2. THE CELL
When we learnt about hierarchy in Biology, we
found out that cells are the simplest collection of matter
that can live
For this reason, Cells are often defined as the smallest
units that exhibit life
They are as important to Biology as atoms are to Chemistry
The Cell Theory is credited to Schleiden and
Schwann and of course Virchow and it states that;
- all organisms are made of cells
- cells are the smallest units of living organisms
- new cells come from pre-existing cells by cell division
3. Characteristics of cells……..
Cells are very small and they cannot be seen
with the naked eye
- so, magnification is absolutely necessary
A microscope is a tool of choice in this regard.
it enables us to study the structure and the functions of
cells
Microscopic examination of objects exploits three
factors;
- magnification
- resolution
- contrast
4. Magnification;
- the ratio between the size of an image produced by a
microscope and the actual size of that image
Resolution
- ability to observe two adjacent objects as
separate objects
- different microscopes have different resolving
ability
- a light microscope has a low resolving
power (0.2mm) than an electron microscope
(0.5nm)
5. Contrast
-ability to see an object of interest as being
different and separate from any other adjacent
object
- this depends on the properties of the
object being viewed
- contrast can be improved by specifically staining
the object of interest so that it can be easily viewed
under a microscope
Much this content has been dealt with during your last
Practical class
6. The need for microscopy is appreciated but;
why are cells small in size???
7. high surface area to volume ratio is necessary
for efficient functioning of cells
- the volume of a cell determines the amount of
chemical activity it carries per unit of time
- The surface area of a cell determines the amount
substances the cell can exchange with the outside
environment
- Organisms grow and their demands for exchange
of substances with the external environment grows
at the same time.
Efficient functioning needed for this is met through
the small size of cells (with their typical high
surface area to volume ratio)
Organisms get big but the cells remain small in size!
8. More about cell volume and shape!
If the red blood cell was not small in size and did not have
a concave shape, would it be as suited for its function as it
is?.........
So, cells have small size and they can have different shapes
9. Other characteristics of cells….
Cells are surrounded by a plasma membrane
This is separates the cell from its immediate
environment
The plasma membrane is made of phospholipids
with proteins and other molecules embedded
within them
Please read page 72 of your textbooks for further
Details about plasma membranes
A detailed coverage of the subject matter about
membranes shall come under Bio 211 in semester 3
11. Cell types……..
At a cellular level, Biologists describe cells as
prokaryotic or eukaryotic (See page 72 of your
textbooks)
-these have similarities and differences
The prokaryotic cell
- small in size
- Found in unicellular organisms
- Cells surrounded by a cell membrane and some
also have a rigid cell wall
- Membrane encloses cytoplasm
- Cytoplasm contains dissolved substances
NB: Cytoplasm is not divided into
compartments
12. - DNA and ribosomes exist free in the medium
- Cell surface of some prokaryotes have pili (for
attachment) and flagella for swimming
The Eukaryotic cell
-Larger than prokaryotic cell
- Found in multicellular organisms:
Plants and animals as well as simple
multicellular organisms (protists, fungi and
plants and animals)
- Cells are divided into membrane-bound
compartments called organelles
- some organelles have comparable size to
prokaryotic cells
13. Cell organelles
- organelles have double membranes and some
contain DNA and the means to use it
- organelles perform specialist functions
- eukarytotic cells mainly store genetic information
(DNA) in the nucleus
We can study cell organelles by microscopy or
chemical means
- microscopy reveals that the shape of organelles is
identical between different organisms
- this technique also reveals that animals cells do not
have a cell wall but plant cells have it
14.
15. Please refer to figure 4.7 in your textbooks for
further details about animal and plant cells.
Overall, Chapter 4 in your texbooks covers the
content of this lecture very well and even
beyond our present needs.
Take advantage!
