibrium of water, gases, nutrients, and waste|
|Examples||phagocytosis, pinocytosis, sodium-potassium pump||diffusion of oxygen and carbon dioxide, facilitated diffusion of hormones|
|Importance||Provides essential nutrients in a timely manner, including amino acids, sugars, and large lipids||Maintains equilibrium within the cell and allows free transport of oxygen, water, and carbon dioxide.|
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.
Membrane Transport Worksheet #1
Membrane Transport Worksheet #2
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