3. Cell Membrane (Plasma Membrane):
The membrane is composed of a phospholipid bilayer
arranged back-to-back. The membrane is also covered in places
with cholesterol molecules and proteins. The plasma membrane is
selectively permeable and regulates which molecules are allowed to
enter and exit the cell.
Cell Membrane
4. A model that describes the structure of cell membranes. In this model,
a flexible layer made of lipid molecules is interspersed with large protein
molecules that act as channels through which other molecules enter
and leave the cell.
Fluid Mosaic Model
5. Phospholipid Bilayer
Two Layers of phospholipid back to back
Cholesterol Molecules
Helps to stabilize phospholipid and keep
them in position
Composition of Cell Membrane
6. Proteins
1. Peripheral Proteins
That only go halfway to the membrane
1. Integral Proteins
Entirely through the membrane
These proteins serve a channel to allow
the molecule to enter and leave the cell
Composition of Cell Membrane
7. Channel Proteins in Facilitated Transport
Facilitated transport moves substances down their concentration gradients. They
may cross the plasma membrane with the aid of channel proteins.
8. Carrier Proteins
Some substances are able to move down their concentration gradient across the
plasma membrane with the aid of carrier proteins. Carrier proteins change shape as
they move molecules across the membrane.
9. Main Function
● to maintain the physical integrity of the cell - that is
to mechanically enclose the contents of the cell,
and also
● to control the movement of particles e.g. ions or
molecules, into and out of the cell
Protection
• The cell membrane protects the cell from some
harmful chemicals in its external environment.
• It also protects the cell from loss of useful
biological macromolecules held within the cell by
its plasma membrane.
Functions of Cell Membrane
10. Selective Permeability
The cell membranes that enclose cells
(inside the cell wall in the cases of plant
cells and prokaryotic cells) are selectively
permeable. That is, the structure of these
membranes is such that they allow
certain particles, incl. e.g. molecules, -
but not others - to pass through the
membrane, hence into or out of the cell
Functions of Cell Membrane
11. Active Transport
Cell membranes, also known as "plasma membranes",
can allow active transport of specific molecules across
the cell membrane in either direction, i.e. either into or
out of the cell.
That is - cell membranes can allow some particular
molecules to move against a concentration
gradient e.g. from a lower concentration outside the
cell to a higher concentration inside the cell, or
vice-versa. Active transport (movement against the
concentration gradient) requires, that is it uses, energy
Functions of Cell Membrane
12. Metabolic Activites
Plasma membranes include as part of their structures
certain proteins and enzymes that are involved in some
of the metabolic processes of the cell.
Functions of Cell Membrane
