Biology 1.3 revision notes

Fluid mosaic model and cell membrane transport. 

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Cell Membrane and Transport
The Fluid Mosaic Model
The Fluid Mosaic Model is a model that was conceived by S.J.Singer and Garth Nicolson in 1972 in
order to describe the structural features of the biological membranes. The plasma membrane is
described as a fluid because of its hydrophobic (hate of water) integral components such as lipids
and membrane proteins that move laterally or sideways throughout the membrane. That means the
membrane is not solid but more like a fluid. The membrane is also described as mosaic because like a
mosaic that is made up of many different parts the plasma membrane is composed of different kinds
of macromolecules such as integral proteins, peripheral proteins, glycoprotein's, phospholipids,
glycolipids and sometimes cholesterol and lipoproteins.
From the model the plasma membrane is said to be a lipid bilayer with proteins. It is this way because
of its phospholipids component that folds over itself to create a double later when placed in polar
surroundings such as water. This structure of the membrane is essential to its functions some of which
are cellular transport and cell recognition.
Features of the fluid mosaic model
Double layer ­ BILAYER of
phospholipids which can move about by
Diffusion in their own monolayer.
Phospholipids tails point inwards
forming a non polar hydrophobic interior. The
phospholipids heads point outwards facing
the aqueous (water containing) medium
surrounding the membrane.
Some phospholipids fatty acid tails are saturated ­ straight so fit together tightly. Some are
unsaturated ­ bent so fit together loosely. The more unsaturated tails there are the more
fluid the membrane becomes. The lower temp, the less fluid.
Most protein molecules float like icebergs in the layers, some are fixed to structures inside
the cell and don't float.
Some proteins are embedded in the outer layer, some in the inner layer and some span the
two layers. Hydrophobic and hydrophilic parts of the protein molecules sit next to the
Hydrophobic and Hydrophilic portions of the phospholipids of the membrane. This ensures
the proteins stay in the membrane.
The membrane is 7nm thick on average.
Some phospholipids have carbohydrates attached to them called ­ glycolipids.
Some of the proteins have carbohydrates attached to them called ­ glycoprotein.

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The membrane also contains molecules of cholesterol.
The cell membrane is describes as a fluid mosaic model. Some of the proteins completely span the
membrane. These are called intrinsic proteins. Other proteins are found only on the inner surface or
on the outer surface of the membrane. These are called extrinsic proteins.
Hydrogen bonds between the polar regions of the proteins and the phospholipids keep the
membrane stable.
Diffusion
During diffusion molecules from an area of high concentration diffuse to an area of low
concentration.…read more

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Active transport
Is the movement of particles from an area of low concentration to high concentration (against the
concentration). This requires energy from ATP.
To remove all waste products.
To absorb all micro/rare nutrients e.g. in root cells.
The cells contain more/ high concentrations of minerals but they still want to absorb more from
the soil they use active transport.
Osmosis
The movement of water molecules through a semi permeable membrane from an area of high
concentration to an area of low concentration.…read more

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Incipient plasmolysis ­ the cell membrane just starts to pull away from cell wall and
p=0
At this point = s…read more

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