ocr biology y1: microscopy

what is cell theory?

• cells are the most basic unit of all life and make up animals and plants

• new cells only develop from existing cells

what does the condenser lens of a microscope do?

focuses the light directly on the specimen

what does the objective and eyepiece lens do?

• produce a more magnified image

• reduce chromatic aberration/purple fringing (colour wavelengths becoming focused at different positions)

what is magnification?

the degree to which the size of an image is bigger than the actual object

what is resolution?

the shortest distance between two structures in a specimen where they can still be seen as distinct objects

how do you prepare a dry mount?

place a thin slice of a dry sample or the whole dry sample onto a slide and cover it with a cover slip

how do you prepare a wet mount?

• use a pipette to drop liquid (which needs to be of a similar refractive index as glass so that light does not refract or diffract) on a slide

• use forceps to place a very thin slice (to let light pass through) of the specimen onto the liquid

• add a few drops of stain to increase contrast and allow you to tell the components apart

• slowly lower the cover slip over the sample at an angle to push out air bubbles

how do you prepare a squash slide?

• same way as a wet mount, but the sample is squashed by the cover slip (using a lens tissue)

• E.g. root tips for cell division

how do you prepare a smear slide?

• smear the sample across the slide using a slide’s edge at an angle, then lower the cover slip slowly at an angle

• E.g. blood

what is gram staining?

• a differential staining technique (a way of staining that involves multiple colours) to tell gram-negative and gram-positive bacteria apart

• add crystal violet stain

• add iodine to fix the dye

• add ethyl alcohol to wash away the dye

• add safranin to stain the cell membrane

• gram-positive bacteria will turn violet, as the crystal violet dye has stained the thick peptidoglycan cell wall

• gram-negative bacteria will turn pink, as the cell wall is too thin to take up the violet dye

• This test is important for prescribing the correct antibiotics

what is the acid-fast technique?

a differential staining technique done to identify mycobacteria like tuberculosis (mycobacteria are acid-fast and will turn red instead of blue)

how do transmission electron microscopes work?

a beam of electrons is transmitted through a specimen and focused to produce an image (similar to light microscopy)

how do scanning electron microscopes work?

a beam of electrons is sent to the surface of a specimen before reflecting back to be collected

what are the features of TEM microscopy?

• 2D

• very high resolution

• highest magnification

• specimen must be dead/non-living as it is in a vacuum

• complex preparation process using harsh chemicals results in artefacts (distortions, fingerprints, air bubbles, etc)

what are the features of SEM microscopy?

• 3D

• low resolution (but higher than light microscopy)

• higher magnification than light microscopy but lower than TEM

• specimen must be dead/non-living as it is in a vacuum

• complex preparation process using harsh chemicals results in artefacts (distortions, fingerprints, air bubbles, etc)

what are the features of laser scanning confocal microscopy?

• type of light microscopy

• uses a focused laser beam to scan specimens that have been dyed with fluorescent dye (so that they give off light)

• 2D (but a computer can create a 3D image out of them by putting together many 2D images from different angles)

• higher resolution + magnification than normal light microscopy (but lower than electron microscopy)