A few substances can diffuse directly through the lipid
bilayer part of the membrane.
The only substances that can do this are lipid-soluble
molecules such as steroids, or very small molecules, such as
H2O, O2 and CO2. It’s a passive process.
Osmosis
Osmosis is the diffusion of water across a
membrane
. Water molecules can diffuse freely across a membrane,
but always down their concentration gradient, so water
therefore diffuses from a dilute to a concentrated
solution.
active
Transport
The protein binds a molecule of the substance to be
transported on one side of the membrane, changes shape,
and releases it on the other side. The proteins are highly
specific, so there is a different protein pump for each
molecule to be transported.
Passive
transport
Passive transport is the transport of substances across a
membrane by a trans-membrane protein molecule.
. There are two kinds of transport protein:
Channel proteins
form a water-filled pore or channel in the membrane. This
allows charged substances (usually ions) to diffuse across
membranes. Most channels can be gated (opened or closed),
allowing the cell to control the entry and exit of ions
Carrier proteins
Carrier Proteins have a binding
site for a specific solute and
constantly flip between two
states so that the site is
alternately open to opposite
sides of the membrane. The
substance will bind on the side
where it at a high concentration
and be released where it is at a
low concentration.
Vesicles
Exocytosis: is the transport of materials into a cell. Materials are enclosed by a fold of the cell membrane, which
then pinches shut to form a closed vesicle. Strictly speaking the material has not yet crossed the membrane, so it is
usually digested and the small product molecules are absorbed by the methods above. When the materials and the
vesicles are small (such as a protein molecule) the process is known as pinocytosis (cell drinking), and if the
materials are large (such as a white blood cell ingesting a bacterial cell) the process is known as phagocytosis (cell
eating).
Endocytosis: is the transport of materials out of a cell. It is
the exact reverse of endocytosis. Materials to be exported
must first be enclosed in a membrane vesicle, usually from
the RER and Golgi Body. Hormones and digestive enzymes
are secreted by exocytosis from the secretory cells of the
intestine and endocrine glands.