Chapter 1Cells: discovery and
exploration
Cells are the basic function units of all living things.
Most cells are too small to be seen by the naked eye, so microscopes give enlarged images of
cells and the structures they contain.
They make it possible to examine cells in great detail.
The recognition that all living things share a common structural unit
– the cell –
provided the foundation for one of the major unifying themes in
biology.
But…how are cells made?
All living things consist of one or more organised structures that are called
cells or of products of cells.Cells are the basic functional unit of life.
Scientists Schwann and Schlieder came up with the first main theory, known as
the Cell Theory.
Several ideas were put forward as to how cells became cells.
Spontaneous generation is the idea that living things could arise from non-living or
dead matter.
Another idea was that living things developed that gathered to form a compact mass, then
becoming organised cells.
Life span of cells
The lifespan of cells in multi-cellular organisms varies greatly, even among human cells
The average life span of some human cells are:
- Stomach cells 2 days- Mature sperm cells 2-3 days- Skin cells 20-35 days- Red blood cells about 120 days
A typical plant cell
A typical animal cell
Tools for viewing cells
Generally, cells are too small to be seen with the naked eye.
Because of this, we need to use microscopes.
Microscopes can be divided into two main categories:
- Light microscopes and - Electron microscopes
Light microscopes (LMs)
- Increase the ability of the human eye to see tiny objects
- Use visible light that illuminates and passes through a specimen
The path of light in LMs
Simple light microscopes are similar to a magnifying glass.
Compound light microscopes have at least two sets of lenses.
Most have several objective lenses, each of a different magnification.
How large the object appears depends on the magnification powers of both the eyepiece and the lenses used.
Characteristics of the lenses also influence a microscope’s resolution.
Resolution is the ability to see two points that are close together as two separate points.
We use microscopes to resolve things that our eyes are unable to see.
Staining
Cells are virtually colourless, so are difficult to see under a standard LM.
Therefore, staining is usually required.Groups of cells are cut into thin slices, only a few cells
thick before staining.
These treatments are often toxic to the cells, and can often distort cell features.
Other LMs
Phase contrast LM (see Fig 1.11b)- A modified compound light microscope (CLM)- Developed to view unstained living cells- The image developed has highly contrasting light and
dark areas
Flourescence microscope (see Fig 1.12)- Another type of CLM- Uses UV light to reveal compounds that have been
stained with flourescent dyese.g. cancerous breast cells
Other LMs
Scanning confocal microscope (see Fig 1.14)- Sort of a cross between a light microscope and an
electron microscope- Uses laser light and special optics to allow a viewer
to look at successively deeper layers of an object- The viewer does not have to cut the object into thin
sections, the microscope does that- Can also produce 3D
Electron Microscopes
Transmission electron microscopes (TEM)(see Fig 1.18)- Invented in the 1930’s- A beam of electrons with a much shorter wavelength
passes through and is used to illustrate specimens- The beam is shone through the specimen,revealing great detail of the internal structure of specimens
TEMs have much greater resolving power than light microscopes.
This is because of the shorter wavelengths of electron beams.
TEMs have revealed the presence of many kinds of cell organelles and have shown the internal structure that
exists within cells.
Scanning electron microscope (SEM)(see Fig 1.19)- Released in 1965- The electrons are bounced off the surface of the
specimen, giving an extremely detailed view of the surface of a specimen.
- Depending on the size, parts of or whole organisms can be scanned
Which type of electron microscope was used to generate these images?
Recent developments - LMs
Differential interference contrast (DIC) microscopes
- Used to obtain 3D impressions of an object- Used in IVF techniques to view the process of the
sperm fertilising the egg (see FIG 1.22)
Automatic scanning of cells- Used to search many cells at a time for defective cells
Recent developments - EMs
Freeze fracture- A small block of living or dead cells are rapidly frozen
in liquid nitrogen- Such rapid freezing reduces changes to the cells, so
that when they are put into the vacuum chamber to be cut, the internal structures can easily be seen
Shadowing (see Fig 1.24)- Fractured pieces of cells are exposed to and covered
by heavy metals e.g. platinum or gold- Specimens are then dissolved away, and the metals
remain with the impressions of the cells distinguishable features
Images were taken from…
http://www.microscopeworld.com/misc/gift-ideas-131.htmhttp://bomi.ou.edu/bot1114/botany1114/elder/cells/acell2.gifhttp://sun.menloschool.org/~cweaver/cells/plantcell/url?q=http://www.seallabs. com/graphics/ant.jpgwww.lps.u-psud.fr/.../ Image/Materiel/Orsay1.jpghttp://www.cas.muohio.edu/~mbi-ws/microscopes/images/
LightDiagram.GIFhttp://www.columbia.edu/cu/record/archives/vol21/vol21_iss6/
record2106.32c.gifhttp://history.nih.gov/exhibits/genetics/images/sect2/9b.jpg
Reference for text…
Kinnear J and Martin M. 2006. Nature of Biology, Book 1, Third Edition. Jacaranda, Q.L.D.