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Diffusion is the movement of ions or molecules from a region of greater concentration to a region of lesser concentration. Osmosis is the diffusion of water through a selectively permeable cell membrane. Channel proteins and carrier proteins can facilitate the diffusion of charged and polar substances, including water, across cell membranes.
learning outcomes
You should be able to:
Predict rates of diffusion of substances under different conditions.
Predict the effect of different environmental conditions on the movement of water into and out of living cells.
Explain the role of channel proteins in facilitating diffusion across membranes.
What properties determine whether, and how fast, a substance will diffuse across a membrane?
The properties that affect diffusion across a membrane are size and mass (smaller is faster), electric charge (less polar is faster), and concentration gradient (the higher the gradient, the faster the rate of diffusion).
What is the effect on red blood cells if the kidney removes too many ions from the blood and makes it hypotonic to the cells?
If blood is hypotonic, water will enter red blood cells, causing them to swell and perhaps burst. (This is called hemolytic anemia.)
Cells in the nervous system may have a higher concentration of K+ ions inside the cell than outside. What is the role of the cell membrane in maintaining this imbalance? How can K+ membrane channels eliminate this imbalance?
The hydrophobic cell membrane lipids are relatively impermeable to charged ions such as K+. A channel will allow diffusion of K+ out of the cell until equilibrium is reached, with equal concentrations inside and outside the cell.
The process of diffusion tends to equalize the concentrations of substances outside and inside cells. However, one hallmark of a living cell is that it can have an internal composition quite different from that of its environment. To achieve this, a cell must sometimes move substances against their concentration gradients. This process requires an input of energy and is known as active transport.