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The Cell Membrane
The cell membrane (or plasma membrane)
surrounds all living cells, and is the cell's most
important organelle. It controls how substances
can move in and out of the cell and is
responsible for many other properties of the cell
as well. The membranes that surround the
nucleus and other organelles are almost
identical to the cell membrane. Membranes are
composed of phospholipids, proteins and
carbohydrates arranged in a fluid mosaic
structure, as shown in diagram.
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Cell membrane structure
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The Cell Membrane
The phospholipids form a thin, flexible sheet, while the proteins "float" in the phospholipid sheet like icebergs, and the carbohydrates extend out from the proteins.
The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Different kinds of membranes can contain phospholipids with different fatty acids, affecting the strength and flexibility of the membrane, and animal cell membranes also contain cholesterol linking the fatty acids together and so stabilising and strengthening the membrane.
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The Cell Membrane
The proteins usually span from one side of the phospholipid bilayer to the other (integral proteins), but can also sit on one of the surfaces (peripheral proteins). They can slide around the membrane very quickly and collide with each other, but can never flip from one side to the other. The proteins have hydrophilic amino acids in contact with the water on the outside of membranes, and hydrophobic amino acids in contact with the fatty chains inside the membrane. Proteins comprise about 50% of the mass of membranes, and are responsible for most of the membrane's properties.
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The Cell Membrane
The carbohydrates are found on the outer
surface of all eukaryotic cell membranes, and
are attached to the membrane proteins or
sometimes to the phospholipids. Proteins with
carbohydrates attached are called glycoproteins,
while phospholipids with carbohydrates attached
are called glycolipids. The carbohydrates are
short polysaccharides composed of a variety of
different monosaccharides, and form a cell coat
or glycocalyx outside the cell membrane.
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Movement across Cell Membranes
- Cell membranes are a barrier to most substances, and this property allows materials to be concentrated inside cells, excluded from cells, or simply separated from the outside environment. This is compartmentalization is essential for life, as it enables reactions to take place that would otherwise be impossible. Eukaryotic cells can also compartmentalise materials inside organelles. Obviously materials need to be able to enter and leave cells, and there are five main methods by which substances can move across a cell membrane:
- Lipid Diffusion
- Osmosis
- Passive Transport
- Active Transport
- Vesicles
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Osmosis
Osmosis is the diffusion of water across a membrane. It is in fact just normal lipid diffusion, but since water is so important and so abundant in cells (its concentration is about 50 M), the diffusion of water has its own name - osmosis. The contents of cells are essentially solutions of numerous different solutes, and the more concentrated the solution, the more solute molecules there are in a given volume, so the fewer water molecules there are. 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.
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Osmosis
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Cells and Osmosis
- The concentration (or OP) of the solution that
surrounds a cell will affect the state of the cell,
due to osmosis. There are three possible
concentrations of solution to consider:
- Isotonic solution a solution of equal OP (or
concentration) to a cell
- Hypertonic solution a solution of higher OP (or
concentration) than a cell
- Hypotonic solution a solution of lower OP (or
concentration) than a cell
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Cells and Osmosis
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Passive Transport (or Facilitated
Diffusion)
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Active Transport (or Pumping)
Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. 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 pumps are also ATPase enzymes, since they catalyse the splitting of ATP ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.
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Endocytosis
Endocytosis 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).
