FACT
Recall the structure
if lipid molecules
from the previous
chapter onthe
chemistry of lifeFACT
WATCH: a video
about the cell
membrane.
See video:
SHORTCODEat
http://www.everythingscience.co.zaCell membrane DUMMY
Thecellmembrane, also called the plasma membrane, is composed of a double layer of
lipids (fats) called aphospholipid bilayer. It physically separates the intracellular (inside cell)
space from the extracellular (outside cell) environment. All plant and animal cells have cell
membranes. The cell membrane surrounds thecytoplasmwhich is part of the protoplasm
and is the living component of the cell.
The lipid bilayer forms spontaneously due to the properties of the phospholipids from which
it is made. The majority of the lipid structure, the fatty acid chain, isnon-polar (uncharged).
Non-polar substances are unable to dissolve in water and are therefore known ashydropho-
biccompounds. Some lipids also containpolar(charged) regions. Polar substances can dis-
solve in water and are known ashydrophilic(water-loving) compounds. The bilayer forms
because hydrophilic heads point outwards and from hydrogen bonds with water, and the
hydrophobic tails point towards one another.
Figure 3.12: The lipid bilayer showing the arrangement of phospholipids, containing hydrophilic,
polar heads and hydrophobic, non-polar tails.
All the exchanges between the cell and its environment have to pass through the cell mem-
brane. The cell membrane isselectively permeableto ions (e.g. hydrogen, sodium), small
molecules (oxygen, carbon dioxide) and larger molecules (glucose and amino acids) and
controls the movement of substances in and out of the cells. The cell membrane performs
many important functions within the cell such as osmosis, diffusion, transport of nutrients
into the cell, processes of ingestion and secretion. The cell membrane is strong enough to
provide the cell with mechanical support and flexible enough to allow cells to grow and
move.
Structure of the cell membrane: the fluid mosaic model
S.J. Singer and G.L. Nicolson proposed the Fluid Mosaic Model of the cell membrane in
- This model describes the structure of the cell membrane as afluidstructure with
various protein and carbohydrate components diffusing freely across the membrane. The
structure and function of each component of the membrane is provided in the table below.
Table 3.2 refers to the components of the cell membrane shown in the diagram in Figures
3.12 and 3.13.
Chapter 3. The basic units of life 73