The key thing here is to note that both diffusion and osmosis are passive processes, and active transport is an active process. A passive process is one which does not require any energy in order to occur, and an active process does require energy. Diffusion is simply the movement of any molecule from an area of higher concentration to an area of lower concentration, down a concentration gradient. These can be any molecules in a gas or a liquid. Think of it as though the molecules are simply moving to somewhere where there is more space for them to spread out. Osmosis is a special type of diffusion and can only be used to describe the movement of water molecules (as oppose to another type of molecule such as a gas). Osmosis occurs where there is a semi-permeable membrane which only allows water molecules through. On one side of the membrane there are more solute molecules than there are on the other side (one side has a high concentration of solute). Water molecules move across the semi-permeable membrane from the less concentrated solution to the more concentrated solution, evening out the concentration gradient. They do this until equilibrium is reached and the concentration gradients on either side of the semi-permeable membrane are the same. Active transport requires energy. This is because in active transport, a substance is moved from an area of low concentration to an area of high concentration, against the concentration gradient. This is the opposite direction to diffusion and osmosis. Think of it like a fish swimming upstream in a river, which requires lots of energy!
Active and passive transport are biological processes that move oxygen, water and nutrients into cells and remove waste products. Active transport requires chemical energy because it is the movement of biochemicals from areas of lower concentration to areas of higher concentration. On the other hand, passive trasport moves biochemicals from areas of high concentration to areas of low concentration; so it does not require energy.
| Active Transport uses ATP to pump molecules AGAINST/UP the concentration gradient. Transport occurs from a low concentration of solute to high concentration of solute. Requires cellular energy. | Movement of molecules DOWN the concentration gradient. It goes from high to low concentration, in order to maintain equilibrium in the cells. Does not require cellular energy. |
| Endocytosis, cell membrane/sodium-potassium pump & exocytosis | Diffusion, facilitated diffusion, and osmosis. |
| Transports molecules through the cell membrane against the concentration gradient so more of the substance is inside the cell (i.e. a nutrient) or outside the cell (i.e. a waste) than normal. Disrupts equilibrium established by diffusion. | Maintains dynamic equilibrium of water, gases, nutrients, wastes, etc. between cells and extracellular fluid; allows for small nutrients and gases to enter/exit. No NET diffusion/osmosis after equilibrium is established. |
| proteins, ions, large cells, complex sugars. | Anything soluble (meaning able to dissolve) in lipids, small monosaccharides, water, oxygen, carbon dioxide, sex hormones, etc. |
| phagocytosis, pinocytosis, sodium/potassium pump, secretion of a substance into the bloodstream (process is opposite of phagocytosis & pinocytosis) | diffusion, osmosis, and facilitated diffusion. |
| In eukaryotic cells, amino acids, sugars and lipids need to enter the cell by protein pumps, which require active transport.These items either cannot diffuse or diffuse too slowly for survival. | It maintains equilibrium in the cell. Wastes (carbon dioxide, water, etc.) diffuse out and are excreted; nutrients and oxygen diffuse in to be used by the cell. |
There are two types of active transport: primary and secondary. In primary active transport, specialized trans-membrane proteins recognize the presence of a substance that needs to be transported and serve as pumps, powered by the chemical energy ATP, to carry the desired biochemicals across. In secondary active transport, pore-forming proteins form channels in the cell membrane and force the biochemicals across using an electromagnetic gradient. Often, this energy is gained by simultaneously moving another substance down the concentration gradient.
There are four main types of passive transport: osmosis, diffusion, facilitated diffusion and filtration. Diffusion is the simple movement of particles through a permeable membrane down a concentration gradient (from a more concentrated solution to a less concentrated solution) until the two solutions are of equal concentration. Facilitated diffusion uses special transport proteins to achieve the same effect. Filtration is the movement of water and solute molecules down the concentration gradient, e.g. in the kidneys, and osmosis is the diffusion of water molecules across a selectively permeable membrane. None of these processes require energy.
Video explaining the differences
Here's a good video explaining the process of active and passive transport:
Examples
Examples of active transport include a sodium pump, glucose selection in the intestines, and the uptake of mineral ions by plant roots.
Passive transport occurs in the kidneys and the liver, and in the alveoli of the lungs when they exchange oxygen and carbon dioxide.
References
- Wikipedia: Active Transport
- Wikipedia: Passive Transport
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Understanding:
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport
Cellular membranes possess two key qualities:
- They are semi-permeable (only certain materials may freely cross – large and charged substances are typically blocked)
- They are selective (membrane proteins may regulate the passage of material that cannot freely cross)
Movement of materials across a biological membrane may occur either actively or passively
Passive Transport
Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)
Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)
There are three main types of passive transport:
- Simple diffusion – movement of small or lipophilic molecules (e.g. O2, CO2, etc.)
- Osmosis – movement of water molecules (dependent on solute concentrations)
- Facilitated diffusion – movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)
Active Transport
Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)
Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)
There are two main types of active transport:
- Primary (direct) active transport – Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport
- Secondary (indirect) active transport – Involves coupling the molecule with another moving along an electrochemical gradient
Types of Membrane Transport