Bio Club – Cell Membranes
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Cell membranes are mainly composed of lipids and proteins (almost 1:1 ratio)
The membrane is a phospholipid bilyaer, with proteins on it. (fluid mosaic model).
o Phospholipids are constantly on a move (fluid), but the proteins stay where
they are, held on by microfilaments within the cytoskeleton of the cell.
o Proteins are located “randomly”. Held in place by microfilaments in
cytoskeleton.
o Phosphlipids are amphipathic molecules (hydrophobic and hydrophilic)
In between the phospholipic layers, there is a lot of cholesterol (which is essential).
Cholesterol keeps the normal fluidity within the membrane (stabilizing it),
preventing the cell membrane from becoming too rigid or too fluid. (cholesterol is
also amphipathic)
o When it’s too cold, cell membranes become more rigid, and cholesterol fits in
between the phospholipids so that they don’t condense together and
crystallize.
o When it’s too hot, cell membranes becomes more fluid, and the hydrophobic
portion of the cholesterol molecule attracts the hydrophobic sections of the
phospholipids, and so makes cell membrane slightly immobilized. Under
normal conditions, this also ensures that the cell membrane isn’t too
permeable to molecules.
o Cholesterol help stabilize position of proteins in the cell membrane to ensure
cell function.
The phospholipids in a cell can also be different: some are unsaturated and some are
saturated. Saturated lipids make the cell membrane more rigid, and unsaturated
lipids do the opposite and make the membrane less viscous.
Cell membranes also have glycolipids (combination of carbohydrates and lipid).
Responsible for cell-cell recognition, receptor sites for chemical signals, and binding
cell to tissue.
Types of proteins
o Integral proteins – proteins that have parts of itself in the layer.
o Peripheral proteins – proteins that are on the surface of the bilayer
(intra/extracellular)
o Transmembrane proteins – proteins that go through the whole membrane
(subset of integral proteins)
o Glycoproteins – similar function as glycolipids.
Six major functions of membrane proteins:
o Transport
o Enzymatic activity
o Signal transduction
o Cell-cell recognition
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o Intercellular joining
o Attachment to cytoskeleton and extracellular matrix (structural support to
animal cells )
There are three main methods for transport: passive transport, active transport and
bulk transport.
Passive transport
o Passive transport means that no work needs to be done by the organism to
carry out the transport mechanism.
o In diffusion, molecules move through a concentration gradient (two regions
with different concentrations). This happens due to the kinetic nature of
molecules – molecules move in random motion, and generally moves from a
high concentration to a low concentration until equilibrium is reached.
o In order to pass through the phospholipid bilayer directly, the molecule must
be small and hydrophobic (non-polar). This is because in the middle of the
bilayer, the tails of the phospholipids are non-polar.
 Examples include Carbon Dioxide, Oxygen.
o And so these small hydrophobic pass through the membrane via diffusion.
o For hydrophilic molecules (polar molecules), they pass through the
membrane through facilitated diffusion. Facilitated diffusion is carried out by
integral proteins that have pores through the middle (channel proteins for
smaller ions or polar molecules; carrier proteins for bigger molecules). These
pores open in the presence of the substrate, and the molecules move
through concentration gradient. Despite the name, this is not diffusion. No
energy is required.
 Examples include glucose and small ions
o Osmosis is a special case of facilitated diffusion. Osmosis is the passive
movement of water molecules, across a partially permeable membrane, from
a higher to lower concentration of water molecules (or lower to higher
concentration of solute). No energy is required.
Active transport
o Most of the time cells require substances that are from outside of the cell,
even when the concentration of that substance is greater within the cell.
Diffusion cannot happen since molecules do not move from a lower
concentration to a higher concentration naturally.
o Therefore, active transport is required to bring these molecules into the cell.
Active transport is a process that requires metabolic energy from the cell, in
the form of ATP.
o Active transport has these characteristics:
 Active transport works against a concentration gradient. This means
that molecules move from a region of low concentration to high
concentration. Normally, within the cell there are some ions and
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molecules that are useful to the cell (e.g. Nitrate ions in plant cells)
and these do not escape the cell. However, when more of the
substrate is available outside the cell, the cell takes in these molecules
and ions via active transport.
 Active transport is a highly selective process. Cells choose which ion or
molecule to uptake based on what is needed. For example, in plants,
if sodium nitrate is available for absorption, the plant cell will take in
more nitrate ions than sodium ions.
 Active transport involves special integral proteins called pump
proteins. The pump mechanism is activated by the reaction of the
protein with ATP; ATP supplies the energy for the pump to open and
close.
o Sometimes these pump proteins (or pump molecules) can work in one
direction, or transport two different molecules in two different directions.
o Most common example of two direction active transport: Sodium and
Potassium ions pump.
 ATP from within the cell is joined to the pump protein to activate the
pump.
 Sodium ions are loaded to the pump from within the cell.
 Protein pumps the sodium ions out of the cell. In this process the ATP
that was joined becomes ADP + Pi, and the Phosphate ion stays joined
to the protein.
 Potassium ions are loaded into the protein, and protein transports
potassium ions into the cell.
 At this time the phosphate ion is released, and the entire process
repeats again.
Bulk Transport
o Bulk transport is used to transport big pieces of matter in vesicles in and out
of the cell. This applies for both solids and liquids. Movement into the cell is
called endocytosis, and movement out of the cell is called exocytosis.
o Bulk transport is possible due to the cell membrane’s strength and fluidity.
o There are two types of endocytosis: for solids, it’s called phagocytosis. For
liquids, it’s called pinocytosis.
o The vesicle of matter first touch the cell membrane when it goes in, and then
the cell membrane pulls inward, sucking in the vesicle into the cell. Example:
macrophages – cells that digest damaged cells or bacteria.
o In exocytosis, it’s the same but in the opposite direction. This is how
chemicals and proteins from rough endoplasmic reticulum and Golgi
apparatus are transported out of the cell.