Prokaryotic and Eukaryotic Cells - animals and plants are only two of the Kingdoms of living things. Biologists recognise at least five different Kingdoms in the living world. More importantly, the five Kingdoms are divided into two fundamental groupings according to the type of nucleus the cells of their members have. We call the two groupings the prokaryotes and eukaryotes. The eukaryotes are animals, plants, fungi and protoctist. Several individual chromosomes are found within the nucleus, which is a relatively large spherical sac, bound by a nuclear membrane. The prokaryotes are bacteria and cyanobacteria (photosynthetic bacteria). The cells of the prokaryotes have no true nucleus, but contain a single circular strand of DNA in the cytoplasm.
Microscopy – there are two main types of microscopes, optical and electron. Optical (light) microscopes are also known as compound microscopes because two lenses (objective and eyepiece) are combined to produce a much greater magnification. The best light microscopes can magnify up to x1400 – 1500. Light microscopes are limited by their resolving power/resolution. Resolution is the ability to distinguish between objects close together. The wavelength of ordinary light is 2µm, which means that a light microscope can only distinguish between objects which are 2µm further apart from each other. Less than this and the object will appear as one when observed using a light microscope.
Light microscopes do however have a number of advantages –
Specimens can be living or dead
Specimens are not usually distorted
Specimen preparation is quick and simple
Natural colour is observed
Cheap and portable
Electron microscopy uses waves of electrons instead of light. The wavelength of electrons is 1nm so the resolving power of these microscopes is much superior to light microscopes. (E.M. resolution = 1nm) The magnification available is x500,000. Magnets are used to focus the beams of electrons. The main disadvantage associated with the use of the EM is due to the fact that the electron beam must be used in a vacuum to avoid the electrons being scattered by air particles. Specimens for EM must therefore be prepared (dehydrated and hence killed and then fixed) so that they retain their structure in a vacuum. Such harsh preparation methods can disrupt the cells and cause artefacts – features that do not naturally occur, appear. There are two types of microscopy – scanning and transmission electron microscopy. A transmission EM passes beams of electrons through a very thin object so you can see detail within a cell. A scanning EM bounces beams of electrons off the surface of an object so you observe a 3D appearance.
Electron Microscopy limitations –
Dehydration of the species causes distortion
Cannot examine living or wet specimens
Electrons cannot be directly viewed / must be projected onto a fluorescent or permanent record made on a photographic plate
From a thin specimen it is difficult to interpret cellular structures
No colour imaging
Pre-treatment may often result in artefacts / distorted structures
1m = 100cm = 1000mm = 1x106um = 1x109nm
1. What is 2.5cm in um?
2. What is 0.3un in nm?
3. An object has a size of 0.5cm in a photograph, and the magnification is 25,000. What is its real life size in um and nm?
4. The scale shown with an EM photograph shows that 1.5cm is equivalent to 0.1um. What is the magnification used in the photograph?
Comparison of Plant and Animal cells – Plant cells are protoplasts bordered by an extracellular cellulose cell wall and possess a vacuole and chloroplasts. Animal cells lack chloroplasts. In contrast to plant cells, animal cells possess centrioles.
Cell Ultrastructure –
1. Cell Surface Membrane – also known as the plasma membrane. It serves as a boundary between the cell and its environment. It consists of phospholipid and protein molecules. The arrangement of these within the membrane is known as the FLUID –