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Transport across membranes
- Lipid diffusuion
- 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.
- 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.
- A few substances can diffuse directly through the lipid bilayer part of the membrane.
- 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.
- . 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.
- Osmosis is the diffusion of water across a membrane
- 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.
- 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
- 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.
- 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.
- Channel proteins
- . There are two kinds of transport protein:
- Passive transport is the transport of substances across a
membrane by a trans-membrane protein molecule.
- 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.
- 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).
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