- Peripheral proteins. Peripheral proteins are on the inner and outer surface
of the cytoplasmic membrane and have the characteristics of a polar and
non-polar regions.
The Function of the Cytoplasmic Membrane
The cytoplasmic membrane regulates the flow of molecules (such as nutrients)
into the cell and removes waste from the cell by opening and closing passages
called channels. In photosynthetic prokaryotes, the cytoplasmic membrane func-
tions in energy production by collecting energy in the form of light.
The cytoplasmic membrane is selectively permeablebecause it permits the
transport of some substances and inhibits the transport of other substances. Two
types of transport mechanisms are used to move substances through the cyto-
plasmic membrane. These are passive transport and active transport.
Passive TransportPassive transportmoves substances into and out of the cell down a gradient
similar to how a rock rolls downhill, following the gradient. There are three
types of passive transport. These are:
- Simple diffusion.Simple diffusion is the movement of substances from a
higher-concentration region to a lower-concentration region (net move-
ment). Only small chemicals (oxygen and carbon dioxide) or lipid-soluble
chemicals (fatty acids) diffuse freely through the cytoplasmic membrane,
using simple diffusion. Large molecules (monosaccharide and glucose) are
too large to enter the cell. - Facilitated diffusion.Facilitated diffusion is the movement of substances
from a higher-concentration region to a lower-concentration region (net
movement) with the assistance of an integral protein across a selectively
permeable membrane. The phospholipid bilayer prevents the movement of
large molecules across the membrane until a pathway is formed using
facilitated diffusion. The integral protein acts as a carrier by changing the
shape of large molecules so the protein can transport the large molecules
across the membrane. - Osmosis.Osmosis is the net movement (diffusion) of a solvent (water in
living organisms) from a region of higher water concentration to a region
of lower concentration.
CHAPTER 4 Prokaryotic and Eukaryotic Cells^75