3.1 AS Unit F211: Cells, Exchange and Transport The cell is the basic unit of all living things. How to use a light microscope Why electron microscopes are so important in biology. Details of cell structure and ultra-structure The roles of cells and organelles.

3.1 AS Unit F211: Cells, Exchange and Transport Candidates should be able to: state the resolution and magnification that can be achieved by: 1. a light microscope 2.a transmission electron microscope 3.a scanning electron microscope

MAGNIFICATION AND RESOLUTION Cells are too small to be seen with the naked eye. The light microscope was developed to produce enlarged and more detailed images of cells. The magnification of an image is how much bigger it appears under the microscope than it is in real life, and is worked out using the following formula: magnification = image size ÷ actual size

MAGNIFICATION AND RESOLUTION Magnification on its own does not increase the level of detail seen, it just increases the size. Resolution refers to the ability to see two distinct points separately.

MAGNIFICATION AND RESOLUTION If the resolution of a light microscope is 200nm (0.2μm), this means it can see any two different points as separate objects if they are 200nm apart or more. If they are any closer than this amount, they appear as one object

THE LIGHT MICROSCOPE Light microscopes use a number of lenses to produce an image that can be viewed directly at the eyepiece. Light passes from a bulb under the stage, through a condenser lens and then through the specimen. This beam of light is passed through an objective lens and then the eyepiece lens.

THE LIGHT MICROSCOPE

The light microscope usually has a number of objective lenses which can be rotated into position; a x10 lens will magnify the image 10 times. The eyepiece lens then magnifies the image again by x10. So using these two lenses the final magnification the microscope is capable of producing is: (which) x40 x100 x400 x1000

THE LIGHT MICROSCOPE Using the light microscope some specimens may be seen directly, live specimens may be seen. Some tissues may need to be stained to add colour to aid seeing detail. The more preparation a specimen needs the more it may be distorted

THE ELECTRON MICROSCOPE Electron microscopes can achieve higher resolutions. Electron microscopes generate a beam of electrons, which have a wavelength of 0.004nm. They can distinguish objects 0.2nm apart

THE ELECTRON MICROSCOPE Transmission Electron Microscope (TEM) The electron beam passes through a very thin sample to which heavy metals have been added (e.g. Osmium tetroxide). The electrons pass through dense, metal, parts less easily than to the sections on which the metal is absent. This gives a ‘shadowing effect’ and produces contrast in the final 2D image produced. Maximum possible magnification of x500,000

THE ELECTRON MICROSCOPE

Scanning Electron Microscope (SEM) The electron beam is directed onto a sample which has been covered with heavy metal. The electrons don’t pass through the specimen, they bounce off. This produces a final 3D image view of the surface of the sample. Maximum possible magnification of x100,000

THE ELECTRON MICROSCOPE