The membrane structure is a fluid mosaic in which
phospholipids move quickly in their own layer and slowly
from one layer to the other resulting in the displacement of
other molecules. The membrane is a diffusion barrier that
controls the exchanges of ions, solutes, metabolites, or
macromolecules between the inside of the cell and its environment. The transport of substances through the membrane
interacts with the cellular metabolism.
Simple diffusion transports concern small inorganic or
organic molecules (O 2 , CO 2 , NH 3 , H 2 O, ethanol, etc.). They
require a concentration gradient on both sides of the membrane and do not allow the intracellular accumulation of
substances (passive diffusion). However, specific proteins
may help the passage of solutes in the presence of a concentration gradient; it is the facilitated diffusion that does not
involve energy and allows passage of molecules such as
glycerol, fatty acids, aromatic acids, etc.
Active transports are used to transport substances when
they are in low concentration in the environment of the cell.
These transports, allowing accumulation of substances
against concentration gradients, require energy. Several
mechanisms are involved:
1. ABC transporters (ATP-binding cassette transporters) use
the binding proteins located on the external face of the
membrane that bind to the substance to be transported
(mono- or disaccharides, amino acids, organic acids,
nucleosides, some inorganic ions). These proteins then
bind to an intramembranous specific carrier. Finally,
energy from the hydrolysis of ATP releases the nutrient
that is transferred into the cytoplasm.
Cocci
Diplococci
Chain of
cocci
Tetrads of
cocci
Cocci in
cluster
Ovoid cells
Chain of
rods
Platelet of cocci
Vibrio-shaped
rods
Spiral cell
Square cells
Bud
Slender
tube
Budding cell
Rod-shaped cells
Stalked cell
Filament of cells
Branched
filaments
Cell with
endospore
Filaments
forming a rosette
Hyphae
and exospores
Cell
without wall
Fig. 3.2 Some shapes and
associations of prokaryotic cells
(Drawing: M.-J. Bodiou)
28
R. Matheron and P. Caumette
phospholipids move quickly in their own layer and slowly
from one layer to the other resulting in the displacement of
other molecules. The membrane is a diffusion barrier that
controls the exchanges of ions, solutes, metabolites, or
macromolecules between the inside of the cell and its environment. The transport of substances through the membrane
interacts with the cellular metabolism.
Simple diffusion transports concern small inorganic or
organic molecules (O 2 , CO 2 , NH 3 , H 2 O, ethanol, etc.). They
require a concentration gradient on both sides of the membrane and do not allow the intracellular accumulation of
substances (passive diffusion). However, specific proteins
may help the passage of solutes in the presence of a concentration gradient; it is the facilitated diffusion that does not
involve energy and allows passage of molecules such as
glycerol, fatty acids, aromatic acids, etc.
Active transports are used to transport substances when
they are in low concentration in the environment of the cell.
These transports, allowing accumulation of substances
against concentration gradients, require energy. Several
mechanisms are involved:
1. ABC transporters (ATP-binding cassette transporters) use
the binding proteins located on the external face of the
membrane that bind to the substance to be transported
(mono- or disaccharides, amino acids, organic acids,
nucleosides, some inorganic ions). These proteins then
bind to an intramembranous specific carrier. Finally,
energy from the hydrolysis of ATP releases the nutrient
that is transferred into the cytoplasm.
Cocci
Diplococci
Chain of
cocci
Tetrads of
cocci
Cocci in
cluster
Ovoid cells
Chain of
rods
Platelet of cocci
Vibrio-shaped
rods
Spiral cell
Square cells
Bud
Slender
tube
Budding cell
Rod-shaped cells
Stalked cell
Filament of cells
Branched
filaments
Cell with
endospore
Filaments
forming a rosette
Hyphae
and exospores
Cell
without wall
Fig. 3.2 Some shapes and
associations of prokaryotic cells
(Drawing: M.-J. Bodiou)
28
R. Matheron and P. Caumette