Movement of Materials into and out of the Cell
We noted earlier that all substances that enter or leave the cell must pass
through the cell membrane in some way.
The permeability of a membrane is its capacity to allow materials to move through it.
Since the cell membrane of animal cells is selective and does not allow all materials to pass through it, we say that it is semipermeable.
Material can move into and out of a cell in these ways:
• Active transport
• Pinocytosis and Phagocytosis
Some materials readily pass through the membrane from an area of higher concentration to an area of lower concentration. This process is called diffusion.
When materials require help to pass through the cell membrane, the process is referred to as facilitated diffusion.
In certain situations, materials pass through the cell membrane against the concentration gradient.
In this case, an expenditure of energy is required. The process is called active transport.
An example is the sodium/potassium pump, in which the sodium ions are forced out of the cytoplasm of the cell and into the surrounding tissue fluid and potassium ions are pumped back into the cell cytoplasm.
Sometimes a substance is not able to pass through the cell membrane.
When the concentration of this substance is greater on one side of the cell membrane than the other, water will tend to pass through the membrane to the area of greater concentration.
This process is called osmosis.
Pinocytosis and Phagocytosis
Sometimes, the cell membrane will engulf a minute amount of tissue fluid and its contents. This process is called pinocytosis.
During pinocytosis, the cell membrane produces a vacuole to contain the engulfed
When the cell membrane engulfs larger particles, such as bacteria or other
cells, the process is called phagocytosis.
After either pinocytosis or phagocytosis, digestive fluids may pass from the cytoplasm into the vacuole.
The end products of digestion are absorbed from the vacuole into the cell cytoplasm.
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