Movement across membranes (Summary)
There are many membranes involved in cell biology.
All biological cells are enclosed by a plasma membrane (often called the 'cell membrane'). Eukaryotic cells, including plant cells and animal cells, also contain membrane-bound organelles that are surrounded by one or two layers of plasma membrane. For example, both mitochondria and chloroplasts are enclosed by two layers of membranes - called the 'outer membrane' and the 'inner membrane'.
See the structure of plasma membranes for more about their components and construction. In general membranes are barriers that separate regions whose chemical and sometimes physical properties are different e.g.
- Chemical Properties:
Different concentrations of certain chemicals result in different pH values.
- Physical properties:
Different chemical compositions and reactions taking place can result in variations in physical properties e.g. in pressure.
Biological membranes are barriers that are semi-permeable.
Semi-permeable means that they allow some substances to pass across the membrane (barrier), but not others.
Which substances can pass across a membrane depends not only on the particles of the substances themselves but often also on additional considerations such as how much of the substance is already on each side of the membrane (i.e. concentration gradient), or on the availability of energy to support the process because some transport mechanisms require energy to be provided by the cell in order for certain types of substances to be moved from one side of the membrane to the other (i.e. in the cases of Active Transport incl. Bulk Transport - as opposed to Passive Transport).
There are two types of transport mechanisms by which particles, e.g. molecules, traverse membranes:
- Passive Transport mechanisms do not require energy from cells (metabolic energy),
e.g. simple diffusion, facilitated diffusion, osmosis.
- Other transport mechanisms do require energy from cells, e.g. active transport and bulk transport mechanisms.
Types of movement across membranes:
Passive Transport Mechanisms |
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Diffusion - Diffusion is the movement of particles, such as molecules or ions, from a region of higher concentration to a region of lower concentration, i.e. "down a concentration gradient". The energy used in the process of diffusion is supplied by the particles themselves so, for example, rates of diffusion increase with increase in temperature. |
1.  |
Simple Diffusion |
The simplest form of movement across biological membranes e.g. cell membranes (see the diagram of a simple phospholipid bilayer above) depends on the size and polarity of particles. Many non-polar molecules can move through biological membranes unaided. This is also possible for some small polar molecules e.g. water and urea. |
2. |
Facilitated Diffusion |
Facilitated diffusion differs from simple diffusion because it involves "facilitation" of the process by proteins that form part of the structure of the membrane. |
Channel proteins have hollow cores that enable ions and small polar molecules to cross the membrane by passing through channel proteins. (Provides a "channel" for passage of particle) |
Carrier proteins bind to a particle, e.g. a large polar sugar molecule, then move it across the membrane, releasing it on the other side of the of phospholipid bilayer. ("Carries" particle) |
Note: These types of facilitated diffusion are "diffusion" because they involve particles moving down their concentration gradients and "facilitated" because the movement is assisted by the role of the proteins. |
3. |
Osmosis |
Osmosis is the diffusion of water across membranes.
Diffusion is the movement of particles down their concentration gradient.
However, as the idea of "concentration of water" is confusing the term "water potential" is used instead. Hence, in simple terms, osmosis involves movement of water from regions in which there are relatively more water molecules per volume, i.e. "high water potential", to regions where there are fewer water molecules per volume "lower water potential".
(Water potential is expressed in units of pressure, usually kPa. All water potential values are negative ; highest possible value=0, lower water potential -> larger negative values.) |
Water can pass through phospholipid bilayers by simple diffusion or by facilitated diffusion through special channel proteins called aquaporins. (Most water molecules crossing membranes do so via aquaporins.) |
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Transport Mechanisms - requiring energy from cells |
4. |
Active Transport |
Active transport is the movement of a substance, e.g. an ion, against its concentration gradient (i.e. from a low concentration to a higher concentration) by carrier proteins. |
5. |
Bulk Transport |
Bulk transport is the movement of substances across a membrane within a small vacuole (a temporary vacuole formed for this process - much smaller than the permanent vacuoles of plant cells). It is the means by which larger molecules and particles move through membranes enclosed by vesicles formed by sections of cell membrane.
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Note: Vesicles are small vacuoles. Vacuoles, including vesicles, are enclosed by a single layer of membrane. |
Exocytosis is the process by which a cell moves the contents of secretory vesicles out of the cell via the cell membrane. |
Endocytosis is the opposite process by which the contents of secretory vesicles are moved into the cell via the cell membrane. |
There are also other bulk transport mechanisms e.g. phagocytosis and pinocytosis. |
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The above is a simple list of how particles move across membranes.
See the individual links for more about each of the transport mechanisms including how they work and when they apply.
Remember : Biological membranes are semi-permeable barriers.
The two types of transport mechanisms are:
- those that don't require energy from cells incl. passive transport incl. diffusion & osmosis, and
- those that do need energy from cells incl. active transport and bulk transport mechanisms.
For more about the movement of substances around the entire bodies of living things see the section about movement of materials within organisms (i.e. transport in biology).
