PinkMonkey Online Study Guide-Biology
3.3 Movement through the plasma membrane
The cell membrane separates the cell from the external
environment. In order to communicate with the external environment for
the purpose of survival (e.g., for the consumption of food, minerals etc.),
there is movement in the cytoplasm and plasma membrane. This movement
occurs through several mechanisms which are listed below:
Osmosis : One method of movement through
the membrane is osmosis. Osmosis is the movement of water. Osmosis
often occurs across a membrane that is semipermeable. A semipermeable
membrane allows only certain molecules to pass through while keeping
other molecules out. Osmosis is really a type of diffusion involving
only water molecules.
Diffusion : Another method of movement through the membrane is diffusion. Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration. This movement occurs due to molecules which constantly collide with each other. The total effectual momentum of the molecules is away from the region of high concentration to the region of low concentration.
Diffusion is the random movement of molecules.
The exchange of molecules (taking place from a higher concentration region
to a lower concentration region) leads to the formation of concentration
gradient. Diffusion phenomena can be seen by letting a drop of dye
into water. The color of the dye gets diffused throughout the water.
Facilitated diffusion : A third method is facilitated
diffusion which occurs across the plasma membrane. This type of diffusion
is very specialized. This occurs only in cases where specific proteins
in the membrane permit only certain molecules across the membrane. These
membrane proteins allow movement in the direction that diffusion would
normally take from a region with a higher concentration of molecules to
a region with a lower concentration of molecules. No energy use is required
for facilitated diffusion.
Active transport : A fourth method for movement
across the membrane is active transport. When active transport takes place,
a protein moves a certain substance across the membrane, usually from
a region of lower concentration to a region of higher concentration. As
you know this movement is against the concentration gradient, hence
energy is required for this movement. Normally the cell gets its energy
from ATP (adenosine triphosphate). For example, in cardiac muscles, active
transport takes place. In these cells, sodium ions are constantly transported
out of the cell. The cellular compartment is a region of high concentration
of sodium ions. Buildup of electrically charged ions allows changes in
voltage over the cell membrane which affects contraction of muscle cells.
Endocytosis : This another mechanism of movement across the plasma membrane. In this type, a small patch of plasma membrane encloses particles or tiny volumes of fluid which are at or near the cell surface. The membrane enclosure then sinks into the cytoplasm and breaks off from the membrane, forming a vesicle that moves into the cytoplasm. When the vesicle contains particulate matter, the process is called phagocytosis. When the vesicle contains liquids or droplets of fluids the process is called pinocytosis.