Similarities & Differences Between Osmosis & Diffusion | Sciencing
Covers selective permeability of membranes, diffusion, and facilitated diffusion ( including channels and carrier proteins). Understanding of Relationship between Phospholipid Membrane Permeability and Self-Diffusion Coefficients of Some Drugs and Biologically Active. The Cell Membrane: Diffusion, Osmosis, and Active Transport otherwise elastic membrane stability and make it less permeable to water-soluble substances.
Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane.
A concentration gradient is a just a region of space over which the concentration of a substance changes, and substances will naturally move down their gradients, from an area of higher to an area of lower concentration. In cells, some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion.
Effect of Membrane Permeability on Diffusion and Osmosis
Selective permeability The phospholipids of plasma membranes are amphipathic: The hydrophobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others.
Structure of a phospholipid, showing hydrophobic fatty acid tails and hydrophilic hea.
A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown. Image modified from OpenStax Biology.
Polar and charged molecules have much more trouble crossing the membrane. Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrophobic core. Water molecules, for instance, cannot cross the membrane rapidly although thanks to their small size and lack of a full charge, they can cross at a slow rate.
Additionally, while small ions are the right size to slip through the membrane, their charge prevents them from doing so. Larger charged and polar molecules, like sugars and amino acids, also need help from proteins to efficiently cross the membrane.
Diffusion and passive transport
Diffusion In the process of diffusion, a substance tends to move from an area of high concentration to an area of low concentration until its concentration becomes equal throughout a space. For example, think about someone opening a bottle of cleaning ammonia in the middle of a room. The ammonia molecules will initially be most concentrated right where the person opened the bottle, with few or no molecules at the edges of the room.
Ultimately, if the bottle is capped and the room is closed, the ammonia molecules will become evenly distributed throughout its volume. The same will happen with molecules of any type: This process does not require any energy input; in fact, a concentration gradient itself is a form of stored potential energy, and this energy is used up as the concentrations equalize.
Image showing the process of diffusion across the plasma membrane. Initially, the concentration of molecules is higher on the outside. There is net movement of molecules from the outside to the inside of the cell until the concentrations are equal on both sides. Each individual substance in a solution or space has its own concentration gradient, independent of the concentration gradients of other materials, and will diffuse according to that gradient.
Other factors being equal, a stronger concentration gradient larger concentration difference between regions results in faster diffusion. Thus, in a single cell, there can be different rates and directions of diffusion for different molecules.
How do diffusion and osmosis relate to the function of a cell membrane?
For example, oxygen might move into the cell by diffusion, while at the same time, carbon dioxide might move out in obedience to its own concentration gradient. Diffusion sees molecules in an area of high concentration move to areas with a lower concentration, while osmosis refers to the process by which water, or other solvents, moves through a semipermeable membrane, leaving other bits of matter in its wake.
For example, oxygen diffuses into red blood cells, and salt placed outside a cell will draw out the cell's water through osmosis, dehydrating it. While they seem similar, they have different mechanisms of action and purposes in Earth's many species. Diffusion Follows a Downhill Concentration Gradient Gases and substances dissolved in a liquid diffuse from an area of high concentration to one of low concentration.
For example, if you spray perfume into the air, the volatile perfume molecules will spread out in the air from the concentrated point of origin.
Diffusion also takes place with or without a permeable membrane in a liquid, such as water. Diffusion of small molecules across plant or animal cell membranes follows a concentration gradient.
For example, if oxygen is higher on the outside of a cell, it will diffuse into the cell until the oxygen concentrations are equal on the outside and inside of the cell. Osmosis Follows an Uphill Concentration Gradient During osmosis, water flows from a low solute concentration across a semipermeable membrane to a high solute concentration. For example, if you add water to a blood sample, consisting of plasma and red blood cells, water will enter the red blood cells and cause them to swell, because the blood plasma has become less concentrated than the inside of the red blood cells.
However, if you add sugar or salt to the blood sample, water will leave the red blood cells and cause them to shrink and pucker.