Introduction - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 2

Cells are the key building blocks of living systems. Some of them are self-sufficient
while others co-operate in multicellular organisms. The human body is composed of
cells of 200 different types. A typical size of a cell is of the order of 10

µ m and its

10 − 16 kg. In its natural state, 70 % of the content
constitutes water molecules. The fluid content of a cell is known as the cytoplasm.
The cytoplasm is the liquid medium bound within a cell, while the cytoskeleton is
the lattice of filaments with a network of attracting proteins formed throughout the
cytoplasm.
Two major types of cells are:

dry weight amounts to about 7 ×

(a) prokaryotic: simple cells with no nucleus and no compartments. Bacteria (e.g.
E. coli ) and blue-green algae belong to this group.
(b) eukaryotic: cells with a nucleus and a differentiated structure including com-
partmentalized organelles as well as a filamentous cytoskeleton. Examples here
include higher developed animal and plant cells, green algae, and fungi. Eukary-
otic cells emerged about 2 billion years ago, and comprise all the life kingdoms
except monera. The Greek meaning of the word eukaryotic is “true nucleus”.

µ

Bacteria have linear dimensions in the 1-10 m range while the sizes of eukary-

µ

otic cells range between 10 and 100 m. The interior of a bacterium experiences

considerable pressure reaching up to several atmospheres due to the presence of a
membrane which, except for Archaebacteria , is composed of layers of peptidoglycan
sandwiched between two lipid bilayers the inner of which is a plasma membrane.
Plant cells have linear dimensions that vary between 10 and 100 µ m. They are

bounded by a cell wall whose thickness ranges between 0 m and is com-

posed of cellulose. Among its organelles, plant cells have a nucleus, endoplasmic
reticulum, Golgi apparatus, and mitochondria. Unique to plant cells is the pres-
ence of chloroplasts and vacuoles. Unlike bacteria, plant cells possess a cytoskeletal
network adding to their mechanical strength. Animal cells tend to be smaller than
plant cells since they do not have liquid-filled vacuoles. The organizing center for
their cytoskeleton is a cylindrical organelle called a centriole that is approximately
0

. 1 and 10 µ

m long. Instead of chloroplasts that are sites of photosynthesis, animal cells
have mitochondria that produce the required energy supply in the form of ATP mole-
cules obtained from reactions involving oxygen and food molecules (e.g. glucose,
sucrose, etc).
Eukaryotic cells possess membrane-bound internal structures called organelles
briefly discussed below. Mitochondria produce energy, a Golgi apparatus (where
various macromolecules are modified, sorted, and packaged for secretion from the
cell for distribution to other organelles) is shaped like a stack of disks. The endoplas-
mic reticulum surrounds the nucleus and is the principal site of protein synthesis.
Its volume is small compared to the surface area. A nucleus is the location of chro-
mosomes and the site of DNA replication and transcription. All of the material within
the cell excluding the nucleus is defined as the cytoplasm whose liquid components
are referred to as the cytosol while the solid protein-based structures that float in it
are called the cytoskeleton. The main component of the cytosol is water. Most of the
organelles are bound within their own membranes. Most of the cell's DNA is stored

. 4 µ