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Cell Structures
Chapter three
Cells and Viruses
• There are basically three type of biological
units: prokaryotic cells, eukaryotic cells and
viruses.
• Eukaryotic cells: these cells make up the
human body and plants. This cells are
distinguished by the presence of a nuclear
membrane surrounding the cell nucleus and
the presence of numerous membrane bound
organelles within the cell.
Cont…
• Prokaryotic cells: unlike eukaryotic cells, these
cells lack membranous structures; bacteria
make up the largest class of prokaryotic cells.
• Viruses: viruses differ from both eukaryotic
and prokaryotic cells in the sense that they
can exist as living and non living entities. They
consist of only a nucleic acid molecule
surrounded by a protein shell.
Cell Compartments
• The cell interior is divided into a number of
compartments, these compartments are
surrounded by membranes. These membrane
bound compartments are known as cell
organelles.
• Each cell organelle performs a specific
function that contributes to the cell’s survival.
• The interior of a cell is divided into two
regions:
Cont…
• (a) Nucleus: this is a spherical structure
usually located near the center of the cell.
• (b) Cytoplasm: this region makes up all the
interior of the cell except the nucleus. The
cytoplasm contains two components: cell
organelles and the cytosol ( fluid surrounding
the organelles).
Cont…
• Membranes: these form a major structural
element in human cells. They are found
surrounding the entire cell (plasma
membrane) and enclosing most of the cell
organelles.
• Functions of the membrane
– They act as a selective barrier to the passage of
molecules, allowing some molecules to cross
while excluding others.
Cont…
• One of the advantages of restricting the
movements of molecules across membranes is
that the products of chemical reactions can
often be confined to specific organelles.
• As we shall discover in later chapters, the
hindrance offered by a membrane to the
passage of substances can be altered to allow
increased or decreased flow of molecules or
ions across the membrane.
Membrane Structure
• All membranes consist of a double layer of
lipid molecules in which proteins are
embedded. The lipid layer prevents the
movement of most polar or ionized molecules
through the membrane, whereas the proteins
provide pathways for the selective transfer of
these substances through the lipid barrier.
• The major membrane lipids are
PHOSPHOLIPIDS.
Cont…
• Phospholipids:
These are amphipathic molecules. By this we
imply that one end has a charged polar region,
and the remainder of the molecule consists of two
long fatty acid chains which is non polar.
The phospholipids in cell membranes are
organized into bimolecular layer with the non
polar fatty acids chains located in the middle and
the polar regions located toward the surface.
Membrane proteins
• There are basically two classes of membrane
proteins. They are; integral and peripheral
membrane proteins.
• Integral membrane proteins
– Closely associated with the membrane lipids and
cannot be extracted from the membrane without
disrupting the lipid bilayer
– They are amphipathic, thus possessing a polar and
non polar region.
Membrane Proteins
• Peripheral membrane proteins
– These proteins are not amphipathic.
– They are located at the membrane surface where
they are bound to the polar regions of the integral
membrane proteins
Cell Organelles
• The cell consists of units called organelles, these
organelles perform specific functions that aid the
survival of the cell. The cell organelles include;
nucleus, ribosome, endoplasmic reticulum, golgi
apparatus, mitochondria, lysosomes.
• Nucleus:
– The primary function of the nucleus is the storage,
transmission, and expression of genetic information
used to synthesize the proteins that determine the
structure and function of the cell.
Cell Organelles
• Ribosomes :
– These are the sites were protein molecules are
synthesized using genetic information sent by RNA
messenger molecules from DNA in the nucleus.
– These ribosomes can be either found bound to an
organelle called endoplasmic recticulum or found
free in the cytoplasm.
– Free ribosomes are released into the cytosol,
whereas those bound are transferred to the golgi
apparatus.
Cont…
• Endoplasmic Reticulum:
– Two forms can be distinguished, the rough surfaced
and smooth surfaced endoplasmic reticulum.
– The rough endoplasmic reticulum is involved in the
packaging of proteins that are to be secreted by cells.
While the smooth endoplasmic reticulum is the site at
which lipid molecules are synthesized. The SER also
stores and releases calcium ions involved in
controlling various cell activities.
Cont…
• Golgi Apparatus:
– This is a series of closely opposed, flattened
membranous sacs that are slightly curved, forming
a cup shaped structure.
– The Golgi apparatus sorts the different types of
proteins received from the rough endoplasmic
reticulum into vesicles that will be delivered to
various parts of the cell.
Cont…
• Mitochondria:
– These are primarily concerned with the chemical
processes by which energy is made available to cells in
the form of molecules of adenosine triphosphate
(ATP).
– Most of the ATP used by the cells is formed in the
mitochondria by a process that consumes oxygen and
produces carbon dioxide. Mitochondria possess small
amounts of DNA that contain the genes for the
synthesis of some of the mitochondrial proteins.
Cont…
• Lysosomes:
– These are spherical organelles that are
surrounded by a single membrane
– The fluid within a lysosome is highly acidic and
contains digestive enzymes
– They act as cellular stomachs breaking down
bacteria , they may also break down cell
organelles that have been damaged and are no
longer functioning normally.
Diagram showing an animal cell